Here is a compilation of essays on ‘Sedimentary Rocks’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short essays on ‘Sedimentary Rocks’ especially written for school and college students.
Essay on Sedimentary Rocks
- Essay on the Pre-Cambrian Rocks
- Essay on Palaeozoic or Primary Rocks
- Essay on the Secondary or Mesozoic Rocks
- Essay on the Tertiary or Cainozoic Rocks
1. Essay on the Pre-Cambrian Rocks:
The oldest or Pre-Cambrian group has so far yielded in our islands only obscure traces of fossils, and so we are at present ignorant as to what kind of animals or plants lived during the accumulation of its deposits, though we feel sure that some life was in existence during their formation.
As these oldest rocks have been covered, often to many thousands of feet, by newer ones, it is only after extensive earth-movements that they have been raised above the present sea-level. They have been subjected, when deeply buried, in very many cases to great heat, and in consequence they have been then completely altered from their original condition, the actual, minerals of which they have been formed having disappeared and new minerals having crystallized in their places.
Wherever such complete alteration has occurred all traces of animal and vegetable remains must have disappeared. Sometimes, indeed, we are ignorant as to whether the original rocks were sediments laid down under water or lavas poured out from volcanoes or molten rocks squeezed up from below which never reached the earth’s surface.
Such rocks are to be seen in the Malverns, Carnarvonshire, and Cornwall, while they cover large areas in the north and west of Scotland and the north and west of Ireland.
They frequently show a banded structure and are chiefly formed of the minerals quartz, felspar, and either mica or hornblende, and it is usually the occurrence of the mica or hornblende which gives them their striped appearance. They are called Gneisses or Crystalline Schists.
Besides these crystalline schists we find in several places a group of Pre-Cambrian rocks which were of volcanic origin; this is to be seen in Pembrokeshire, Shropshire, Warwickshire, and Leicestershire.
In Scotland, on the west coast near Loch Torridon and Loch Maree, there is a true sedimentary deposit of Pre- Cambrian age resting on the earlier Pre-Cambrian gneisses. This is a red sandstone which has so far yielded no recognisable fossils, as the remains of plants or animals in rocks are called. From the locality, where it is well exposed, this bed is called the Torridon Sandstone, but it can also be seen in the Island of Skye and elsewhere.
At the base of the Torridon Sandstone are found pebbles of the older gneisses and schists. Now these latter rocks were turned into gneisses and schists by pressure and heat, and the presence of fragments of them in the Torridon Sandstone proves that this conversion had taken place before the sea of Torridonian time had got to work on the gneisses and schists, which must have been therefore upraised to the surface in order that the sea might attack them, break off fragments, roll them round and round) and finally embed them in the sandy deposit it was forming.
Hence our island’s geology we are forced to look back to a far remoter time. It seems probable that beds were formed below a far earlier sea, into those beds molten rock was squeezed from below, and the whole mass sunk and was buried by later deposits.
Then great earth – movements occurred, the beds were subjected to such enormous pressure and heat that their mineral structure was entirely altered, and after that they were raised up above the sea-level, the superincumbent rocks were removed by denudation, and then the sea got to work at the altered rocks below, broke them up and washed their fragments to and fro below its waters.
Not till this last phase occurred did the Torridon Sandstone begin to form, and we must draw the conclusion that enormous ages must have passed between the formation of the earlier Pre-Cambrian schists and the later Pre-Cambrian sandstone.
The Torridon Sandstone:
When this very ancient sandstone is carefully investigated, we find that it was formed just like any sandstone of to-day. It shows over considerable areas layers of deposits differing in small characteristics just like the sandstones of later times. One layer may differ in colour from another, or one layer may be coarser grained than another, and so evidence of what is called Stratification is given.
Again where currents have been strong the sand beneath sea-water is washed now in this direction, now in that. Here it accumulates in a heap, there it is scooped out into a hollow, and a change in the current may sweep away the top of a heap and deposit another set of layers of sand, which do not conform in their arrangement to the layers in the lower part of the original heap. This gives rise to what the geologist calls Current-bedding.
Now current-bedding is well seen in the Torridon Sandstone, and gives us evidence of sea-currents in those far-distant times. At the end of Pre-Cambrian times there was a general upheaval of the British area, and what has been termed the First Continental Period began. How long it lasted we cannot tell, what kind of life existed on its surface no one knows, but when submergence at length took place we find in the shallow-water beds of the succeeding period remains of living things.
The upraising of the Pre-Cambrian rocks and the following submergence produced some alteration in their relative levels, and so when the next set of beds, the Cambrian, came to be deposited on the older rocks they did not always lie on the same layer of Pre -Cambrian rock but crossed from one to the other. This is what is generally meant by Unconformity, and its presence is always a proof that a lapse of time occurred between the deposition of the upper rock and that of the rocks it rests on.
In some places, as in the west of Ireland, no Cambrian rocks are found resting on the Pre-Cambrians, and it is a matter for discussion whether the Cambrians were never laid down there because that particular tract of Pre-Cambrians never went under the sea, or whether after the Cambrians were laid down they were in subsequent ages removed.
2. Essay on Palaeozoic or Primary Rocks:
It is in the earliest Palaeozoic rocks, those of the Cambrian system that we find in our islands the first traces of living creatures. The Cambrian beds, when seen resting on the earlier Pre-Cambrian rocks, are always separated from them by a pronounced unconformity.
That is to say, the Pre-Cambrians were raised up from their original position of formation, planed off by denuding forces, and then lowered down and covered by the succeeding Cambrians. So the lowest Cambrian rocks are shallow- water deposits of the nature of conglomerates or coarse hardened sandstones called grits derived by denudation from the older rocks.
The land of early Cambrian times probably occupied part of the area of the present Atlantic and stretched eastwards as far as the west of Ireland, but the east of Ireland was beneath the Cambrian sea, for we find sandstones and grits of this early age near Bray to the south of Dublin, and on the opposite coast of the Irish Sea a similar type of rock is to be seen forming the high ground above Harlech.
The Cambrian rocks of this Welsh area have been bent into a huge arch called the Harlech Anticline, the word anticline being the geological expression for a fold in the form of an arch.
The Cambrian Beds:
Besides sandstones and grits there occur in the Harlech anticline slates, which were, when they were first laid down, muds.
Farther north these lowest Cambrian beds rise up again horn beneath a covering of newer rocks and strata of the nature of slates are seen at Llanberis and are worked in many quarries, of which the Penrhyn quarries are the most famous. These slates rest on sandstones and conglomeratic beds. In Pembrokeshire the St. David’s area gives a very similar picture of early Cambrian times, conglomerates and sandstones followed by beds which were originally muds covering Pre-Cambrian rocks.
Hence we may look on the area comprised by the east of Ireland, the Irish Sea, and West Wales as being in early Cambrian times part of a shallow sea into which sediment was brought by rivers flowing from land which then existed to the west; but as time went on the sea-floor sunk and as the land’s edge got farther off, mud was deposited in a deeper sea over the coarser deposits of a period in which the sea was shallower.
For many miles to the east the Cambrian beds are covered by later deposits, but they are seen again in the Malvern Hills and in Shropshire, and again in a few scattered localities in the Midlands, such as near Nuneaton.
Speaking generally, a sandstone is seen at their base, and this is covered by muds.
Over these Lower Cambrians come flaggy beds and slates seen in North and South Wales and in the Lake district, while their equivalents in the border counties of Wales and in the English Midland district are shales, that is to say, fine-grained somewhat sandy muds, compacted by pressure and still splitting along the original planes of deposition.
Since there is a very similar distribution of Cambrian rocks in America stretching westwards, the shallow-water Cambrian beds and the Pre-Cambrian rocks being found along the East American coast, it is possible that in Cambrian times a continent stretched over the present North Atlantic, and that the denudation of this continent furnished the conglomerates, grits, and muds of East Ireland and Wales on the east and of East Canada on the west, while farther off were formed the deep-sea deposits.
In the north-west of Scotland Cambrian beds consisting of sandstones and muds are also found, and tell much the same story as the English and Welsh deposits.
As has been mentioned above, the early Cambrian rocks give us the first glimpse into the life which has existed in past times, and some of the Cambrian deposits are full of fossils.
The Life of Cambrian Times:
The life of this early period was not nearly so advanced as the life of to-day. There were none of the Mammalia which take so striking a part in the life of the world now, no birds, no reptiles, no fish. There were shell-fish, corals, and worms. The shell-fish were either those possessing one shell and called Gastropods, or those possessing two and called Bivalves in consequence.
Of all the teeming life of the period only one of the kinds of animals has any representative living now.’ In Cambrian times a bivalve, named Lingula, is found frequently. So frequently indeed in one set of beds that they are therefore called the Lingula flags, and in the warmer waters of the Pacific a species of Lingula still lives.
Even the kind of animal of Cambrian times is often utterly unlike any that we see living today. The two most important to the geologist are the Trilobite and the Graptolite. The nearest modern representative of the former is considered by some to be the king crab, but there is nothing living one can compare to the graptolite.
Their great use lies in the fact that they tend to exist in certain beds and then die out. Thus a trilobite called Olenellus lived in early Cambrian times, later on one called Paradoxides, and later still one called Olenus.
Consequently when new Cambrian beds are discovered to contain fossils, as has recently been the case with the Nuneaton shales, when Olenellus is found in the lowest shales and Paradoxides in the higher ones, we can at once say what beds in Wales correspond to each of these sets of shales in age.
The Cambrian beds of North Wales are perhaps best seen between Barmouth and Portmadoc. The lowest sandstones are seen in the hill country to the east, and as this deposit dips northward, the higher beds of slate come on and are to be seen to the south of Maentwrog. The upper Cambrian beds, the Lingula flags and overlying rocks, are well seen round Tremadoc.
After Cambrian times a further submergence took place, and the sea of Ordovician times spread over more of the land to the west, and as it crept westwards and pounded at the cliffs it built up deposits of coarse conglomerates and coarse sandstones, while occasional mud layers were thrown down in which numerous graptolites are now found preserved. Limestones were produced in parts where there was clear water, and this West Irish district was also the scene of intense volcanic activity.
In Great Britain and the east of Ireland, since the shoreline was too far away, we find no coarse conglomerates and the sediment laid down gave rise usually to coarse sandstones and to shales, while limestones also occur, but not frequently. Volcanoes built up deposits of great thickness over West Wales, Cumberland and Westmorland, and South Scotland, while in Dumfriesshire we find deep muds.
The life of the Ordovician period is of a very similar type to that of the Cambrian. Trilobites and graptolites still form the most useful fossils, the graptolites in particular occurring in extraordinary abundance. Abereiddy Bay, to the north of St. David’s, is a very famous place for collecting Ordovician graptolites.
The limestone beds where they occur, as in the Bala district of North Wales, the Keisley district of Yorkshire, the Coniston district of Westmorland, and the Kildare and the Portraine districts in Ireland, contain abundant corals, bivalves, and trilobites.
The Ordovician volcanic beds are so hard that they have given rise to some of our most rugged mountain scenery.
Thus Snowdon and Cader Idris in Wales and much of our Lake district mountainous tract are formed out of Ordo- vician rocks of volcanic origin.
Following on the Ordovician beds come the Silurian strata. They form a regular continuation of the Ordo-vicians, but in parts of Wales and along the Welsh border there were local interruptions of deposition.
The Silurian Period:
In one respect the Silurian times were markedly different from those of Ordovician age, for there was an almost complete cessation from volcanic outbursts. Only in Kerry, in Pembrokeshire, and near Bristol is there any evidence of Silurian lavas and ashes; everywhere else sedimentation went on quietly.
The Silurian rocks are seen in a map to form a huge curve stretching from Carmarthenshire through Central Wales to Denbighshire, while they are also exposed in Monmouthshire and along a line stretching north from Tortworth, in Gloucestershire, a little to the east of the Severn, through May Hill and the Malverns to the Wenlock district in Shropshire.
They also occur in the Lake district and from the southern uplands of Scotland. No doubt they were laid down over much of Central and East England, but at present they are covered by later deposits.
The Silurian beds are usually of the nature of sandstones and shales, though limestones are found and also mud deposits.
In the Mendips and in the Tortworth areas lavas and volcanic ashes are found associated with sandstones and limestones.
In Monmouthshire the Usk district shows the upper two divisions of the Silurian beds which are chiefly sandy shales and sandstones. One band of limestone is found, called the Wenlock Limestone, which consists chiefly of fragments of the stem of an animal called an Encrinite—a so-called sea-lily.
The stem consisted of a number of plates shaped like circular discs; when the animal died the discs fell apart and formed thick deposits. Amongst these are found numerous fossils, but most of them are of a fragmentary nature and were probably broken by wave- action.
In the May Hill area all three divisions of the Silurian are seen, a coarse and a fine sandstone with accompanying shales at the base, and above it sandstones and shales with a marked limestone band just as in the Usk district, only the limestone is not made up of encrinite fragments, but is full of large and small corals. In fact we have here the relics of a coral reef. Other fossils are abundant, trilobites and bivalves being found.
The same types of Silurian deposits are seen at Malvern and in the Woolhope and the Wenlock areas. But in Mid Wales, North Wales, and in the Lake district we come across beds which were muds, and have given rise to mudstones and slates. These yield copious remains of graptolites, the trilobites, and bivalves not being so numerous as in the other type of deposit.
In South Scotland the Silurian beds are well seen at Girvan on the Ayrshire coast and near Moffat in Dumfriesshire. One of the interesting points about the fauna of Silurian times is that in our highest Silurian beds, the upper beds of the Ludlow age, remains of fish occur for the first time in the history of our islands. This bed, called the Ludlow Bone Bed, has been found near the town of Ludlow, and in Shropshire and elsewhere it is full of small scales, spines, and teeth of fish.
At the close of Silurian times we come to an epoch of elevation and mountain building. To the south of England open sea continued to exist over France, and the northern portion of it extended over Devonshire and Cornwall. In consequence the marine sandstones, shales, and limestones of this age found in this country are called Devonian. Very probably a long land-locked arm of this sea extended westwards over the south of Ireland, where coarse grits of great thickness are found.
Over South and Mid Wales, however, we find a very different type of deposit, a red sandstone, which was deposited in a large lake cut off by a narrow barrier from the southern sea and hemmed in to the north by land which stretched through Central Ireland and the English Midlands and extended up into Scotland. Over a great part of Scotland and the north of Ireland another great lake extended, but seems to have had no connection with the southern lake.
The deposits formed under the waters of these huge lakes are chiefly of the nature of sandstones, though occasionally somewhat sandy muds have given rise to beds called marls.
As the prevailing colour of these beds is a deep red, the name Old Red Sandstone has been given to them. At the same time there are districts, such as Fifeshire, where beds of other colours are found.
The marine beds of Devon are very fossiliferous and show a fauna intermediate in character between that of the preceding Silurian and of the succeeding Carboniferous periods. The Old Red Sandstone, however, is very un- fossiliferous, the waters of the great lakes proving very unproductive of living things. Whether they grew too salt, or whether too full of iron, we find that after extensive searches for fossil remains only a few fish teeth and scales have, as a rule, rewarded the searchers.
It is true that the Old Red Sandstone is at times a coarse conglomerate made up of well-rolled fragments of rock, amongst which it is almost certain that shells and fish remains would get smashed to atoms. But there are thousands of feet of fine-grained sandstones in which one would think traces of former animals would easily be preserved.
The more northerly of the two lakes has in a few places yielded a number of fossil fishes, and in the hunt for these several noted Scotch geologists have made their names. Perhaps the most famous was Hugh Miller, who in 1841 published a book on the Old Red Sandstone.
A famous spot for Old Red Sandstone fishes is Dura Den, near Cupar in Fife. Here there is a layer in a yellow sandstone simply crowded with fish remains. These occur not as scattered fragments and scales, but the whole bodies with the heads and fins attached are of common occurrence.
Nor are we only able to get a peep into the fish life of the great lakes and the multitudinous fauna of the southern sea, but we can tell something of the life which went on the extensive land tracts which separated the sea from the lakes and the lakes from one another.
There are places where the vegetation which grew on the land, after being washed down into the waters of the Old Red Sandstone times, sank, and was afterwards buried by more sand accumulations. So abundant are these remains in some places that thin seams of coal have been formed. The types of plant life are numerous, the ferns being particularly abundant, and the most ancient remains of insects are found in American beds of this age.
The Old Red Sandstone beds are of enormous thickness in places, thus in the west of England there are some 10,000 feet of the deposit. It produces excellent pasturage, and can often be used as a first-rate building stone.
The Carboniferous Period:
After Old Red Sandstone times the main mass of the country once more sank beneath the sea, and over most of England and Wales a marine deposit was formed on the Old Red Sandstone, and on the older rocks which separated the basins in which that sandstone had formed.
Thus in Gloucestershire the beds of the succeeding Carboniferous series rest conformably, i.e. without any interruption of deposition, on the Old Red deposits, while in Yorkshire they rest on Silurian and older beds, thus giving an excellent instance of what is called in geology an unconformity.
The conformable passage of the Old Red Sandstone up into the Carboniferous beds is well seen in the sides of the Avon gorge. The passage beds between the rocks of the two systems are sandy beds of various colours, which pass up into shales, and these are covered by a massive grey limestone, some 2000 feet thick, called the Carboniferous Limestone.
In Yorkshire the Silurian and older beds are covered by a conglomeratic layer and by sandy beds, and these by the Carboniferous limestone, which in Derbyshire reaches some 4000 feet in thickness.
The important Carboniferous limestone is found over extensive tracts in South Wales; it rims the South Wales coalfield and forms much of the cliff overlooking the Bristol Channel; it rims the Forest of Dean Coalfield, forming the high ground at Mitcheldean and elsewhere. To the south of the Bristol Channel it forms the hills above Weston, and runs south-eastwards along the Mendips, while it occurs round the Bristol Coalfield, though it is in parts hidden by newer deposits.
But it is in the north of England that it occurs most extensively. Here it forms the southern end of the Pennine Chain, extending north from near Ashbourne to Buxton, and extends from the Ribble valley in Yorkshire northwards to Northumberland.
It will be noticed that wherever this bed occurs there we find high ground, thus the Mendips, the Pennine Chain, and some of the hills in Denbigh and Flint all owe their prominence to Carboniferous limestone.
But when we trace this deposit northwards, we find in Scotland that it passes laterally into freshwater deposits. Much of Scotland, therefore, which had been land during the preceding age continued to form the land which fringed the northern boundaries of the Carboniferous limestone sea.
The purity of the limestone makes it very valuable as a rock which can be burnt for lime. It is thought that much of it was deposited in vast lagoons into which no sediment was brought.
The life whose remains are found fossilised in this limestone is very different from the life of the previous ages, gone are the graptolites so common up to the end of Silurian times, while the trilobites only exist as a very few genera, and are never found in beds of a later date. Corals are abundant and built up numerous and extensive reefs, bivalves in plenty occur, and fishes are in places abundant, but no further advance in the types of beings seems to have been made.
Probably at different times various parts of the Carboniferous limestone sea became invaded by a coarse sandy shallow-water deposit called the Millstone Grit. Its most extensive development is in Yorkshire and Derbyshire, where it forms the backbone of those counties and covers the limestone which emerges from beneath it near Buxton on the south and near Skipton on the north.
It forms the well-known Yorkshire moors, and on it comes the still shallower water deposits of the coal-measures. These contain not only marine beds, but estuarine deposits as well.
Besides being found to such a wide extent in England, the Carboniferous beds are found in Ireland, and there cover almost all the central Irish plain. At the close of the Carboniferous period a very general upward movement of our islands took place and brought on Continental conditions.
Though in Ireland this uplift took place without any pronounced folding, for there the Carboniferous rocks are either horizontal or only bent into gentle undulations, this was far from being the case in England and Wales.
The rocks of the north of England were bent into a gigantic arch, whose axis ran north and south, and has given us the Pennine Chain, while the rocks of South Wales and South England were bent into a series of troughs and arches of which the largest was the trough in which the coalfields of South Wales and the Forest of Dean now lie, while the Mendips form the southern edge of another trough.
How long this Continental period continued we do not know, but it must have been for an enormous length of time; the coal measures were denuded away from the top of the arch in the north of England, and also a great deal of the millstone grit, so that the next deposit, called the Permian was laid down some times on the coal-measures, sometimes on the millstone grit, and sometimes on the Carboniferous limestone.
The Permian Rocks:
The Permian beds are so called because they are well developed in the district of Perm in Russia. In Britain they are best developed on the two sides of the great arch of Carboniferous beds in the north.
Near Penrith and in the Eden valley the rocks of this period are red sandstones, shales, and a limestone called the Magnesian Limestone, and similar rocks are seen on the eastern side of the Pennines from the mouth of the Tyne to as far south as Nottingham.
Beds of this age occur extensively in the English Midlands, and stretch up the borders of Wales to Liverpool and farther north, while some of the red beds of Devon are considered to be of the Permian period. The fossils of our Permian rocks are not very numerous, and as they show greater affinities to those of the preceding than to those of the succeeding rocks, the Permian beds are usually classed with the Palaeozoic rocks.
Perhaps the greatest point of interest about the Permian animals lies in the fact that the first fossil reptiles are found amongst them.
On reviewing the rocks of the Pre-Cambrian and Palaeozoic periods it is obvious that the types of deposits are strictly comparable with present day accumulations. The vast bulk of the rocks are conglomerates, sands, and muds, but limestones occur, and amongst these various rocks come lavas and volcanic ashes.
Throughout the enormous period of time which elapsed during all this deposition of rock material, the earth’s crust was not always at a constant level. At times it sank in some places, at times it rose, and at times it remained stationary. Large tracts of it when they rose maintained their general horizontality, while equally large tracts show much folding.
While the sea-waters teemed with marine life, and absence of fossils in the lacustrine beds seems to show how difficult it was for life to exist in their waters, we have evidence that the land-surfaces were covered by immense tracts of vegetation. But vegetation is, as a rule, throughout all geological periods, not preserved in surface deposits, but in deposits beneath water to which the plants were washed.
Naturally when a land-surface sinks below the sea-level, the waves and currents will find the’ loose surface-deposits an easy prey. It will be only under rather special circumstances that such beds will be preserved. But if the general type of sedimentation is just that of to-day, the same is by no means true of the life of these early days.
In the Pre-Cambrian beds we have as yet no good information as to the life which then existed, but as the Cambrian life is so abundant, so varied, and so highly organised, we cannot doubt that there was a preceding fauna, which may have stretched back to the times during which some Pre-Cambrian beds were accumulated.
Hence there is still hope for the geologist who attacks such sedimentary pre-Cambrians as the-Torridon sandstone, that he may become the discoverer of types of life which will be new to geology.
At the same time we must remember that the earliest living beings may have been mere animated jelly with no inside skeleton and no outside shell to remain fossilised in after ages.
During the Palaeozoic period we see that there was a constant rise in the type of life. The lower types continue to exist right through the Palaeozoic beds, but whereas at first no fishes were in existence they are found in Britain in Silurian times, and whereas at first no reptiles occurred, in the Permian times they inhabited the world.
So far, however, there was no conquest of the air, and no land animals of the usual types that are found now. The Carboniferous forests resounded with the hum of insect life and in their waters were fish, but none of the other types of life we are accustomed to associate with forests were then in existence.
Even the trees were of- a very different kind to those of to-day in, the northern hemisphere, though the Australian vegetation is in some ways not unlike that of the Carboniferous forests. In England one finds in marshes and damp places a small plant called the horse-tail, which seldom grows to a height of over a foot and a half, and is, when full grown, about half an inch thick at its base. In Carboniferous times there were huge trees of very much the same type of plant as our horse-tail of to-day.
So we find on careful examination that though some types of life died out in the Palaeozoic rocks altogether, we can discern amongst the living things of our present world direct descendants of that early life.
Since these early rocks have been covered, often to a very great depth, by succeeding deposits, they have frequently become compacted by pressure and by infiltration of material which has acted as a cement. They therefore have become hardened during the course of ages, and where they are exposed they tend to form, in consequence, hilly country.
Very nearly all Ireland and Scotland and Wales are formed of them, while the English hill – masses of the Pennines, the Lake district, the Welsh borders, and of the Midlands are all formed of these early rocks.
Since they have often sunk to great depths and been covered by a thick blanket of newer deposits, they have in several places been much altered by heat, and they have been subjected to all the earth movements which have affected our island regions. Hence they frequently show mineralogical change where they have been subjected to intense pressures, or to the heat Which has come from a later injection of molten rock.
And they often show much folding and faulting, and in regions where these have occurred it is only by careful fossil collection that the age of the various beds can be determined, and by comparison of the rocks of a disturbed area with those of a region less affected by earth movements.
3. Essay on the Secondary or Mesozoic Rocks:
The secondary or mesozoic rocks are divided into the following systems:
The conditions under which the earliest secondary rocks, called the Trias, were laid down were very similar to those which obtained during the Permian period, but the Continental nature of the deposits is even more marked.
There must have been an arid desert over most of our islands, and the hillsides were covered by rock fragments detached by the effect of the sun’s heat, which caused the rocks to expand and crack just as they do in desert countries to-day.
When torrential rains did come these fragments were swept down on to lower ground or into any fissures which there happened to be in the bare rocks of the country.
In this way was formed the so-called Dolomitic conglomerate which rests on the Primary rocks near Bristol. This is an accumulation of blocks of Old Red Sandstone or of Carboniferous limestone or of Millstone grit, and in many cases it is found filling up fissures in the Carboniferous limestone near Bristol, and on the south Welsh coast near Tenby, the blocks being embedded in a red quartzose sand.
Higher up in the Trias comes a thick bed of well-rolled pebbles of a rock called quartzite, which were no doubt derived from a hardened sandstone of Primary age, probably in Brittany. This bed is well seen in the cliffs at Budleigh Salterton in the south Devon coast. Above this comes red sandstone sometimes mixed with a certain amount of clay so that it becomes a marl, while at other places beds of red clay occur in the sandy deposit.
Our English Triassic beds are as a rule unconformable to the rocks below, though there are places where there is no sign of any break between them and underlying Permian rocks.
They seem very generally to pass over the edges of the Permian beds on to the older rocks, such an occurrence being called an overlap. The higher beds of the Trias also overlap the lower ones, and so it appears that these beds were not deposited beneath an open sea, but in shallow basins which were not occupied by the sea, and were not freshwater lakes.
Fossils are very uncommon in them, false-beading is frequent, and in many cases the beds run up against older rocks which must have been islands or hills when the Trias was formed. Gradually the sandy deposits, either washed or blown down from the hills, filled up the hollows in the land-surface and spread over the higher ground.
The shallow conditions which must have obtained during the accumulation of much of the Triassic deposit are proved by many lines of evidence.
In the muddy layers there are to be found preserved sun-cracks, and the little marks made by rain-drops, these showing that the mud must have been left exposed to the weather, then hardened by the sun’s heat, and then covered by another layer of mud which gently filled the hollows in the layers on which it was deposited.
Then, again, there are in these Triassic beds the marks of animals’ footprints, and the grooves which were made by their tails being dragged along over the slime.
Some of the sand deposit must have been borne along by the wind, for its grains are in places perfectly rounded, this being the characteristic form of desert sand. In Charn-wood Forest the older rocks on which the Trias sandstones rest are well polished by the sand which was blown over them.
In the hollows which existed in the plains, which were in the main sandy deserts, water accumulated and evaporated, and in consequence various salts crystallised out from solution.
It is the occurrence of these salts that makes, the Triassic beds so useful to man. In the Western Midlands of England, and in Ulster, common salt is found in abundance. In Cheshire it is found in two beds of which the lower one is sometimes upwards of 100 feet thick. Not only is common salt found in beds of this age, but bands of gypsum or calcium sulphate occur, and near Bristol there are deposits of celestine or strontium sulphate which are worked, and the material is exported in some considerable quantity.
The red sandy deposits of Triassic age have been given the name of the New Red Sandstone to distinguish them from the far more ancient Old Red Sandstone. The term New Red Sandstone is, however, one which is sometimes used to include the red sandstones of the Permian as well as those of the Triassic age.
The Trias occupies an extensive area in England and is also seen in north-east and in south Wales, while smaller areas of it are found in Ulster and in Scotland.
On the south coast of England red Triassic sandstones are seen along the cliffs of Devon near Sidmouth, and in Dorsetshire near Beer Head. They continue in a northerly direction to the Bristol Channel, giving rise to some of the best pasture-land in Devon and Somerset. They run up the Severn valley and broaden out to form the great central plain of England, from which they continue northwards along the two sides of the Pennine Chain.
The eastern line forms the fertile plain of Nottingham and the Vale of York and runs into the sea at the estuary of the Tees, while the western line passes through Staffordshire and Cheshire and across the mouth of the Ribble to Lancaster Bay.
It will be noticed that many of our great rivers wind for many-miles over New Red Sandstone rocks. The Severn from near Kidderminster to its mouth runs almost wholly over rocks of this age, while the Trent, the Dee, the Mersey, and the Tees all traverse considerable stretches of Triassic beds.
As might be expected in a deposit largely formed under desert conditions, fossils are very scarce. But beds of this age in the Alps have yielded a very varied fauna. The animals which existed in these times were very different from those of Palaeozoic ages, though some of the earlier types still survive.
Amongst the higher animals the reptiles show an increasing development, and many of them were of vast size, and by their bulk and uncouthness must have, to a certain extent, corresponded to the elephants and rhinoceroses of to-day.
Some had three bird-like toes, others four or five toes, and cases of a single footprint measuring 20 inches in length have been noticed.
The earliest crocodiles are of Triassic age, and, what is of great interest, the earliest mammals. One kind is called Microlestes, and has been thought to be distantly related to the banded ant-eater of Australia.
Towards the end of Triassic times a downward movement began and the salt waters of the ocean crept in over the Triassic deposits, and over some of the surrounding lands.
The rocks deposited beneath the waters of this sea are in this country of the nature of green sandstones, black shales, and limestones.
Since beds of this age are extensively developed in the Rhaetian Alps they have been called the Rhaetic beds. They are by some classed amongst the Triassic, and by others amongst the Jurassic rocks. As the country sank, the lowest member of the Rhaetic beds would be deposited quite conformably on the latest members of the New Red Sandstone, but quite unconformably on the older rocks which had projected above the sandy covering, while some of-the Triassic beds might also be unconformably overlain by the newer deposit.
Thus near Chipping Sodbury the Rhaetics lie almost horizontally on a platform of Palaeozoic rocks which slope steeply westwards, those exposed near Chipping Sodbury being Old Red Sandstone, while those farther to the west are the beds of the overlying Carboniferous series.
Most of the beds of the Rhaetic series are in striking contrast to those of the underlying New Red Sandstone both in their colour and in their fossil contents.
One noteworthy point is that it is very usual to find in the Rhaetics a bed called a Bone Bed, and sometimes more than one such deposit is found.
The Silurian bone bed has been already mentioned, and these Rhaetic bone beds seem to be a later production of much the same type. Owing to some cause, whose nature we can only guess, vast numbers of fish over wide areas died and sank to the sea-floor; very little other deposit came with them, for they form a deposit only a few inches thick.
In the Sodbury section this bone bed is found resting on the floor of Carboniferous and Old Red Sandstone rocks, but is not continuous, as it is absent over some patches of this old sea-floor.
The Rhaetic beds, though occurring over a very long stretch of country from north to south, owing to their thinness, for they are seldom as much as 150 feet thick, never occupy a wide tract of country on account of their usual easterly dip. They tend, however, to form a low ridge overlooking the softer New Red Sandstone, and so have some effect on the scenery of the country.
The vertebrate animals of the Rhaetic beds are of great interest; they include fishes and reptiles. Amongst the fishes is one closely allied to a fish now living in the Queensland rivers. Amongst the reptiles the Ichthyosaurus with its lengthy tail and the Plesiosaurus with its enormous neck are common and continue to live in the succeeding period.
The next two systems of the secondary rocks are called the Jurassic and the Cretaceous.
The former name is given to the lower beds because they are well exposed in the Jura mountains, while the latter is given to the succeeding beds because one of them is the thick limestone deposit called the chalk.
Owing to uplifts during times when the Jurassic and lower Cretaceous rocks were deposited, parts of our islands became dry land, and so we find a great variation in beds which were laid down at one and the same time in different parts of our islands.
The Jurassic Beds:
These contain three great clay deposits called the Lias, the Oxford Clay, and the Kim- meridge Clay, which are separated from each other by beds which are in the main limestones.
But while in Gloucestershire the Lias clay, which is the lowest of the Jurassic deposits, is covered by marine limestones and subordinate sands, in Yorkshire it is overlain by estuarine deposits, thus yielding evidence of land to the north.
The Lias is a greasy clay which forms heavy land, and is found from Lyme Regis on the Dorsetshire coast to Whitby in Yorkshire. There is but little of this deposit seen on the west of the Pennine Chain, but small patches – of it are seen near Carlisle and in the north of Shropshire.
No doubt in former times it spread much farther to the west than where it is now found covering the older beds, but was afterwards denuded away. It is largely used for brick-making, and in consequence it may often be well seen in brick-pits. In places, especially in the south of England, there are layers of limestone in amongst the clay, and both the limestone and the clay are usually very fossiliferous.
The important shells found in the Lias are those of the Ammonite. This animal was allied to the Nautilus, and is useful to the geologist in the same way as the trilobite and graptolite of the Palaeozoic rocks. Different kinds of ammonites are characteristic of different layers in the Lias, and so distant exposures of the rock can be correlated by observing the kinds of ammonites which can be found in them.
But the Lias period was especially the period of huge marine reptiles, the Plesiosaurus and the Ichthyosaurus abounding, and complete skeletons are not uncommonly found in quarries.
Over the Lias clay come the so-called Oolite beds. These are, in the south of England, marine limestones and sands. The limestones often present an appearance like the roe of a fish, and hence their name of oolite.
The lowest beds are grouped together as the Inferior Oolite, and above them comes the Great Oolite. The lowest member of the Great Oolite series is called the Stonesfield slate and has yielded many reptilian and some mammalian remains.
These oolite beds form hilly country overlooking the plains of soft Lias clay. Thus the Cotswolds are made of fairly hard oolitic limestone and stand sometimes 750 feet above the Lias of the Vale of the Severn. The colour of the beds is yellow or grey, and they are often full of fossils. Bivalves are numerous, corals are at times abundant, while sea-urchins and ammonites are not uncommon.
The little round grains which give these rocks their name of oolite are in many cases due to the growth of some microscopic form of life which extracted carbonate of lime from the sea-water and built up the grain from this material. Sections through some oolites have been made, and when examined under the microscope there are often seen to be tubes running round and round in the material of which the grain is built.
This Jurassic period is a particularly interesting one, for it was while it was passing that the conquest of the air began. There were two ways apparently by which development of existing life might progress and give rise to flying animals. There was a reptilian animal which grew a membrane between its very much lengthened little finger and its body, thus being endowed with the power of flight. This animal was the Pterodactyl. It had no feathers, and its wings may be compared to the bat’s wings of to-day.
There was also the reptile-like animal which developed feathers on its forearm and other parts of its body, and grew a feathered tail. This animal was the Archaeopteryx. Both these bird-like .animals were very different from the birds of to-day. They both had teeth, and they both had fingers on their forearm.
The tail of the Archaeopteryx also was quite different from the bird’s tail of to-day. Instead of being a bunch of feathers growing out from the hinder part of the body, it was the jointed end of the backbone from each joint of which came a pair of feathers.
Speaking generally the fauna and flora of the Jurassic period show considerable similarity to the Australian fauna and flora of to-day.
The middle beds of the Oolitic series consist of the Oxford Clay below and limestone beds above. They too are divided by geologists into different divisions by the particular kinds of Ammonites which are found in them. Corals abound in some of the calcareous beds, which must have been ancient coral-reefs.
The upper oolites also consist of a clay below, the Kimmeridge Clay, and calcareous beds above, the Portland and the Purbeck limestones. The three places which give their names to these beds are all in Dorsetshire, and it is in that county that they may all be well seen, and are very fossiliferous.
The Portland stone has been largely used in England for building purposes; St. Paul’s Cathedral and part of London Bridge are made of it.
The Lowest Cretaceous Beds:
In the south of England the lowest deposits of the succeeding Cretaceous period follow on the Jurassic rocks with little or no break, and consist of beds laid down in a lake or estuary. They are for the most part lacustrine sands and are covered by marine beds above, and since they are all well seen in the area of the Weald in Kent, Surrey, and Sussex, they have been given the name of the Wealden beds.
Long after their deposition this area was bent into a mighty fold, and over Kent, Surrey, and Sussex a huge arch was formed which in after times had its crest worn away so that the underlying Wealden beds are now exposed in north Kent and Surrey where they slope or dip to the north, and in south Kent and Sussex where they dip to the south.
Their fossils are not only the remains of animals which lived in the waters of the Wealden lake, but also those of animals and plants which were washed into that lake from the surrounding country. The plant remains are relics of conifers and ferns, while the animal remains are those of insects, reptiles, fish, and shell-fish.
The Pterodactyl still existed, and there were some huge reptilian monsters of which one is called the Ignanodon, and measured occasionally as much as 40 feet from the tip of his tail to his snout.
The Wealden beds pass up into the marine deposits called the Lower Greensand, so well seen on the cliffs between Hythe and Folkestone. These beds are exposed round the north and south edges of the Wealden area, dipping northwards and southwards off the Wealden beds. From Reigate westwards they are chiefly of a sandy nature, while eastwards calcareous beds come in.
In a deep boring at Kentish Town in London there was no trace of Wealden or of Lower Greensand beds, so that we may assume the former presence of a London ridge of older rocks which formed the northern boundary of the Wealden Lake, and afterwards continued to exist above the waters of the Lower Greensand sea.
To the north of this ridge we find Lower Greensand beds in Buckinghamshire, and at Sandy and Potton, not far from Cambridge, they are much worked. They are here full of fossils, many of which have been washed out of older deposits and rolled about a great deal before they were finally entombed. Such fossils are called derived fossils, and correspond to those which the sea is now washing out of our cliffs and embedding in the sand and mud round our coasts.
In Yorkshire the beds which correspond with the Lower Greensand of the south of England are well seen on the coast to the north of Flamborough Head. They are of the nature of marine clay, and are given the name of the Speeton Clay.
The deep borings in and around the London district not only prove the existence of a buried ridge of Palaeozoic rocks, but also that the Lower Greensand deposits come on to this ancient land, but do not cover its higher portions, and we must look on the sea of this period as deepening to the north of the ridge, for the rocks laid down beneath its waters over Lincolnshire and Norfolk are shallow-water deposits, while those in Yorkshire are deep-water marine clays.
The geography of this Lower Cretaceous period is fairly plain. At first a huge fresh-water lake existed over much of the south of England and was drained towards the south-east by a river which flowed through France to an open sea to the south. Along the northern shore of this lake rose the London ridge of Palaeozoic rocks, which separated the lacustrine waters to the south from a gulf of a northern sea.
The waters of this gulf covered part of North Germany and Denmark, and stretched into Norfolk, Lincolnshire, and Yorkshire. Thence its shores passed out into what is now the North Sea, and returned westwards over part of the north of Scotland.
Following on this Wealden period came subsidence. The northern and southern oceans joined across France and England, and the Lower Greensand beds were deposited over the fresh-water deposits in the south of England, and the marine deposits to the north of London. But the London ridge still remained above this later sea.
For although the Lower Greensand beds crept over the edges of the Wealden deposits and rested on much of the Palaeozoic rocks which formed the shore of the earlier time, yet they do not overtop the ridge and much of it has no Greensand beds above it.
This deposition of the Lower Greensand beds on the rocks of a far earlier age is another good example of what is called in geology an unconformity.
But subsidence still went on, and over the Greensand of South and Central England, and right over the top of the London ridge, a marine clay, called the Gault, is now found.
The Gault and Upper Greensand:
This bed is well seen on the sea-coast near Folkestone, and forms the foreshore slightly to the east of that town. In this exposure multitudes of fossils may be easily found, bivalves and Ammonites of great beauty and variety yielding a rapid harvest to the collector.
This clay runs along the foot of North and South Downs, and to the north of London is seen in Cambridgeshire. It is very commonly burnt for bricks, so that exposures are often seen. On the Norfolk coast, however, near Hunstanton, at the time when so much of the country to the south was being covered by the mud now called Gault, a red chalk deposit was formed, which extends northwards into Yorkshire.
The thick, greasy Gault clay gives a very definite type of scenery. Coming as it does between the porous sandy beds below it and the porous chalk above, it gives rise to a low-lying damp strip of country on which streams flow and ponds form. The vegetation on it is quite different from that on the chalk hills and on the sandy tracts close by, so that when one passes over it by rail or road its presence is at once to be marked.
Resting on the Gault in our southern counties comes a deposit of no very great thickness called the Upper Greensand. This is seen occasionally along the base of the North and South Downs and in the Isle of Wight, while to the west it is found in Berkshire and Wiltshire, where it fringes the Marlborough Downs from near Swindon to Devizes.
Along the south coast it may be seen crowning the cliffs near Charmouth in Dorset, and as it is here a bright yellow sand it is very conspicuous and has given the name Golden Cap to one of the hills cut into by the sea. It is seen farther to the west at Beer Head, and probably at one time extended over part of Devon to the north, as remains of a rock which was probably Upper Greensand are found in gravels near Bideford.
Over the Upper Greensand comes the well- known deposit of the Chalk, which occurs so extensively and forms such marked features in our country. No doubt it extended over a far wider area than it now occupies, but denudation has removed it from a vast extent of country.
It now forms the great high land of Salisbury Plain and the Marlborough Downs, from which it runs out eastwards to form the North and South Downs, south-westwards to form the high land of Dorsetshire, and north-eastwards to form the range of the Chilterns, and then passing along the hill country of Royston near Cambridge expands into Suffolk and Norfolk, and passing up through Lincolnshire runs into the sea at Flamborough Head.
In the extreme south of Dorset a sharp bend in the strata brings up the chalk, and in a continuation of this line more chalk is seen in the Isle of Wight. Away far to the north-west of the English exposures is an area of chalk forming the northeast corner of Ireland, and we cannot doubt that much of the intervening tract was covered also with this deposit.
As it is a soft rock it would not be likely to remain as fragments in any bed formed by the denuding agents of after years which might attack it. But it contains an enormous quantity of flint which is very hard, and the amount of flint debris in succeeding rocks is so large that we know from this that a vast amount of chalk has disappeared.
When we go south from England we find chalk again in the north of France, and it continues till we get to the Loire. Beyond there, however, the character of the deposit changes into a hard limestone in which there is a common fossil which is rarely found in the northern beds.
Eastwards the story is the same, so that we may regard the chalk as having been deposited in a gulf opening into a western ocean, this gulf being bounded by a ridge near the Loire on the south, and by East Germany on the east, for when we go to Dresden we find the beds of the Chalk period there are sandstones and show that we are nearing the land of the period.
The chalk is divided up into zones, each zone being characterised by certain fossils. Thus the Lower Chalk has the zone of Rhynconnella Martini, a bivalve, at its base above the bed known as the Chalk-marl, while the Upper Chalk has the zone of Belemnitella mucronata at its top in Norfolk, Kent, and the Isle of Wight, but this zone is absent from Yorkshire, Cambridgeshire, Surrey, and other localities, owing to its removal by denudation. This fossil is the remains of a cuttle-fish type of animal.
The Lower Chalk is frequently grey in colour. The Middle Chalk has two hard beds in it, the lower one called Melbourne Rock, after a Cambridgeshire village of that name, and the upper one the Chalk Rock, which is well seen from Wiltshire to Hertfordshire.
The fossils of the chalk, though never very numerous at any one spot, are not uncommon. The commonest are usually sea-urchins and bivalves, while Belemnites and Ammonites are sometimes abundant. Quarries are frequent in the chalk, as it is so commonly extracted in order to be made into lime.
As a rule it forms bare downs with woods often in their hollows, the beech growing splendidly on the limestone soil. Owing to its being traversed by joint planes it tends to form vertical cliffs, where it comes to the sea. It has been mentioned that it is found in the Isle of Wight. Here it has been bent into a great arch, and the crest of the arch has been worn through so that lower strata are seen throughout the centre of the island.
The southern portion of the arch remains on St. Catherine’s Down, the northern portion forms the backbone of the island from Culver Cliff on the east to the down above Freshwater on the west.
This northern limb of the arch plunges sharply down so that the originally horizontal beds of chalk are now almost vertical. The western point of the island is subjected to the attacks of the seas brought up by south-westerly gales, and so rapidly has marine denudation eaten away the soft sands and clays which occur on both sides of the chalk that the latter is left as a long westerly promontory, and the extreme end of this has been cut through at various points, so that a line of pinnacles has been left jutting out into the sea called the Needles. These are doomed to gradual extinction, but as the outer ones get removed newer ones may be formed to the east of the one nearest the shore.
This line of chalk was at one time continuous with the chalk hills of Dorsetshire, but the intervening rock has been worn away in past times, and the breach is now being gradually widened.
The chalk is usually of a pure white colour though it is sometimes stained yellow by iron oxide, and in its upper parts it is usually characterised by parallel layers of flints. These, when broken, are usually black inside and so give a very characteristic striped appearance to the view of a chalk quarry or chalk cliff.
Flints are made of a material called silica, and sometimes they have formed round a sponge or a piece of a sea-urchin or other fossil, and at times there may be a complete shell surrounded by the flint. The flints are often of very fantastic outlines, so much so that it appears impossible for us to imagine that they were formed on the old sea-floor while the chalk was accumulating.
Hence it is thought that originally the silica which now forms the flint was disseminated through the chalk deposit, and that it afterwards collected in more or less definite bands often round sponges which were originally made of silica, but also round other bodies such as shells or pieces of shells.
Since at the close of the Chalk period elevation occurred and denudation took place the uppermost beds which were originally deposited have been largely removed, the highest beds being seen in Norfolk and parts of Kent.
When the English chalk is examined carefully under a microscope it is found to contain multitudes of very tiny shells. The bulk of these are perforated by small holes, and so have been called foraminiferae (Lat. foramen = a hole). These shells are made of carbonate of lime, and the chalk is built up of incalculable numbers of these tiny remains of animals.
As there is no detrital deposit intermingled with them we must conclude either that the margin of the gulf in which the chalk was formed was far from the masses of chalk which now remain, or that any material which was washed into this gulf quickly subsided, and has been removed from the neighbourhood of the old land margin by after-denudation.
It is interesting to note that quartz grains do occur in the Lower Chalk to the east of Devon, and they may have been derived from the granite in Devon, then part of a land- surface which drained eastwards. In Ireland conglomerates of the Chalk period give better evidence of the nearness of the land, and there is a rapid passage from them to the typical white chalk.
Chalk in Ireland and Scotland:
It has been mentioned that chalk extends into Antrim where it has been protected from denudation by later lavas which were poured out on to the top of it, and on solidifying protected it from being worn away. Cretaceous beds of the nature of hard grey limestones are found in Skye, of no very great thickness it is true, but useful as evidence that the chalk sea once extended to this region.
Between Antrim and Skye there is no known locality where the chalk is found in situ, but owing to a fortunate discovery in 1900 we know that it once extended over the Island of Arran, 100 miles to the south-east of Skye.
In Arran there was a large volcano in times long after the Chalk period, and the throat of this volcano must have opened through older rocks up into overlying Rhaetic, Lias, and Chalk beds, and some of these overlying beds cracked off and slid down into the volcanic vent so that they came to rest far below the position at that time of the beds from which they came.
Now all the Rhaetic, Liassic, and Chalk deposits have been swept away from Arran and the country round, but these sunken fragments have been found, and prove the former existence of the covering of Rhaetic, Lias, and Chalk which once spread over the island. That the blocks are of the Liassic, Rhaetic, and Chalk ages is proved by the fossils they contain, and they bear silent witness to the power of denuding agents.
Just as one can point to deposits being formed to-day which resemble the sands, clays, and gravels of past times, so one can indicate a deposit now forming like the chalk. Dredging from the depths of the Atlantic and Pacific proves that foraminiferae are living in their millions in the waters of our clear oceans, and slowly falling down to the bottom are building up a grey ooze, which is almost an exact repetition of the chalk of earlier times.
No detritus from the European shores gets very far from the coast-line, and the deposition of carbonate of lime in the form of minute shells goes on year after year, and century after century slowly, very slowly, building up a thick deposit, which, when it is raised above the sea-level, may perhaps only be distinguished from the more ancient chalk by its contained fossils.
4. Essay on the Tertiary or Cainozoic Rocks and the Post-Tertiary Deposits:
We now come to the shallow-water phase of the Third Marine period, during which Tertiary sediments were deposited in the south of England.
The Eocene Beds:
All the north-western portion of our islands formed a land area during this time, and early in the Tertiary period there welled up through numberless fissures in the west of Scotland and in the north-east of Ireland a molten rock called basalt. Extremely little explosive action occurred, and the continuance of the outpouring of the lava gradually built up a plateau of enormous thickness which extended from Antrim along the west coast of Scotland and farther to the north.
The southern portion of the land area to the north drained eastwards or south-eastwards across England into a gulf which extended over the south-east of England from Norfolk to Dorset. Long after their deposition in this area the Eocene beds were raised up, and at the same time bent into two gigantic troughs separated by an arch.
The crest of the arch runs in an east and west direction through the North Hampshire Downs and the centre of Kent, and since the Tertiary beds have been removed from the arch the remaining Tertiary deposits are now seen forming two huge troughs separated from one another by the chalk in Hampshire.
The northern trough is roughly in the shape of a triangle now having its apex at Hungerford, and extending to the east coast from Norfolk to North Kent, and is called the London basin. Owing to its trough-like shape the beds along its northern side dip to the south, while those on its southern side dip to the north.
The southern trough is called the Hampshire basin, and extends from near Salisbury on the north to the Isle of Wight on the south, while it runs east and west from Bognor to Studland Bay.
The magnitude of the period which elapsed between the deposition of the highest Chalk beds and that of the lowest Tertiary beds must have been immense. Not only was the chalk raised so that in places its highest bed – were removed by denudation, but the type of life in Tertiary times will be seen to be quite different to that of the end of the Cretaceous epoch.
When deposition did begin in Tertiary times a marine sand formed over Kent and Surrey as far west as Leather- head. It is called the Thanet Sand. It is not fossiliferous towards the west, but at Herne Bay and in Pegwell Bay near Ramsgate, there are fossils to be extracted from it.
Later on there is plenty of evidence that the chalk and its contained flint layers were being attacked by the sea of the Eocene period, for we find vast accumulations of well-rounded flint pebbles, which were washed into shingle banks, and these often allowed of the formation behind them of lakes and marshes, where clay deposits accumulated and plenty of plant remains became fossilised.
These beds are called the Woolwich and Reading beds from their occurrence near these two towns, but the types of bed found at these places are very different—in fact there are three very distinct kinds of Woolwich and Reading beds. In East Kent they consist of Marine sands; in West Kent, East Surrey, and South Essex of Estuarine sands and fine grey clay, often containing layers of oyster shells; while in the Hampshire basin and along the northern margin and the western end of the southern margin of the London basin, they are formed of irregular layers of fresh-water clays and sands, the former generally of varied and bright colours, the latter also are brightly coloured and frequently contain flint pebbles.
Sometimes these flint pebbles are cemented together, and the resulting bed is then often called a pudding-stone, as the round dark flints in the rock look like currants and raisins. From the various natures of the Woolwich and Reading beds it is clear that when they were deposited a river ran from a western land and opened out into a sea which lay to the east of Kent, and since the beds contain flints we have in them proof that the chalk was then being denuded away.
It is in such Estuarine beds that we would expect to find the remains of land animals preserved, for, especially in times of flood, carcases of drowned animals would float down – stream until they sank and became entombed. In accordance with this idea we find fossil bones of tapir like animals and many other mammals, of turtles and many other reptiles, while a bird has been found at Croydon. Shell-fish are abundant, and plant remains fairly common.
Above the Woolwich and Reading beds, in some parts of Kent, there are sands and beds of flint pebbles, the Old- haven beds, which must have been laid down as shoals on the earlier deposits, and on the top of these, or on the Woolwich and Reading beds, comes a thick clay deposit known as the London Clay.
This is the most important member of the Tertiary deposits; it is a marine clay, and so points to the sea covering the wholeof the London and Hampshire basins when it was deposited. At the same time the land was not far off as remains of terrestrial plants and animals are found in it.
The Isle of Sheppey is a very noted place for fossils from this stratum, and there have been found there the remains of the palm, acacia, and mimosa, of crocodiles, turtles, and fishes, of birds, and of tapir-like animals, some of them related to the ancestors of the modern horse.
Many of the genera of shell-fish found in it continue to exist at the present day. The climate must have been very warm, though perhaps not tropical.
Gradually this Eocene sea began to silt up, and we get evidence of a river from the west bringing sands on to the clay, called the Bagshot Sands, and clay beds called the Bracklesham and Barton beds.
Wherever Bagshot sands remain now they produce a very typical kind of country. It is somewhat hilly, and on these sand-hills grow firs, gorse, heather, and bracken, while the vegetation on the surrounding clay is largely of oaks and elms.
The country near Aldershot is of Bagshot sands, and Hampstead Heath, to the north of London, owes its character to its being on these beds.
The Oligocene Beds:
Following on the Eocene beds come the deposits known as the Oligocene beds. The uplift of the western parts of our islands, which had previously cut off communication with the warm western ocean, was continued so that deposition only went on in lagoons, in estuaries, and in fresh-water lakes. On the north of the present English Channel we only find Oligocene beds in the Isle of Wight, and at Bovey Tracey in Devonshire.
The life of the Eocene and Oligocene times is markedly different from that of the Cretaceous period. Only one species (a bivalve) goes on from the one to the other. The most abundant fossils are no longer Ammonites, Belemnites, and the kinds of bivalves of previous ages, but shellfish without a bed, and bivalves like those of to-day, and many of them belong to genera which inhabit warmer seas than now surround our shores.
The Lower Eocene beds contain plants which show a temperate climate, such as the poplar and the hazel, but the London clay and Bagshot beds contain firs, palms, and cactuses, which point to much warmer conditions. The mammalian remains are important, amongst them being those of early forms of the tapir, rhinoceros, pig, deer, and monkey. It is owing to the appearance for the first time of such animals as these that the period has been called the Eocene, which means the dawn of recent life (eos = dawn, kainos = recent).
The Miocene Period:
The Miocene period (meion = less, kainos = recent) succeeds the Oligocene, and during the whole of it our islands seem to have been above sea-level, for there are no miocene deposits that have been found in them It was, however, a period of profound importance, for very pronounced earth-movements occurred in many regions of the world, and our islands were noticeably affected.
In the Alps Eocene beds were raised 3000 feet above sea-level, while in the Himalayas they are found still higher.
Naturally such gigantic uplifts could not go on without the earth’s crust being affected at a great distance. We find the Eocene, and of course the underlying beds also, of the south of England bent into great folds. In the Isle of Wight the chalk and the overlying Eocene deposits are actually vertical, as may be seen at the Needles and Alum Bay, while the still lower oolite beds of the Dorsetshire coast are also vertical and much bent, as may be seen near Lulworth.
Farther to the north the Eocene and underlying beds are bent into the two troughs of the Hampshire and London basins. Later denudation has separated the Hampshire Eocene beds from those of the northern trough, and has also removed much of the underlying cretaceous arch in Kent, Surrey, and Sussex, so that the chalk of the North Downs is no longer continuous with that of the South Downs.
We may say that the present physical geography of Great Britain largely dates from Miocene times. Beds which have been deposited since then are, as a rule, mere scattered accumulations of continental denudation, and as they do not necessarily he over one another their relative ages are not always easy to determine.
In France and America there are deposits of Miocene age which enable us to get a glimpse at the life of the period, and we find that the likeness of the higher forms of animals to those of the present day becomes more pronounced, and at the same time the lower forms also get more and more alike to those we now find round our shores.
When deposition did occur again in Britain it was chiefly in East Anglia, though deposits of the same age are also known at Lenham in Kent and at St. Erth in Cornwall. The period is called the Pliocene (pleion = more, kainos = recent).
The Pliocene Beds:
The St. Erth beds are clay, loam, and sand, and form a deposit only about 8 feet thick lying about 100 feet above sea-level. Some ninety species of Mollusca have been found in these beds, and none of these now live in northern seas, though some inhabit the southern waters of to-day.
The Lenham beds in Kent largely owe their preservation to their having fallen into hollows in the chalk, which have been formed by solution. The fauna includes a few species which are unknown out of the Pliocene of East Anglia, but have been found in the Miocene of the Continent. Hence these Lenham beds are supposed to be the oldest of the Pliocene beds of England.
In East Anglia the Pliocene deposits are in the main formed of shelly sandstones commonly called Crag. The shells are frequently fragmentary, but plenty of whole specimens are common. The various crags are deposited at various spots, and do not rest on one another in a definite succession. But by examination of the junctions of one crag with another, and by careful investigation of their fossils, it has been found possible to draw up a list of the various Pliocene beds in the order of their deposition.
The oldest one is the Coralline Crag seen near Aldeburgh, its fossils being largely bivalves and Gastropods, nearly 40 per Cent of which are extinct, while of the existing species about 20 per cent are no longer found round our coasts, but are Mediterranean forms.
The Red Crag is seen between Walton and Aldeburgh, and is usually deeply stained by iron oxide. Still fewer species are now extinct than in the case of the Coralline Crag, and there are also fewer southern existing forms.
The Norwich Crag begins at Aldeburgh and runs along the coast past Southwold and Lowestoft to the river Bure. Its fauna is very distinct from that of the Red Crag, and contains far fewer existing southern forms, while Arctic species occur.
West of Cromer, near Runton, is seen another Pliocene bed, the Weybourn Crag which seems to contain no southern forms of Mollusca at all. Still newer than this Weybourn Crag come a very interesting set of beds called the Cromer Forest-bed series.
At its base comes a bed with fresh-water shells, then the Forest-bed proper, and then another fresh-water bed. The Forest-bed consists of beds of clay, sand, and gravel, in which occur rolled stumps and stems of trees and smaller fragments of branches, and also bones and teeth of mammals.
Elephants’ teeth and bones can be picked up on the shore near Cromer, especially after a gale which has swept the beach partly away. And the remains of a very large number of animals such as deer, rhinoceros, etc., have also been found. There seems little doubt that this is a deposit laid down in the estuary of a huge river which flowed down northwards, and may have been the river whose diminished representative is now the Rhine.
Above this Forest-bed comes a bed of marine sand, not very fossiliferous but containing northern forms, and then a fresh-water clay containing the remains of plants, of which some, such, as the dwarf willow and dwarf birch, now live only within the Arctic circle.
Although, as is obvious, we can only get occasional peeps into what was going on in Britain during Pliocene times, yet the story is a fairly consistent one.
The climate shows a steady fall in temperature. In Eocene times it had been distinctly warm, possibly tropical, but in Pliocene times, though at first it must have been very much like that of to-day, we find that it got colder and colder. The forms of life which now live in northern seas get more and more common, and at last we find traces of a vegetation which is now considered Arctic.
This decrease in temperature did not stop, but, going on, assisted in the production of vast fields of snow which gave rise to vast sheets of ice and to the formation of deposits which belong, not to the Pliocene, but to the Pleistocene period.
The Pleistocene Period:
Not only was our country affected by the ice which accumulated on its own hills, but ice came over the North Sea from Scandinavia, bringing with it boulders of Scandinavian rocks and stranding them in what is now the east of England. This period of intense cold is called the Glacial Epoch, and is one during which but little life occurred.
There were, however, before the ice and snow finally vanished, so-called inter-Glacial periods, when warmer conditions prevailed. When at last the temperature rise was permanent and the ice had retreated, an enormous stretch of country was left covered either by a thick greasy clay in which boulders and stones of various sizes are embedded, or by deposits of sand and gravel.
The Clay deposit is well seen in East Anglia, and often covers the .high ground only, since the present rivers have cut through it and removed it from their valleys.
Glacial gravels and sands are finely exposed along the cliffs near Cromer.
There seems no certain evidence that man lived in our islands in pre-Glacial times, but when the ice had cleared away there are abundant traces of his existence in caves and in river-gravels.
At the close of the Pliocene period England was united to France by a stretch of land covering the present Straits of Dover. Gradually, however, the sea attacked that stretch of country and, assisted by a certain amount of subsidence, finally cut its way through and severed England from the Continent.
One result of this union of Britain with the Continent in Pleistocene times was that various animals, which otherwise could not have got here, walked across the land connection.
When our country was covered with ice it was no fit place for the animals which afterwards lived in it. But as the ice retreated to the mountains and as the climate got warmer and vegetation increased, the European animals followed the vegetation and wandered northwards in search of food.
But the moment we became separated from Europe by the sea, this immigration was stopped, and one result of this is that now we are poorer in mammals and still more markedly poorer in reptiles, with their smaller powers of dispersal, than our continental neighbours.
The same fact is seen when we compare the mammals and reptiles of Great Britain and Ireland, and points to these two countries having been separated at a still earlier date.
Thus the number of species of Mammalia in Germany is about 90, in Scandinavia 60, in Britain 40, in Ireland 22, while the number of species of reptiles and amphibians in Belgium is 22, in Britain is 13, and in Ireland is 4.
When, however, we consider winged animals, many of which can fly over considerable stretches of sea, we find a much less marked difference. Ireland has 7 bats, Britain 12; Ireland has about no land birds, Britain 130.
The Pleistocene period passes into the period whose deposits are generally called Recent.
They are largely of the nature of river-deposits, and contain the remains of animals washed down the rivers. But there are here and there beach-deposits round our shores and accumulations of materials in caves which by their contents enable us to see how the life of the Pleistocene period gradually merged into that of to-day.
No doubt one of the great causes of the disappearance of certain Pleistocene animals was man himself. He passed through a stage when nothing but stone weapons were used to one in which bronze replaced stone as the material with which he fought, and finally the Bronze Age passed and the Iron Age began, in which we now live. But whether his weapon was stone, bronze, or iron when he was attacked by any fierce animal he would try and slay it, or if he was hungry he would try and kill, not through fear but through love of food.
Accordingly most of the wild animals which lived here with man in early days have now been swept away. We can find bears in Austria and wolves in Russia, but man has killed the last wolf and bear in England very many years ago.
Thus a general survey of the rocks of past ages leads one to the conclusion that the geography of any part of the world is constantly changing. The land becomes sea and the sea land, a fresh-water deposit is covered by a marine one and vice versa, and the general aspect of a land area is never constant. During the lifetime of a generation but little alteration may be noticed, but if a sufficiently long period of time be allowed to elapse the slow processes of nature produce their inevitable results.