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Essay on Glacier
Essay Contents:
- Essay on the Meaning of Glacier
- Essay on the Motion of Glacier
- Essay on Ice-Falls and Crevasses on Glacier’s Surface
- Essay on Avalanches and Moraines
- Essay on Glacial Erosion
- Essay on the Ice Scratches of Glacier
- Essay on the Evidences of Ice Action in our Islands
- Essay on Post-Glacial Life and Climate
Essay # 1. Meaning of Glacier:
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In the Arctic regions it is difficult to see the effects of a covering of snow and ice on the rocks below; but in the Alpine regions it is easy, for their, there has been for some time to retreat of the ice, and so in summer-time one can see what has been its effect on rocks which it covered at no very distant date.
In such arctic regions the snow collects on the highest ground and, becoming compacted by its weight, is converted into masses of homogeneous ice, and this, pressed on by the weight above it, makes its way down the valleys as constantly moving ice-streams, called glaciers.
Essay # 2. Motion of Glacier:
That these glaciers do move down their valleys has been proved by driving a quantity of stakes in a straight line into their surfaces and watching them day by day. It required no great length of time for the straight line to become a curved one, for the centre of a glacier moves faster than the sides, owing to the greater friction against the rocky side of the valleys.
Tyndall, in 1857, experimented in this way on a famous glacier in Switzerland, the Mer de Glace, and found that a stake near its edge at one place moved 12 inches in twenty- four hours, while a stake nearer the centre moved through 33 inches in the same time.
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In the winter of 1859 he again experimented and found again that the glacier was moving every day, but that the motion was very much less than during the summer, being indeed only about half the summer movement.
Other experiments on other glaciers gave similar results. The lower Grindelwald glacier moved near its edge at the rate of 18 inches in twenty-four hours, and near its centre at 22 inches in the same time, while the largest glacier in Switzerland, the great Aletsch glacier, gave the numbers 2 inches and 19 inches at points near its edge and centre.
Other experiments showed that a river of ice moved very much like a river of water, and there is one point of importance to notice, namely, that the steeper the fall of the valley floor the quicker the motion.
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When a glacier gets below a certain level it begins to disappear, owing to melting proceeding faster than the glacier can be fed by fresh ice from above. It follows that glaciers with steep falls can push their snouts, as their lower ends are called, lower than glaciers with slight falls, other things being equal.
It is in consequence of this that such a glacier as the steep Glacier des Bossons pushes down amongst the vegetation of the Chamounix valley, while the Mer de Glace, with its more gradual fall, ends much higher up on the mountain-side.
Essay # 3. Ice-Falls and Crevasses on Glacier’s Surface:
There is one effect of great importance, which is seen when a glacier comes to a steep part in its valley. When a river comes to a vertical cliff it may happen that it is broken into fragments, but a steep part at the bottom of a valley filled with water causes rapids. In the case of ice there is also an extra rapid movement over the steeper part, and in consequence the ice there is torn away from the more slowly moving mass higher up the valley.
In other words, a crack is started. This often occurs with a noise like that of an explosion, and although the crack is at first very small, as time goes on the unequal motion widens the crack, and what is called a Crevasse is formed.
That part of a glacier where crevasses exist is a very dangerous one, and when a climber comes to it he always, if possible, takes to the rocky side above the crevassed part, as offering him a less dangerous path to traverse.
But if the glacier proceeds on from the part where the fall was rapid to a part where the fall becomes less, it always shows a mass of ice uninterrupted by crevasses over this latter part. This is one of the most striking points in glacier scenery, and is due to the fact that when the more rapidly moving ice over the steep part catches up the more slowly moving ice on the less steep part the edges of the crevasse are forced together till they touch, and the two sides after being pressed together freeze once more into a solid mass of ice.
These crevasses give evidence of the unequal pace of a glacier’s surface, for they do not stretch as straight lines from side to side of the valley, but are curved, with the curve pointing downwards near the centre of the glacier.
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Moreover, when accidents have occurred and men have fallen down a crevasse, their bodies have been found many years afterwards near the snout of the glacier, far below the position of the crevasse where the accident took place.
Owing to the sun’s heat much ice is melted during the day, and so streams are formed on the surface of the ice, but these streams sooner or later come to a crevasse and pour down it, reach the valley floor, and eventually emerge from below the end of the glacier as a river.
But it is not only the surface of the glacier that is melted during the day. Some ice and snow on the mountainsides above is melted, and the water thus formed makes its way into cracks in the rocks. At night this water freezes, and during the freezing of water expansion takes place and a considerable force is exerted. In consequence of this, fragments of rock are separated from the mountain-sides, but are still bound to them by a cement of ice. In the morning, however, this cement melts and the rock fragment may fall down on to the glacier below.
Essay # 4. Avalanches and Moraines:
Avalanches:
Sometimes a big fall of rock results; this is called an Avalanche, and is a great danger to mountaineers. In consequence, certain localities are never passed during the morning hours lest an avalanche should sweep the climbers to destruction.
Moraines:
The rocks which have fallen on to the glacier are of course carried down on its surface, with the result that a heap of stones lines its sides, and this is called a Lateral Moraine. When a crevasse is formed rocks may slip down it to the bottom of the glacier and will then be carried along under the superincumbent mass of ice. When two glaciers meet their inside edges become the middle of the resulting glacier, and their inside lateral moraines become a Median Moraine on this larger ice-river.
At last, however, all these moraines are brought to the end of the glacier and build up a confused mass of angular stones, called a Terminal Moraine.
As the rocks forming all these moraines have had very little rubbing against one another, they are in general angular and unscratched, and as there is little fine material amongst them to bind them together, they are usually quite loosely packed. They are often therefore very dangerous places on which to scramble about, for the stones so easily turn when one steps on them that one runs great risk of spraining one’s ankle.
But the case of the stones which fall down a crevasse, or which slip down between the ice of the glacier and the rocky side of the valley, is a very different one. These are dragged along by the ice and make the lower surface of the glacier resemble a huge piece of sandpaper.
Essay # 5. Glacial Erosion:
Just as the grains of sand scrape and wear down any material softer than themselves, so the rocks beneath a glacier scrape and wear down any softer rock they are dragged over on the surface below the glacier, while if they pass over any harder rock they may themselves get their edges rounded and their surfaces scratched.
All this attrition which goes on produces a great deal of fine material, and this is washed along by the streams which flow beneath the glacier, with the result that the river which emerges from below the glacier’s snout is not clear water formed from melted ice, but is of a milky colour, owing to the enormous amount of fine material in suspension.
This is gradually deposited lower down the valley, but a rapid glacier river may retain the milky colour for many miles of its course. Where the river flows into a lake deposition is quicker owing to the rapid checking of the pace of the waters, and thus it is that the Rhone, which is fed by glacier rivers for the ninety miles of its course before it enters the Lake of Geneva, comes in white and issues forty miles away at the town of Geneva perfectly clear.
The deposition of glacial mud at the upper end of the lake has built up a deposit, and this grows at such a pace that Port Valais, which was a port in Roman times, is now about a mile and a half from the edge of the lake.
Essay # 6. Ice Scratches of Glacier:
The ice scratched stones are brought to the end of the glacier and are there mingled with any which have been brought down in and on the ice to form the terminal moraine, while, if for any cause the glacier retreats, the scratched rocks over which it has passed in days gone by remain as evidences of its former presence.
Since the stones which are dragged along beneath a glacier are not rolled over and over as they would be beneath a running river, they are not so regularly rounded as those which are water-worn. Their shape is quite a peculiar one, and is called sub-angular, and their marked feature is that they are scratched in fine lines.
As the direction of the movement of a glacier in general is a constant one the scratches on the valley floor over which it moves are usually parallel, but as the blocks beneath the glacier may get shifted so that now one part of the block points down the valley now another, we find that the scratches on the blocks are not all parallel, but cross one another in various directions.
Plenty of these ice-scratched stones may be picked out of any of the terminal moraines of Swiss glaciers, and plenty of ice-scratched rock faces may be seen at their ends and along their sides.
It is to be noted, however, that not every kind of rock will show scratching. A limestone or a slate is a moderately soft rock and will keep its scratches, but a sandstone, though it is soft and may be scratched, will soon have its scratches removed by weathering action after the ice has retreated. Other kinds of rock again may be so hard that the rocks they move over may not be hard enough to mark them.
If a period comes when for some cause or other the glacier retreats up its valley after having come down nearly to the same point for many years, the jumble of blocks called the terminal moraine and the lateral moraine will be left as proofs of the former extension of the glacier to the places where they occur.
Moreover, during the retreat of a glacier masses of rock on its surface may be carried to some spot along the side of the valley, and then, when the ice melts, they may be stranded there; these are called Perched Blocks, and have sometimes been left in most extraordinary situations.
We have noticed, therefore, the following results of glacier action which are being produced to-day :
(1) Moraines,
(2) Ice-Scratched Blocks,
(3) Ice-Scratched Rock Surfaces,
(4) Perched Blocks.
If we find these results in any part of the world we may confidently assert that though no glacier has ever been seen there, yet in times gone by a glacier did exist there, and that these are the relics left after its death.
Essay # 7. Evidences of Ice Action in our Islands:
Now every one of the above results of glacier action can be seen to-day in all the mountainous districts of England, Wales, Scotland, and Ireland. Moraines line the valley-sides of the Connemara district.
Ice-scratched rock surfaces are to be seen up the west coast of Scotland, and amongst the finest examples are those round Loch Coruisk in the Island of Skye. Ice-scratched boulders may be collected from the coasts of Dublin and from the Cumberland valleys, while perched blocks are common in North Wales and elsewhere.
Erratics:
When an ice-stream has passed by, or over, an exposure of a peculiar type of rock it becomes easy to trace its former track by the blocks of this rock—Erratics, as they are termed—which it carried along and dropped when it melted. This tracking of the former path of a glacier by the debris it has left is analogous to the tracking of the hares by the hounds in a paper-chase.
Those coming after the hares judge of the course which was taken by the bits of paper left on the surface, and in the same way we now, when we find a string of boulders resting on the surface, argue that that line represents the passage of an ice-stream in days gone by; for no other agent than ice, as far as we know, was capable of depositing these blocks in their present positions.
As the North-Western Railway makes its way from Lancashire to Carlisle it has to climb up the Westmorland Fells and reaches the summit of its rise close to the village of Shap. On the hill-side to the west of the railway, not far from Shap village, may be seen evidences of quarrying, and the stone which is extracted there is the famous Shap granite, which forms such a handsome building stone that it is seen in buildings throughout the length and breadth of England.
It is marked off from all the other granites in our islands by the presence in it of very large red crystals of a mineral called felspar, and in consequence a trained eye can unfailingly recognise any fragment it sees as a piece of Shap granite.
The rock is only found in situ in one small oval, which measures about two miles by one, but a stream of erratics, all made of this peculiar granite, is found extending from Shap to Flamborough Head. This stream does not follow the course of any valley, but travels down from its parent hill, 1600 feet high, over a limestone ridge 1000 feet high, across the Eden valley, over another limestone ridge more than 1400 feet in height, down into the vale of York, over the oolite ridge and over the chalk hills to the east, to the sea at Flamborough.
We have clear evidence here, therefore, of a vast body of ice which moved eastwards from Westmorland across Yorkshire, overriding the valleys of the country, which must in those days have been full of ice, and when a warmer climate came on we must think of this ice-stream as gradually melting from east to west and depositing on the hill ridges and on their sides the blocks of rock which it had brought with it during its time of motion.
Another rock which can be easily recognised is the white granite from the Criffel in Kirkcudbrightshire, and blocks of this are found forming a stream stretching south over the Isle of Man, part of Cumberland, Lancashire, Cheshire, North Wales, and County Dublin.
Boulder Clay:
The finer sediment produced by the rasping action of ice-masses on the country beneath it has given rise to a thick clay in which are embedded small and large blocks of stones, many showing scratching. This clay is called Boulder Clay or Till, and is found in most parts of England except in the southern counties.
The Cotteswolds and the Severn valley to the north of Gloucester show no boulders or boulder clay, and it therefore seems probable that the ice-sheet which covered the north, east, and centre of England did not extend into Gloucestershire. No doubt as the rest of England was so cold there was snow on the Cotteswolds, but the area of high land could not have been large enough to produce glaciers.
Still the action of the frost must .have assisted in breaking up the limestone of these hills, and the streams which flowed out towards the Severn must have brought down the debris which is now seen in the river gravels of the Cotteswold streams.
When the great ice-sheet, owing to the climate getting warmer, retreated from the lower ground, ice would still be formed amongst the mountains and push its way down the, valleys, and then this ice too would gradually diminish, and finally the last trace of summer snow vanish from the highest peak. Careful investigation of glacial deposits, however, shows that the course of events was not quite so simple.
In between glacial beds come beds containing the remains of plants, which indicate a more or less temperate climate, so that we are bound to come to the conclusion that there occurred a waxing and a waning of the Arctic conditions more than once in the history of what can be comprehensively called the Glacial Epoch.
When the ice and snow finally retreated and vegetation spread over the land, the types of plant and animal life were those which are now found in Arctic regions. Some, like the woolly elephant and woolly rhinoceros, are now extinct, though their carcases have been found preserved in frozen ground in Siberia; but others, like the musk ox, still inhabit northern lands.
Essay # 8. Post-Glacial Life and Climate:
Then, as the average temperature rose, the tops of our high hills were the final refuge of the life that loved the cold. In consequence we still find some special types of plants which only live on our mountain-tops, in Arctic regions, and near the snow-line elsewhere.
The summer warmth in those post-Glacial days, when snow still accumulated on our hills, would cause large swollen torrents to rush down valleys, which are now perhaps almost or quite streamless. It is probable, therefore, that denudation may have been quicker in certain localities then than it is now.
The temperature of our islands was in early post-Glacial days so low that animals no doubt crept into caves for warmth, and these natural houses became the store-houses of their bones and of those of their prey. As a limestone district is often full of caves, and as a limestone cave gradually gets a deposit of carbonate of lime formed over its floor, we frequently find animal remains beneath the present floors of such caverns.
The cave bear, the tiger, and the hyena are thus proved to be some of the many animals which inhabited our islands and are now extinct. Man also came wandering into these islands and ousted the animals from their dens, leaving in them in his turn his bones and his implements, and the remains of the animals he killed for his food.
The Glacial period therefore gradually merges into the human period, the earliest portion of which is known as the Prehistoric age.