ADVERTISEMENTS:
In this article we will discuss about the geological maps and its uses.
Maps which are coloured so that they show different kinds of rock are called geological maps. They are published by the Government on the scale of an inch to a mile and should be bought by anyone who wants to find out about the geology of any district. Besides showing the kinds of rock which are beneath the surface-deposits, they show other points.
The slope of a bed of rock in any direction is called its dip, and the angle of dip at any spot is the greatest angle the bed makes with the horizontal at that spot. In a map the direction of the dip is represented by a small arrow, and the magnitude of the angle of dip is shown by a small figure by the arrow, thus ↓ 30 means a southerly dip of 30°.
ADVERTISEMENTS:
If the bed is bent up and down into a trough and an arch, the trough is called a syncline and the arch an anticline. Sometimes the pressure to which a bed has been subjected has not only caused it to bend, but has actually broken it; in this case either of two results may have happened.
If the fracture has brought the beds A and B into the above position, the crack is known as a normal fault. It is to be noticed that in this case no vertical shaft can pass through the same succession of beds twice over.
But if the fracture has taken place like this:
it is called a reversed fault, and it will be seen that a vertical shaft ab passes twice through the beds CD. All faults are shown on a geological map by a thick line, generally in white.
ADVERTISEMENTS:
The line at any place at right angles to the dip along the horizontal plane is called the strike of the beds, but owing to denudation this need not be the line along which the bed is exposed, which is called the line of its outcrop. If a bed is striking north and south, and an east and west valley cuts into it and exposes its lower edge, though the strike is north and south the line of outcrop may be east and west along the sides of the valley.
By noticing the direction of the dips in a geological map one can at once tell the order of succession of the beds from the lowest to the highest, and one can also tell how they are bent.
ADVERTISEMENTS:
It is often a good thing to represent what is shown on a map by a geological section. This is a view of what one would see in a vertical plane if one imagined the country cut through vertically and everything in front of the cut removed. In many maps one will see dotted lines called contour lines, each of which passes through spots at the same height above sea-level. Thus the line with 300 printed close to it means that all the places through which that particular contour line passes are 300 feet above sea-level.
When one draws a geological section through any part of the country represented on a map it is very important to notice these contour lines, as one can then represent the slope of the valleys and hills in the section. It often happens that the rock which may be regarded as making the country is covered in places by surface-deposits, as they are called.
This does not mean the soil beneath the grass, but that over the country rock is some deposit of sand or gravel or clay which has been brought over the country rock in recent times. In the case of river-deposits these are coloured a yellow colour in our survey maps. But in the case of other recent deposits their presence is generally represented by scattered small black dots on the colour which represents the rock below it.
If one looks at a map before going to a district in order to decide on what spot to visit, one had better therefore settle to go to a part not dotted in the map. Also there is nearly certain to be an exposure at any place where a dip- arrow is printed, as there must have been rock exposed there which enabled the original surveyors to be sure of the dip-angle.
With a geological map in one’s hand, one can tell at once to what division of the rocks any exposure belongs. The exposure may be in a cliff-face, in a stream or in a quarry, and the position can be found on the map, and the colour of that part of the map will enable one to find out to what period and age the rock belongs.