The exposed surface of a quarry, railway- or road-cutting shows that soil in the agricultural sense is a very thin skin on the earth’s surface. Often at no great depth the more or less loose material ends abruptly on solid rock. In favourable instances, where natural agencies have not exerted a mixing effect on the soil layer, it is possible to trace the transition from rock to soil in well-defined stages starting from unchanged rock in the lower reaches, and finishing with a surface layer which appears to bear little relationship in colour and texture to the bed-rock below. Overlying the solid rock there may be a layer of rock fragments separated by intrusions of material that is little more than soft rock. In this the details of structure of the mother rock will still be visible, particularly if the rock is crystalline and not very homogeneous in character. The nearer to the surface the examination is made, the greater the changes that are encountered till, at the surface, there is a layer which is recognisable as soil in the agricultural sense.
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