CoachA geological map shows, at every point of the ground surface, the rock unit that lies there beneath the soil, with the contacts — the surfaces separating two units — and the structures (faults, fold axes, attitudes of layers) that affect them. A map unit is a body of rock that can be recognised and followed in the field and is drawn in one colour; its outcrop is the area where it lies at the surface.
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Example 4.18 (The history of the Varden Hills)
On the map of the Varden Hills: the schist is the oldest unit; the granite cuts it, so it is younger; the sandstone rests on both, cutting off the granite’s outline: an unconformity, after a long erosion. The sandstone, limestone, shale and chalk follow one another without a break and are folded together into an anticline (sandstone and basement in the core, near ) and a syncline (chalk in the core, east of the map’s centre): the folding came after the chalk was laid down. The fault offsets the folded contacts: it is younger than the folding. Finally the river has cut down and spread its alluvium, which no fault breaks.