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82 showed clearly that in Cornwall, as elsewhere, old land-surfaces can be found far below the sea-level. The shape of the valley-bottom, and the rapid lessening of its fall as the coast is approached, in several cases point clearly to the proximity of the sea, and show that its ancient level must have been about 70 feet below present tides.

Here it may be pointed out that as the sea-level is approached the steady seaward fall of a rocky V-shaped valley quickly lessens, changes to a gentle slope, and then to a flat, more or less wide according to the length of time during which the river has been kept at the same level, and could only swing from side to side, without deepening its bed. In Cornwall there is a definite limit below which the erosion of the valleys has not gone, and at this level the valley widens and flattens as it does elsewhere.

The eastern border of Cornwall is formed by the extensive harbour which receives the Tamar, Tavy, and Plym, and this harbour is obviously nothing but a submerged seaward continuation of the combined valleys eroded by these rivers. The rivers, it must be remembered, though short, receive great part of the drainage of Dartmoor, where the rainfall is excessive; they are therefore very liable to floods. These streams also bring down much coarse gravel and sharp granitic sand, so that their erosive power must be exceptionally great during floods. It seems therefore