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 that the scour would always have been sufficient to keep open a channel well below low-water level. No stream tin has been worked in the Plymouth estuary, so that we cannot point to any continuous sections in the ancient alluvial deposits, such as are found further west. These tin deposits, however, date in the main from a period somewhat earlier than that with which we are now dealing. They were probably swept down from Dartmoor when floods were far more severe, during the annual spring melting of the snow during the Glacial epoch. Unfortunately, also, the lately finished harbour works at Devonport proved the existence of only modern alluvium, without any submerged forests. Before dealing with the rivers which flow into Plymouth Sound it is necessary, however, to say a few words about the harbour itself and its origin. Plymouth Sound and the various submerged valleys which open into it illustrate well both the continuity of geological history, and the great difficulties which await us when we deal with valley erosion which in part dates far back into Tertiary times. The Sound is not merely a submerged continuation of the Pleistocene valleys, and between this wide gulf and the narrow valleys there is a curious want of continuity. We do not know the true depth to the rocky floor; but at two places just outside the mouths of the estuaries deep hollows are scoured through the sands. One of these, just outside the Hamoaze, or estuary of the Tamar, shows a rocky bottom at 150 feet, probably the true rock-floor of that part of the Sound. The other hollow, 132 feet, is just outside the Cattewater; but does not reach rock.