There was formerly a sheet of water near Twyford named Ruscombe Lake, which had some claim to be called a natural lake, in that it was a low-lying bit of ground which was flooded owing to the absence of a good outlet. Its natural outlet was into the river Loddon, and there is a patch of alluvium extending from its site through Stanlake Park to that river. It was eventually drained by making a deep channel called the "Cut," draining a considerable area into the Thames near Bray. It has been asked why the river Thames did not follow the line of Ruscombe Lake and the Bray Cut, all soft clayey soil and low ground, instead of cutting the great and deep valley through the chalk by way of Wargrave, Henley, Great Marlow, and Maidenhead. The explanation probably is that the river Thames existed before any of these valleys, and that its course was determined by local features which have long since been destroyed by rain and streams, and by the river itself. 6. GEOLOGY AND SOIL. Before giving further account of the physical geography of the county it is necessary to learn somewhat of its geology, as the physical conditions are to a large extent dependent upon geological structure. By Geology we mean the study of the rocks, and we must at the outset explain that the term rock is used by the geologist without any reference to the hardness or compactness of the material to which the name is applied; thus he speaks of loose sand as a rock equally with a hard substance like granite. Rocks are of two kinds, (1) those laid down mostly under water; (2) those due to the action of heat.