Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/164

70 top surface of the bed of stones on which it lies, just as the last bed is almost parallel with the mean angle of the surface of the rock on which it is spread by the action of water (see E). This section is to natural scale.

The exposure of rock at many different points of the horseshoe bend near the piers of the viaduct, and in the bed of the river, adds to the interest of the sections at Quaker's Yard. I shall now describe the character of this large rock-basin in which the gravel under consideration is contained. The first place at which I observed the rock was near the river, where a small quarry is worked close to one of the piers of the viaduct. The dip is slightly south-east, and the stone is divided by nearly vertical joints; there is another quarry of the same character on the opposite bank, between the tramway-bridge and the south-east end of the railway-viaduct, but close to the latter. In following the tramway round the horseshoe bend the rock is frequently visible; in the river-bed it is concealed by a coating of gravel as far as the junction of the stream at the north-east point of the bend. The gradient of this small stream is steep, and the water enters the river with some velocity, running nearly parallel with the Taff near the junction. The water of the Taff, up to the junction, is smooth, and flows quietly over a gravel bottom; but the lateral stream, coming in with added velocity, causes the stream of the Taff River almost to split into two different paths, one taking the east, and the other, or larger portion, crossing the bed of the river, and hollowing out a deep channel in the rock close to the west bank. The river flows below the junction at H entirely over rock, the gravel not being able to rest on account of the velocity of the current.

The action on the river-bed corresponds with what I have represented in other sections that I have made, which show that the work of denudation, or cutting out of the solid rock, is principally due to running water, either in the lateral streams themselves, or in the main river immediately after the side-stream has emptied itself into it. Of this I shall bring forward other examples at another