Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/165

Rh time, to show that the work of denudation performed by the main stream flowing at the bottom of a deep valley becomes insignificant compared with that of the side-streams flowing into the main river.

The river itself only operates on a narrow slip of ground at the bottom of the valley, although, by the momentum it possesses and its constant action, it has a powerful denuding force. The side-streams flowing into the river, on the contrary, although more intermittent, act on the whole of the remaining surface of the valley, either through themselves or their smaller tributaries. They thus denude an area twenty times as great (or even more) as the river itself can touch. The side-streams are less fixed in position, and run with many times the speed of the river itself. Their power as an agent of denudation is not in proportion to the average amount of water which they contain, but to their speed in flood-times.

The side-streams occupy channels of infinitely greater length than that of the main river. The points where the greatest force of water is employed now at the bottom of the Taff valley are where the water in the side-streams and the main stream unite, as at the north-east corner of the horseshoe bend at Quaker's Yard at H.

At the south-eastern corner is a large quarry close to the road, in which 60 feet of massive-bedded sandstone is seen. It is also seen in the river-bed.

As the rails near the viaduct are 96 feet above the level of the river, and as rock is seen 120 feet above the rails, and 96 feet below them, we have at least 216 feet of solid sand-rock exposed in these sections, without any intermixture of clay or shale. This is an important fact, as we may observe that a horseshoe bend of as perfect a form as in alluvial soil may be excavated in solid rock. Mr. Fergusson has attempted to prove that the sizes of the bends and curves in rivers depend upon the velocity of the river, and that a curve in a river falling 10 feet in a mile will be smaller than one in a river falling 10 feet in a mile; but at Quaker's Yard we have a well-formed large horseshoe bend in a river falling 50 feet in a mile. The laws which regulate the flow of rivers may be different in sand-rocks from what they are in alluvial soil; but I still hold the view as to the cause of these curves which I mentioned to the Society in 1852, but which it is unnecessary to discuss at the present moment.

At A, Plate IV. fig. 1, the gravels are exhibited very clearly in the cuttings at and near Quaker's-Yard Station; these are 200 yards long and 40 feet deep, and are represented at the north end of the section. The local sand-rock furnishes the largest, most numerous, and least-rolled pieces, many of them 8 and 9 feet long. The boulders of Mill-stone-grit are generally well rolled, and not more than 6 feet long; those of Old Red Sandstone are throughout well rolled, and not often more than 2 feet. Not more than one stone out of 500 is derived from the Old Red Sandstone or Millstone-grit.

The gravel is supposed to be from 30 to 40 feet thick at A, covering the side of the hill to a height of 200 feet or more above the river.