Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/171

Rh cause of this great fall of rock and clay is due to the position of the permeable and impermeable beds, and the influence of the immense rainfall occurring, I believe, at former periods, which has detached large portions from the high ground on the side of the valley, and let it fall into the channel of the river below.

The character of the deposit in this valley is very simple; it is sand-rock, clay, and shale, derived from the adjoining higher beds. In the pit sunk near C all the stones were either weathered or rolled, including one block of several tons weight. As the Rhondda valley is separated by a hill 800 feet high in one part, and 600 feet from Hirwain in the lowest part, above the river-course, which flows off from the Millstone-grit and Old Red Sandstone, there are no boulders of these rocks in this valley; all is purely local.

In sinking the pit at C, which was 20 feet in diameter, the rock was reached on the west side of the pit 8 yards before solid ground on the east side was met with, so that the angle of the escarpment of the ancient bank of the river or side of the valley was about 50° to the ancient river.

By means of this sinking it is known that the gravel there is 101 feet thick; and, taking the average thickness to be 20 yards, and the width 1700 yards, there would be 34,000 cubic yards in each yard run (supposing that section C represents the average amount of gravel in the valley), which, at l$1⁄2$ ton to the cubic yard, represents 89,760,000 tons of gravel to the mile. The river flows over at least 50 feet of gravel, and probably much more.

The section of gravel passed through in the pit is as follows:—

The fall of the Rhondda river in the distance of a mile before it reaches C is about 500 feet, in which distance there are two waterfalls.

The vast extent and the character of the Rhondda gravels proves that the denudation of the Rhondda valley was not dependent upon large rivers, or marine or glacial action, but might have been caused by intense pluvial action.

In a wet season, water pours over the escarpment along the edge of the hills into the Rhondda valley, and is not confined to a few distant watercourses; but there is no evidence of any accumulations of gravel now; perhaps the fall of stones along the escarpment equals the quantity carried down the river in floods.

The Salisbury pits are celebrated for their mammalian remains,