TYLOR QUATERNARY GRAVELS. 77 



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 im- 

 mense 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 ofi' 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|-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 : — 



ft. in. 



Turf or peat , 3 6 



Blue clay with large stones 23 6 



Sand 18 



Blue clay 5 6 



Gravel with large stones 26 6 



Grravel with clay 13 



Cliff 5 



Grravel 6 



101 



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 water- 

 faUs. 



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. 



