Notes upon the Diluvial and Allunlal Deposits of the Taffe f^ulley. 273 



Between Merthyr and Castel Cocli several tributary streams, each flow- 

 ing down its peculiar valley, fall into the Taflfe ; of these the Cynnon and 

 Rlionddha are the principal on the west, and the Bargawd or river springing 

 out of the Taffe, the only considerable one on the eastern side. The heads of 

 these subordinate valleys incUne more or less obliquely towards the north. 



The upper twenty miles of country, composing this valley, consist of 

 pennant sandstone and coal measures ; above Merthyr, the carboniferous 

 limestone appears at Morlais, and below the ravine the Lower Garth and 

 Castel Coch mountain present a similar band. 



The rocks skirting the plain are, for the most part, dolomitic conglome- 

 rate, new red sandstone, marl, and lias. The rocks of the carboniferous 

 series dip at an irregular, but for the most part considerable, angle ; those 

 of later origin are more generally horizontal. 



Such being then the rocks, of which this valley and the skirts of its em- 

 bouchure are composed, we shall next pass to its contents. 



These are of four kinds. 



I. The diluvial gravel, occupying the bottom or trough of the principal 

 and some of the subordinate valleys. 



II. The debris, by which the sides of the mountains are masked, and the 

 edges of the gravel more or less covered up. 



III. The gravel beds now occupying the beds of the existing streams. 



IV. The silt and alluvial mud occurring above all these, in various parts 

 of the valley. 



I. Of the diluvial gravel. 



This gravel varies considerably, both in the thickness and figure of its 

 beds, in the proportion between the sand and pebbles of which it is com- 

 posed, in tlie bulk and figure of those pebbles, in their geological character, 

 and in the direction of tlieir dip. 



1. The tiiickness and figure of its beds depend obviously upon the phy- 

 sical features of the basin in which the gravel is deposited ; upon points be- 

 tween the confluence of two valleys, as at Quakers' Yard, it forms a tongue 

 extending up the mountain to a considerable height ; above sudden contrac- 

 tions, it forms beds of immense thickness, of which that intersected by the 

 road under Craig-yr-alt may be cited as a magnificent example; and where 

 the valley expands, in which case the plain is formed usually of this gravel. 

 Where the sides of the valley are contracted and nearly parallel, and the 

 mountains steep, gravel does appear to have been deposited. 



Tills gravel does not, under ordinary circumstances, ascend very high up 

 tiic mountains; and where it does, its escarpment is usually steep. In the 

 bottom or at the mouth of a valley it forms a plain, which, though compa- 

 ratively level, is not to be confounded with the surface of an alluvial soil, 

 seeing that it is often thrown up into tumps of various figures, and not un- 

 frccjuently, as near Velindre, stands out in a long lunated tongue, with a 

 steep escarpment oa the concave side. 



