72 PROCEEDIIfGS 0? THE GEOLOGICAL SOCIETY. 



The stratiiication is generally flat where the sand is bedded at all ; 

 but the junction of the sand-rock and gravel is rarely seen at any 

 part of this section north of the viaduct. 



The gravel of the Taff retains its character as far as the sea-coast, 

 and resembles that of the iS^eath river, described by Mr. Moggridge, 

 Quart. Jo urn. Geol. Soc. vol. xii. p. 169. During the excavations for 

 the Swansea Docks, Mr. Moggridge found part of this river-gravel, 

 which he identified by the presence of river-rolled Old-Ked-Sandstone, 

 Limestone, and Millstone-grit boulders (which had come from the 

 parent-rocks, more than twenty miles distant) above stratified beds 

 of marine clay alternating with peat. The marine clay contained 

 Scrohicularia piperata, a shell which now lives abundantly in the 

 sands of the adjoining shore. 



The river Taff and the Swansea and Neath rivers expose sections 

 of gravel all along their course to the sea-coast ; and the ancient 

 banks and river-beds of these rivers consist of well-rolled boulders 

 from the higher districts, deposited with those of local rocks in every 

 part. Their ancient banks have a contour and escarpments like 

 those of the Aire (see Plate lY. figs. 4 & 5), The gravel becomes 

 rather smaller as the sea-coast is reached, and has then, no doubt, 

 been removed and redeposited many times. The gravel near the 

 level of each river is of similar age throughout its course, and must 

 be considered very recent. 



Mr. Godwin-Austen (Quart. Journ. Geol. Soc. vol. vii. p. 134, pi. 7) 

 also describes this part of South Wales under the heading " Insulated 

 Areas in periods of greatest depression," from evidence of a very 

 different character from that offered by me. 



Fig. 7 is accompanied by a scale of heights in feet, and of length 

 in chains (66 feet each), and is a reduction by the pentagraph of a 

 section taken for me in 1867 to determine exactly the thickness 

 and position of the Hirwain gravel. (See Map, fig. 3, page 67.) 

 The thick deposits of gravel on Hirwain Common are cut through by 

 several rapid streams flowing from the lofty range of hills separating 

 Hirwain Common from the Ehondda Yalley. I have selected the 

 section, fig. 7, along the course of one of the Hirwain streams, as an 

 instance of the continuity of gravel from high ground to the bottom 

 of the valley, and of the regularity of the action of denudation and 

 also of deposition. 



Fig. 8 is a reduction from a drawing made of an excavation in 

 the gravel and the underlying coal-seam. The gravel is tinted to 

 represent the lower, middle, and upper gravels. The lowest portion 

 consists principally of the clays and shales of the Lower Coal-measures, 

 slightly moved from their outcrop down towards the river Cynon. 

 The middle division is stronger clay than the upper ; but as the 

 gravels approach the river they become sandy, and have been worked 

 over for the limestone boulders they contain, and for sand for the use 

 of the furnaces at the ironworks. 



This sketch gives an actual view of the passage of the soft beds of 

 the Coal-measures into gravel, at the end of the gravel-period. This 

 excavation gives a better illustration of the actual process of denu- 



