The stratification 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 Neath river, described by Mr. Moggridge, Quart. Journ. 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-Red-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 Scrobicularia 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 IV. 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, pl. 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 Rhondda Valley. 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-
