Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/163

Rh bited in the centre of the cutting. From this section it is evident that the Taff river flowed at one time in a straight line from near Quaker's-Yard Junction to Aberdare Junction, excavating the small valley shown in fig. 5, before it formed the horseshoe bend shown on the plan (Pl. V.), through which the river now reaches the same point, G; this valley at D is about 40 feet deep, and 200 yards across, and is excavated out of the wedge-bedded pennants, which dip south-east at a small angle, but have both horizontal and vertical joints well marked, dividing the rock into cubical masses at angles varying from 80° to 85°.

The railway engineers have taken advantage of this natural valley, and cut through it to a depth of 60 feet below the bottom of the gravel.

I wish to call attention to the position of this well-rolled loose gravel, containing many pieces of Old Red Sandstone and Millstone-grit, brought from above Merthyr and deposited at a very high level in the small valley cut out of the sandstone rock at Quaker's Yard. The rails are 393 feet above the datum-line at Cardiff, and the gravel is about 460 feet above the datum-line, and 163 feet above the river Taff, close by. If the horseshoe bend (Plate V.) had been gorged with ice at the gravel-period so as to prevent the passage of the river round the valley B H C, Plate V, then the river might have risen to a height which would have enabled it to pass along the line B D E F C; and the rolled gravel, which is a loose superficial deposit, may have been left by the last great flood.

Returning to Plate IV. fig. 1—before reaching E the rails cross a small lateral valley on an embankment 20 feet high, the surface of the ground dipping at right angles to the rails at an angle of 15°, and so reaching the river at an angle of 30°. The great gravel-cutting E F is now entered, and shows a maximum thickness of gravel of 70 feet.

The great cutting, E F, nearly in the line of the old incline, commences with a series of large blocks of sandstone from 6 to 9 feet long, laid at an angle of 1° N., and forming the last or topmost layer of the gravel. These great pieces of rock have evidently only descended a few yards from the mass from which they were detached. The gravel becomes 55 feet thick at a point (E) where the sand-rock is exposed (see fig. 6, p. 70). The sand-rock in the cutting is visible for nearly 60 yards to a height of 7 feet above the rails at the maximum. This gravel may be observed to be roughly parallel to the top surface of rock, which slopes gently to the south. The greatest thickness of gravel with large boulders occurs near F, reaching 70 feet. The slope of the surface is given at different points.

The larger blocks are often arranged in lines or planes of deposition, forming a distinct measurable angle. In the larger gravel-cutting, E F, they are not in any case more than 15°; near the viaduct they reach an angle of 35° near the rock. These planes follow, and are nearly parallel with, the surface of the rock which crops out at E, and are so arranged that the last bed of gravel forming the surface, being also the last superficial deposit, is also almost parallel with the