Vol. 54.] IN SOTJTH WALES, DEVOIT, AND CORNWALL. 255 



The borings and trial-shafts for a tunnel were sunk at the mouth 

 of the river below Briton Ferry, some 2 miles below the bridge. In 

 the deepest shaft, 350 yards west of the present low-water channel, 

 rock was reached at 53 feet below the level of low water. Tho 

 borings east and west of this shaft reached rock at 33 and 35 feet 

 below the level of low water, but though 53 feet may be about the 

 greatest depth of this particular rock-channel, there is ample room 

 for a deeper one on either side of it.^ 



The deposits above the rock were found to be gravel and clay, 

 and sand and clay in layers under a varying thickness of sand. 



Low-water mark is now nearly 2J miles to the seaward of this 

 section, and a depth of 53 feet at low water is not reached for more 

 than 2 miles farther out. 



The valleys of the Tawe and the Neath lie parallel to each other, 

 about 5 miles apart, separated by ground 1000 feet high. They 

 extend in a N.IST.E. direction for 22 miles to the Old Red Sandstone 

 mountains of Brecknockshire, which rise 2,300 to 2,600 feet above 

 the sea. The drainage-area of each valley is about 100 square 

 miles. 



To the eastward of the Neath river the Ogmore, the Taff and 

 Ely, the Rumney, and the Usk flow into the Bristol Channel, the 

 first-named in a well-defined valley, but the others through a wide 

 alluvial flat beneath which the position of submerged rock-valleys 

 has not been traced, though at Cardiff and Newport considerable 

 thicknesses of deposits over the rock have been proved, often 

 having a coarse gravel at the base. An interesting section of a 

 submerged land-surface was exposed in the construction of Barry 

 Docks. It was described by Mr. Strahan,^ who showed that a 

 subsidence of at least 55 feet must have taken place since the 

 formation of the lowest peat-bed. 



The Wye. 



Before and during the construction of the Chepstow railway-bridge, 

 the bed of the river Wye was explored by sinking a 3-foot trial- 

 cylinder, by borings, and by the sinking of twelve cylinders of 6 and 

 8 feet diameter to the rock, through 44 to 58 feet of overlying deposits. 

 The general section thus obtained is illustrated in fig. 3 (p. 256). 

 The eastern or left bank of the river is a precipitous cliff of Mountain 

 Limestone, 125 to 130 feet high from low water. The rock was 

 traced for 100 feet from the face of the cliff; it is less precipitous 

 near low-water level, and generally horizontal beyond, where it is 

 22 feet below the level of low water or 63 feet below high-water 

 spring- tides, and is overlain by a thin stratum of red clay. Against 

 the foot of the cliff there is an accumulation of rock-fragments as 



^ The writer is indebted to Mr. Yockney, M.Inst.C.E., the engineer of the 

 railway, and to Mr. Groldwyer, M.Inst.C.E., the resident engineer, for par- 

 ticulars of these two sections. 



2 Quart. Journ. Geol. Soc. vol. lii (1896) p. 474. 



Q. J. G. S. No. 215. u 



