Vol. 54.] SUBMERGED ROCK-VALLEYS IN SOUTH WALES, ETC. 259 



Then comes the deep channel called ' The Shoots,' 550 yards wide 

 at the level of low water, and 280 yards wide at the bottom, 

 which is as much as 60 feet below the level of low-water of spring- 

 tides. The sides are steep, on the western side almost precipitous in 

 places. This deep channel is excavated in the Pennant Beds of the 

 Coal Measures. West of ' The Shoots ' the rock is above low water 

 for about 300 yards, and near the western shore is a second low- 

 water channel 12 feet deep. 



The rock-bottom is bare all across the present river, but in the 

 cutting at the eastern end of the tunnel an old channel was laid 

 open, the beds in which are described as follows by Mr. C. Richardson, 

 the engineer of the tunnel : — The New Red Sandstone rock, which 

 in the bed of the river is considerably above the level of low water, 

 at the end of the tunnel is 13 feet below that level, from which it 

 rises gradually eastward. Fpon it lies a coarse gravel with large 

 masses of Old Red Conglomerate, Millstone Grit, and Mountain 

 Limestone, over which is a finer gravel, with clean river-sand over 

 it. It is stated by Prof. Sollas ^ that the lowest gravel contained 

 glaciated stones. These beds, which were quite free from mud, are 

 covered with silt or muddy sand, over which is a bed of peat below 

 half-tide level, or about 20 feet below ordinary high-water spring- 

 tides ; then comes a pale-coloured clay, then another peat-bed 2 feet 

 thick, over which is blue clay up to the surface of the ground at 

 about high-water level. These beds preserve their general level, 

 undulating slightly, all along the cutting, for more than | mile. 

 They are cut through down to the silt beneath the lowest peat-bed 

 by four old rivercourses which are filled up with mud. The trunk 

 of a large oak-tree lay below the upper peat. Many years before, 

 when the Bristol & South Wales Union Railway was being con- 

 structed, a peat-bed 2 to 7 feet thick was found in five places east of 

 this cutting, resting directly upon the red marl, under 11 to 14 feet 

 of clay. 



The cylinder-foundations of the Severn Bridge, 14 miles above 

 the tunnel, furnished another section of the bed of the Severn 

 (fig. 6, p. 260). Twenty-one piers, composed of forty-eight cylinders, 

 were sunk in the bed of the river to the marl-beds of the Old Red 

 Sandstone. The deepest part of the rock-bottom is near the western 

 side, where it is 42 feet below the level of low water, and is covered 

 up by 24 feet of sand and silt. The rock rises thence gradually 

 to the eastward, and reaches the level of low water at the eastern 

 shore, rather more than ^ mile distant. A little to the east of the 

 deepest part of the rock-section, there is a bed of peat about 2 feet 

 thick immediately over the rock, 39 feet below the level of low 

 water, and 68 feet below high- water spring-tides. The peat, which 

 is covered by about 3 feet of clayey gravel and stones, was found 

 only in one pair of cylinders, but a bed of clayey gravel and stones 

 was found overlying the marl nearly to the eastern bank, with one 



^ Quart. Journ. Geol. Soc. voL xxxix (1883) p. 622 



