Vol. 54.] SUBMERGED EOCK-VALLETS IN" SOTJTH WALES, ETC. 261 



break. Sand and silt, from 14 to 26 feet thick, cover the gravel 

 for the whole width of the river. Borings made for a proposed 

 earlier bridge, J mile lower down the river, seem to indicate that 

 the peat-bed extends in that direction, widening out considerably.^ 



Near Gloucester, the two branches of the Severn are crossed by 

 the South Wales Eailway, and trial-pits were sunk before the 

 bridges were constructed. Six were sunk at Over, giving a section 

 extending 100 yards on each side of the river, and five on the other 

 branch, giving a section of about the same length (fig. 7, facing this 

 page). The Liassic clay was reached at from 15 to 28 feet below 

 the level of low summer-water at Over, and at from 18 to 31 feet at 

 the eastern branch of the river. At Over, about 2 feet of rough 

 gravel overlies the Lias, with 2 feet of red clay and sand over it. 

 Then comes a bed of black peat from 3 to 10 feet thick (the bottom 

 of which is 33 feet below the level of high water), and then 4 to 

 8 feet of sandy mud, over which is a second peat-bed 2 to 4 feet 

 thick, covered up by blue clay, sand, and mud to the ground-leveL 

 At the eastern branch of the river, the gravel immediately over the 

 lias is 6 to 18 feet thick, and the red clay over it 2 to 5 feet thick. 

 A bed of black peat, the only one in this section, overlies the red 

 clay ; it is 2 to 4 feet thick, and in thickness and level appears to 

 correspond with the upper peat-bed at Over. Sand and mud cover 

 up the peat to the ground-level. 



The Avon. 



About 7 miles below the mouth of the Wye, the Avon enters the 

 Severn estuary through the remarkable Clifton gorge, upwards 

 of 200 feet deep, and more than 700 feet wide where the Suspension 

 Bridge crosses it. The improvement of the channel in recent years 

 has not revealed any complete cross-section of the rock-bottom, of 

 the river, but the sides have been found to slope down beneath the 

 mud as steeply as 1 in 1, and to 27 and 32 feet below the level of 

 low water. At the Zigzag ^ just above the Suspension Bridge, 

 nearly 7 miles from the Severn, the rock-bottom was traced to a 

 depth of 23| feet below the level of low water on each side, leaving 

 72 feet in the middle unexplored. The mud is 24 feet deep : gravel 

 and large stones are usually found overlying the rock, which is 

 Mountain Limestone. 



West of the Avon an alluvial flat constitutes the southern shore 

 of the Severn estuary as far as Highbridge. As to the rivers beyond 

 that point I have no special information. 



^ Mr. Or. W. Keeling, M.Iast.C.E., has kindly supplied the particulars of 

 the Severn Bridge section. 

 ^ The writer is indebted to Mr. McCurrich, M.Inst.O.E., for this information. 



