266 ME. T. CODRINGTON ON SUBMERGED ROCZ-V ALLEYS [Aug. 1 898, 



Hamoaze. 



Inside Devil's Point, on the Devon side of Hamoaze, is Stone- 

 house Pool, continued in a narrow creek for a mile inland, in 

 which there is a considerable depth of mud, but about which informa- 

 tion is wanting except at the inner end, where the creek is crossed 

 by the Cornwall Railway. There, at l^ mile from the mouth, occurs 

 o feet of silt over a rock-bottom 9 feet above low-water level. 



Passing up the eastern side of Hamoaze, we next come to Keyham 

 Lake, the rock- bottom of which was proved by borings through silt. 

 The greatest depth to the rock reached was 25 feet below the level of 

 low water, at nearly | mile from the mouth of the creek. 



The piles of the railway-viaduct across Weston Mill Creek showed 

 the rock-bottom to be a valley with sides sloping at the same inclina- 

 tion as above the sea-level, namely, 1 in 6 to 1 in -i^, and with the 

 bottom 6Q feet below the level of low water in the creek, or 60 feet 

 below that of low-water spring-tides in Hamoaze, where the deepest 

 ■sounding opposite the creek is 72 feet below the level of low water. 

 The mud is 72 feet deep at the deepest place on the section. 



At the crossing of Tamerton Lake by the South- Western Railway 

 the rock-bottom was traced only to a depth of 15 feet below the 

 level of low water, where it was shelving downwards, the deepest 

 part of the rock- valley being north of the bridge. 



The cylinder-foundations of the viaduct across the Tavy gave a 

 complete section of the rock-vailey of that river (fig. 10). From the 



:3!f. 



Pig. 10. 

 River Tavy 

 H. W. S. T. 



[Scales : Horizontal, 400 feet=l inch ; vertical, 100 feet=l inch.] 



south the bottom slopes downward gradually to a depth of 68 feet 

 below the level of low water, and then rises abruptly at a slope of 

 1 in 1|. In every cylinder the rock (Devonian shale) was found to 

 be covered with a deposit described as ' stiff yellow clay, including 

 small granite-boulders,' or ' granite-boulders in hard yellow clay,' 

 from 2 to 4 feet in thickness. Above this bed, which thinned out 

 against the sides of the valley, is sand, about 68 feet thick at the 

 deepest part, and reaching 8 or 10 feet above low- water level on 

 the north side.^ 



^ The particulars of this section were kindly supplied to me by Mr. R. H. 

 Worth, who published it, and sections of some of the creeks round Plymouth, 

 in Trans. Plymouth Inst. 1890-9] . 



