268 ME. T. CODKINGTOl^ ON SUBMERGED ROCK-VALLEYS [Aug. 1 898, 



shale) was soft and broken up. The infiUing of silt is about 

 40 feet deep. 



Forder Lake, a creek over ^ mile long, is crossed by the railway 

 near its mouth. The deepest part of the rock-valley was found to 

 be 66 feet below the level of low-water spring-tides, the sides sloping 

 at 1 in 2 and 1 in 5, corresponding with the slopes above the sea- 

 level. The infilliug of silt is 70 feet deep. The deepest sounding 

 opposite the mouth of this creek is 22 feet at low water, with a 

 mud bottom. 



Wiveliscombe Lake furnished a similar section, reaching 46 feet 

 below the level of low water, with a depth of 56 feet of silt and mud. 



Nottar Creek is tidal for 2| miles ; it is crossed by a viaduct at 

 about 3 mile from its mouth. The rock-bottom was found to reach 

 46 feet below the level of low-water spring- tides, and the silt is 

 55 feet deep. 



At the crossing of the St. Germans river the rock-bottom was 

 proved to be 41 feet below the level of low-water spring-tides, with 

 about 50 feet of silt over it. 



Farther on in this direction the railway crosses rock-valleys 

 above the sea-level, which have sections altogether similar to those 

 of the valleys now partly below that level. 



The drainage-area of the Tamar and Tavy above Saltash is about 

 450 square miles, extending nearly to the north coast, to Dartmoor 

 (1971 feet), and to the Brown Willy range (1360 feet). The 

 drainage-area of the Plym and Torry, flowing into the Laira, is 

 about 70 square miles, rising to 1600 feet; that of the Lynher, 

 entering jS[ottar Creek, about 50 square miles, rising to about 

 1300 feet. The Tidi, entering St. Germans river, drains only 25 

 square miles, rising to about 900 feet above the sea, and the other 

 creeks are fed by streams of quite insignificant length, which 

 drain only a few miles of country, though they rise in comparatively 

 high ground. The streams running into Coombe Lake and Forder 

 Lake flow from land 250 to 300 feet above the sea, that into 

 Weston Mill Creek from land 300 to 350 feet, and those into 

 Stonehouse Creek and Millbay from land 200 feet above the sea. 

 It is evident that neither in area of cross-section nor in depth do 

 the rock-valleys bear any proportion to the present drainage-areas. 



The deep-water channel from Hamoaze follows the western shore 

 to opposite Drake Island, and then bends round to the north of it, 

 passing by Eastern King, Millbay, and the Hoe to the mouth of 

 Catte water, where it turns south. There are soundings of 20 to 23 

 fathoms along this line to opposite Millbay, and of 14 to 15 fathoms 

 opposite the Hoe ; outside that, to the Breakwater, the Sound is 

 nowhere deeper than 9 fathoms, with a bottom of blue clay, which 

 probably covers a deeper channel in the rock. Outside the Break- 

 water the depth increases gradually to the 10-fathom line, which 

 lies f mile outside the Sound. Seaward of this the depth increases 

 more rapidly to 14 and 22 fathoms, but it is quite 1| mile outside 

 the Sound before a depth as great as that at the entrance to Hamoaze 

 is reached. 



