264 MR. T. CODEINGTON ON ST7BMEEGED EOCK-VALLEYS [Aug. 1898^ 



and there above it. Outside this line the depth increases, and the 

 10-fathom line is but 200 to 600 3'ards distant, passing close to the 

 Mewstone, thence to Berry Head and outside Torbay. The depth 

 of the rock-bottom at Maypool is not reached until nearly as far out 

 as the 20-fathom line, about 2 miles outside the mouth of the Dart. 



It may be noticed here that pai't of the town of Dartmouth 

 (Butter E-ow, etc.) appears to be situated on a silted-up creek. The 

 depth of the silt is considerable, and when a new quay-wall was 

 built a few years ago the rock-sides of the creek were found to slope 

 downwards to such a depth that the bottom was not reached by 

 borings carried down from low-water spring-tides. 



The drainage-area of the river Dart is 200 square miles, extending 

 to Dartmoor (1971 feet above Ordnance datum). Waterhead Creek 

 and Long wood Creek, where they join the Dart, are only 1| mile 

 from the water-parting between that river and the coast between 

 the Mewstone and Berry Head. The ground at the dividing ridge 

 is 480 to 580 feet above the sea. 



About 12 miles south-west of the Dart is the Salcombe estuary, 

 which enters the sea between Prawle Point and Bolt Head, passing 

 through the Archaean (?) rocks in a narrow channel. Inside that,, 

 in the Devonian shale, the estuary widens considerably and branches- 

 into seven creeks. There are no sections, but appearances lead me 

 to think that there are submerged rock-valleys of considerable depth. 

 The streams running into the creeks are short, rising in ground 

 from 300 to 450 feet above the sea-level. The whole area now 

 draining into the estuary does not exceed 50 square miles, but in 

 former times the upper part of the drainage-area of the Avon may 

 have been included. Parther west the estuaries of the Avon,Erme,. 

 and Yealm, \^'hich rise on Dartmoor at 1500 to 1600 feet above the 

 sea, present similar features, 



Plymouth and Neighbourhood. 



A remarkable series of rock-valleys below the sea-level has been 

 explored in the neighbourhood of Plymouth. 



The general configuration of Plymouth Sound and the waters 

 communicating with it will be seen on reference to a map. At the 

 inner end of the Sound, east of Plymouth, is the entrance to Catte- 

 water and the Laira, a section of the latter being afforded by the 

 cylinder-foundations of a railway-bridge. Opposite the entrance tO' 

 Cattewater is Sutton Pool. 



West of the town of Plymouth is Millbay, occupied by the Great 

 Western Docks, the bottom of which was thoroughly explored 

 by borings. Parther west is the narrow entrance to Hamoaze 

 through Devonian Limestone, inside which the water widens out 

 imd branches into many creeks or lakes, all in Devonian shale- 

 Eight of these lakes are crossed by timber viaducts of the Cornwall 

 liailway, in spans of 66 feet, the pile-foundations of which gave 

 accurate sections of the rock-bottom. The Tamar is crossed by 

 Saltash Bridge, and Tamerton Lake and the Tavy by viaducts of 

 the South-Western Eailway. 



