272 ME. T. CODEINGTON ON SUBMERGED EOCK-V ALLEYS [Aug. 1 898, 



flank of the more southera valley, about 100 feet lower. Yet 

 another group of large blocks is passed through by the railway near 

 Gawton, between Beer Alston Station and the tunnel, about 4 miles 

 higher up the valley. They lie near the water-parting between the 

 Tavy and the Tamar, which are there not much more than a mile 

 apart, at about 400 feet above the sea. Other blocks strew the 

 slope of the Tamar Yalley. 



In the bottom of the Tamar Yalley at Eumleigh, about a mile above 

 Calstock, is a Boulder Clay deposit worked for'brickmaking. The 

 upper 4 to 6 feet, reaching to 10 or 12 feet above high water in the 

 river, is lighter in colour and less clayey than the lower, which is 

 visible to a depth of 20 feet. The matrix is a mixture of clay and 

 small stones, and contains angular slate, rounded boulders of granite, 

 white opaque quartzite, and a hard, dark, jasper-like rock with 

 brown veins. A block of granite with large felspars, subangular, 

 but smoothed, measured 2x2x2 feet, and many other boulders 

 nearly or quite as big remain in the bottom of the pit. 



It would be interesting to trace the origin of these boulders to 

 the high ground drained by the present river, but it is more to the 

 present purpose to lay stress upon the marked glacial character of 

 the deposits. Mr. R. H. Worth, who brought this and the similar 

 deposit near Weir Head to my notice, assured me that what was 

 called ' stiff yellow clay with granite-boulders ' and ' granite-boulders 

 in hard yellow clay' found in the cylinders of the Tavy bridge- 

 foundations corresponded in all respects with these deposits. 

 There appear thus to be here, as in South Wales, glacial deposits in 

 rock-valleys now below the sea-level, the valleys of course being 

 older than the deposits laid down in them. In this connexion the 

 observation of Colenso, that the rock-bottom under the stream-tin 

 deposit at Pentuan appeared to be worn by friction, made some 

 years before Agassiz drew attention to glacial action in this country, 

 is interesting. 



It is difficult to suppose that the rock-valleys now many feet below 

 the level of low water were scooped out, by whatever cause, or moulded 

 by land-ice at that level. If those in South Wales were to be 

 raised above the sea-level, the sea-bed, as far out as the 10-fathom 

 line, would be laid dry. This would include all Milford Haven 

 except a few narrow pools, all Caermarthen Bay inside Caldy and 

 Worm's Head, all Swansea Bay, and all the Bristol Channel to the east 

 of Barry, except a narrow channel extending up to the Plat Holme, 

 and a few pools higher up. The sections which have been described 

 would thus be far from the sea, and it may be reasonably supposed 

 that the valleys would have been considerably above it. An elevation 

 of 20 fathoms would lay dry all the Bristol Channel east of Swansea 

 and Ilfracombe, and leave the mouth of Milford Haven more than 

 a mile from the sea. 



To raise the rock-vaUey of the Dart at Maypool above the sea, 

 everything out to the 20-fathom line must be laid dry. This line 

 passes 1| mile outside the mouth of the Dart, 1 to 2 miles outside 



