116 T. M. KEADE ON THE DKIFT-BEDS OE THE 



C. Bookleaf clay, greyish brown in colour, containing a level bed of 

 sand (shown by dotted line). The beds are perfectly level, containing 

 no stones or gravel. The interlaminations were as if dusted with 

 sand as fine as emery powder. 



D. Brown clay of very fine texture ; it contained some few glaciated 

 blocks of white limestone. The top of the bed is irregular. 



E. Blue clay, powdery, full of slate fragments, some long and 

 polished like a hone, most of them much striated along their longer 

 axis. No limestone was found in this bed. The junction with the 

 bed below is obscure ; one shades into the other. Nodules were found 

 in this bed. A patch of white clay with gravel occurred between the 

 brown and blue clay at h. 



¥. Alluvium composed of the washings of all the other beds. A 

 cutting in the direction of the valley to drain the brick-croft, showed 

 a rude bedding of pebbles a few feet below the surface. The brown 

 clay in the brick-croft is about 7 feet below the surface ; but it 

 varies much in depth, and, I was informed, rested upon gravel. In 

 the alluvium a good-sized oak was met with. 



Vymwy Water-works, Oct. 2, 1882. 



Since the preceding was written, I have had the opportunity of 

 examining with Mr. G. F. Deacon, joint engineer with Mr. 

 Hawksley, the trench excavated for the construction of the masonry 

 dam of the Yyrnwy reservoir which is to supply Liverpool with 

 water. It is not exactly in the district I am describing, as the 

 watershed really contributes to the supply of the river Severn; but 

 I give a description of it as helping to throw light upon the subject 

 of the mountain-drift. 



The trench, when I saw it (Sept. 28 and 29), was opened out across 

 the valley through the drift to the rock about a distance of 700 feet, 

 or the whole way across the bottom of the valley (fig. 33). The 

 trench was over 120 feet wide at the bottom — an open cutting with- 

 out timbering ; and such an opportunity of examining a valley- 

 section may never occur to me again. . 



The river Yyrnwy is one of the affluents of the Severn ; and the 

 gathering-ground above the dam, about 7 miles S.E. of Bala Lake, 

 contains 23,500 acres. The valley above the dam is remarkably flat- 

 bottomed ; for with 80 feet depth of water at the dam a lake of 1115 

 acres area and 5 miles long will be formed. The bottom of the 

 valley opposite the Hirnant tunnel, which is to tap the intended 

 lake, is 760 feet; and the top water of the lake will be 825 feet 

 above ordnance datum. 



But one of the most interesting facts made evident by the works 

 is that the alluvium and Drift forming the plain of the valley lie 

 in a true rock-basin ; for borings through these deposits proved that 

 the rock inside what will ultimately be the lake or reservoir, is con- 

 siderably below any part of the rock in the bottom of the trench. 



