380 MR. A. STRAHAN ON THE GLACIATION OF 



however, for the northern and the Welsh drifts to intermingle along- 

 this line. The ballast-pit at Colwyn Bay, for example, shows that 

 the latest deposit of northern derivation, a reddish-brown clay with 

 few boulders*, is jumbled up with a very stony and gravelly deposit, 

 derived entirely from the waste of the Silurian hills by which the 

 bay is enclosed. Similarly the underlying sands, which probabl}^ form 

 part of the northern series of drifts, are interstratified with well- 

 bedded gravels of Wenlock-Shale fragments t. These deposits rest 

 upon an intensely hard and stony basement-clay, composed chiefly, 

 if not entirely, of fragments of "Welsh parentage, and extend but a 

 short distance inland, the valleys leading up among the hills^ 

 containing a drift of purely local origin, which may or may not 

 be continuous with the basement-clay of the coast. In the Vale of 

 Clwyd also it is not possible to assign an exact southern limit to the 

 drift of northern origin, the difficulty being here increased by the 

 fact that both the far-travelled and the local drift are of a red colour. 

 The occurrence of chalk flints, noted by Prof. Hughes as a charac- 

 teristic feature of the drift a])out St. Asaph t, indicates that the 

 currents from the north extended as far south at least as shown on 

 the map. The most southerly boulder of Scotch granite noted by 

 Mr. Mackintosh occurs about one mile south of Denbigh, on the 

 Ruthin road §. In Plintshire, Criffel granite has been found by 

 Mr. Mackintosh about a mile west of Mold, and Eskdale granite 

 with chalk flints on Halkin Mountain, indicating that here also there 

 is tendency for the northern drift to inosculate with the local deposit 

 of western derivation. 



As a general rule, however, the far-travelled drift is found to' 

 overlie that which is made up of the rocks nearer at hand. Thus 

 the northern drift at Colwyn Bay, though mixed up with beds of local 

 derivation, lies on the top of the almost purely local basement-clay. 

 Again at Rossett the northern Boulder-clay of the low ground was 

 clearly deposited after the great banks of local gravel at Gresford 

 had come into existence. 



As in the case of the Lancashire striae, so in that of the Welsh, 

 it is always a Boulder-clay or a gravelly deposit, corresponding in 

 its want of stratification and in its containing scratched stones to 

 a Boulder-clay, that is found to rest on the striated surfaces. The 

 Boulder-clay on the Welsh border resembles that of the low grounds 

 of Lancashire and Cheshire in being interstratified with sands and 

 gravels, but differs from it in being very variable in composition ; 

 for it consists of Silurian, Carboniferous, or Triassic debris principally, 

 according to local circumstances. There is, moreover, a less sharply 

 marked distinction between Boulder-clay and gravel at the higher- 

 levels, the former, when made up of sandstone debris, seeming occa- 



* The lower-level brown Boulder-clay of Mr. Mellard Eeade : Quart. Journ. 

 Geol. See. vol. xH. p. 104 (1885). See also Hall, " On the Geology of the 

 district of Creuddyn," Proc. Liverpool Geol. Soc. (1867). 



t Geology of the Coasts adjoining Ehyl, Abergele, and Colwyn (Geol. 

 Survey Memoir, 1885). 



I Proc. Chester Soc. Nat. Sci., part 3 (1885). 



§ Quart. Journ. Geol. Soc. vol. xxxv. p. 432 (1879). 



