.# 



DEIKTS OE THE YALE OF CLWYD 79 



in the case of the lower-level sands such as those of the Vale of Clwyd ; 

 but this distinction would be difficult to maintain, and the manner 

 of occurrence of the North-Wales Marine drift along well-defined 

 terraces and with current-bedding and horizontal stratification 

 bearing a definite relation to the physical geography of each district 

 renders this explanation improbable in that case. Besides, the 

 character of the shells is not consistent with the idea of such extreme 

 glacial conditions. The Moel Tryfan deposit, as pointed out by 

 Gwyn Jeffreys (Quart. Journ. Geol. Soc. vol. xxxvi. 1880, p. 355), 

 is not strictly glacial ; the fauna has a Norwegian rather than an 

 Arctic facies. 



So, again, in the case of the marine drifts of the plains of Cheshire 

 and Lancashire, Shone draws attention to the mixture of forms 

 (Quart. Journ. Geol. Soc. vol. xxxiv. 1878), and suggests in expla- 

 nation that the Scandinavian shells in the sands and gravels were 

 derived from an older Boulder-clay (p. 389). Even in the case of 

 the clay-drift, which he thinks was dropped in quiet deep water, he 

 shows that many of the shells must have been carried into it from 

 a sandy shore, and explains this (p. 388) by reference to existing 

 conditions in the estuary of the Dee, where shells with sand in 

 them now get carried out by thin shore-ice into deeper water. In 

 this case it is clear that scratched boulders of granite &c. from the 

 northern land-ice drift must get dropped into the same clayey 

 deposit without having their striae obliterated by being rolled along 

 a shingly shore. 



Bearing all this in mind, we may now pass on to consider the 

 second division of the drifts of the Yale of Clwyd, a stage in which 

 the deposits were derived in part from the old western drifts above 

 described, and also in part from the boulder-clays which were 

 formed at the end of the ice from the Lake-district, and from the 

 shingle which travelled along the shore from the flint-bearing drifts 

 of other areas. 



Paet TIL 



The St. Asaph Drift. 



In a paper read before the Chester Society of Natural Science in 

 1880* I spoke of this as the Clwydian .Drift ; but as further sub- 

 divisions seem to be already possible, I now use the name St. Asaph 

 Drift as more precise for the stratified beds on which the Cathedral 

 of St. Asaph stands. It might be called the Sea-drift, being the only 

 drift in the vale which we know to have been of marine origin ; or 

 we might speak of it as the Newer Drift, to distinguish it from that 

 older deposit on which it rests irregularly wherever the two are 

 found together, and which, on other evidence, seems to belong j^o a 

 previous state of things. It might be referred to as the Northern 

 Drift, seeing that in it we find for the first time in the history of the 

 vale fragments of northern origin. 



* Proc. Chester Soc. Nat. Sci. no. 3 (1884) 



