Vol. 68.] POST-GLACIAL CHANGES IN THE LOWEK DEE VALLEY. 181 



II. The Composition of the Glacial Deifts. 



The region in which the River Dee leaves its deep valley in the 

 Welsh hills and enters the comparatively flat country formed by 

 the Coal-Measures has been found to be a critical area in connexion 

 with the post-Glacial history of the remainder of its course. In 

 this district the Glacial Drifts are a thick and important deposit 

 forming the greater part of the surface. They have been derived, 

 roughly speaking, from the Irish Sea Ice-Sheet and from the Welsh 

 Ice-Sheet, as may be seen from their distinctive compositions. 



(a) The Irish Sea Drift. 



Except in the Llangollen Valley and in the upper part of the 

 Ceiriog Valley, the Irish Sea Boulder Clay forms the base of the 

 drift-deposits. It is a stony till, usually red, though sometimes 

 blue, and contains a number of Lake District erratics. The most 

 abundant and easily recognized are Eskdale granite, Ennerdal? 

 granophyre, and various dyke-rocks and lavas which are not as a 

 rule specifically identifiable. At the same time Coal-Measure 

 sandstone and Carboniferous-Limestone boulders are usually pre- 

 sent, while Welsh erratics are rare. The following species of marine 

 shells, occurring as derivatives, have been kindly identified by 

 Mr. Clement Eeid : — 



Turritella communis. 

 Psammohia. 



MyaO). 



Cardium edule. 

 Trophon. 

 Tellina balthica. 



East of the mouth of the Ceiriog this basal Boulder Clay is often 

 overlain by Glacial sands ; while these in turn are succeeded by 

 an upper, almost stoueless, claj-. This is sometimes laminated, 

 and may be a re-sorted boulder-clay which has been accumulated 

 on the land side of the retreating ice-sheet in much the same way 

 as that in which, according to Mr. Wedd, a similar clay originated in 

 North Staffordshire.^ Lake District boulders and shell-crumbs are 

 found also in this clay. 



West of the mouth of the Ceiriog and south of the Dee the 

 basal Eed Drift (Irish Sea Drift) occurs below the AVelsh Drift, and 

 has been recognized in the Ceiriog Valley as far up as Castle Mills. 

 In the Brookside tributary also, it has been traced up to 1000 feet 

 above sea-level. Judging from the abundance of northern pebbles 

 in the gravels throughout the district, one might well consider 

 that the Eed Drift formerly extended over aU the ground east of 

 the Millstone Grit escarpment. At the same time, it must be 

 borne in mind that if the Irish Sea and Welsh ice had mingled, 

 Welsh Boulder Clay and northern erratics might be deposited at 

 the same time. The occurrence of northern boulders in the Welsh 

 gravels might thus be accounted for ; but the former explanation 

 seems to be more reasonable, and is supported by a section of sand 



^ W. Gibson & C. B. Wedd, ' Geology of the Country around Stoke-upon- 

 Trent— Explanation of Sheet 123 ' Mem. Geol. Surv. 2nd ed. (1905) p. 63. 



