ANCIENT GLACIERS IN EASTERN HEMISPHERE. 149 



earth' which was overlaid by bedded deposits of shell- 

 bearing drift, with erratics of the northern type. 



" It has been supposed that the drift-deposits were ma- 

 rine accumulations ; but it is inconceivable that the cave 

 could ever have been subjected to wave-action without the 

 complete scouring out of its contents. 



" To resume the delineation of the limits of the great 

 Irish Sea Glacier : From the Vale of Olwyd the boundary 

 runs along the range of hills parallel to the estuary of the 

 Dee at an altitude of about nine hundred feet. As it is 

 traced to the southeast it gradually rises, until at Frondeg, 

 a few miles to the northward of the embouchure of the 

 Yale of Llangollen, it is at a height of 1,450 feet above 

 sea-level. Thence it falls to 1,150 feet at Gloppa, three 

 miles to the westward of Oswestry, and this is the most 

 southerly point to which it has been definitely traced on 

 the Welsh border, though scattered boulders of northern 

 rocks are known to occur at Church Stretton. 



" Along the line from the Vale of Olwyd to Oswestry the 

 boundary is marked by a very striking series of moraine- 

 mounds. They occur on the extreme summits of lofty 

 hills in a country generally almost drif tless, and their ap- 

 pearance is so unusual that one — Moel-y-crio — at least has 

 been mistaken for an artificial tumulus. The limitation 

 of the dispersal of northern erratics by these mounds is 

 very clear and sharp ; and Mackintosh, in describing those 

 at Frondeg, remarked that, while no northern rocks ex- 

 tended to the westward of them, so no Welsh erratics could 

 be found to cross the line to the eastward. There are 

 Welsh erratics in the low grounds of Cheshire and Shrop- 

 shire, but their distribution is sporadic, and will be ex- 

 plained in a subsequent section. 



" Having thus followed around the edges of this glacier, 

 it remains to describe its termination. It is clear that the 

 ice must have forced its way over the low water-shed 

 between the respective basins of the Dee and the Severn. 



