474 ME. F. W. HARMEK ON THE [Nov. 1907, 



mountains of North Wales towards the south, carried vast numbers 

 of Scottish and Lake-District erratics into the northern part of the 

 basin of the Lower Severn, heaping them also upon Cannock Chase, 

 and upon the high land near Wolverhampton. 



In South Wales, Dr. Strahan and his colleagues have shown 

 that ice descended in great thickness from the Brecknock Beacons 

 towards the Bristol Channel, reaching the shores of the latter near 

 Swansea, filling the Neath and Taff valleys to overflowing, and 

 rising to a great height on the intervening hills. 1 



Evidence has also been found of the invasion of the southern 

 part of this district by ice from the Irish Sea, which is supposed to 

 have travelled up the Bristol Channel from west to east, and to havo 

 crossed the Pembrokeshire peninsula from St. David's Head towards 

 Gower and to the neighbourhood of Cardiff: erratics believed to have 

 been derived from the first-named locality having been found nearly 

 100 miles to the eastward of their probable source. 2 



The depth of St. George's Channel between St. David's Head and 

 Ireland, however, exceeds 50 fathoms, and the natural course of 

 the Irish-Sea glacier, joined by those descending the western slopes 

 of the Welsh mountains, should have been southward along the 

 deep submarine valley opening out to the Atlantic. The distribution 

 of the erratics just mentioned seems therefore to indicate that 

 the volume of ice, approaching the narrowest part of St. George's 

 Channel, was too great to enable it wholly to escape in that direction, 

 some of it being forced by lateral pressure to travel eastward up the 

 Bristol Channel. 



It seems worth considering whether so important an ice-stream 

 would not have blocked the entrance to the estuary of the Severn, 3 

 the result being an accumulation of sedentary ice in the valley of 

 that river, derived partly from the glaciers of Central Wales 4 

 and partly from Atlantic blizzards, which I think, for meteorological 

 reasons, may have prevailed at that epoch. Such blizzards might 

 have filled portions of the Severn basin, then unoccupied by moving 

 ice, with masses of snow that would have eventually consoli- 

 dated. 



This view may possibly throw light on the origin of the great 

 alluvial and lake-like plain of Glastonbury, and of the gorge at 

 Clifton, to be discussed hereafter. It may explain also why Arenig 

 boulders have been piled up on the Clent Hills, south- west of 

 Birmingham, to a height of nearly 900 feet. 5 It is difficult to 



1 ' Summary of Progress of the Geological Survey for 1898 ' 1899, p. 164. 



2 F. T. Howard & E. W. Small, Trans. Cardiff Nat. Soc. vol. xxxii (1899) 

 p. 44; also 'Geology of the South Wales Coalfield, pt. viii — The Country 

 around Swansea ' Mem. Geol. Surv. 1907, p. 139. 



3 See also as to this, Dr. H. C. March, Proc. Dorset Nat. Hist. Field-Club, 

 vol. xix (1898) p. 136. 



4 Dr. Strahan considers that a great body of ice was generated in Central 

 Wales, ' Summary of Progress of the Geological Survey for 1900 ' 1901, p. 137. 



5 Mr. W. J. Harrison states that they have been traced to a height of 897 

 feet O.D., Proc. Geol. Assoc, vol. xv (1898) p. 401. 



