R. M. Deeley — Ice-flows in the Trent Basin. 69 



as cordierite, came from a higher horizon than is now left exposed. 

 It may perhaps be noted tliat tourmaline is comparatively rare in the 

 rock, although the sediments at the immediate margin are much 

 tonrmalinised and have been so altered in the process that all the 

 original andalusite has disappeared. Its rarity may well have some- 

 thing to do with the freshness of the cordierite, as I had already 

 been led bj^ a study of the Cape Town granite to attribute to 

 processes connected with tourmalinisation the conversion of that 

 mineral into aggregates of ' pinite '. 



1 



V. IcE-FLOWS IN THE TrKNT BaSIN. 



ByE. M. Deeley, M.Inst.C.E., F.G.S. 



N 1866 I communicated a paper to the Geological Society of London 

 on the Pleistocene Succession in the Trent Basin. The boulder- 

 clays and outwash deposits of this district are of two distinct kinds, 

 the one containing rocks from the west and north-west, and the other 

 boulders etc., from the east or north-east of the district. In all 

 cases, except where they have been ploughed up and re-arranged by 

 the ice itself, the drifts containing westerly rocks only are the lowest, 

 and the drifts with easterly rocks have been spread over them. We 

 thus have two distinct ice-flows to deal with. 



I contented myself in 1886 with describing the nature of the 

 deposits, the conditions under which they were formed, their succession 

 and the ice-flows they indicate ; no attempt being made to explain 

 how the variations in the direction of the ice-flows arose. For some 

 years doubts were felt by many as to the constancy of the succession 

 described, it being considered that the Trent Yalley was an area in 

 which the easterly ice-flow met the westerly flow, and that the 

 deposits merely indicated that in this area the ice-flow from the west 

 overpowered the flow from the east and vice versa. The mapping of 

 the area, however, by the officers of the Geological Survey confirmed 

 ray general conclusions, that the westerly drifts were the first to be 

 laid down and that the easterly drifts were subsequently spread 

 over them. 



Harmer suggested that the westerly ice passed down the Trent 

 Valley in the first instance, and that it was at a later date thrust 

 back b)' the great ice-sheet which crossed the German Ocean from 

 Scandinavia. Such an explanation at first sight appears very feasible ; 

 but the nature of the deposits, and their arrangement with regard to 

 each other, does not appear to me to admit of such an explanation ; 

 for had the area been entirely covered by ice, the direction of the flow 

 of which varied, boulder-clays and mixed gravels only would have 

 been formed ; whereas each kind of boulder- clay is accompanied by 

 its own kind of outwash gravels and sands. 



Since 1886 our knowledge of the nature of the deposits formed by 

 glaciers, and the method of their formation, has been very greatly 

 increased. Alaska, Greenland, Spitzbergen, and the Antarctic area 

 have all been more or less explored, with the result that glacial 

 problems have had a good deal of light thrown upon them, lint even 



