R. M. Deeley — Ice-flows in the Trent Basin. 71 



consist of two distinct kinds of boulder-clay and gravel, the upper 

 series being derived from the east and the lower from the west 

 and north-west. This great disparity in the nature of their rock 

 contents induced me in 1896 to regard them as belonging to two 

 distinct epoclis of glaciation. I am now, however, of the opinion that 

 tliey all belong to one great period of ice advance and retreat, wliich 

 was probably marked by considerable but minor oscillations of tlie 

 ice-front ; for although the area might appear to be anything but 

 a marginal region of the ice-flow it really occupies such a position. 



As far as England is concerned, the distribution of the boulder-clay 

 and moraines seems to show that the ice-margins paused, on two 

 separate occasions, along difl^erent lines. These two lines are regarded 

 by many, and for good reasons, to be the margins of the maximum 

 extension of the ice-sheets of two different periods. Whether this be 

 the case or not, these two ice-margins throw very considerable light 

 upon the problem with which we have to deal. 



In tlie Figure is shown the probable ice-margin in the British. 

 Isles and the North Sea during the time the Purple Boulder-clay was 

 formed. The British ice is then considered to have filled the Irish 

 Sea Basin and mounted the watershed between the Trent Valley and 

 the Cheshire Plain. If there were several periods of ice advance 

 they probably all went through this stage before reaching a maximum. 

 With increasing cold we maj' imagine that the Irish Sea ice at a (see 

 Figure) passed into and down tlie Trent Valley before the Scandi- 

 navian ice-sheet {b) reached it from the other direction. Indeed, tlie 

 Irish Sea ice may have collected rapidly, being in the track of the 

 moist Atlantic winds, and arrived at its maximum development and 

 formed the Early Pennine Boulder-clay long before the Scandinavian 

 ice-sheet reached the Trent Valle}' area. 



But upon this Early Pennine Boulder-clay there rests the thick 

 bed of false-bedded Quartzose Sand. This deposit, there is good 

 reason to suppose, was dejDosited in open water which was flowing 

 from the west. The retreat of the ice-margin to the west again, the 

 Quartzose Sand indicates, may have been due mainlj' to a slightly 

 warmer interval supervening. We must, however, assume that the 

 water-level in the Trent Basin was considerably raised, or the fine 

 sand beds would not have been formed. Such a ponding up of the 

 water-level might have been the result of the blocking up of the 

 North Sea by the Scandinavian ice-sheet. Whether the Straits of 

 Dover then existed and the water-level was raised over a large area 

 of the south-east of England, or whether the ice merely ponded up 

 the water in the Trent Valley, is not certain. The creation of a glacial 

 lake in the Trent Valley would of itself tend to melt off the westein 

 ice-lobe, issuing from the basin of the Irish Sea, and cause it to 

 retreat, in addition to the melting ])roduced by a temporary 

 amelioration of the climate. The latter was probably the most 

 effective agent of change. 



At the end of the Quartzose Sand stage the climate became colder 

 and the western glacier-lobe readvanced. A sand-pit in the Quartzose 

 Sand at Aylestone, south of Leicester, showed a thick bed of fine 

 current-bedded sand covered by reddish boulder-clay containing 



