488 MR. F. W. HAEMEB ON THE [NoV. I907,, 



of Chalky Boulder-Clay of the East Anglian type, several miles in 

 length, between Blyton and Northorpe, to the north-east of Gains- 

 borough ; simultaneously, therefore, with the extension of the 

 Teesdale ice-stream to Doncaster, the North-Sea ice must have 

 blocked the Humber gorge, and any depression that may then 

 have existed between the Lincolnshire and the Yorkshire Wolds. 



An ice-barrier would thus have come into existence at the stage 

 represented by the Boulder- Clay of Doncaster and Northorpe, 

 extending from the Pennines to the Lincoln ridge, the latter pro- 

 bably somewhat higher than at present, as indicated by the vast 

 amount of Oolitic detritus present in the Boulder-Clay of the region 

 farther south. Unless the Lincoln gap be pre-Glacial, which seems 

 to me improbable, this ice-dam would have impounded the drainage 

 of the Trent basin and have formed a lake, its overflow escaping 

 eastwards over the Jurassic ridge into the plain of the Lower 

 Witham, and eventually excavating the deep notch at Lincoln. 



Assuming that the ice-stream travelled up the Trent basin 

 beyond Lincoln, the gap would have been blocked as it moved 

 southwards, and the erosion of a second channel, that of Ancaster, 

 would have become necessary. The Ancaster notch resembles the 

 notch at Lincoln. It is roughly parallel to the latter, crossing the 

 Jurassic ridge at right angles, and has a similarly recent and 

 sharply-cut character. It has not been excavated to a sufficient 

 depth, however, to capture permanently any part of the drainage of 

 the Trent basin. 



By the time the Ancaster gap was in its turn blocked by the 

 advancing ice, the Trent glacier must have met that descending 

 from the valley of the Derwent, of the former extension of which 

 into this region there seems to be satisfactory evidence, and the 

 suggested lake would have ceased to exist. 



During the retreat of the ice-sheet, lacustrine conditions would 

 have again obtained in the Trent basin, the Ancaster gorge first, and 

 then that of Lincoln, being cleared of ice and possibly deepened. 

 It is to this stage that I assign the low-lying gravels mentioned by 

 Mr. Jukes-Browne, with those north of Lincoln, and the Bunter- 

 pebble drift found south-east of the Lincoln gorge — the latter 

 occurring in great force in sheets on both sides of the present course 

 of the Witham, along a tract of country 5 or 6 miles in width, as 

 shown in PI. XXXIII; these gravels seem to indicate floods of 

 far greater volume than any which now obtain. 1 



The view here taken suggests an explanation of a difficulty. 

 As is well known, the Lincolnshire plain, east of the Jurassic 

 ridge, is more or less covered with Boulder-Clay, and ice no doubt 

 stretched thence continuously, at one period, across the Penland 

 into Norfolk and Suffolk. On the contrary, although a sheet of 

 Chalky Boulder-Clay extends on the west of the ridge for well nigh 



1 See also, as to this, A. J. Jukes-Browne, Quart. Journ. Geol. Soc. vol. xxxix 

 (1883) p. 609, 



