Notices of Memoirs — Papers read at British Association. 471 



Lincolnshire Wolds by the North Sea ice, as explained in a former 

 paper, must have impounded the drainage of the Trent basin and 

 caused the formation of a lake, the overflow of which could only have 

 escaped over some part of the Lincoln ridge. Unless the Lincoln 

 and Ancaster gaps were already in existence, which seems to the 

 author improbable, some such overflow must have been initiated at 

 that time. The continuous advance of the Trent glacier southwards 

 would eventually have blocked the Lincoln gorge, probably with 

 drift, and the Ancaster gap would have been originated, being 

 afterwards similarly blocked, in its turn, as the ice moved on. 

 These channels, however, would have been reopened successively, 

 and probably deepened, when the ice retreated. 



Referring next to the case at Goring, we find scattered over the 

 low country round Oxford a number of isolated hills, generally 

 capped by gravel, the origin of which, it is not easy to explain on the 

 hypothesis of the fluviatile erosion of the Oxford plain ; they present 

 no such difficulty, however, if we regard the latter as the site of an 

 ancient lake, the bottom of which has been gradually lowered. 



It has been long known that the gravels in question contain 

 Triassic pebbles, but it is still more important to notice the presence 

 in them, often in great abundance, especially as they are traced 

 towards the gap, of grey Lincolnshire flint. 



This flint drift connects itself with a great trail of such detritus 

 extending continuously from Buckinghamshire to the Wolds, being 

 exceedingly common both in the Chalky Boulder-clay of the Ouse 

 basin and in the gravels into which the latter passes towards the 

 south-west. 



The grey flints occur in the highest part of the Oxford gravels, 

 at elevations exceeding 400 feet, as, for example, on a hill immediately 

 to the south-west of that place, and at Basildon, near Goring, above 

 the narrowest part of the gorge. The erosion of the Oxfonl plain, 

 and of its outlet below that level, cannot, therefore, have commenced 

 until after the arrival of the glacial drift in that region. 



Other gravels, also containing Triassic pebbles and Lincolnshire 

 flint, occur at a somewhat lower level, representing a later stage in 

 the deepening of the bed of Lake Oxford and of the Gap. 



The south-westerly advance of the Chalky Boulder-clay glacier up 

 the Ouse basin, preventing any possible drainage to the east through 

 the Stony Stratford Valley, must have caused the formation of a lake 

 over the comparatively low ground which probably then existed 

 between the Chilterns, the White Horse, and the Cotswold Hills. 

 That the drainage of this lake was from the first in the direction of 

 the present gorge is shown by the presence of flint gravel immediately 

 above it, near the 400 foot contour; it occurs also within it at 

 a lower level. Once started, the drainage has continued to run in the 

 same direction to the present day. The swirl of the water, swollen, 

 especially in summer, by the melting of the ice-sheet which lay close 

 at hand, converging constantly to one point, eventually produced the 

 trumpet-shaped opening which formed such a marked feature of 

 the Gap. 



