Yd. 63.] OKIGIN OP CERTAIN CANON-LIKE VALLEYS. 503 



Lake Oxford would have taken place over the latter, possibly at 

 first by one of the narrow, sharply-cut valleys, which connect, 

 over a low col, the Oxford plain with the basin of the Thames. 

 The position of Goring Gap, however, nearer to the points at which 

 the northern affluents of the Thames (or lsis) enter the Oxford 

 plain would have enabled it to secure the larger share of the 

 effluent water, favouring the more rapid erosion of a gorge in 

 that locality. When the latter had been cut down to a level 

 of about 400 feet O.D., escape through the other channels would 

 have ceased. 1 



Prof. J. W. Gregory has drawn attention to the trenches above 

 referred to in a well-known paper, attributing them, however, to 

 the action of rivers at a time when the Chalk-escarpment extended 

 farther north-westward. 2 Their modern appearance, and more or 

 less rectilinear channels, seem to me opposed to that view. If, 

 moreover, it can be shown that the great valley of the Thames at 

 Goring is not pre-Glacial, it seems unlikely that these relatively 

 unimportant gorges are so. 



Some observations made by Mr. Pocock seem to me to be in favour 

 of the view that lacustrine conditions formerly obtained in the Oxford 

 region. He traced some terrace-gravels, occurring at isolated 

 positions near the tops of the lower elevations bordering the north 

 side of the plain, being most strongly marked where the affluents 

 of the Thames (the Windrush, the Evenlode, and the Cherwell) 

 open into it. These gravels maintain, he says, a uniform elevation 

 of about 290 feet O.D. ; from 70 to 100 feet above the existing 

 flood-level. They do not descend as they are traced down- stream 

 in the main valley, although they rise as they are followed upward 

 along the lateral streams. He regards them as delta-gravels of the 

 tributaries, rather than as deposits of the main river. 3 I think 

 that they may be deltas ; but their uniform height seems to indicate 

 they may have originated under lacustrine as well as under fluviatile 

 conditions. 



Mr. Osborne White, moreover, has called my attention to some 

 flint-gravel terraces, banked up against the north-western face of 

 the Chalk-escarpment, extending more or less continuously from the 

 neighbourhood of Ipsden to Brightwell, the position of which he 

 has kindly marked on the map (PI. XXXIY). They occupy for the 

 most part a strip of land between the contours of 300 and 400 feet, 

 being found in a few places, however, at a higher as well as at a 

 lower level. They can hardly represent the course of a former river 

 or a succession of river-terraces. The flint-drift of which they are 

 composed is mainly of local origin caused, as Mr. White thinks, by 

 torrents running down the combes of the escarpment. These gravels 

 seem to me similarly to indicate the littoral accumulations of a lake, 



1 Lake Oxford must have derived a vast amount of water, especially in 

 summer, from the melting of the great ice-sheet, the termination of which lay 

 near at hand. 



2 'Natural Science' vol. v (1894) p. 101. 



3 ' Summary of Progress of the Geological Survey for 1904 ' 1905, p. 21. 



