Physical Features 19 



main streams indicates an older drainage-system of pre-glacial denudation, 

 leaving channels to which the water returned after the melting of the ice. 1 



In the excessive denudation of this epoch, the whole land-surface was 

 mobile, and torrential streams have swept out broad river-valleys, leaving 

 gravel deposits on their sides to mark the strength of the current, and giving 

 rise to a residual system of water-courses, which at the present time 

 dominate the surrounding country and its recovered flora. 



The primary river-beds are of Pre-glacial and Tertiary origin, following 

 the direction of the dip of the strata to the SE., and mark furrows between 

 ridges 6-7 miles apart, of which traces are left in the Wytham-Boar's Hill 

 and Baldon series, and the Shotover-Garsington-Milton line bounding the 

 lower valley of the Cherwell, and again NE. in the hills at Brill. 



The Thames itself is of later origin, as cutting across the line of the 

 Oxford Clay from its source to Yarnton, at right angles to the preceding 

 lines of flow, and hence ultimately reaching these streams ; the great loop 

 of the Upper Thames (Isis), north of the district, being formed by the river 

 cutting into the course of the older Evenlode at Eynsham, and then into 

 the Cherwell valley at Kidlington, to spread out over a broad area, 2-3 miles 

 in width, and leaving gravel deposits dropped in successive steps to mark 

 its ancient course, as also the periodicity of its torrent-flow,with alternating 

 periods of less active action and deposition of alluvial mud only, as charac- 

 teristic of modern times. The older the stages and the broader the valleys, 

 the more vestigial are the traces remaining, and the deeper hollows of the 

 valleys are now spread over by alluvial mud in which run the present diminu- 

 tive streams. 2 



Plateau Gravel. Special interest attaches to the highest deposit of 

 gravel at elevations of 100-350 ft. above the present river-bed ; since it 

 does not consist of local material, and was not laid down by existing streams. 

 It consists predominantly of siliceous quartzose pebbles derived from more 

 Northern sources, and is hence distinguished as Plateau Gravel of the 

 Northern Drift, left as water-worn or glacial detritus in pockets and patches 

 on old valley slopes, without any bedding or terrace-distribution. Such 

 constituents are large Buntcr Pebbles of the New Red Conglomerate from 

 the Midlands, traced to the valley of Moreton, white quartz, variously 

 coloured quartzose, sandstone of the Millstone Grit, and residual flints 

 from the Chalk. The stones are rounded or sub-angular : but do not pre- 

 sent any glacial striae. Smaller pebbles of this drift may have been washed 

 into lower gravel deposits, and mingled with them ; but all are to be dis- 

 tinguished by being non-calcareous, as opposed to the river-drift from oolitic 

 rocks of marine coral-formation. 



Erratic pebbles (up to 8-10 in.) occur abundantly on Boar's Hill ; but 

 there is no such drift on the Shotover side of the valley. A bed of such 

 pebbles resting on Gault at Cumnor Hurst is 12 ft. thick, and 8 ft. on Boar's 

 Hill ; these spots being 290-350 ft. above the river. Patches in Bagley 

 Wood have determined the direction of the higher Abingdon Road : the 

 stones are imbedded in a reddish clay. The drift is regarded as glacial in 

 origin ; 3 but it probably represents the debris of older mountain torrents of 

 the primary river-valleys, collected and distributed by later ice-sheets of the 

 glacial epoch. At any rate, it is clear that if the high valleys of this period, 

 so far little cut, were filled with ice to this level, i. e. 500 ft. above the sea, 

 the protruding tops of the highest hills, even if much higher than at present 



1 Pocock (1908), loc. cit., p. 106. 



2 Pocock (1908), loc. cit., p. 82. The depth of the alluvium in the centre of the valley is abont 

 that of the river, the main stream running on a gravel bottom with mud at the sides. 



5 Pocock, loc. cit., p. 102. 



B 2 



