PEBBLES ON SUMMITS AS WELL AS IN VALLEYS. 253 
through which these rivers now flow; and compels us to refer then- 
excavation to the denuding agency of the same diluvial waters which 
imported the pebbles. It seems probable that the first rush of these 
waters drifted in the pebbles within the great escarpment of the 
oolite, and strewed them over the then nearly continuous plains ; and 
that the valleys were subsequently scooped and furrowed out by the 
retiring action of these same waters; for it is not easy to imagine any 
explanation of the fact of the pebbles being heaped together on the 
tops of the insulated, steep, and nearly conical hill of Wytham, and 
of the elevated ridge of Bagley Wood near Oxford, or on the highest 
crest of the oolite ridge of Witchwood Forest, and the chalky summits 
near Henley, unless we suppose the transport of the pebbles to those 
summits to have been anterior to the excavation of the valleys that 
now intersect and surround them. Xor is this hypothesis unsup¬ 
ported by the fact, that it is on the elevated plains that flank the 
vales of the Evenlode and Cher well, no less than in the lower regions 
which form their present water-courses, that the quartzose pebbles 
are scattered in an almost uninterrupted line, marking distinctly the 
course by which they have been propelled from Warwickshire into 
the valley of the Thames. (See map, Plate XXVII.) 
There is another strong fact tending to prove the excavation ol 
the valleys of the Evenlode and Cherwell, and of the Thames (in 
part) near Oxford, to have been subsequent to the transport of the 
Warwickshire pebbles, namely, the absence of pebbles of oolite in the 
beds of gravel just mentioned as crowning the summits of Wytham 
Hill and Bagley Wood. Hence we may infer that the destruction 
of the oolite strata, as far as concerns these valleys, was not so 
much the effect of the advancing deluge as of its retiring waters, 
