254 VALLEYS EXCAVATED BY SUBSIDING DILUVIAN WATERS. 
cutting out deep gullies and furrows in the table-lands, and sides of 
the higher ridges, and covering their bottom with gravel, composed 
partly of the wreck of the strata immediately inclosing them, and 
partly of pebbles, which their first rush had transported from more 
distant regions; and thus it will appear that the lower trunks of the 
valleys of the Thames, Cherwell, and Evenlode, (i. e, those portions 
of them which may be fairly attributed to the exclusive action of 
denudation, and which lie below the average level of the table-lands 
which flank their course,) did not exist at the time of the first ad¬ 
vance of the waters, which brought in the pebbles from Warwickshire, 
but were excavated by the denuding agency which they exerted 
during the period of their retreat *. 
If we examine the geological structure of that large portion 
of England which lies south-east of the escarpment of the oolite 
formation, along its whole extent, from the coast of Dorset to that of 
Yorkshire, we shall find in it no one stratum that has the smallest 
* The excavations produced by the waters entering the low point of the oolite 
escarpment near Moreton have been so great, (see map, Plate XXVII.) that the head 
springs of the Evenlode, taking their rise from the lias strata in the vale of Moreton 
beyond the termination of the oolite, flow south-eastward toward Oxford, instead of 
falling by the much shorter course of the Stour into the valley of the Severn: and it is 
of importance to observe, that the Evenlode and Cherwell are the only rivers of all 
those which flow into the Thames, which have not their head-springs within the escarp¬ 
ment of the great oolite. The sources of the Cherwell, and a few of its earliest tribu¬ 
tary streams, being similarly circumstanced to those of the Evenlode, owe their exist¬ 
ence to similar denudations cut through the oolite strata into the clay beds of the sub¬ 
jacent lias, even as far south as the town of Banbury. The lowness of the oolite 
escarpment at the lip or gap above Banbury appears still further from its having been 
selected as the line by which the Oxford Canal is conducted out into the sandstone 
plains of Warwickshire. This was probably also the lowest point along the N. W. fron¬ 
tier of the oolite formation antecedently to the rise of the diluvian waters; which, ad¬ 
vancing from the north with great velocity, would enter more abundantly, and produce 
larger deposits of gravel from the central counties along this line of lower depression, 
than in any other part of the oolitic area. 
