EAST ANGLIA DI7RIXG THE GLACIAL PERIOD. 193 



PI. XV.), that many of the valleys running S. and E. from the chalk 

 escarpment to the sea are, almost to their source, cut through the 

 Upper Boulder- clay, and expose these gravels beneath it. They 

 are thus proved to be very persistent over a definite area, within 

 which it is a rare occurrence for the clay to be seen resting on the 

 older geological formations. It is probable that the gravels extend 

 beneath the clay over nearly all the intervening area, in greater 

 force along the lines of the larger valleys (which I believe to have 

 been carved into nearly their present form in preglacial times), 

 and in a more attenuated, or even patchy condition under the higher 

 grounds. 



But the gravels can in no instance of which I am aware be 

 traced up to the escarpment of the Chalk, or, in other words, beyond 

 a certain definite level. It is not that they disappear beneath a 

 great thickness of Boulder-clay to reappear at its opposite boundary; 

 on the contrary, it is evident that they gradually thin out ; and a 

 few miles before the escarpment is reached we find the Boulder-clay 

 overlapping them and resting directly on the Chalk (fig. 2, PI. XV. j. 

 This is well seen in many sections, also along the N*W. face of the 

 scar]), where the junction of the Chalk and the clay forms for many 

 miles a well-marked line, the absence of any intervening sand or 

 gravel being constant and remarkable. The fact of the gravels not 

 running up to the escarpment was noticed, but no inference drawn 

 therefrom, in 1835, by Mr. Caleb Burrell Bose, who writes * : — 

 " The general surface of the chalk must have suffered prodigious 

 abrasions from the violence of the elements, as evidenced by the 

 immense quantity of gravel formed and collected in various 

 situations, as well as by the different altitudes at which the chalk 

 is found, it appearing immediately beneath the vegetable soil, even 

 on the highest ground ; and at a level of not less than 50 feet lower 

 it may be found covered by more than 150 feet of sand and clay 

 containing boulders." 



In the thinning-out of the Middle Glacial beds against the Chalk, 

 and their not rising beyond a certain height, we have a clue to 

 the conditions under which they were formed. The great chalk 

 escarpment, as a long and narrow ridge, standing at this time well 

 above the sea, was an important feature in the physical geography 

 of the period ; and the Middle Glacial sands and gravels, as such, 

 are entirely owing to its existence. For it formed a barrier op- 

 posing itself to the strong current which must at that time have been 

 sweeping round from the Xorth Sea to the Atlantic Ocean. By 

 this current were brought down the materials of which the gravels 

 are composed, and which consist of pebbles derived from the rocks 

 of the northern and eastern coast along which it travelled, mixed 

 with a large percentage of flints from the chalk barrier itself. 



In the gravels are intercalated occasional masses of Boulder-ciay 

 which were brought down by icebergs, and which, heavily descend- 

 ing from them, have distorted the gravels wherein they now lie 

 without any approach to order or arrangement. Further evidence 

 * " Geology of West Norfolk,'' Phil. Mag. vols. vii. and viii. 



o 2 



