Pl. 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, Pl. XV.). This is well seen in many sections, also along the N.W. face of the scarp, 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[1]:— "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 opposing itself to the strong current which must at that time have been sweeping round from the North 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-clay which were brought down by icebergs, and which, heavily descending 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.
