[Plate XV.].
Introduction.
In submitting to the consideration of the Society the following observations, I would remark that they are intended to form, not a full description, but a sketch of the physical geology of East Anglia during the glacial period. The evidence upon which they are founded is derived from deposits that either have been, or will be, mapped by the officers of the Geological Survey. Eull descriptions will appear in their published maps and memoirs; therefore the subject will here be treated generally, and all details omitted.
The object I have more especially in view, is to offer an explanation of the origin of a somewhat puzzling series of gravels and sands, classed by Mr. S. V. Wood, Jun., as "Middle Glacial," to give a reason for their occurrence in certain areas and non-occurrence in others, and to account, on mechanical grounds alone, for the almost total absence therefrom of any (except derived) fossil remains. I wish also to remark briefly on the probable means of formation of the so-called "Denudation gravels."
The subject is divided, for reasons that will appear, into two parts; the first relates to the district south and east, and the second to that north and west, of the great Chalk escarpment.
By the term "East Anglia" is meant a tract of country, north of the Thames, which may be considered to be bounded on the west by a line passing from London through Hertford, Royston, and St. Ives to the estuary of the " "Wash," and which includes all Norfolk, Suffolk, and Essex, with part of Cambridgeshire, Hertfordshire, and Middlesex. The general contours of the southern part of the district at every hundred feet above the sea are shown by the dotted lines on the map (fig. 1, Pl. XV.), the smaller and later-formed features being omitted as immaterial to the argument.
The lowest tract in the district may conveniently be termed the "Cambridge Valley:" a large portion of it is fen-land bounded on the south and east by Gault, which is, with trifling exceptions, the lowest geological formation that comes to the surface in the area in question. On the right side of the Cambridge valley runs the chalk escarpment, divided as usual into two main lines, one of the Lower and one of the Upper Chalk. The latter forms some of the highest ground in the district[1], and may be traced as an uneven ridge, with rounded hills and hollows, from Buntingford, by Saffron Walden, Haverhill, Thetford and Swaffham, to the sea. The ground slopes gradually away from this ridge towards the London Basin, where the Tertiaries set in, one of the series (the London Clay)
- ↑ At Tharfield (S. of Royston) 550 feet, and in West Norfolk 450 to 650 feet above the sea.
