ON THE GLACIAL GEOLOGY OF EAST ANGLIA. 191 



22. Notes on the Physical Geology of East Anglia during the 

 Glacial Period. By W. H. Penning, Esq., F.G.S. (Bead 

 December 15, 1875.) 



[Plate XV.]. 



Introduction. 



In submitting to the consideration of the Society the following ob- 

 servations, 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 explana- 

 tion of the origin of a somewhat puzzling series of gravels and 

 sands, classed by Mr. S. V. Wood, Jim., 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, 

 aud Middlesex. The general contours of the southern part of the 

 district at every hundred feet above the sea are shown by the dot- 

 ted lines on the map (fig. 1, PI. 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 *, and may be traced as an uneven ridge, 

 with rounded hills and hollows, from Buntingford, by Saffron 

 Walden, Haverhill, Thetford and SwatTham, 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 Thai-field (S. of Koyston) 550 feet, and in West Norfolk 450 to 650 feet 

 above the sea. 



Q. J. G. S. No. 126. o 



