1867.] WOOD — SOUTH-EAST OF ENGLAND. 40^ 



edge of tlie chalk in this part formed lofty ground, which after suh- 

 mergence received this deposit on its summit at a higher level than 

 the parts forming the extremities of the section, which at the outset 

 of the Postglacial denudation were swept away ; the denudation 

 eroding the vale of Aylesbury succeeded to this. 



The section, fig. 9, displays a structure that is constant along 

 arc 3 for about six miles on the north side of the Chelmer, and again 

 for three or four miles on the south side of that river, for which dis- 

 tances the Glacial clay lying on the western slope of the arc forms a 

 narrow strip extending along the flank of the arc-crest. Were it not 

 for this feature and the fact that, as the arc declines in altitude, 

 the Middle Glacial series becomes continuous over the crest (the 

 denudation of the crest being proportional to the extent of its ele- 

 vation), I should be disposed to doubt that the hill was formed 

 principally from the rolling character of the movements giving rise 

 to the surface-impress, from which the arc- structure has arisen, 

 as I conceive, by the denudation ensuing upon it. 



The part traversed by this section, being a portion of the general 

 tract of Glacial beds whose southern lip stretches along the heights 

 of the north side of the Thames, is one which, having emerged at the 

 earliest part of the Postglacial period, has been subjected to no 

 denudation save that which it underwent in its elevation. The dis- 

 trict traversed by the sections, figs. 7 & 8, on the other hand, not only 

 underwent this denudation, but was, in the part formed by the vale of 

 Aylesbury, subjected, as before observed, to a further denudation 

 produced by the arm of the Postglacial sea which stretched up, 

 and wore out, that vale. Now, if we suppose the part traversed by 

 the next section, fig. 9, to have been subjected to this subsequent 

 denudation, so as to have scored out the trough in the mode indicated 

 by the dotted line, we seem to get a section identical in structure 

 with fig. 8. I infer, in the cases of all these arcs, that there was 

 an inequality of level at the commencement of the Glacial- clay period, 

 but that this was exaggerated by the disturbances causing the arc- 

 structure, and the line of it deeply defined by the Postglacial denu- 

 dation. One thing seems clear to me from the general structure of 

 the area — namely, that the arc-like or curvilinear form taken by the 

 scarp is wholly due to the direction taken by the Postglacial denu- 

 dation induced, as it seems, by the disturbances that put an end to 

 the deposition of the Upper Glacial clay. 



If the Index Map of the memoir be examined, it will be seen that 

 the deeply denuded troughs intersected by Sections 7 and 8 are divided 

 from each other by an isthmus at Hitchin, where the two prin- 

 cipal tracts of Glacial beds approach close to each other, and where, 

 by the cessation of the denudation, it is to be inferred that these 

 two tracts were at a very early part of the Postglacial period joined 

 to each other. Now both the trough intersected by Section 7, and 

 that intersected by Section 8, rapidly expand as they recede from the 

 Hitchin isthmus, each opening out into the main Postglacial sea — 

 the former into that indicated by the extensively denuded tract before 

 mentioned as extending from the northern lip of the Glacial beds 



