166 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



ing, is a sTmmetrical continuation of the long curvilinear sweep of 

 the Yorksliire and Xorth Lincolnshire Wold-scarp ; so that it seems 

 to US to follo-vr, that the currilinear contour of the western TTold- 

 edge is (in some degree at least) due to the causes which imparted 

 direction to the Postglacial denudation *. Moreover this clearly 

 defined trough is pan of one continuous line of Postglacial ero- 

 sion, which, continuing the "Wold-scarp from the northward, is 

 itself continued by the valley of the Wensum and Tare to the south- 

 east (in Norfolk) — a valley throughout formed out of the Glacial beds, 

 and departing far away from the Subcretaceous outcrop. The whole 

 thus forms a great arc or curve of denudation, which, between the 

 Steeping and Wensum, is interrupted by the Pen-country and 

 Wash, but which, from the termination of the scarp at Castor to 

 the end of the curve where the Tare enters the sea, omitting 

 the interrupted part, exhibits the clearest evidence of having been 

 formed by the earlier Postglacial denudation. This interrupted por- 

 tion ai)pears to us to have been occuj)ied by that one of the arms of 

 the later Postglacial sea which stretched from the eastward up to 

 Hitchin, where it was divided from a similar arm coming up from 

 the west by an isthmus indicated by the close approach in that pai't 

 of the two piincipal tracts of Glacial beds in England. These arms 

 are represented by the troughs crossed by the sections, figs. 7 & S, at 

 page 402 of the 23rd volume of this Journal. 



The Glacial clay (a of our sections, figs. 6, 7, & 8) is almost exclu- 

 sively composed of degraded chalk — veryhttle of other material, and 

 comparatively few boulders of distant rocks, being present. Unlike 

 the purple clay of Yorkshire, however, where, in addition to the 

 large blocks, the small erratics of PalEeozoic and older Secondaiy 

 rock swarm, the fewer erratics, other than flint, of the clay of 

 mid-Lincolnshire are generally of large size, rivalling the enormous 

 blocks which, derived by coast-waste from the lower part of the 

 purple clay, strew the Holderness coast. This extremely chalky 

 clay appears (from information obtained by us from persons employed 

 in land- drainage and well-sinking) to rest in places on the lead- 

 coloured Glacial clay with chalk of the lower grounds ; but in others 

 we found it resting directly on the Oolitic clay. Lii some parts, as at 

 Bolingbroke, it clearly passed downwards into the less chalky clay 

 which preserves so constant a character over eastern and east- 

 central England, which ia mid-Lincolnshire itself is the suiface- 

 bed where greater denudation has taken place, and which on the 

 Holderness coast underlies the purple clay. As the Upper Glacial 

 clay in parts of Hertfordshii^e and Essex, and in one part of Xorfolk, 

 is nearly as chalky and white as this clay a' (although more tena- 

 cious), there appears to us nothing by which the latter can be dis- 

 tinguished from the general mass of the Upper Glacial clay of these 



* Tt is "by no means iinlikely that this great are of denudation, as well as 

 those referred to at page 400 of the 23rd volume of this Journal, may have had 

 their forms determined hy the influence exerted by some prior condition of the 

 Subcretaceous outcrop upon the forces producing the Postglacial denudation, 

 giving rise to the direction which was imparted to it. 



