162 PBOCEEDINGS OF TR~E GEOLOGICAL SOCIETY. 



sections, each stretching from a great tract of imdenuded Glacial 

 clay into the troughs occupied by the Postglacial sea, show the ex- 

 tensive denudation of the Glacial beds which has taken place towards 

 the Trent valley (that is, in a westerly direction), both from the 

 northern and from the southern extremities of the area under con- 

 sideration. 



The cliff-like (or continuous) scarp presented by the edge of the 

 chalk everywhere north of Castor is illustrated by the part of the 

 section fig. 5 which crosses it, the only difference being the greater 

 elevations which it attains north of the Humber. Now the sec- 

 tion across mid Lincolnshire (fig. 6) shows a structure of which 

 fig. 5, owing to the greater and different denudation of the area tra- 

 versed by it, affords no trace (namely, that the Glacial clay occupied 

 the extensive depression formed by the eastern slope of the Oolitic 

 ridge, and by the western slope, not scarp, of the chalk Wold), and, as 

 it seems to us, will clearly indicate that the cliff-hke (or continuous) 

 scarp of the chalk, presented in fig. 5, has been, if not produced, 

 yet augmented and modified by a denudation supplementary to that 

 which has formed the valleys traversed by the section in fig. 6. Pig. 5 

 (which in this respect may be regarded as illustrating the condition 

 of the entire Wold-scarp from the Purple-clay edge, near Speeton, to 

 the commencement of the Glacial-clay tract of mid Lincolnshire) 

 exhibits no trace of the chalky (or basement) part of the Upper Glacial 

 clay, or of the purple (or upper) part of that clay, or of the more 

 feeble, and Postglacial, Hessle clay, either on the Wold-top or on the 

 western slope ; while associated with that feature occurs the cliff-like 

 scarp in question. 



Let us now contrast the features of fig. 5 with those afforded 

 by fig. 6, carried across mid Lincolnshire, and intersecting the 

 Wolds where they have no chff-like scarp. 



This section shows that before the Glacial clay was swept away 

 by denudation, the slope formed by the outcrop of the chalk base 

 and of the subcretaceous series was occupied by this clay in great 

 thickness, the occurrence of outliers of it upon the chalk itself 

 proving that it also spread to some distance over the western edge 

 of that formation *. 



It is out of the chalk and subjacent deposits, with the Glacial clay 

 bedded up to and over them in solid mass, that the valley-system of 

 mid Lincolnshire has been cut, as from one common bed, by the 

 Postglacial denudation. To all intents, as far as the formation of 

 the valleys goes, the Glacial clay of this part may be regarded as the 

 same bed for the action of denudation as the subcretaceous sands and 

 sandstone whose place it has taken ; and as this feature has a special 

 interest in connexion with the structure of the Upper Glacial clay in 

 other parts where similar features prevail, we give the following de- 

 tailed sections in illustration of it (p. 161) : — 



Pig. 7 is carried across the valley of the Bain and across that 

 of a tributary of this river; but the latter valley, although its 

 watershed falls into the Bain, owing to a rise in the bottom between 

 * See fig. 10, post 



