152 



PROCEEDINGS OF THE GEOLOGICAl SOCIETY. 



degradation of the purple clay, an admixture of material having been 

 introduced from the chalk which had been laid bare of the Glacial clay 

 by the previous early Postglacial denudation. The appearance of 

 this slight chalk admixture is quite dissimilar to that so profusely 

 dispersed in the basement clay and in the equivalent of that clay 

 over the central and eastern counties. In examining the structure 

 of the latter in central Lincolnshire, we have been led to the con- 

 clusion that the enormous preponderance of chalk in this portion of 

 the Upper Glacial clay has been caused by the grinding down of the 

 "Wolds by a capping glacier ; and the contrast between this deposit 

 and the Hessle clay at once suggests that the chalk in the latter 

 has been introduced by no such Glacial action, but by the mere 

 coast-erosion of the Hessle-clay waters, assisted by the transporting 

 agency of ordinary shore-ice. The Hessle clay occupies the gorge 

 of the Wolds through which the Humber passes ; but, so far as we 

 have been able to learn (and we have assiduously searched for it), it 

 has no place on the western side of the Wolds, the later part of the 

 repeated westerly denudation having so sharply swept it off on the 

 west of this gorge, that its boundary there is but a continuation of 

 the line of the Wold-scarp from Yorkshire into Lincolnshire. The 

 following section illustrates its position within this gorge, and the 

 manner in which the Humber (with which river the deposit has no 

 connexion whatever) has cut through it. 



Fig. 2. — Section across the Hiver Humber (5 miles). 



End of the 

 Yorkshire Wold. 



1. The chalk. 



d. The Hessle sand and gravel. 



The Hessle clay. 

 /. Gravel. 



When we consider the height to which this clay rises along the 

 eastern Wold-slope, and over the highest of the purple-clay hills of 

 Holdemess, and the manner in which it o ccupies the Humber gorge, 

 looking, as it were, into the great trough on the west of the Wolds — 

 indeed, slightly entering it — there is no room to doubt its former occu- 

 pation of that trough ; while the manner in which (prior, necessarily, 

 to the existence of the Humber river, whose stream the line of denu- 

 dation crosses at right angles) it has been cut off by a denudation that 

 has swept north and south along the western Wold-foot indicates 

 that the sea had possession of that trough after the country on the 

 east of the Wolds had entirely emerged from the waters which depo- 

 sited the Hessle clay. 



At the only spot on the Lincolnshire coast which affords a section, 

 namely the low cliff of Cleethorpe, the Hessle clay caps the purple ; 

 while the latter is shown by borings there to exist to a thickness of 

 60 feet, resting immediately on the chalk, without (as is the case also 



