WOOD AND ROME LINCOLNSHIBE AND S.E. YORlfaHIRE. 179 



ther progress of an arm of the denuding Postglacial sea when, by 

 the emergence of the land, it had sunk into those troughs after its 

 earlier denudation (or that over the crests and that forming the 

 general valley-system) had been accomplished, which is presented 

 by the tract of Glacial clay still occupying the mid- Lincolnshire de- 

 pression. Nothing shows so readily to an observer how little our 

 present seas represent those of the Postglacial period than to view 

 from the Wold-top above Speeton the sea close at hand on the east, 

 and barred out in this way by the purple clay from the troughs occu- 

 pied by the Postglacial sea, and to realize in a coup d'oeil the grand 

 westerly sweep of denudation which the latter sea has effected. 



It is the mapping of the Glacial beds that best brings out these 

 features and makes them intelKgible ; but with that before us, though 

 only in the more general way that we have been able to accomphsh 

 it, we see that the Glacial clay at Rasen (in mid-Lincolnshire) and 

 that below Speeton, sixty miles apart, are respectively the points 

 where terminated two separate tongues that parted from the larger 

 arm of the Postglacial sea which occupied the great vale of York, and 

 in which the outlying ridges of purple clay at Gate Helmsley, Plaxton, 

 and Barton Hill before mentioned formed islands or shoals. The 

 connexion of this larger arm with the main sea is more difficult to 

 trace ; but as the other arms stretching southward, such as that which 

 occupied the trough through which the Trent and upper waters of 

 the Witham now run, seem barred in from the south by the great 

 tracts of Glacial clay that stretch from near Corby, in South Lin- 

 colnshire, across Leicestershire into North Warwickshire, it would 

 seem to be northward, in the direction of the Tees, that we should 

 seek its outlet. Not, however, having examined the country north- 

 west of York, we forbear to speculate upon the precise direction in 

 which these arms of the Postglacial sea had their outlet to the main 

 waters. 



It only remains now to point out the distinction between the 

 valleys north of Flamborough Head and those of the country south 

 of it. Although the position of the Glacial beds shows that the 

 Glacial sea occupied great depressions, such as that of Lincolnshire, 

 traversed by the section, fig. 6, as that of the great vale of York, or 

 as that on the north of the high chalk country of Hertfordshire and 

 Bedfordshire*, yet wherever the Glacial beds by remaining furnish 

 direct means of proof, we see that the valleys (as distinguished from 

 the great scarped depressions) are wholly newer than those beds every- 

 where south of Flamborough, the only exception known to us being 

 the long dry vaUey through which the railway runs from Hitchin 

 by Stevenage and Welwynf ; so that, although we must naturally 



* This depression is indicated by the lower position occupied by the Glacial 

 clay on the north-western sides of the sectipns, figs. 7 and 8 at page 402 of the 

 23rd vol. of this Journal. 



t This intrgeglacial valley widens near Hertford, and becomes the plain out of 

 which two existing parallel valki/s, those of the rivers Lea and Miran, are exca- 

 vated (see a paper lately read before the Society by Mr. Hughes). It was filled 

 with the Middle Glacial beds, out of which these two river-valleys are in that part 

 formed. 



VOL. XXIV. PART. I. 



