KENDALL : THE GLACIATION OF YORKSHIRE. 
317 
and round the north-westerly slopes of the Cleveland hills, as a piece 
of work likely to throw much light upon this question, for it is quite 
conceivable that some ice from seaward may have made its way in. 
The condition of the Vale of Pickering at this time offers 
another problem of great interest for solution, and it is to be hoped 
that some one may be stimulated to grapple with it. A very cursory 
glance at the map would suffice to show that some radical change in 
the drainage had been brought about at a geologically recent period. 
The broad opening seaward and the extremely flat and swampy con- 
dition of so much of the Vale must mean that the natural outlet was 
towards the sea, but that the line of morainic hills extending across 
the eastern end had obstructed and leversed the drainage which is 
now carried off to the westward by the River Derwent, which flows 
through a very steep and obviously recent gorge. * 
So much of the low ground within the Vale is covered by 
"warp" and other like materials that it is exceedingly difficult to 
determine whether or not any ice penetrated from the eastward, but 
from what I have already said it might be inferred that I regard this as 
far from improbable. Several patches of boulder clay are mapped in 
the western portion of the Vale of Pickering, but I should regard 
these with a good deal of suspicion until some account had been 
given of the precise nature and contents of the alleged boulder clay. 
It appears to me probable that when the Scandinavian ice closed the 
natural outlet the Vale of Pickering was converted into a great extra- 
morainic lake, into which muddy water would flow from the melting 
ice-front, and the precipitation of this mud would form those clays 
called " warp." Bergs would break from the ice and raft out boulders 
which might be ultimately entombed in any deposits forming oil the 
bed of the lake. From the oolitic hills and the chalk wolds streams 
would descend producing delta deposits round the margin of the 
lake, and the surplus water would overflow through a notch which 
would be gradually deepened and cut back to produce the existing 
gorge of the Derwent. This is what one would expect ; and it has 
yet to be shown how far the facts accord. 
* A reference to the Geol. Surv. Mem. on the Jurassic Rocks of Britain, 
Vol I., Yorkshire, by Mr. C. Fox-Strangways, shows that the problem has 
been considered by an able observer, who has, however, left many questions 
still unsettled. 
