BOULDER-CLAY IN LINCOLNSHIRE. 409 
gravels ; but wherever Boulder-clay has been seen beneath them it 
is either the white or the dark blue variety of the Upper Glacial 
Clay. 
About Revesby the chalky or “ white clay ’’ is seen to pass down- 
wards into the blue variety as it descends into the fen-basin ; and 
the occurrence of this clay in the very place of the Hessle Clay 
appears to show that the former has in some way been connected 
with the absence of the latter. 
It has occurred to me as a possible solution of the question that 
the great Glacial Clay may then have existed in such mass over the 
northern part of the fen district as to have prevented the influx of 
the Hessle-sea waters, presenting a line of low cliffs against which 
the later Boulder-Clay was banked up in the same way as it is 
against the eastern and southern slopes of the Wolds. 
We know that the Glacial Clay did descend into, and probably to 
a great extent filled up, the great hollow of the Fens; the north- 
ward declination of its base-line in Cambridgeshire * has heen in- 
dicated by Mr. Penning and myself; and we now find it sinking 
southward in a similar manner from the Lincolnshire highland. 
Whether the subsequent period was one of emergence or submer- 
gence, it was undoubtedly a period of great denudation; and tne re- 
excavation of the fen-basin seems have been one of its results; the 
position of the Hessle Clay therefore in the north part of the Fen- 
land may indicate one stage of this process. 
The only alternative explanation which suggests itself is that the 
re-excayation of the fen-basin had been completed before the de- 
position of the Hessle Clay, and that an open bay existed over the 
site of the present Fenland, and that, as the climate again increased 
in glacial severity, the inner and shallower part of this bay became 
blocked up with ice, which presented a lofty ice-wall on the seaward 
side, and that the Boulder-clay was accumulated on the outside of 
this wall. This, however, is a mere speculation; and although it 
would account for the difficulty, there is no collateral evidence to 
support it; while the former supposition connects itself with two 
ascertained facts—(1) the occupation of the fen-basin by the Great 
Chalky Clay, (2) the termination of the Hessle Clay at the point 
where the older clay descends into the fen country. J. am inclined 
therefore to look upon the first hypothesis as the most probable ex- 
planation of the facts. 
I also concur with Mr.S. V. Wood in thinking that both the in- 
ternal structure and general behaviour of the Hessle Clay furnish 
evidence in favour of the supposition that it was formed along a 
coast-line by the action of shore-ice. This is a question on which 
there is great difference of opinion, other writers, notably Dr. James 
Geikie, holding that this clay in common with all other Boulder- 
clays is the product of land-ice. Let us therefore reexamine the 
characters presented by the Hessle Clay throughout its extension in 
East Lincolnshire. 
* Quart. Journ. Geol. Soc. vol. xxxv. p. 198, and ‘The Posttertiary De- 
posits of Cambridgeshire,’ Cambridge, 1878, pp. 28 and 77. 
