BOULDER-CLAY IN LINCOLNSHIRE. 407 
he believes that in early times the river Witham flowed through 
this gap to the outfall at Wainfleet. 
At Sibsey Norland the Boulder-clay sets in again, and appears on 
the map in the form of a long island rising out of the flat silt- 
lands which surround it on every side. Mr. Skertchly mentions a 
well section at Sibsey*, showing Boulder-clay (31 feet) with a vein 
of sand below yielding a water-supply, as at Stickford. 
South of Sibsey the Hessle Clay would appear to sink finally 
below the level of the fen-deposits, following probably the increasing 
slope of the floor of the fen-basin in that direction. Mr. Skertchly 
remarks + that ‘‘around Boston Boulder-clay of the dark blue 
character can be occasionally seen in several of the brickyards at 
depths of from 15 to 26 feet from the surface. The fen-beds around 
are much thicker than this; and Boston seems to stand on a sub- 
merged bank of Boulder-clay, of which the islands of Sibsey and 
Stickney are the only parts which appear at the surface.” 
It will be seen therefore that he regards the Boulder-clay under- 
lying Boston as a continuation of the Sibsey Clay, which now proves 
to be referable to the Hessle Clay, and is therefore much younger 
in age than the Boulder-clay underlying the rest of the fen to the 
west. How far the Hessle Clay may once have extended to the 
southward cannot at present be ascertained ; but it is very unlikely 
that the deposit should have terminated thus abruptly in the midst 
of what is now the fenland ; for this area seems to have existed, with 
much the same configuration as now, in times long anterior to the 
formation of the Hessle beds. We may therefore reasonably ex- 
pect that they will eventually be recognized in more southerly parts 
of the fen-basin ; but it is very problematical whether any of the 
gravels in those regions, supposed by some to be contemporaneous 
with the Hessle Sands, ever had any actual connexion with them. 
Mr. Skertchly gives a well section at Fossdyke in which 37 feet 
of yellow sandy clay intervene between beds that are clearly deposits 
of fen and Boulder-clay of a light blue colour. It would be idle to 
speculate upon the probability of this belonging to the Hessle Clay ; 
but the possibility is perhaps worth pointing out. 
It may, however, be assumed as more than probable that where 
Boulder-clay is found to underlie the more recent beds in the fen 
district east of the bank or ridge above described, such Boulder-clay 
belongs to the Hessle and not to the older chalky clay. 
§ 3. The Origin and Mode of Formation of the Hessle Clay. 
The geographical disposition and extension of the Hessle Clay in 
Lincolnshire are suggestive of some remarks regarding the conditions 
under which it appears to have been accumulated ; and in this con- 
nexion its relations to the Chalky Boulder-clay or Upper Glacial of 
Searles Wood become of great importance. 
The contrast between the character and behaviour of the two 
clays is very conspicuous. The configuration of the country during 
* Geology of the Fenland (Mem. Geol. Sury.), p. 275. 
t Op. cit. p. 211. 
