416 A. J. JUKES-BROWNE ON THE HESSLE 
these gravels abut against the Boulder-clay, the obscurity of the 
ground renders it dangerous to pronounce a decided opinion on the 
question*. They have not yet yielded any organic remains; but 
their mode of occurrence near Kirkby and Revesby is suggestive of a 
fluviatile origin ; moreover their inland and upland continuations 
appear to be true river-gravels, comparable to those long ridges of 
gravel in Cambridgeshire which have been described by Mr. Penning 
and myself—records of a river-system which is older than the 
valleys of the present streams}. ‘The great outspread of sand and 
eravel round Tattershall may be the estuarine termination of these 
ancient Lincolnshire rivers: this is the conclusion to which 1 am led 
by the mapping of the district, in its present stage ; but the comple- 
tion of the survey will doubtless throw more light on the matter. 
(3) The March Gravel. There is no direct connexion between 
this and the gravels on the north edge of the Fen, as Mr. Wood’s 
language would lead us to suppose. If, however, we assume what 
I have just indicated as probable, viz. that the remarkable series of 
ancient river-gravels which occur in Cambridgeshire and Lincoln- 
shire are coeval, then we may naturally conclude that what appear 
to be their estuarine terminations are likewise of the same age; but 
as regards the particular epoch of Post-tertiary time to which they 
belong, we have as yet no testimony which is worthy the name of 
evidence. We only know that they are of later date than the Great 
Chalky Boulder-clay ; whether they belong to a period anterior or 
posterior to that which witnessed the formation of the Hessle Clay 
we have no means of deciding. The probabilities, however, are in 
favour of their being younger than this clay. 
(4) The Barnwell Gravels. The mapping of the country round 
Cambridge demonstrates that these form part of a much newer de- 
posit than the old series of gravel ridges which appear to find their 
termination at March. They belong to the present, and not to the 
former yalley-system. If therefore the older gravels should even- 
tually prove to be in any way connected with the Hessle series, it is 
clear that the Barnwell gravel cannot also be of that age; yet it 
contains Cyrena fluminalis im abundance. Thus it becomes evident 
that this shell cannot be depended on as a criterion of age. 
(5) The Nar-valley beds. Myr. Skertchly looks upon these as 
newer than the oldest gravels of the Fenland (March &c.) and pro- 
bably contemporaneous with the beach-grayvelst. This view is an- 
tagonistic to that of Mr. Searles Wood, Junior, who thus speaks of 
them § :—“ In all these gravels [Kelsea, Hunstanton, March], as well 
as in the Nar Brickearth, Ostrea edulis, which is absent from all 
the East Anglian, and, indeed, from all the English Glacial beds, is 
abundant ; and there can be little doubt that the four are synchronous 
x A trench carried across the junction-line of the clay and gravel would pro- 
bably decide this important point. 
1+ Penning, Quart. Journ. Geol. vol. xxxii. p. 191; Jukes-Browne, ‘ Post- 
tertiary Deposits of Cambridgeshire,’ p. 46. 
t Geology of the Fenland (Mem. Geol. Survey), p. 235. 
§ Introduction to Supplement to the ‘Crag Mollusca’ (Pal. Soc.), p. xxviii. 
