Geoscience Reference
In-Depth Information
A Greensand Ridge Walk has been established along the length of this feature,
linking a number of estates and nature reserves where the acid-soil vegetation and
fauna are distinctive.
Northeast of Sandy, under the Cambridge Western Plateau, and further northeast
across the rest of Area 13, the Lower Greensand below the surface blanket becomes
thinner and has only rarely produced low ridges, being generally completely obscured
by Anglian ice-laid material or younger alluvium.
The rest of the Early Cretaceous succession, above the Lower Greensand, consists
of mudstones of the Gault, about 75 m in thickness. The Gault tends to be a uniform,
relatively soft material, so topographic slopes that occur within the area underlain by
Gault bedrock are generally due to valley incision or slope retreat, rather than to the
picking out of bedrock material contrasts.
Phosphate-rich
coprolites
caused an economic boom in Cambridgeshire, particu-
larly in the nineteenth century, when they were extracted from the Cretaceous rock at
a number of different levels. One of these, labelled the Cambridge Greensand, occurs
at the junction between the Gault and the Chalk, where a few metres of chalky mater-
ial contain a scatter of black and grey fossils, pebbles and nodules that are enriched in
phosphate and valuable as agricultural fertilisers. Some of the phosphate is the result
of mineralisation in the groundwater of the excreta of Cretaceous organisms, particu-
larly fish. This phosphate-rich material was named coprolite, for polite purposes, us-
ing a word from classical Greek (
kopros
, faeces). Areas of low-lying Cambridgeshire,
wherever the Cambridge Greensand, Gault or Lower Greensand were found to be rich
in coprolites, were subjected to a coprolite 'rush' (Fig. 239). Open-cast digging and
washing of the coprolites involved large numbers of local and imported workers.
FIG
239.
Coprolite digging in the late nineteenth century, between Orwell and Barrington,
11 km southwest of Cambridge. (Photo held at the Sedgwick Museum, Cambridge)
