LOWER CHALK—CAMBRIDGE AND SUFFOLK. 
193 
CHAPTER XVI. 
THE LOWER CHALK IN CAMBRIDGE AND SUFFOLK. 
General Description. 
The thickness of the Lower Chalk in Cambridgeshire is between 
160 and 170 feet, but owing to the low elevation and undulating 
character of the ground it covers a considerable area ; there are 
also several outlying tracts of it on the north-western side of the 
Cam valley. 
The Zone of Ammonites varians , or Chalk Marl, has the Cam¬ 
bridge Greensand, or “ Coprolite Bed,” at its base throughout the 
county. This passes up into soft bluish-grey marl, succeeded by 
firmer blocky marl, or “ clunch,” as it is locally termed ; the complete 
thickness of the zone being from 60 to 70 feet. Whether this bed 
is the precise equivalent of the Chloritic Marl of the South of England 
is difficult to say, because neither Stauronema Garteri nor TurrHites 
Morrisi have yet been found in it. 
The Tottevnhoe Stone continues in full force all across the 
county, and has been quarried in many places for building-stone. 
It varies in thickness from 12 to 20 feet, and its outcrop forms a 
distinct feature or rise in the ground. 
The rest of the zone of Holaster subglobosus is about 80 feet thick. 
As in the other counties north of the Thames, it consists of greyish 
chalk in the lower part and nearly white chalk in the upper part, 
the change from one to the other being rapid. The whole consists 
of blocky chalk with curvilinear jointing, and without any clear 
bedding planes. The Belemnite Marls at the summit exhibit the 
usual Midland aspect, consisting of layers of marl with an inter¬ 
vening course of white chalk. 
Stratigraphical Details. 
The Cambridge Greensand, or Coprolite Bed. 
This lies at the base of the Chalk Mail, and rests on an uneven 
and eroded surface of the Gault. A full account of this bed and 
its contents is given on the memoir on the “ Neighbourhood of 
Cambridge,” * to which the reader is referred for details, only a 
resume of the more important points being here given. 
* Geology of the Neighbourhood of Cambridge, Mem. Geol. Survey 
1880, by W. H. Penning and A. J. Jukes-Browne. 
