468 
THE CRETACEOUS ROCkS OF BRITAIN 
CHAPTER XXXVIII. 
THE MIDDLE CHALK IN CAMBRIDGESHIRE AND 
THE NORTH-EAST OE ESSEX. 
General Description. 
The Middle Chalk occupies a tract of considerable width from 
the neighbourhood of Royston on the west to the valleys of the 
Cam and the Lin below Chesterford and Linton, lying partly in 
Cambridge and partly in Essex. Thence it rises into the Gog Magog 
Hills, and spreads north-eastward below the higher Chalk escarp¬ 
ment to Dullingham and Newmarket. For the sake of conveni¬ 
ence we shall describe some sections near Royston in this chapter, 
though that town is really in Hertfordshire. 
The zones of this area were established in 1880,* and were fully 
described in the Geological Survey Memoir on the Neighbourhood 
of Cambridge (1881). It was then that the name of Melbourn 
Rock was given to the rocky beds at the base of the zone of Rhyncho- 
nella Cuvieri , the name being taken from the little village of Mel¬ 
bourn, near Royston, where there are good sections of the beds 
that lie at the junction of the Middle and Lower Chalk. It must 
be remembered, however, that the Melbourn Rock of the Cambridge 
Memoir includes the sub-zone of Actinocamax plenus, which was 
subsequently separated. 
A line of section drawn by Mr. W. H. Penning and myself through 
the Gog Magog Hills showed that the thickness of the beds from the 
base of the Melbourn Rock to the base of the Chalk Rock was about 
230 feet, of which some 60 or 70 feet may be assigned to the lower 
zone and about 160 feet to the Terebratulina zone. 
In 1881 the higher zone was divided into two parts, because 
the upper beds contained Holaster planus, Micraster corbovis, 
and Spondylus spinosus , which linked them, as was thought, more 
closely to the beds above than to those below. More recent re¬ 
searches however have shown that these species occur not infre¬ 
quently in the upper part of what is generally recognised as the 
Terebratulvna zone, and the bearing of this on the zonal nomen¬ 
clature has been discussed on p. 360. 
Throughout the greater part of Cambridgeshire the Middle Chalk 
lies almost horizontally, its easterly inclination being so slight as 
to be inappreciable in quarry sections, but south-east of Royston 
there is a remarkable line of flexure, probably accompanied by a 
* The Sub-divisions of the Chalk, Geol. Mag., Dec. 2, Vol. vii., p. 248 
(1880). 
t See “ The Melbourn Rock and Zone of Belemnitella plena,” by W. Hill 
and A. J. Jukes-Browne, Quart. Journ. Geol. Soc., Vol. xlii p. 216 (1886). 
