tion is
alluvial Fenland. The general strike of the rocks is along a south-west
and north-east line, the dip is south-easterly. The oldest rock is the
Jurassic Oxford Clay, which appears as an irregular strip of elevated
flat ground reaching from Croxton by Conington and Fenny Drayton to
Willingham and Rampton. Eastward and northward it no doubt forms the
floor of the Fen country, and at Thorney and Whittlesea small patches
rise like islands, through the level fen alluvium. The Coralline Oolite,
with the Els worth or St Ives rock at the base, occurs as a small patch,
covered by Greensand, at Upware, whence many fossils have been obtained;
elsewhere its place is taken by the Ampthill Clays, which are passage
beds between the Oxford and Kimmeridge Clays. The latter clay lies in a
narrow strip by Papworth St Agnes, Oakington and Cottenham; a large
irregular outcrop surrounds Haddenham and Ely, and similar occurrences
are at March, Chatteris and Manea. Above the Kimmeridge Clay comes the
Lower Greensand, sandy for the greater part, but here and there
hardened into the condition known as "Carstone," which has been used as
an inferior building-stone. This formation is thickest in the
south-west; it extends from the border by Gamlingay, Cuxton and
Cottenham, and appears again in outliers at Upware, Ely and Haddenham.
The Gault forms a strip of flat ground, 4 to 6 m. wide, running roughly
parallel with the course of the river Cam, from Guilden Morden through
Cambridge to Soham; it is a stiff blue clay 200 ft. thick in the
south-west, but is thinner eastward. At the bottom of the chalk is the
Chalk Marl, 10 to 20 ft. thick, with a glauconitic and phosphatic
nodule-bearing layer at its base, known as the Cambridge Greensand. This
bed has been largely worked for the nodules and for cement; it contains
many fossils derived from the Gault below. Several outliers of Chalk
Marl lie upon the Gault west of the Cam. The Chalk comprises all the
main divisions of the formation, including the Totternhoe stone,
Melbourn rock and Chalk rock. Much glacial boulder clay covers all the
higher ground of the county; it is a stiff brownish clay with many chalk
fragments of travelled rocks. Near Ely there is a remarkable mass of
chalk, evidently transported by ice, resting on and surrounded by
boulder clay. Plateau gravel caps some of the chalk hills, and old river
gravels occur at lower levels with the bones of mammoth, rhinoceros and
other extinct m