Geology and Physiography 15 



is white chalk with some nodular bands. The junction between the 

 labiattis- and Terebratulina-zones is difficult to define, partly owing to lack 

 of exposures; the total thickness of the Middle Chalk, however, is about 

 200 ft., the upper part consisting of white chalk with marl seams and flints.' 

 Characteristic fossils of the Terebrattiliua-zonc are Terebratulina lata, Sterno- 

 taxis planus, Micraster corbovis and a variety o£ Echinocorys scutatus, a species 

 elsewhere diagnostic of the Upper Chalk. There are exposures near 

 Dullingham Station, at the Linton Whiting Works, and east of Great 

 Chesterford. 



As in the Thetford district, the Chalk Rock of the Cambridge district 

 occurs above the base of the Upper Chalk. At the best exposure (Under- 

 wood Hall) about 6 ft. of white blocky chalk with tabular flints underlies 

 the Chalk Rock, which consists of hard patches of chalk embedded in soft 

 white chalk, often with no clear-cut demarcation. Green-coated nodules 

 are absent. Ammonoids and gasteropods are common (they are almost 

 absent between the Chalk Rock and the Chalk Marl), and a typical 

 Hyphantoceras reussianum fauna occurs. The lithology of the Chalk Rock 

 reappears towards the top of the plainis-zone in the Top Rock. This is 

 distinguished from the Chalk Rock by the occurrence at its upper surface 

 of a hard limestone with pinkish brown nodules, about one foot thick, 

 with a definite top crowded with green-coated nodules, some of which 

 are internal casts of Micraster cortestudinarium. The best exposure of the 

 Top Rock is near Westley Waterless ; it can also be seen south of Higham 

 in West Suffolk. 



The Micraster-zones of the Chalk are largely concealed under Boulder 

 Clay; the cortestudinarium-zone is seen north-west of West Wratting, and 

 the coranguinmn-zone near Shudy Camps and Saffron Walden. The Chalk 

 is white and flints are plentiful. 



THE PLEISTOCENE DEPOSITS OF THE 

 CAMBRIDGE DISTRICT" 



In Cambridgeshire no exposures have been found of beds comparable in 

 age with the Crag and Early Pleistocene deposits of the eastern parts of 

 Norfolk and Suffolk. Coarse gravels and sands, fan-wash fmgering out 

 from the Lower Chalky Boulder Clay ice sheet, can be seen underlying that 

 Boulder Clay on the high ground south-east of Cambridge — on the 

 Gogmagog Hills and at Haverhill in Suffolk. The ice sheet advanced over 

 a fairly deeply dissected landscape as far south as the London Basin. 



' H. Dixon Hewitt has shown that part at least of the famous Brandon Flint 

 Series (Suffolk) belongs to the Terebratulina-zone. 

 ^ By T. T. Paterson, M.A. 



