2 Geology and Physiography 



Around Mildenhall, Brandon, Thetford and Lakenheath, the character- 

 istic features of the Chalk upland are modified by a covering of gravels and 

 sands that occupy the area known as Breckland. In places, deep circular 

 depressions formed by solution reveal the presence of the underlying 

 Chalk. Some of these depressions are very large and contain water more 

 or less permanently, e.g. the Devil's Punch Bowl, 4 miles north of 

 Thetford. Other depressions, which are probably also solution hollows, 

 hold the well-known meres of the Thetford district, remarkable for the 

 fluctuations of their water levels, and even for their complete desiccation 

 at certain periods. 



(ii) The Western Plateau Hes south of Madingley along the Cambridge- 

 Bedford road. From Eltisley, the plateau extends southwards for about 

 7 miles and occupies a considerable area; the surface stands between 

 200 and 250 ft., and, when viewed from a distance, looks remarkably even. 

 Originally, it had a much wider extent both eastwards towards the lower 

 part of the Chalk scarp, and westwards: its present limits are the result of 

 dissection by the streams of the Ouse and Cam drainage systems. Large 

 areas have, however, escaped dissection. The plateau is due in the main to 

 a covering of Chalky Boulder Clay which overlies rocks ranging from 

 Oxford Clay to Chalk. These rocks are exposed only on the dissected 

 slopes of the plateau. 



(iii) The Feiilaiid occupies the northern part of the district. At one time 

 the Chalk uplands of Norfolk were continuous with those of Lincolnshire, 

 stretching across what is now the Wash. This ridge of Chalk was worn 

 away by the action of rivers and the sea, and behind the ridge the Fenland 

 was carved out of soft Jurassic clays — the Oxford, Ampthill and Kimeridge 

 Clays. The surface of this Jurassic plain was uneven, and its liigher portions 

 projected above the general level to become the "islands" of the historical 

 period. The whole region emerged from the various phases of the Ice Age 

 with its islands capped with Boulder Clay, but with its basin nature 

 unchanged. Subsequent time has witnessed the filling up of this basin. 



The modern surface, therefore, is composed of post-glacial deposits 

 consisting of alternating layers of peat and silt or clay (Buttery Clay), 

 which rest on a foundation of formations ranging from Oxford Clay to 

 glacial or post-glacial sands and gravels (March Gravels, etc.). 



RIVER SYSTEM 



Broadly, the Cambridge district may be regarded as an immature pene- 

 plain, in which the influence of the varying resistance to erosion of the 

 formations has not yet been obliterated. The area was invaded more than 



