4 Geology and Physiography- 



products from an ice sheet with an ice-contact slope along the south-west 

 margin (Linton and Hildersham). 



In the south of the area, the drainage consists of the tributaries of the River 

 Cam which join to form the main river above Cambridge (Fig. 3). The 

 Rhee, also known as Cam or Rhee, rises in the group of powerftd springs 

 which issue below the road at Ash well (Herts), and flows as a strike stream 

 mainly on the Gault to Trumpington, where it is joined by the Cam (also 

 known as Cam or Granta), which rises about 4 miles south of Newport, 

 and flows northward along the Chesterford-Newport depression.^ This 

 depression is underlain by a deep, narrow channel eroded into the Chalk 

 to a depth of at least 20 ft., and possibly even to 100 ft., below o.d. It is 

 filled largely by loams and sands, apparently laid down in water. A third 

 tributary, known as the Granta, descends across the face of the Chalk 

 escarpment from the neighbourhood of Bartlow and Linton. Of these, 

 the Rhee and the Granta are related to the dip and strike of the rocks, 

 whereas the Cam has been determined by the events that formed the 

 above-named depression. The Bourn brook, which drains the western 

 plateau, rises near Eltisley and enters the Cam above Cambridge, just 

 below its junction with the Rhee (see Fig. 3). 



It is not improbable that the three major depressions which traverse the 

 Chalk escarpment were eroded by streams flowing from the north and 

 west as consequent streams down the general dip slope of the Chalk, and 

 that the headwaters of these streams were captured by the development of 

 consequent or strike streams (e.g. the Rhee), leaving these depressions as 

 wind gaps. The Chesterford-Newport deep channel was probably eroded 

 further by an overflow from a glacial lake occupying the ground near 

 Cambridge which was hemmed in between the ice on the north and the 

 Chalk escarpment on the south. The Little Ouse-Wavency gap may also 

 have been an overflow channel. In various parts of the district borings 

 have revealed the existence of chamiels or holes eroded to considerable 

 depths below O.D., but the origin of these is as yet unexplained. The valleys 

 of the main rivers Cam and Ouse are occupied by river gravels in which 

 certain well-defined terraces can be observed. 



UNDERGROUND STRUCTURE^ 



Although no rock older than part of the Great Oolite Series immediately 

 beneath the Combrash outcrops within the area, older rocks are known 

 from borings, particularly near JVIethwold where Middle and Lower Lias 



' The Upper Cam has apparently brought about the capture of streams which 

 formerly flowed southwards to the London Basin. 

 » By E. C. Bullard, Ph.D. 



