406 PROFESSOR HUGHES 



gravel is found at a much higher level, and indeed is finally 

 cut out altogether by a boss of gault capped by chalk marl 

 between the Abbey and the Junction Station. 



If we extend our enquiries we find further evidence of the 

 subsidence of the area in (geologically speaking) recent times. 

 When sinking for water at Impington Park Mr Macfarlane 

 Grieve found in one place Boulder Clay, with a gravelly bed 

 at its base, to a depth of 88 feet 6 inches, and at Button 

 Bridge, north of Wisbech, marine alluvium was proved to a 

 depth of 65 feet resting upon Boulder Clay. Widespread 

 depressions since glacial times are shown, according to the 

 observations of Prestwich, Whitaker, and others, by deep 

 troughs of Boulder Clay in many parts of East Anglia. It is 

 clear, therefore, that there has been a subsidence affecting the 

 level of the river at Cambridge which has been going on since 

 the glacial age, has been continued to the time of the deposi- 

 tion of the river-silt at the " Backs " of the colleges, and may 

 be going on still. 



For various reasons, therefore, the actual height of the 

 ground above O.D. is not of great importance in our present 

 enquiry, but the relative height of different portions of it and 

 the fall towards the river, does help us to detect what were in 

 old times the waterlogged areas, and the districts unsuitable 

 for houses. 



The height about the railway station is over 50 feet above 

 O.D. This drops to 45 feet at the south-east end of Parker's 

 Piece, and to 42 feet on the opposite side by Park Terrace. 

 From 47 feet in Brooklands Avenue it falls to 41 feet near 

 Brookside, a level maintained on either side of Downing 

 Grounds to near Downing Street, where it has fallen to 

 37 feet. 



Now if we examine the natural features along this line of 

 country we can trace first of all a marked terrace projecting 

 from the east side of the valley down which Vicar's Brook 

 runs. This terrace is lost sight of near Brookside. A good 

 deal of water is obviously delivered, where the gault rises 

 underneath it, from the higher terraces along St Andrew's 

 Street to the lower part of the gravel which forms the sub- 



