Geology and Physiography 



23 



The post-1935 work has enabled water-table contours to be constructed 

 for three different dates in each year (October, March and June), and 

 Fig. 8 is an example of the results obtained. The October level of 1935 was 

 selected as it came after the drought of 1934-35 and represents, therefore, 

 an exceptionally low level. The March level of 1936 represents the spring 

 maximum of that year, and although this was lower than the maxima for 

 1937 and 1938, the two readings chosen represent the greatest range within 

 any of the three years. Although the best wells have been levelled from 

 bench-marks, too great reliance should not be placed on the details of the 

 contours. The errors inevitable with a party of nearly eighty students are 

 partly counterbalanced by the large number of observations taken; but 



Ft«t 



4oo 



20O 



O.D. 



Scale 

 



1 Mile 



_i 



Chalk Rock. 



Uppac Ct\atk 



Vertical Exa^g'eratlan 26 timis 



Malboutn 

 Rock 



MiddLU Chalk 





TottirnVioc Stone ~ 



^»>\i;.Jl»5t^ 





\ 



Fig. 9. 

 Water Table: Section along a line running north-west from Balsham (see Fig. 8). 



on the other hand, the wells are irregidarly distributed, and pumping, 

 together with the recent closing of many wells, prevent great accuracy. 

 This appUes particularly to the neighbourhood of the town of Cambridge. 

 A feature of note is the fall of the water table below the level of the 

 Granta near Abington and Linton during October 1935; an impervious 

 bed of alluvium presumably enabled the river to flow perched above 

 the neighbouring Chalk water table. 



The section (Fig. 9) covers the same period as the map and shows the 

 geological formations that influence the water table. The relation between 

 surface relief and the water table is clear, particularly in the sudden rise in 

 level north-west of Balsham. The seasonal range is also seen to increase 

 with the depth of the water table. The section is terminated on the west at 

 the outcrop of the Totternlioe Stone near Cambridge, and on the east at 

 the occurrence of several perched water tables in the Boulder Clay near 

 Balsham. In this latter area, wells, witliin 40 yards of each other, diflfer 

 by more than a hundred feet in their water levels. 



