20 Geology and Physiography 



In the middle and later phases of the Sub-Boreal period, semi-marine 

 conditions were replaced by those of peat-formation. Locally, pine and 

 birch woods developed (e.g. Wood Fen at Ely, and Woodwalton), and the 

 fen surfaces became dry enough to have encouraged dense occupation by 

 Bronze Age man. Along the fen margins, freedom from flooding allowed 

 sphagnum peat to form incipient raised bogs. 



The Sub-Atlantic Period. From 500 B.C. This period is generally recognised 

 as colder and wetter than the Sub-Boreal. At this time the Fenland became 

 very inhospitable, and was, apparently, shunned by Iron Age man. The 

 period is marked by two stratigraphical events. In the peat fens were 

 formed the shallow lakes which persisted into the last century (e.g. 

 Whittlesey Mere, Ugg Mere). Their sites are stiU recognisable by the 

 deposits of lake marl that formed on their beds. The seaward side of the 

 fens was built up during the Roman times by the deposition of fine silt 

 above the upper peat. These silts formed a broad belt round the Wash 

 and they extended to high-tide level. They were densely occupied in 

 Romano-British times,' especially in the last stages of their formation, and 

 this is true also of the raised banks of the tidal rivers which form landward 

 extensions of the sUt country. These levees are now recognisable as 

 "roddons", raised banks standing above the peat fens. They become 

 increasingly evident as drainage causes the wasting of peat which formed 

 over their flanks after the Roman period. The courses of the extinct 

 waterways of the Fenland have been mapped by Major Gordon Fowler, 

 and Fig. 7 summarises the available information about their extent and 

 distribution. They are mostly of Romano-British age. 



THE CHALK WATER TABLE SOUTH-EAST 

 OF CAMBRIDGE' 



The Fenland and much of Cambridgeshire which Ues below the 50 ft. 

 contour hne are not well adapted for a water-table survey. On the Chalk 

 uplands in the south-east of the County, however, a considerable amount 

 of work has been done since 1935 by undergraduates of the University 

 Department of Geography. The water levels in about 120 weUs, within the 

 region covered by Fig. 8, have been measured three times a year; and 

 fortnightly observations have been taken (during term) at about fifteen 

 reliable and widely distributed wells. Previous to 1929, similar measure- 

 ments had been made by Prof. W. B. R. King and also by officers of the 

 Royal Enguieers.3 



' See p. 92 below. ^ By W. G. V. Balchin, B.A., and W. V. Lewis, M.A. 



3 See H.J. O. White, "The Geology of the Country near Saffron Walden" 

 {Mem. Geol. Surv. 1932), p. 109. 



