accumulation of the Gamhridyeshire Pleistocene Deposits G5 



and undoubted marine deposits containing sea-shells are found 

 to a height of at least 50 feet above sea-level in the Nar Valley, 

 and deposits up to 80 feet above sea-level have been claimed as 

 marine. Unfortunately no exposure of these Nar Valley beds 

 has been seen for a very long time, and their exact upward limit 

 is a matter which must remain unsettled until new excavations 

 are made. It is held, with good reason, that the beds of March 

 and the Nar Valley are geologically contemporaneous in the sense 

 that they belong to the same period of sea-invasion, which was 

 subsequent to the accumulation of the chalky Boulder Clay ; and 

 as there is good evidence that much of the fenland was low-lying 



Nar Level 



Fig. 1. 



AB. Slope of ground before marine gravels were deposited. 

 CD. ,, „ after ,, ,, ,. 



a. Tract of marine gravels. 



b. ,, interdigitating marine and fluviatile gravels. 



c. ,, fluviatile deltaic deposits. 



d. ,, erosion in valley towards its head, during period of deposit of 



a, b, c. 

 1, 2, 3. Order of formation of deposits in tracts c and d respectively. (1 is oldest.) 

 Vertical scale greatly exaggerated. 



ground after this boulder-clay was formed, it would appear prob- 

 able that the March gravels are earlier than those of the Nar 

 Valley, and therefore that a gradual silting up of a bay of the 

 sea took place, until the sediments reached a height of at least 

 50 feet above present sea-level. 



During this period of silting the rivers Ouse, Cam and others 

 would build delta-deposits along the lower parts of their courses, 

 with interdigitation of marine and fluviatile deposits in an inter- 

 mediate belt of ground as shewn in figure 1. In this delta- 

 material, the chronological sequence of deposit would be from 

 below upward, as shewn by 1, 2 and 3 in the belt c. The upper 

 waters of the rivers would still be eroding, and the sequence 

 would be from above downwards (see figs, in tract d). 



After submergence had ceased, it would be replaced by re- 

 emergence, as shewn by the erosion of the rivers to their present 



