March and Barnwell Gravels 45 



bedded gravel resting on a very uneven surface of ordinary 

 Boulder- clay which here contains no shells. The shells from 

 the gravel are almost always a good deal worn. The general 

 aspect of the fauna is considerably more arctic than that of the 

 present North Sea. The commonest forms are Tellina balthica, 

 Turritella terebra, Buccinum undatum, Littwina littorea, and 

 Cardium edule, and with these occur occasional fresh water 

 species including Cyrena fluminalis. 



The Gravels of the Present River System present less 

 complex problems than do the older ones, but even they 

 exhibit phenomena not yet understood. They occupy the low 

 ground along the larger valleys often to a height of twenty or 

 even fifty feet above the present river level, and they cover 

 nearly all the ground on which Cambridge and the adjoining 

 villages are built. 



Three stages, indicated by successive terraces, are dis- 

 tinguishable, and the lower course of the river as thus indicated 

 seems to have varied considerably. 



The gravels of the highest and oldest stage are generally 

 spoken of as the Barnwell Series and from their included 

 fossils are the most interesting. 



From Barnwell the gravels have been traced along the 

 Cam valley to Chesterford, up the Lin to Linton, by Barton 

 and Comberton to Toft in the Bourn valley, and along the 

 Rhee to Foxton and Barrington. Below Barnwell a wide 

 spread of similar gravel extends across to Histon and one 

 or two patches of it occur further to the north. 



Exposures in the Barnwell Terrace are fairly abundant 

 and are often interesting. 



At Barnwell the ground is mostly built over but occasional 

 building excavations are made, and when found, they afford 

 a fine collecting ground to the conchologist. Cyrena fluminalis 

 abounds in several of the loamy beds, and teeth and bones of 

 large mammalia and especially of Mammoth are quite common. 



In the angle between the Milton and Victoria roads at 

 Chesterton several large gravel pits are worked and in these 



