48 The Geology of Cambridgeshire 



expand to occupy wide areas. Exposures in this terrace are 

 uncommon but the fine gravel which caps several of the 

 Shepreth cement pits seems to belong to this series. Fossils 

 are rare and uninteresting. " ". v 



Later than all these Gravels is the alluvium of the valleys 

 and the various deposits which form the Fenland. These mark 

 the completion of geological change in Cambridgeshire and show 

 that the rivers had ceased from active erosion and that their 

 lower courses had passed into the condition of water-logged 

 swamps at or about the level of the sea. 



The deposits of the Fenland may be divided into three 

 main types, Alluvium, Peat, and Marine Silt, but these from 

 the nature of the case are so mixed up that no boundaries can 

 be drawn between them, and no order of succession is ever 

 maintained. 



As might be expected, Alluvium is dominant in the 

 southern part of the area along the river banks ; Silt in the 

 northern districts near to the sea ; and Peat in all places 

 where the temporary or permanent absence of the other two 

 types has left it free to grow. 



The Peat of the Fens is an accumulation of plant remains 

 and includes many species of plants which only grow with 

 roots and stems beneath the water, and indeed it seems 

 probable that it was only when such plants had already 

 prepared a platform that the more usual damp-loving land 

 plants could begin to grow. 



Fen Peat, therefore, must have grown outwards quite as 

 much as upwards, so the relative height or depth of particular 

 layers at different localities cannot be any criterion of their 

 relative age and the many small areas which remain as Meres 

 are only areas not yet reached by the outgrowing plants. 

 Whittlesea Mere is a good example. 



The best sections of Peat are to be found in areas preserved 

 for Turf-cutting, and of these, Burwell Fen is the most access- 

 ible. In the cuttings there, the lower peat consisting of 

 Juncus and water plants is generally under water, but the 



