GEOLOGY 



The intercalation of sand and gravel with the Chalky Boulder Clay 

 is best explained as a marginal phenomenon produced at different stages 

 in the advance and retreat of the ice-sheet. 



In the ancient hollow along the Cam valley the Boulder Clay was 

 seen to be much mixed with sand and gravel in alternate irregular 

 masses. This is also the case at Elsenham and elsewhere. Gravel over- 

 lies Boulder Clay in some places on the higher ground, although not to 

 any considerable extent, as at Mountnessing and near Chipping Ongar. 



Loam also associated with the Boulder Clay occurs over a large area 

 in north-eastern Essex ; it rests mostly on gravel and sand, and from its 

 mode of occurrence it might be taken to replace the Boulder Clay. It thus 

 extends over the heaths of Boxted, Ardleigh, Crockleford and Elmstead, 

 and around Wicks and Great Bentley, where the soil a ' fertile loam ' 

 is regarded as good turnip-land. 



Minor patches of loam occur on the Boulder Clay between Peb- 

 marsh and Twinstead, at Finchingfield, south of Terling, and on Hat- 

 field Heath : in such situations the loam is usually a decalcified portion 

 of the Chalky Boulder Clay. 



In former days the Boulder Clay was much used for claying or 

 marling the ground for agricultural purposes ; hence numerous old pits, 

 now for the most part ponds, may be noticed in the fields. 



Sands and gravels occur beneath the main mass of Boulder Clay, 

 but their occurrence though widespread is uncertain. They were distri- 

 buted during the earlier stages of the period of great glaciation, and 

 occasionally, as in Hertfordshire, seams of Boulder Clay occur in the 

 gravel. They may outcrop between Boulder Clay and London Clay on 

 one side of a valley and be absent from the other side. Hence in sink- 

 ing a well through Boulder Clay supplies of water cannot always be 

 depended upon, although in many localities a good supply is obtained. 



These sands and gravels occur along the Cam valley at Quendon, 

 along the Stort Valley at Clavering, Harlow and Roydon, below Sudbury 

 in the Stour valley, along the Colne from Great Yeldham to Halstead and 

 Gosfield, along the Pant from Radwinter, the Pods Brook from its source 

 near Great Bardfield, and the Chelmer in places from Thaxted. 



In some of these valleys, as along the Chelmer, it is not easy to 

 separate the Glacial gravels from the newer valley gravels which are 

 principally derived from them. Occasionally, as near Roxwell, the 

 gravel is cemented into a conglomerate. 



Gravel extends over the high grounds of Danbury, being perhaps 

 in part the wreck of an outlier of older pebbly gravel. Towards the 

 south and west Boulder Clay is intercalated with the gravel, while on 

 the east, at Maldon and on Hazeleigh Common, Boulder Clay over- 

 lies it. 



Glacial gravel occurs around Colchester, at Brightlingsea and St. 

 Osyth ; and there are patches on Mersea Island, at Tolleshunt and on 

 Tiptree Heath. Some of these tracts of gravel are not clearly to be 

 separated from the range of old valley gravels which in east Essex 



