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 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