198 THE GEOLOGY AND SCENERY OF THE CLUB'S VOYAGE 



be 234 feet thick. On the other hand, at the waterworks at 

 Moulsham, near Chehiisford, the surface beds consisted of 63 1 feet 

 of Glacial Drift, which rested upon 86^ feet of London Clay. The 

 river-gravel and alluvium, which occupy the flat ground close to the 

 streams, are the work of the rivers, and are consequently confined to 

 their valleys. Rivers are perpetually tending to change their 

 courses, to eat into the bank on one side and to deposit gravel, sand, 

 or loam on the other. The nature of the material deposited in this 

 way at any given spot depends partly on the force of the current, 

 partly on the nature of the rocks higher up the stream. These 

 valley-beds between Maldon and Chelmsford probably seldom 

 exceed twenty feet in thickness, and average less. They, in all 

 probability, rest everywhere upon the London Clay. 



As we leave Maldon, a broad flat of river-gravel appears on the 

 northern bank of the Blackwater between Heybridge and Langford, 

 and a small patch surrounds the railway station. It is slightly 

 higher in level than the alluvium of the marshes. Old river-gravel 

 has always been a favourite site for human habitations, whether towns, 

 villages, or isolated dwellings, while houses on marshes are extremely 

 rare. As we ascend the river, few patches of gravel of any size are 

 seen, while the alluvium of the marshes occupies a belt of ground 

 bordering the stream, and having an average breadth of rather more 

 than half a mile throughout our voyage. It forms excellent pasture 

 land. 



As already stated, the higher ground on each side consists of 

 London Clay capped by Glacial Beds, the latter being hereabouts 

 almost wholly gravel. Between Chelmsford and Maldon, on the 

 southern bank, this Glacial Gravel covers a considerable area, and 

 the overlying Boulder Clay is seen only here and there in small 

 patches. Between Little Baddow and Chelmsford this gravel is 

 shown on the map of the Geological Survey (i N.E.) as coming 

 down to the level of the alluvium on both sides of the stream. Mr. 

 Whitaker, however (Geology of London, vol. i., p. 316), is inclined to 

 think that the wash of sand and gravel down the slopes may have 

 proved deceptive. No doubt there is glacial sand and gravel low 

 down on these slopes, where it is depicted as being, but it is not 

 where it was originally deposited. The material belongs to the 

 (ilacial Period, but all of it below a certain level has been washed 

 down the hillsides during the ages in which the Chelmer was cutting 

 its way downwards to its present level, and thus forming the valley 



