198 THE GEOLOGY AND SCENERY OF THE CLUB'S VOYAGE
be 234 feet thick. On the other hand, at the waterworks at
Moulsham, near Chelmsford, the surface beds consisted of 631/2 feet
of Glacial Drift, which rested upon 861/2 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 (1 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
Glacial 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