THE GEOLOGY OF THE LEA VALLEY.
199
If we note the formations traversed by the southern tributaries
of the Thames east of Windsor, the Wey, Mole, Darent, and Med-
way, we find that they all rise in the Lower Greensand or Wealden
districts, beyond the Chalk of the North Downs. Only the smaller
streams, such as the Cray, Wandle, and Ravensbourne, have their
sources within the Chalk and Tertiary area.
But the northern tributaries of the Thames in the district
between Windsor and the sea are all Chalk and Tertiary streams.
The chief of these are the Colne and Lea (with the smaller rivers
which fall into them), and the Lea is decidedly the more important
of the two. It "rises from the lowest part of the Chalk, north-east
of Dunstable" (Whitaker : "Geology London Basin," part i., p. 3),
very close to the Chalk boundary. Its chief tributaries, the
Mimram, Beane, Rib, Quin, and Stort, also originate in the Chalk,
which, in the uppermost part of their courses, is either bare or
covered by Glacial Drift, which consists of sand and gravel overlain
by Boulder Clay. The sand and gravel appear in the flanks of the
valleys which have been cut by the various streams, while the
Boulder Clay forms the surface of the higher ground between them.
Along a somewhat irregular line, which may be roughly described as
ranging from Sudbury to Bishop Stortford, and thence towards
Hatfield and Watford, Woolwich and Reading Beds have been
detected here and there beneath the Glacial Drift. But the belt of
ground occupied by them is a very narrow one, while the overlying
London Clay may be seen either forming the surface, or more or
less covered by superficial beds, over four-fifths of Essex. It forms,
with the occasional addition of a capping of some superficial bed
of gravel or Boulder Clay, all the higher ground on each side of the
Lea valley below the junction with the Stort, beyond the alluvium of
the marshes, and the older and slightly more elevated flats of gravel
and brick-earth close to the river. These last-named deposits are
entirely due to the action of the stream, and have been formed in
the following manner.
In addition to the action of rivers in cutting their beds deeper
and deeper, and further and further back, they also tend to change
their courses laterally, to eat into the bank on the side where the
current is strong, and to deposit material on the other where it is
sluggish. This, in the case of the Lea, has resulted in the destruction
of much high ground, consisting chiefly of London Clay, and the
deposition, on the planed-down surface, of a comparatively thin bed