150 THE ESSEX NATURALIST.
to south along the Lower Lea and is part of a long flexure across
the Thames into the Croydon area and quite distinct from the
foregoing.
These folds have produced variety in structure which has
given rise to a complex drainage system and a varied landscape.
The other factor was the Ice Sheet which advanced from
the north, ploughing up the surface and leaving the Chalky
Boulder Clay behind when it finally melted. To the west of the
Lea Valley is an extensive plateau scarcely cut into by rivers
and a small but well defined escarpment from Ware to Rick-
mansworth. In marked contrast, the escarpment on the east
has been overridden by the ice, which extended far into Essex,
leaving Boulder Clay nearly as far south as the Thames Plain.
To summarise the evolution of the Lea Valley :—
1. The downfolded London Basin gave the Lea its south-
eastward course from the Chalk Hills.
2. The lesser folds have given it changes in direction.
3. The chief fold determined the north to south lower
valley, and led to the broadening of this valley as the river
worked its way eastwards.
4. S.W.—N.E. folds have influenced the configuration of
the land. The weakened rocks of upfolds have been re-
moved by streams such as the Cobbin's Brook, while the
compressed rocks in downfolds have resisted denudation
and so formed ridges, as in Epping Forest.
5. Differential movement, combined with glacial action,
have made the western side higher than the eastern.
Such structural features would probably produce a similar
drainage system, whether the rocks were hard or soft. Since
scenery depends on the rock in which it is moulded, there will
be several scenic types in the Lea area corresponding to the
main rock types. These four are the most striking :—
I. The wooded plateau of the Upper Valley (with Chalky
Boulder Clay on Chalk) exposing Chalk on the deep cut
valley sides.
II. The Middle Lea Valley floor with its swift current and
water meadows.
III. The marshy flat land on the Lower Valley floor, where
the soil is fertile but not heavy.
IV. The mud flats of the lowest reaches.