ORIGIN OF THE LEA VALLEY.
151
Each rock type, with its characteristic soil, is a fundamental
factor in vegetation, water supply and economic development.
Vegetation. Originally there was marked contrast between
the chalk plateau, the forestal area, and the marsh which occu-
pied the surface of the alluvium. In the forest itself there must
always have been differentiation. Nowadays, the sand and
gravel soils support ever-increasing numbers of birch with heath
and bracken.
On the Claygate Beds at the top of the London Clay, beech
is found, while on the clay proper hornbeam and oak occur, the
oaks only where the clay is heavy. There is one other type—
the vegetation of the Boulder Clay—which bears so marked a
resemblance to the true chalk flora.
Water Supply. Some soils are porous, others are not.
Water accumulates where porous gravels lie upon clay until
it flows down the clay as springs.
In a similar way springs are found above hard bands in the
Chalk and above the Claygate Beds, where ponds usually form,
especially in winter. This is very well shown in the high level
ponds along the Epping Forest Ridge. To these springs the
district owes its irregular surface. The sides of the main valleys
are cut up by a series of smaller valleys which are emphasised
in the contours of the roads which cross them.
From the standpoint of Economic and Human Develop-
ment the Lea Valley offers several advantages. The first, and
probably the greatest, is the open route northwards from London
which helped to make that city important. The others can be
grouped as :—
I. Those depending on the land—defence, forest assets,
soil development, building material ;
II. Those derived from water—navigation, food, water for
household purposes ;
III. Aids to settlement and communication.
(I.) Chalky and gravelly uplands with a good constant
water-supply give ample opportunity for defence. Notable
examples are Ambresbury Banks, the British encampment
against the Romans ; Stane Street, the Roman military road
from Watling Street to Colchester ; Bishop's Stortford Castle,
built upon an island in pre-Norman times ; and the heights
which were used as outer defences of London during the late war.