148
THE ORIGIN AND DEVELOPMENT OF THE
LEA VALLEY.
BY ANNIE R. HATLEY, B.Sc.
[Read 25th October, 1924.]
THE Lea Valley is part of the triangular-shaped "London
Basin," that area of downfolded rocks lying between the
Chiltern Hills and the North Downs.
Throughout the whole area the drainage from rain and
springs on the marginal heights has sent streams southward
and northward into the Thames, whose course lies in the central
line of the basin. The Lea joins the Thames on the north side,
therefore the rocks in its valley will have a southerly tilt. The
oldest will occur at the surface in the north, and younger ones
farther south.
Although the Lea follows the dip of the strata in its general
direction, it is not simply a "dip stream." There are four
well-marked changes in its course, giving five divisions in the
valley :—
(i.) from the source to How Green, S.W. of Hertford;
(ii.) thence to Ware ;
(iii.) thence to the confluence with the Stort ;
(iv.) thence to Walthamstow ;
(v.) thence to the confluence with the Thames.
Of these (i.), (iii.), and (v.), follow the general dip-direction,
N.W. to S.E.
From this brief statement it will be seen that the origin
of the Lea Valley is somewhat complex. The valley has been,
cut out mainly by river water, which is drawn from the Chalk
itself or from the surface of impermeable beds. Most rain falls
during autumn and winter, when plants use less of it than in
the growing seasons, so that the plane of saturation in the
Chalk rises to give maximum flow in the rivers in spring and
minimum in autumn.
The water-parting of the Lea and Ouse river systems is the
hard band called "Chalk Rock," but this is cut through by
nearly all the tributaries of the Lea, and its neighbour, the Colne,
so giving evidence of an increase in the area of the Thames
drainage. The water-parting lies at about 800 feet above