ON THE LEA VALLEY.
229
and one also the result of erosion in either Pre-Glacial or early
Glacial times. In this case in the valley of the Lea there is
quite as strong a probability against the agency of a fault as in
that at Littlebury; while local chemical dissolution, which
formed a possible cause on the Cam, either of making or deepening
the channel, is out of the question with the London-clay of the
Lea.
I have already remarked that at Littlebury glacial deposits
are seen only on the high ground east and west of the river
valley, Boulder-clay alone having been noted there. But at
Newport, also on the Cam, and about three miles southward, the
Glacial sand and gravel is found in the sides of the valley
between the river-deposit at the bottom and the Boulder-clay
above. In the valley of the Stort this is the case everywhere
from its source to its junction with the Lea at Hoddesdon.
From Hoddesdon southward the Glacial Drift, whether gravel
or Boulder-clay, or both, is seen only on the high ground at
some distance from the river. But both on the eastern and
western sides of the Lea it has been noted as far south as
Hendon, Finchley and Muswell Hill in Middlesex, and Wood-
ford and Romford in Essex.4 So that there can scarcely be any
doubt as to the Post-Glacial age of the valleys of the Cam about
Littlebury and the Lea at Tottenham and Walthamstow. Of
course I use the word Post-Glacial in its only legitimate sense
as meaning of later date than the Chalky Boulder-clay of Essex
and Middlesex.
In the north of England, as Topley remarked in the dis-
cussion on Mr. Whitaker's paper, many examples are known of
the Pre-Glacial and Post-Glacial valleys of rivers and the
relations between them. In illustration he stated that in
Northumberland the Blyth was in Pre-Glacial times a tributary
of the Wansbeck, and a deep Pre-Glacial valley, which was
filled with Glacial Drift, occurred between the present valleys.
Many other cases might be mentioned in which the Pre-Glacial
drainage system, having been in its general features like that of
Post-Glacial times, the river valleys in the two periods have
here and there coincided. In this part of the south-east of
England the Glacial deposits have covered the Pre-Glacial
4 Q.J. Geol, Soc. vol. lxviii., p. 365 (1892); Q J. G. 5., vol. 1., p. 443 (1894) Also Essex
Naturalist, vol vii. (1893).