246 THE ESSEX NATURALIST.
and by J. W. Gregory for the high-level quartzite gravels at
Danbury, for which a Miocene age is claimed. At the same
time the full consequences of this view have not been explored
and in particular its bearing upon the former course of the
Thames remains an open question.
Views on the pre-glacial course of the Thames have been
rather diverse, but there is a group of theories containing a
common clement which directly concerns us here. It will be
well briefly to state these theories before developing our present
argument.
Sherlock has urged that the Thames, in pre-glacial times
followed the northern drift route from Bourne End (where it
leaves the present Thames valley) via the Vale of St. Albans
to the neighbourhood of Hertford. Though at one time he
considered it possible that it turned south at this point along the
line of the lower Lea, further consideration of the question led
him to claim that it turned northward out of the London Basin
via the well-known gap which lies between the headwater
streams of the present Stort and Cam. Hawkins, on the other
hand, in a general study of the morphology of the Lower Thames
system, stressed the resemblance, amounting almost to identity,
between the well-known "Henley loop" in the present Thames
valley and the composite loop made by the valleys of the lower
Colne, the lower Lea and the Vale of St. Albans, which joins
them along the foot of the Eocene escarpment. He suggests
that at a former date the Thames flowed "up the Colne and down
the Lea," a conception readily appreciated from a study of the
map. It is perhaps well to remind ourselves that if such a
course was followed, it would lie high above the present valley
bottoms in most places. The reversal of flow in the western
end of the loop would be referred to a later date, and hence a
lower level. Prof. J. W. Gregory1 suggests still another
variation on this plan. He regards the headwaters of the Ouse,
the Upper Lea and the Chelmer as forming dissevered parts of
a formerly continuous stream, entering the sea at Maldon and
receiving in Mid-Essex an important affluent from the south-
west, roughly along the line of the present lower Thames valley.
In so far as this reconstruction is based on the constitution of
1 The Rivers of Essex, Benham's, 1923.