68 THE ESSEX NATURALIST.
melt-water from the ice, their post-glacial ancestors must have
picked out the old valley lines made apparent by sags in the
Boulder Clay. The north-western Boulder Clay area owes its
deep "V"-shaped valleys to the rapid excavation of these
streams, whilst in the unglaciated south-eastern area the streams
approached a mature stage with broad "U"-shaped valleys.
These differences in the maturity of the several valleys are
brought out in the profiles of four typical rivers—the Colne,
Pant, Chelmer and Crouch. The first three exhibit young profiles.
The Crouch, beyond the glaciated area, shows an almost mature
stage, although in its longitudinal profile the 50 ft. and other
"knick-points" can be detected. Only the steps corresponding
to the 50 ft. terrace can be detected in the courses of the former
three rivers. The rivers mentioned also show important dif-
ferences in their valley shapes. It may be observed that the
upper reaches of the three rivers, Colne, Pant, Chelmer, have
very steep sides in the extreme north-west as at Bocking, Little
Waltham and Halstead; the lower parts of their courses, near the
margin of the glaciated area, appear to be of a much more mature
type.
One district has, so far, received little attention in the descrip-
tions of the maps of the successive channels of the Proto-Thames.
This is the region lying to the south-east of the main stream.
During the Boyn Hill period it became of some importance, and it
is necessary to delineate its outlines with as much accuracy as
possible. The greater part of the outline of this area in fig. 6
is conjectural, but acquires some corroboration from the outlines
of the offshore banks of to-day. These latter, paralleling the
north-east to south-west land ridges, seem to be features of some
antiquity. In view of the riverine gravels found along the
presumed path of the main stream, it is apparent that there must
have been land separating this main stream from the sea. The
path of the stream to that sea, at a little later date, implies that
this land area was rather narrow. It seems probable that the
Medway pursued a course directly to a former North Sea and that
one of its tributaries was responsible, by a process of capture,
for the ultimate path of the Thames to that Sea.
Gravels at an altitude of 40 to 50 ft. O.D. indicate that the
Thames in the later Taplow times followed a very similar course,
flowing a little further eastward along the coast of Essex and leav-
ing large areas of coastal gravels as witnesses of its channel.