158 THE ESSEX NATURALIST.
In the W. these gravels cap the Wargrave Hills (475 O.D.),
where Bunter pebbles, which do not occur elsewhere, are
abundant. To the E. the same drift is found on the Epping
Forest ridge, and beyond.
It must not be supposed that the first course of the Thames
corresponded with that of the present river. Sherlock [1912,
1924] and Wooldridge [1938] have shown reason to conclude
that the early Thames, after running from Goring Gap to Bourn
End, swung N.E. through the Vale of St. Albans and thence by
Hertford and Ware. From this point the course is more
difficult to infer : Sherlock carries it N. to the Wash, Wooldridge
E. by Harlow and Chelmsford to the Blackwater. As far as
Chelmsford I agree, but one should not overlook the fact that
beyond Chelmsford we are approaching the country where the
Pliocene emergence was beginning to fade out towards the
sinking area of Suffolk, and it seems to me that the first Thames
would be free to wander over the N.E. gradient in the direction
of the Suffolk border and the shoreline of the Crag Sea.
The shallow-water estuarine deposits of the Norwich Crag,
which have been proved down to 230 ft. below sea level, must
include Thames drainage as well as material from the Ardennes
brought in by the Meuse, or Rhine-Meuse,
In connection with the lower course of the Pliocene Thames
it may not be irrelevant to state that the Cromerian or Forest
Bed channel deposit [Warren, 1940] that I discovered near
Harwich was rich in Kentish chert.
It therefore seems probable that it is a Medway deposit,
for if the Thames had then flowed along the Blackwater the chert
stream would not have carried on to Harwich. So that it seems
more in accordance with the available evidences to place the
Pliocene Thames W. of Harwich.
Many years ago Prestwich [1890] noted the abundance of
"Cretaceous Stones" in the high-level gravels to the N. of the
Thames, and he included this in his idea of a "Southern Drift."
The explanation is simple : the north-flowing tributaries (conse-
quent streams), transported the "Cretaceous stones" beyond
the present Thames when the river flowed along the Hertford-
Ware route. But the contribution of chert carried by different
streams was unequal, and Wooldridge has traced the course of
certain chert-bearing streams.
Epping Forest (360 to 385 ft. O.D.) [Prestwich 1890,
Wells and Wooldridge 1923].—I am indebted to the military
authorities for allowing me to examine the anti-tank ditches
which have exposed about two miles of sections through the
most interesting part of the Pebble Gravel. One trench passes