THE PLIOCENE PERIOD IN WESTERN ESSEX. 253
inevitably fails us, and the course followed by the river must
remain a matter of inference.
The only other locality in Essex where high-level gravels
with abundant chert occur is on the summit of the Laindon Hills.
Its characters may be examined in a small pit near West Lee
Hall. The dominant flints are accompanied by a strikingly
large amount of Lower Greensand Chert and some small quartz
pebbles, the whole being set in a coarse quartzose sand, showing
current-bedding. The elevation is 370-380 ft. O.D., and there
can be no doubt that the gravel is of the same age and origin
as its analogues in the west of the county. The chert of Hert-
fordshire and the Wake Arms outlier must have crossed the
line of the North Downs at or near the present Mole Gap, and
the stream concerned may be not inaptly termed the "Proto-
Mole." The situation of the Laindon gravels opposite the Med-
way Gap suggests another southerly affluent of the river system
—the Proto-Medway. It is probable that the chert occupied
quite a narrow N. to S. zone at Laindon, as it does in the vicinity
of the Lea Valley. Eastwards, it is true that no definite limit
can be set to its former extension, for no ground reaches the
required elevation, but westwards the limit is set by its virtual
non-occurrence at Billericay, etc., in gravels presently to be
noticed.
We may thus regard it as probable that a stream with two
major affluents from the south, and perhaps to be regarded as
the Proto-Thames, crossed the north-western corner of the county
of Essex at a date not long after that of the Lenham Beds. The
acceptance of this idea carries us one stage forward in our enquiry,
but leaves at least one difficult problem untouched, viz., the age
and origin of the "Warley Gravels," formerly known as the
"Bagshot Pebble Bed." As the older name implies, this
formation was at one time grouped with the Bagshot Beds, but
there is now a general concurrence in the view that it is a poly-
genetic gravel of later date. Since it has constant and wide-
spread features, it deserves a name; the designation "Warley
Gravels," originally used by Prestwich and latterly revived by
the officers of H.M. Geological Survey, is in all respects appro-
priate and will be used in the following account.
A brief but excellent account of the Warley Gravel has