THE PLIOCENE PERIOD IN WESTERN ESSEX. 255
no entirely satisfactory solution can yet be propounded. We
must rest content with stating the facts and pointing the
possibilities. Of the latter only three need be considered.
Granted that the Warley Gravel is post-Eocene, it must be (a)
pre-Lenhamian, (b) Lenhamian, or (c) post-Lenhamian. A few
remarks will be made under each of these heads.
(a) The assumption of a pre-Lenhamian origin, from the
wreck of local Eocene pebble-beds, is not probable, but it violates
none of the facts. The heavy minerals of its matrix are of an
Eocene character on the whole, and since pebble-beds occur
in the Bagshot Beds of the vicinity, a source for the dominant
flint pebbles is to hand. The wreck, in situ, of such pebble-beds
might well produce a structureless accumulation such as the
formation in question. As against this, there is the occurrence
of stones other than rounded flints. Moreover, it would imply
that the Lenham Beds had been deposited above and had since
been completely removed (see however, p. 256).
(b) The assumption of a Lenhamian age is faced with the
difficulty of a complete lithological and petrographical disparity.
It is of course conceivable that the Lenham Beds along the
centre of the channel might differ in some respects from the
marginal deposits; such would seem to be the case with the
Stanmore deposits, referred to the Lenham Beds. In pebble-
constitution the Warley Gravels compare fairly well with the
Stanmore shingles, and the pebbles show the same high degree
of weathering. A surer test, that of the heavy detrital minerals,
fails altogether to support the comparison. The suite from the
Warley Gravel is definitely Eocene in character, and the writer
has been unable to detect any sign of affinity whatever with the
Lenhamian suite. This is not sufficiently explained by the fact
that the beds rest on an Eocene horizon, for elsewhere the Len-
hamian suite is largely independent of the nature of the sub-
jacent formation.
(c) In pursuance of the ideas stated above as to the existence
of a post-Diestian river-system we should expect to find the
deposits of the Proto-Darent in the vicinity of Brentwood.
It is in some respects suggestive that the Warley Gravel occupies
an area intermediate between those of the two chert-bearing
series of gravels (see map, fig. 1). The River Darent crosses
the Greensand belt in a region where the chert-phase is largely