250 THE ESSEX NATURALIST.
Lenham Beds, nor are they readily to be reconciled with a marine
origin. As already noted, the Lenham Beds yield a very definite
suite of heavy minerals which permits of their recognition over
wide areas. The Pebble Gravel does not yield this suite, though
some of the characteristic minerals of the Diestian assemblage
occur sparingly in it, as though derived through the break-up
and re-deposition of the Lenham Beds. Again, the character and
the mode of assemblage of the pebble-constituents are compatible
with a fluviatile rather than a marine origin. In particular,
the distribution of Lower Greensand Chert in the Pebble Gravel
is a serious obstacle to a hypothesis of marine origin. It is
abundantly present on the eastern side of the Hertfordshire
plateau, but fails abruptly west of a line running from High
Barnet to Newgate Street. It should be noted that Greensand
Chert occurs very sparingly in the Lenham Beds, even in localities
close to the present chert-outcrops, so that its abundance in
Hertfordshire would seem to indicate a later date for the Pebble
Gravel. Moreover, the curious narrow localization of this con-
stituent points to the existence of a river from the south;
wave-agitation on a shallow sea-bottom could hardly fail to
distribute the chert more widely.
A consideration of these facts has led the writer to view the
Pebble Gravel as a fluviatile accumulation, but little younger
than the Lenham Beds, and referable to the work of a river-system,
ancestral to that of the present Thames, which came into being
on the floor of the uplifted Lenham Channel. The fact that the
Pebble Gravel derives largely from the Lenham Beds in respect
of some of its mineral constituents is thus readily explained,
while the distribution of the pebble-constituents also receives a
consistent and feasible explanation. This view is supported
by the fact that remnants of a slightly older mass of shingle,
referable in all probability to the true Lenham Beds, are
preserved within the region occupied by the Pebble Gravel.
This older shingle occurs around Stanmore and Elstree and is
of simple constitution, consisting largely of flint-pebbles, with
small proportions of quartz and Lower Greensand Chert. It
is distinct both in content and situation from the mass of the
Pebble Gravel, and its claims to be regarded as of Lenhamian
age are very strong. If this be accepted the age of the Pebble
Gravel is fixed as post-Diestian.