THE PLIOCENE PERIOD IN WESTERN ESSEX. 249
impossible; there can be no doubt that the gradient diminished
northward, and it is clear that the surface so projected must
have passed but little above, if it did not actually coincide
with, the summit-plane of the hills in the centre of the Basin.
Turning our attention first to the western environs of the
County, we note that the hills that stretch along the Hertford-
shire—Middlesex border have an exceptionally even summit-
plane, a fact readily appreciated when looking westwards from
High Beach. The region is indeed a dissected plateau, whose
general surface is about 400 ft. above O.D., and it is covered
by a thin accumulation of shingle appearing in the maps of the
Geological Survey as "Pebble Gravel." For so relatively in-
significant a formation, the Pebble Gravel has a most extensive
literature, both its age and its origin having been matters of
question. In the light of our present knowledge the question
of age may be regarded as settled, for the deposit cannot be other
than early Pliocene in age, separated at most by a short interval
of time from the Lenham Beds. At first sight it seems natural
to regard it as equivalent to the Lenham Beds and therefore
marine, but closer inspection shows the idea to be untenable
for the greater part of the Pebble Gravel at least. The chief
facts leading to this conclusion must be noted here, because
of their bearing on corresponding problems in Essex. They are
as follow:—
(a.) The surface on which the Pebble Gravel rests is in-
dependent of longitudinal warping (i.e., of folding along the
main axis of the London Basin), maintaining its elevation un-
changed from Hampstead northwards to Ayot, near the foot
of the Chiltern dip-slope. The Lenham Beds show a definite
longitudinal warping, though it is small in amount.
(b.) On the Chiltern dip-slope the relations of the Diestian
platform to the lower 400 ft. platform are clearly shown and
make it evident that the latter is a distinct and later feature.
For instance, between the Misbourne and the Wye Valleys,
Diestian4 shingle occurs at 560 ft. at Coleshill, while the Pebble
Gravel at its normal elevation occurs close by on the south.
(c.) Neither the lithology nor the petrography of the greater
part of the Pebble Gravel are comparable with those of the
4 The term "Diestian" is used as synonymous with "Lenhamian," in the present com-
munication.