28
NOTES ON DRIFT MAPS,
sides of that ancient town. Elsewhere in Essex they occupy but
little ground.
Here and there, on the tops of the hills of London Clay or Bagshot
Beds, are patches (coloured red on the Drift Maps) of what is styled
"Pebble-Gravel." This bed, though possibly of Pre-Glacial date, is
like the Glacial deposits ignored on the driftless maps, as newer in
age than the Bagshot series and unlike the Pliocene Crags. The
evidence as to its age is discussed by Mr. Whitaker in his "Guide to
the Geology of London." He remarks that it is certainly older than
the Boulder Clay, which has been seen to rest upon it. Whether
older than the underlying Glacial sand and gravel is a question yet
undetermined. It is, however, foreign to my present purpose to
discuss the evidence bearing upon its age, my desire being mainly
to notice it as one of the deposits seen on the drift maps and
ignored on the others. Though these patches of "Pebble-Gravel"
occupy but a very small part of the surface of Essex, they are
probably better known than some other beds occupying a larger
area. For they crown the summit at High Beach, Jack's Hill, and
Gaynes Park in the Epping Forest district ; at Langdon Hill north
of Tilbury, and at the villages of Hadleigh and Rayleigh. South
of the Thames the "Pebble-Gravel" crowns Shooter's Hill in Kent.
Thus, though of little economic importance, the lover of fine scenery
is sure to remember with pleasure a visit to a patch of Pebble-Gravel.
The views from Langdon Hill, especially, are, on the whole, probably
the finest in Essex, and deserve to be much better known than
they appear to be. And it is, perhaps, worth noting that the most
prominent height seen from Langdon Hill is the pebble-gravel-capped
Shooter's Hill, which commands the finest views obtainable in the
district of Kent in which it stands.
On the drift map of Essex are also to be seen other patches of
gravel of doubtful age (coloured orange). This gravel, like the
pebble-gravel, occupies but a few spots of no great size, but differs
from it, as regards age, in being either glacial or post-glacial, not
glacial or pre-glacial. Examples of it may be seen between Brent-
wood and Chelmsford, and a little eastward of Harlow and Saw-
bridgeworth.
Other patches of somewhat later date, being considered to be of
post-glacial age (coloured pinkish), occur at Buckhurst Hill, Wood-
ford, and elsewhere.
We now come to the post-glacial drifts of the river valleys, those