On the Sand-Pit at High Ongar, Essex. 81
1881 (Plate I.), compiled from the evidence of borings,
inclusive of that of the Asylum Committee, which he had
collected in his official duty as Geological Surveyor of that
part of Essex, shows that the flexure in which this ridge has
originated is precisely what by these dotted lines I had
hypothetically anticipated, though I of course could not tell
at what depth from the surface this flexure would bring up
the base of the London Clay.
Now, since the line of the Roding River from Woodford
to Ongar follows that of the arc of the Canterbury series, of
which the rolling ridge at Wickham Bishop is a part, and the
angles made by that river in its course are clue to the crossing
of the arcs of that series by those of the Isle of Wight and
North Sea centres in combination, as already mentioned, it
appears to me that if the sand of the High Ongar Pit be the
Bagshot, it must owe its low position to a similar flexure or
roll to that giving rise to the Wickham Bishop ridge, and
caused by the lateral displacement of the strata which resulted
from the radiating thrusts in which the arc-configuration
originated.
With this the high inclination exhibited by the stratifi-
cation of the High Ongar Sand corresponds; and this
inclination being from E.N.E, to W.S.W., it would show
that it is to the thrust from the northern centre that the
flexure or roll is due. Although the present case is one in
soft strata, yet the diagram given in the plate to the paper in
the 'Philosophical Magazine' to illustrate the removal of the
folds by denudation in hard strata best illustrates it; and to
afford the means of comparison I annex a line of section
(Fig. 2), drawn from the Lower Bagshot of Norton Heath,
through the High Ongar Pit, to the Roding, in which also
the position of the Glacial Beds occurring there is shown.
I do not know the precise elevation of the pit, but estimate
its bottom as upwards of 100 feet below the base of the
passage beds from the London Clay to the Bagshot Sand,6
6 In mapping geologically the Ordnance Sheet 1, I included these
passage beds in the Bagshot; but the gentlemen of the Geological
Survey prefer to include them with the London Clay, which causes a