40
THE ESSEX NATURALIST.
rock surface upon which this surface stratum rested conformably,
and which has not been exposed to eroding elements. The
Chalk provides the required example. From the contours of
the surface of the Chalk stratum and the evidence afforded by
existing streams, the Mid-Pliocene riverine scheme may be
reasonably conjectured.
A description of the surface of the Chalk pavement and
its relation to the existing watercourses is, therefore, necessary,
and will, perhaps, render the position a little clearer.
Figure 1 shows the upper surface of the Chalk, whether
Fig. i. Map Showing Upper Surface of the Chalk,
contoured in hundreds of feet above or below datum.
exposed or concealed, contoured in hundreds of feet, the signs
denoting height above or below O.D. The map is constructed
from the evidence of well, and other, borings which are sufficient
in number throughout the district to allow contour lines to be
drawn with some degree of confidence. The smoothly-flowing
lines lay no claim to detailed accuracy, but the main trend may
be regarded as correct. Five faults appear on the map, and,
with one exception, are not drawn from the evidence of adjacent
borings, but are inferred, with some degree of certainty, from
neighbouring groups of figures showing large discrepancies.