114 THE UNDULATIONS OF THE CHALK IN ESSEX.
of the Chalk occurs, one, two, three, or more, hundred feet above or
below the sea-level, the ciphers being omitted for the sake of distinct-
ness, and the plus and minus signs respectively indicating height
above and depth below the Ordnance Datum. The zero, of course,
implies that the Chalk at or near that line is just at sea-level. The
straighter lines are faults whose existence is imperceptible on the
surface of homogeneous clay, even where not concealed by drift, but
which are sufficiently established by their effect on the Chalk contour-
lines.
The Chalk outcrops from beneath the Tertiary sands along lines
running from Sudbury to Bishop's Stortford, and from East Tilbury to
near Wennington. To the north and south respectively of these
lines of outcrop occur isolated patches of the Tertiary beds, only the
more important of which can be shown on so small a map.
The Chalk is not everywhere at the surface in the spaces de-
nuded of their original Tertiary covering, for Glacial and Post-
Glacial drifts mask both Tertiary and Cretaceous areas, and much
of the ground that is shown as Chalk in the map consists of these
gravels and clays, extending to depths of sometimes more than 100
feet.
I have considered it impracticable to attempt to make out any
undulations in the Chalk beyond the Tertiary boundary, for the
simple reason that where the eroding forces of the Glacial sea laid
bare the Chalk, they dealt with it as erratically as with the Tertiary
beds, cutting it into deep and shallow, at the will of the changing
currents. Instead, therefore, of an undulating plane whose position
may be calculated with a fair approximation to accuracy, as is the
case under the Tertiary area, we cannot safely pronounce on the
position of the Chalk under the Drift fifty yards beyond where it is
seen, or proved by boring. Mr. Whitaker has shown us2 how in the
Cam valley, between flanks of Chalk, the alluvium, barely 300
yards wide, conceals a drift-filled fissure of great depth, showing that
calculations from exposures in such an area are liable to be
completely erroneous. When East Anglia becomes the scene of
numerous collieries, perhaps we shall learn from the undulations
of the under-surface of the Chalk that which we cannot gather from
the open surface, which was lost in the Glacial period. Accordingly
I have left that region untouched, and dealt only with the area where
the Glacial erosion has not succeeded in reaching the Chalk.
2 Essex Nat. vol. iii pp. 140-142, 1889 ; Quart. Journ. Geol. Soc. vol. xliv. pp. 333-340 [1890],