26 NOTES ON DRIFT MAPS,
to which the Coralline and Red Crags belong. Of all these the
London Clay is the only formation occupying any considerable pro-
portion of the surface of the county, for the Chalk crops out only
between Purfleet and East Tilbury in the south, in the neighbourhood
of Saffron Walden in the north-west, and at Sudbury on the north.
Patches of Bagshot Beds exist here and there in the southern parts of
the county, as at Brentwood and Rayleigh ; and in the north-east of
Essex are some inconsiderable exposures of Red Crag. The London
Clay covers a good many square miles in the south and east of Essex,
but in the north and west is largely covered by glacial deposits, and
shows itself for the most part in the river valleys. It is known, however,
to exist beneath the drift as far north as a line drawn from Bishop
Stortford, through Thaxted to Sudbury, while east of Sudbury it may
be seen here and there both within and beyond the county boundary
to the sea coast. Sheet 126 of the horizontal sections of the
Geological Survey shows the geological structure of north-west Essex
and the comparative thicknesses of the various formations. The line
of this section is from the river Chelmer, four miles below Chelms-
ford, by Felsted, Thaxted, and Radwinter, to the county boundary
between Bartlow and Linton, and thence into Cambridgeshire. We
learn from it that glacial deposits, consisting either of sand and
gravel covered by boulder clay, or of sand and gravel or boulder clay
alone, cover almost all the surface between Chelmsford and Bartlow.
And a glance at Sheet 47, across which this section ranges, shows
that any other line of section, as, for example, one from Harlow to
Sudbury, would give much the same result, so far as the amount of
surface covered by glacial drift is concerned. It is true that
inspection of this section-sheet shows that the glacial drift,
and other superficial deposits are, on the whole, inconsider-
able in thickness when compared with the Chalk and Eocene
beds on which they rest. But as regards agricultural and resi-
dential purposes it will be obvious on examination that they are
thick enough to be of the utmost importance. If I turn to the
Geological Survey Memoir, explanatory of map Sheet 47, I find that
at a well at Thaxted, a mile west of the town, 28 ft. 6 in. of Boulder
Clay rested upon 30 ft. of gravel. At Halstead there were nearly
40 ft. of Glacial Drift above the London Clay. At Rickling Green,
between Bishop Stortford and Saffron Walden, there were 25 ft. of
Boulder Clay above 35 ft. of sand and gravel of Glacial age; at Saling
75 ft. of Glacial beds. And we learn (p. 61) that:—"From infor-