CHELMSFORD WATER SUPPLY. 83
water-tower we had a pleasant and extensive view, the uniformly
flattened contours of the ground, characteristic of the drift-covered
portion of Essex, forming the chief drawback to our enjoyment of the
scene. At and north of Chelmsford the highest ground consists of
a not very elevated plateau capped by Boulder Clay, the sand and
gravel of Glacial-Drift age, which underlies the Boulder Clay, appear-
ing on the slopes of the river valleys, while alluvial deposits of vari-
able breadth form level meadows close to the streams. The forma-
tion known as the London Clay underlies all the above strata, and
Geological Section from a spot S. of Writtle (S.W.) to Nabbots, N.E. of Springfield Hall.
Length of section 4 miles. (See Geol. Survey Map 1.N.E.)
a. Alluvium. a'. Older Alluvium.
b. Boulder Clay, c. Glacial Sand and Gravel.
d. London Clay.
may be seen here and there in the valleys, just above the alluvium, though
usually hidden by the stones from the Glacial gravel which have rolled
downhill. The Boulder Clay which forms the greater part of the surface
is impermeable. Many little streamlets, however, take their rise in
it, and unite to form others of greater size which flow through val-
leys, the sides of which are chiefly the sand and gravel of the Glacial
age. But the Boulder Clay may often be extremely thin, or wanting
entirely, in spots on the plateaux, too small to be mapped by any geo-
logical surveyor who is not prepared to devote a life-time to the
minute investigation of a dozen square miles. And as the Glacial
sand and gravel frequently forms a considerable breadth of ground
along the river valleys (and occasionally of the higher ground, as be-
tween Chelmsford and Maldon), a somewhat large quantity of rain
sinks through it, and is thrown out here and there as springs. These
springs occur at the base of the sand and gravel, and are the result
of the existence of the impermeable London Clay beneath, which
prevents the further percolation of the water downwards. At Rains-
ford End one of them has been utilized for the supply of Chelms-
ford. The district around being thinly populated, it is possible that
the water may remain sufficiently pure for drinking purposes for