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THE GEOLOGY AND SCENERY OF THE CLUB'S
VOYAGE FROM MALDON TO CHELMSFORD,
AUGUST 8th, 1891.
By T. V. HOLMES, F.G.S.
(Read August 8th, 1891.)
IT is now three years since our very pleasant voyage on the
Blackwater estuary, from Maldon to the sea off Mersea, took
place. On that occasion we sailed (or were becalmed) on a broad
sheet of water having low shores composed of London Clay, gravel
or alluvium. During our voyage to-day we are towed along a
narrow stream, the banks of which are bright with flowers, through a
rich valley, bordered by hills of considerable height. At Maldon we
leave behind us the broad tract in southern and south-eastern Essex
which is wholly, or almost entirely, free from Glacial Drift, and enter
the district in which that formation covers almost the whole of the
surface, except that occupied by the valleys of the various rivers and
streams. In these valleys the underlying beds appear, that which
everywhere exists beneath the Glacial Drift and the Valley Deposits
during our course to-day being the London Clay. Indeed, could we
prolong our voyage up the Chelmer as far as Dunmow, or ascend the
other streams, which, when united with the Chelmer, form the
Blackwater, as far as Braintree or Coggeshall, we should still find
London Clay in the sides of the river-valleys, and Glacial Drift
capping the plateaux between them. In this district the Glacial
Drift generally consists of gravel covered by Boulder Clay, as we
saw during our excursion to Rainsford's End and Writtle on the nth
of July. Sometimes, however, the gravel is absent, sometimes the
Boulder Clay; and more rarely, as in the new railway cutting at
Maldon, a little Boulder Clay may be seen underlying the gravel.
The full thickness of the London Clay in Essex is perhaps 450 feet,
but of course this is only attained where it is capped by the conform-
able Bagshot Beds. Where it is covered by the highly unconformable
Glacial Drift, as between Chelmsford and Maldon, or is exposed at
the surface, as between Brentwood and Rayleigh, its thickness is
much less. Thus, beneath Valley Gravel, near Maldon railway
station, were 130 feet of London Clay, there being 21 feet of
gravel. And at Maldon Waterworks the London Clay is said to