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formation, the London Clay, although to a great extent covered
by Drift deposits, forms by far the larger portion of the beds on
which our district stands. The hills in many parts of Essex—
such for instance as the ridge extending from Chingford to
Waltham Abbey and the range about Havering-atte-Bower
and Brentwood—are entirely of London Clay, in many cases
capped with outliers of Bagshot sand and Drift formations.
We have but to call to mind High Beech in our own neighbour-
hood to see that the most picturesque features of the county
are due to this formation. Sections of the London Clay,
which is about 420 feet thick in the neighbourhood of the
metropolis, have been exposed in Essex at Buckhurst Hill,
Theydon, Brentwood, Stifford, Upminster, Warley, Ac, and
sections of this formation showing its junction with the
Woolwich and Reading beds have been exposed at Bishop
Stortford and Roydon. The Middle Eocene period is repre-
sented in our county by the Bagshot Sands already alluded
to, which form outliers capping many of our hills, such
as at High Beech, Crabtree Hill near Lambourn, South
Weald, Havering, Blackmore, and large patches stretching
northwards from Warley Common to beyond Brentwood
and again about Kelvedon Hatch. One other very interest-
ing formation of Pliocene date—the Crag—just commences
to appear in the north-eastern corner of the county at Man-
ningtree, south of the Stour valley, and Walton-on-the-Naze
has furnished Red Crag fossils of special geological interest.
At about that period of the earth's history when the Crag
formations were deposited, our globe, owing to a certain com-
bination of astronomical events, began to experience those
great climatic changes which resulted in the Glacial Period,
during which the whole of the northern portions of Europe
and America were laid under an icy covering of great glaciers
which flowed down from all the mountain slopes and high
lands, levelling up the valleys, and becoming confluent, formed
a gigantic ice-sheet, which extended southwards into regions
which now enjoy a temperate climate, whilst floating icebergs
and rafts drifted even into tropical seas, and there thawing,
scattered their accumulated burdens of rock fragments and
miscellaneous debris over the sea bottom. The Glacial Epoch,