308
THE ESSEX NATURALIST
ing plain descending from heights of 60 m (200 ft.) to 105 m (350
ft.) in the north, to near sea level in the south.
For geological reasons it is convenient to consider the region
as two physiographic regions, both being closely related. The
higher region to the north is characterised by a series of ridges
of which the Epping Forest Ridge, the Chigwell Row Ridge and
the Brentwood-Kelvedon Hatch Ridges are the more important.
To the north-east of the region, the southern portion of the Stock-
Billericay Ridge is included. The intervening rivers flow generally
south (the Rom and the Ingrebourne) while the Roding flows
south-west and the Wid to the north-east.
To the south the flat nature of the plain is more apparent
especially in the east where not obscured by housing. The region
comprises the terraces of the Thames, and the rivers mentioned
as flowing south pass across this area to reach the Thames. The
Ingrebourne and the Rom have their upper reaches in the ridges
but the Mar Dyke to the east draws its headwaters from the
south-eastern portion of the plain. There are several regions that
rise above the general level of the plain; at Westley Heights to
the south-west of Basildon spot heights reach 116 m (380 ft.) and
in the extreme south around South Ockendon and Stifford, the
land is marginally higher than the surrounding plain.
Geology.
The surface geology of the region consists of a variety of
sandy and pebbly deposits, the geologic succession being as
follows:
Period Deposit Characteristics
Post-Glacial Thames Terraces Gravels and Brickearths
Glacial Tills Boulder Clays
Pre-Glacial Pebble Gravel Pebbly gravels
Tertiary Bagshot Beds Sands
Claygate Beds Sandy Clays
London Clay Stiff Clays
Blackheath Beds Sands and gravels
The sands and gravels of the Blackheath Beds crop out in a
small area only, but the succeeding strata occupy a wide area.
The London Clay when fresh is steel blue in colour but in our
region it is always highly weathered and a chocolate brown
colour. Towards the top it becomes sandy and the transition
beds between the clay and the sands of the Bagshot Beds are
known as the Claygate Beds. The Bagshot Beds appear to be
entirely of fine or medium grade quartz sands.
The surface deposits and also the physiography of the area
were determined during the Pleistocene when the surface of the
land was carved by the melt waters of the approaching glaciers
and later modified by the ice itself. The melt waters brought
rounded pebbly deposits (the so called 'Pebble Gravel') while