The lithology and stratigraphy of the Anglian deposits of the Lea basin
A revised lithostratigraphy and palaeoenvironmental
history of Anglian sediments in Hertfordshire and west
Essex is based on field observations and laboratory
analysis of samples from 79 sites within the Lea basin and
adjacent areas. Analyses included particle size, acid
solubility and small clast lithology of tills, pebble
counts of gravels, small clast lithology of sands, till
macrofabric and palaeocurrent determinations of sands and
gravels. Multivariate analyses of the petrographic
properties showed that the till samples could be divided
into four main groups. Similarity networks, augmented by
field relations, identified four till lithostratigraphic
units, which merge north-eastwards.
Four gravel lithostratigraphic units were also
identified from multivariate analysis of petrographic data.
The oldest unit, the Westmill Lower Gravel Member, was
deposited by the proto-Thames, and can be followed from the
middle Thames region into the mid-Essex depression.
Eastward flow was disrupted by the advance from the
north-east of ice depositing the first (Ware) till.
This caused the formation of ice marginal lakes. A
temporary southward spillway was initiated in the
lower Lea valley from the Watton Road Laminated Silt
lake, but permanent diversion of the Thames was effected
by a spillway from the Moor Mill Laminated Clay lake,
dammed by Ware Till ice near its southwesterly maximum in
the Vale of St. Albans.
Three readvances followed, depositing lodgement tills
named the Stortford, Ugley and Westmill Till Members. Of
these, the Stortford Till is the most extensive, reaching
the southern margin of glaciation at Finchley and
Hornchurch. Gravels deposited during or between these
three advances are the Smug Oak Gravel in the Colne Basin,
and the Westmill Upper Gravel in the Lea basin. The
latter is divided into the chalk-poor Hoddesdon Gravel Bed
and the chalky Ugley Gravel Bed.