93
limit of the till in the Chelmsford area coincides with the southern limit of the sands and gravels
of the Kesgrave Formation, south of which London Clay dominates. Locally the southern limit
of the till may be determined by the change of geology but elsewhere, as outlined below, other
factors were responsible.
Deformation of the substrate by ice has not been studied systematically in the county. As the
glacier entered the county, it had to surmount low chalk hills. Shearing and transport of rafts of
chalk arc described in adjacent parts of Hertfordshire (Hopson, et al., 1996), but such
descriptions are lacking in Essex. It is highly likely that similar structures are present in the
county. At its southern limit, the ice abutted against the Danbury-Tiptree Ridge. The Ridge has
a core of London Clay. Large quantities of outwash gravel from the glaciers arc present
indicating that water was plentiful, which would have greatly weakened the Clay and made it
particularly susceptible to deformation by the glacier. Passing reference is made to ice-push
structures by Clayton (1957a, b) in the Danbury area. At St Cleres Hall pit, at the western
extremity of Danbury Hill, the London Clay forms a series of north-south trending ridges with
infills of glacial gravel in the intervening troughs, but these are due to loading and sinking of the
gravel on the weakened, saturated London Clay. Had they been of glaciotectonic origin, they
would have trended east-west. Glaciotectonically deformed till is present currently at Roxwell,
but no descriptions or interpretations are available. On a much smaller scale, at Newney Green,
Great Waltham and Broomfield, the periglacial structures of the early Anglian, described above,
can be seen to be deflected by ice drag. This is an area of Essex geology, which warrants further
attention.
The Anglian ice advance is particularly associated with the diversion of the Thames (Fig. 2).
Whilst the key elements of the story are known (Bridgland. 1994; Gibbard, 1994), much of the
detail of the diversion cannot be ascertained. The most obvious evidence for the diversion lies
in Hertfordshire, where gravels of the early Thames (Kesgrave Formation) can be traced flowing
northeastwards through the Vale of St Albans to Ware. Above these is a till found between
Ware and Watford (Ware Member/Eastend Green Member) which passes laterally into lake
deposits near London Colney and at Moor Mill near St Albans (Moor Mill Member). These are
overlain by glacial gravels (Smug Oak Member), indicating flow in the opposite direction that
merge with the Black Park Member, the first course of the modern Thames. The till indicates
that ice penetrated the Thames valley of the time, blocked it and ponded the Thames (lake
Deposits). Thereafter flow was reversed (Smog Oak Gravel) down to the modern Thames.
There is no evidence for a pre-existing valley through London, so the contention is that the lakes
became deeper with time and eventually overspilled from tributary to tributary until an
integrated valley system was developed from which the modern Thames evolved. There is some
evidence to support this in Essex. Gravels capping the Langdon Hills (Wooldridge and
Berdinner, 1922; Wooldridge, 1927) are rich in Tertiary flint (83%, 11.2-16 mm) and. more
importantly, Greensand chert (10%) from the Weald (Bridgland, 1999). This suggests that an
early Pleistocene extension of the Darent crossed southern Essex as a west bank tributary of the
extended Medway (Fig. 2) (Bridgland, 1999). It is likely that Illis was a precursor of the valley
that the diverted Thames was to occupy.
The till sheet extends further south, to Chigwell, Havering and Brentwood, Danbury and
Tiptree. The lulls at these points would have blocked further advance. Beyond this, small
outcrops of till are found in several Essex valleys, in the Lea Valley as far south as Chingford, in
the Roding to Loughton, in the Ingrebourne to Hornchurch, suggesting that fingers of ice
extended forward from the main limit down these valleys (Fig. 2). The last example is
particularly important as the till is directly overlain by Orsett Heath Gravels of the modern
Thames (marked as Black Park Terrace Deposits on 1:50.000 Geological Sheet 257) in the
Essex Naturalist (New Series) 16 (1999)