97
Gibbard et al. (1996) also propose that the lacustrine event was linked with the formation of the
modern English Channel. Prior to the Anglian, the Straits of Dover may not have existed, so
that when the Anglian ice blocked the North Sea, a large pro-glacial lake developed between the
ice-front and the higher land to the south (Gibbard, 1995b; Gibbard et al., 1996). It was into an
arm of this lake that the delta described above prograded. Lake water escaping through, and
deepening, a col near Dover, may have led to the initiation of the Strait.
Very different evidence for the influence of the Anglian ice on drainage is available in the form
of tunnel valleys, now buried. These are deep valleys formed or enlarged by meltwater flowing
beneath the ice. There are two major examples in Essex, the Cam-Stort (Woodland, 1970, Lake
and Wilson. 1990) and the Stour (Woodland. 1970) (Fig. 2). There are minor examples also,
e.g. at Sible Hedingham (Marks and Murray, 1981), east of Chelmsford and north of the Tiptree
Ridgc (Bristow, 1985). These valleys arc quite remarkable. They have irregular bottom profiles
cutting down to depths below the present ground surface of 104 m (340') at Newport Grammar
School and 126 in (414') at Cavendish. The subglacial waters that formed them were part of a
closed hydrological system, flowing under immense hydrostatic pressure, so they were able to
cut to great depths in places and then rise again to higher levels. These valleys quickly filled
with till, sands and gravels, and silts and clays as the ice decayed, so that today they do not show
their dramatic size and origin.
Elsewhere during the melt phase, small lakes formed in hollows in the valleys such as at
Danbury (Clarke, 1975) and Quendon (Lake and Wilson, 1990) and the decay of the permafrost
may have given rise to cambering and the development of gulls (Whiteman & Kemp. 1990)
Summary
In less than 100.000 years, the geography of Essex changed significantly. Prior to the Anglian,
tills was an area of low relief dominated by the gravel terraces of the west to east flowing
Thames in the central part of the county, to which tributaries flowed northwards. It is now
known that a prolonged period of arid periglacial conditions pre-dated the Anglian glacial
advance. During this period, arid tundra ground structures formed on the terraces. These are
rare in Britain, but they have been well displayed in Essex, particularly in the Chelmsford area.
The ice advanced over this flat landscape, laying down till of broadly uniform, 15-30 m,
thickness (Woodland, 1970). Because of this uniformity, the till surface reflects the nature of
the underlying terraces, giving the flat landscape typical of much of central and northern Essex,
as between Ongar and Dunmow. Further south, however, the ice was very disruptive and
brought about profound change. The original course of the Thames was blocked and diverted.
The earlier north-flowing tributaries were replaced by south-flowing rivers, many occupying
freshly filled deep valleys, which fed into the first course of the modern Thames in the south of the
county.
The contribution of evidence from Essex has been crucial in elucidating this very important
period. Some of the evidence is unique to the county. This is reflected by the fact that five Sites
of Special Scientific Interest occur within the county covering the Anglian geological history.
Some points have a poor record of investigation and need further work, such as glaciotectonics.
and others are in contention and need to be the focus of further evaluation, such the Asheldham
Gravels of the Tillingham-Bradwcll area. The stratigraphies of Bridgland (1988, 1994, 1995)
and Gibbard (1994, 1995a, 1999) are difficult to reconcile. The nature of this problem was
outlined by Allen (1998). This is a particular area needing resolution.
Essex Naturalist (New Series) 16 (1999)