THE 2001 AGM ADDRESS
The retreat of the Late Pliocene and Lower Pleistocene Crag sea and the
development of the Early Pleistocene Thames - a review
PETER ALLEN
13, Churchgate, Cheshunt, Hertfordshire EN8 9NB
Abstract
In this review, the history of the retreat of the Red and Norwich Crag sea from the London Basin is
outlined and the role of tectonic activity in this history is explored. Possible subdivisions of the Crag
basin in northern East Anglia are proposed and their possible significance is examined. As the sea
retreated the Thames was established in the London Basin. The earliest presence of the Thames is
inferred from palynomorphs and stone counts from the Crags. Its pre-Anglian course, known from
outcrop, quarry and borehole evidence, was across the northern limb of the London Syncline. Changes
in the gravel composition its deposits indicate important changes in its catchment area.
Introduction
During the Late Pliocene and Early Pleistocene, southern Britain was subject to tectonic instability
which eventually led to the withdrawal of the Crag seas and the establishment of the Thames drainage
system. The history of this period is one with many unresolved problems. One is that our understanding
of these tectonic movements is limited, indeed we are only just coming to terms with the concept.
Another problem is that we are dealing with both marine and terrestrial environments and the latter
has a poor record in the geological archive. Erosion has removed much of the evidence. Burial by
later deposits makes other parts of it difficult to elucidate. However, the main points of the geological
history are known. A provisional stratigraphic framework is provided in Table 1.
Tectonic History of the Crag Seas
The Tertiary history of southern England records a changing palaeogeography, with the North Sea
Basin at times being connected to the Western Approaches and the Atlantic (Curry 1992). The
connection was severed at least twice. This would have been partly, even mainly, due to the infilling
of the local seas; the deeper sea sediments such as the Jurassic limestones and the Chalk were
replaced by shallow water and delta sands of the Tertiary deposits. However, this was also a period
in which tectonic activity was building up as Africa rotated to complete its collision with southern
Europe, culminating in the formation of the Alps. These pressures created the Wealden Anticline
and uplifted northwestern Britain, but left the London area as a syncline, through which the connection
between the North Sea and the Western Approaches was maintained for a time.
In the Red and Norwich Crags, a Lusitanian (southern) element in the fauna (e.g. the Foraminfer,
Pararotalia serrata) suggests that the link with the Western Approaches was open until after the
Norwich Crag was deposited (Meijer & Preece 1996). Although the marine faunal diversity had
fallen from 160 species in the Red Crag to 140 in the Norwich Crag, probably associated with the
shallowing of sea, that number was still high and far from indicative of a restricted environment such
as the modern North Sea.
In East Anglia, the Red and Norwich Crag Formations were deposited in a funnel-shaped basin,
Essex Naturalist (New Series) 18 (2001)
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