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railway cutting at Emerson Park (Holmes, 1892a, b. c. 1893, 1894; Whiteman, 1987; Bridgland,
1994; Whiteman and Bridgland, 1995). In each of these valleys, the till can be traced, albeit
sporadically, to the margin of the Orsett Heath Gravel. Possibly it was this early course of the
modern Thames in Essex that halted these extensions from the Anglian ice front. There is an
obvious irony in that the glacier that caused the diversion of the Thames was later to be limited
by that diversion.
There is an assumption that the rivers of south Essex initially drained northwards to the early
course of the Thames across mid-Essex. There is indirect evidence for this in that the presence
of Greensand chert from the Weald in the early gravels increases at certain points, such as Ongar
and Chelmsford (Green et al, 1982). The diversion of the Thames would have caused reversals
of drainage in these valleys.
In the eastern part of the county, there is further evidence for the diversion (Bridgland, 1994,
1995a) (Fig. 2). In quarries at St Osyth and the cliffs at Holland-on-Sea. stone counts from
lower gravels respectively indicate that the early Thames (Lower St Osyth Gravel) was still
crossing central Essex and that the Medway crossed eastern Essex and that the two were
confluent in the area (Lower Holland Gravel). However, the Upper St Osyth Gravels have a
greater proportion of sand and smaller stones, indicating a significant reduction in the power of
the river. More importantly, the stone counts change. There are fewer of the Thames lithologies
and new lithologies related to the Anglian till, such as Rhaxella chert, occur. This indicates that
the Thames route was no longer bringing in the same range of lithologies as before, because it
was blocked, and the newer lithologies suggest an association with the Anglian ice. the cause of
the blockage. The Upper Holland Gravel is broadly similar to the Upper St Osyth Gravel,
thereby telling the same story, but at some localities the proportion of southern lithologics, such
as Greensand chert from the Weald, increases, reflecting the greater importance of the Medway
tributary once the Thames had been cut off. The Lower and Upper St Osyth Gravels can be seen
also at Fingringhoe.
The nature of subsequent developments is in contention. Gibbard et al. (1995, 1996) describe a
sequence in the Asheldham Member seen in several quarries between Tillingham and Bradwell.
The sequence comprises horizontally bedded gravels, followed by fine grained sands, silts and
clays, in turn overlain by large-scale sandy cross-sets and then horizontally bedded gravels.
Gibbard et al. and Gibbard and Boreham (1995) propose that the basal gravel belongs to an
early Thames-Medway crossing eastern Essex shortly after the initial diversion of the Thames
from its mid-Essex route. The succeeding fine deposits indicate quiet water conditions, relating
to a large lake caused by the Anglian ice advancing down the North Sea basin and blocking the
exit of the confluent Medway-Thames (Fig. 5a). The overlying cross-sets represent a sand delta
prograding northwards into the ice-dammed lake. The succeeding horizontal gravels suggest
that normal fluvial flow returned with the melting of the ice sheet.
Bridgland (1995b) disagrees with this interpretation and presents a different sequence of events
(Fig. 5b). He proposes that at the end of the Anglian cold stage, possibly associated with
deglaciation, fluvial downcutting would have occurred, creating a deep channel from Southend,
through Bradwell, to Clacton. Later in the cold stage gravels would have started to infill the
channel, the Southend/Alsheldham/Clacton Channel Gravel. These gravels arc considered to be
the equivalent of the Orsett Heath Member (Lower Gravel). During subsequent warm period
(OIS 11) interglacial deposits were laid down along the channel, those at Clacton being
particularly famous (Soufhend/Asheldham interglacial deposits; Clacton Channel Deposits).
Sea-level would have been higher and possibly the cross-sets should be associated with this.
Bridgland argues that the cross-beds, being sandy, arc unlikely to be delta foresets deposited
Essex Naturalist (New Series) 16 (1999)