Retreat of the Late Pliocene and Lower Pleistocene Crag sea
primarily climatically controlled. Thus, the cold Pre-Pastonian and Beestonian stages showed lowered
sea-levels and the warmer Pastonian and Cromerian stages are associated with marine transgressions.
However, there were notable exceptions to this, with marine transgressions during the Pre-Pastonian
and Beestonian when cold stage lowered sea-levels would be expected. This may have been the
result of local tectonic or eustatic influences. Ice advances into the North Sea during the cold stages,
though not reaching Britain, may have slightly depressed the earth's crust. Another possibility is that
Scandinavian glaciers blocked the northern North Sea, creating an ice-dammed lake in the south.
Either could have caused raised sea-levels.
Indirect Evidence for the Early Thames
The earliest indications of the existence of the Thames and other major rivers from the Midlands,
the Bytham and Ancastcr Rivers (Fig. 3), can be inferred from palynomorphs and gravel lithologics.
Palynomorphs
Palynomorphs are microscopic organic bodies. They can be pollen, spores, acritarchs (microfossils
of unknown/uncertain origin, probably varied, many of algal affinity), chitinozoans (microfossils
within chitin-like walls, unknown biological affinity, now extinct) or dinoflagellates (unicellular
flagellated organisms, mostly marine).
Palynomorphs from the Chillesford Church (Crag) Member at Easton Ba vents, from the Chillesford
(Clay) Member at Easton Bavents, Covehithe and Sudbourne and from clay within the Westleton
Beds at Thorington show a range of sources from the Carboniferous, Jurassic, Cretaceous, Palaeogene
and the local Quaternary; in addition Silurian acritarchs were recovered from the Clay at Sudbourne
(Riding et al. 1997). The palynomorphs are argued to be derived directly from their source areas as
there are not the dilutions that would be expected if they had been resident in younger deposits
before being reworked into the Norwich Crag. From this it is argued that the palynomorphs must
have arrived by fluvial transport. Longshore drift is ruled out as it would not tap appropriate host
rocks and anyway would also abrade, weather, dilute and homogenise the microfossils. Possibly
there were two, or more, river catchments tapping the range of source rocks (Rose 2000; Rose et al.
2000). The Silurian acritarchs at Sudbourne suggest an early Thames, originating in the Welsh
borderlands, reached that area. However, Silurian acritarchs are lacking further north, in the clays at
Easton Bavents, Covehithe and Thorington, and Riding et al. imply from this a different fluvial
route, again supplying indirect evidence for an early Bytham River from the Midlands (Fig. 3a).
Sudbourne is south of the ridge dividing the coastal Crag basin. Thus the area to the south of the
ridge, the Ipswich Basin, may have been served by the Thames and the area to the north by the
Bytham River through the Stradbroke-Sudbury Basin.
Further palynomorphs collected from sands above the Chillesford Clay at Covehithe comprised
only Eocene dinoflagellate cysts, probably derived from the southern margin of the Crag basin. The
lack of Palaeozoic and Mesozoic forms suggests there was no longer a fluvial input. This may
indicate a major, but temporary, change in the palaeogeography, but it could be an artefact of the
change in the depositional environment.
Stone lithologies
Although the sediments were deposited in a marine or littoral environment, their stone contents
Essex Naturalist (New Series) 18 (2001)
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