Geoscience Reference
In-Depth Information
Fig. 3.12 P680273 (1907) Pit of Waltonian Crag at Little Oakley, northeast Essex from which
Harmer obtained over 600 species of mollusca within an area of 20 yards square. Harmer's car is
visible on the left middle ground and his chauffeur appears to be digging out the material for sorting.
(CP13/050 Reproduced by permission of the British Geological Survey NERC. All rights reserved)
3.3.3 The Norwich Crag
Further north (following the Red Crag in the geological succession) the Norwich
Crag beds, which lie unconformably on the Upper Cretaceous Chalk, were
believed to be part of the estuarine deposits of a proto-Rhine, a large northward-
flowing river that entered the Crag Sea in the region of Cromer today (Harmer
1902
). In many respects these newer deposits were found to differ considerably
from those of the Red Crag. Whereas the latter seldom attain more than about 6 m
(20 ft.) in thickness, the Norwich Crag deposits increase rapidly in depth north-
ward being found 45 m (147 ft.) thick, for example, in a boring at Southwold only
twelve miles from Aldeburgh. As all the shells discovered at this site were of the
shallow-water type, it may be inferred that subsidence of this area had occurred
contemporaneously with the deposition of the sediment. Although fewer in number
than those of the Red Crag, the character of the Norwich Crag molluscan fauna
was found to be generally more or less uniform throughout the whole area covered
by the deposits from Aldeburgh to the Bure valley. However a significant regional
difference was the presence of the arctic shell, Astarte borealis (one of the latest
arrivals in the Crag Sea) in northern parts of the area.
The sandy material comprising the Norwich Crag appeared to have originated
mainly from a southern source as it contained many pebbles of white quartz, a
characteristic feature of Dutch gravels, together with mica, possibly derived from
the Devonian schists of the Ardennes. The proto-Rhine with its affluents seems to
have been an important factor in the later Pliocene history of East Anglia. This
