Geoscience Reference
In-Depth Information
Figs. 3.34, 3.35 and 3.36 P680286, P680287 and P680288 (1907) Harmer's geological
excursion to Norfolk. An inferred view of cliffs in northeast Norfolk between Weybourne and
Happisburgh showing contorted glacial deposits. Folding takes place by the slumping of material
within and on the ice sheet during melting, and by the action of the ice pushing and contorting
sediment near its margin. (CP13/050 Reproduced by permission of the British Geological Survey
NERC. All rights reserved)
b
During the decline and melting of the North Sea ice sheet, valleys in central and
western Norfolk were deeply excavated by the action of torrential melt-water
streams issuing from the margin of the retreating ice. Harmer would have
recognised that the valley in the Ringstead Downs, south of Hunstanton Park, is
the result of a former drainage overflow channel that was cut by the melting ice
waters during the glacial period. The chalk which forms the bedrock of much of
Norfolk is usually covered with drift deposits but in this dry valley white track-
ways indicate that the chalk outcrops at the surface.
Later in the Pleistocene and apparently separated from the earlier glaciation by
a considerable interval of time, possibly representing one of the mild interglacials
proposed by Penck and Brückner, East Anglia was again invaded by ice, this time
by a large inland ice sheet from the northwest termed the Great Eastern Glacier by
Harmer. The ice of this glacier which originated in the uplands of northern Eng-
land, such as Teesdale, flowed down the Vale of York, and then divided into two
ice streams, one branch moving up the valley of the Trent and the other across
Lincolnshire and the Fenland, having been reinforced by lateral glaciers
descending from the Pennines together with ice which had flowed through the
Lincolnshire Wolds or the Humber gap from the North Sea (ice would have lain in
considerable thickness off the Lincolnshire coast during that period) (Fig.
3.37
).
This gave rise to a widespread deposit, the Chalky boulder-clay, forming the
moraine profonde of the second glaciation covering a large part of East Anglia as a
more or less continuous sheet. It contained a great variety of Jurassic and Creta-
ceous erratics, especially Kimmeridge Clay, Neocomian sandstones, hard Chalk,
and distinctive tabular grey flints from Lincolnshire. This glacier ploughed up and
incorporated in its own deposits much of the earlier North Sea Drift, so that the
westward extension of the latter is ill-defined.
As a result there are clearly two types of boulder-clay in East Anglia divided by
sands and gravels; the most notable feature of the upper one (comprising Chalky
boulder-clay with a matrix which varies in different districts according to the
various Jurassic rocks over which the ice had passed) contains Pennine and Oolitic
detritus and blocks of the characteristic tabular grey flint and hard chalk of the
Lincolnshire Wolds. The presence of large quantities of Kimmeridgian detritus
could only be explained by its having been transported by ice from the northwest
that is in a direction more or less at right angles to the flow of the earlier North Sea
Glacier.
A study of the relation of glacial drift in the valleys of the region showed that
there had been a great deal of denudation between the depositions of the two
boulder-clays. This was of great interest to Harmer in connection with the
