Geoscience Reference
In-Depth Information
Welland and Nene) have eroded widened valleys and may be largely responsible for
the removal of the cover of Anglian ice-laid material.
Local slopes must have been modified and moved by slumping and collapse under
Ice Age conditions of frequent freezing and thawing. Some of the flatter surfaces in
this Landscape may have formed during cold, but not ice-covered, periods of the Ice
Age via the removal of sediment by meltwater streams.
On the edge of the Fens south of Peterborough, the slope bounding the Fens may
have been formed, at least partly, by coastal erosion during high-stands of the sea, per-
haps during the Ipswichian interglacial just over 100,000 years ago. Another sugges-
tion that has been made is that a Fenland lake existed here when the Devensian ice was
melting, and that shoreline erosion at the margins of this lake may have been respons-
ible for the Fen-edge slope line.
FIG
272.
Map of slopes in the Oakham area, sub-area II. Located on Figure 268.
Recent examination of the shapes of some of the depressions in the Jurassic up-
land bedrock surface, to the west and south of this Fen edge, has suggested that the
depressions may have formed as thaw lakes that grew on the surface when the ground
was frozen (Fig. 273). These features, generally 1 km or so in diameter, have been re-
cognised as the probable products of
thermokarst
processes (because of the key roles
of heat and ice-melting - see Chapter 2), and they provide a valuable insight into the
mechanisms that have led to the lowering of these clay flat-lands.
Landscape C: The Fens
The Fenland Landscape is defined by the presence of a remarkably flat and extensive
surface blanket of silts and peats that date from the last few thousand years (Fig. 274).
This surface blanket was deposited during the Flandrian rise of sea level following the
worldwide melting of ice that marked the end of the last cold phase of the Ice Age.
Below this cover of surface blanket is Late Jurassic bedrock (Fig. 268). The Oxford
Clay and the Kimmeridge Clay are the best-known layers in the Late Jurassic succes-
sion, which consists primarily of mudstone. The relative ease with which this mud-
