Geoscience Reference
In-Depth Information
FIG
231.
Hillshade map for Area 13, showing bedrock, Landscapes
A
to
E
and localities
(
a1, b1
etc.) mentioned in the text. For more detailed maps of sub-areas I, II and III, see
Figures 237, 240 and 241.
The Early Jurassic mudstones (the Lias, Fig. 225) contain a thin occurrence of the
Marlstone Rock Bed - although, compared to Area 9 further west, this bed has little
importance in Area 13, either topographically or economically. The Early Jurassic is
overlain by Middle Jurassic sandstones, mudstones and limestones belonging to the In-
ferior and Great Oolite Series. These Jurassic strata slope very gradually downwards
to the southeast, at much less than 1 degree overall, although there are local variations
in tilt and small faults, often due to the settling of the bedrock as the edges of valleys
have been eroded.
The present scenery in the northwest of Area 13 consists of valleys eroded into
these Triassic and Jurassic materials, with Middle Jurassic sandstones and limestones
capping hills between valleys that are floored by earlier mudstones. On top of many
of the hills are sheets of Anglian glacial deposits, often with river sands and gravels
that may have been closely linked to ice sheets. Much of the 100 m relief of the val-
ley topography has been created since the glaciation by erosion into and through this
surface blanket. A plateau of this material forms the Yardley-Whittlewood Ridge (
a1
)
between the Nene and Great Ouse valleys (Landscape
B
). An important conclusion is
