Geoscience Reference
In-Depth Information
Landscape B: Jurassic hills and valleys
This Landscape encompasses the area of low hills immediately west of the Fens. Its
Jurassic bedrock is reviewed first.
The closing episode of Triassic times was marked by the flooding of this land-
scape by the sea and the deposition of the Rhaetic layer of distinctive limestones. This
flooding heralded the opening of the Jurassic and Cretaceous seaway (Fig. 227), which
lasted for more than 100 million years. These Rhaetic limestones have locally resisted
recent erosion to produce small escarpments in the scenery. The limestones were fol-
lowed by a general accumulation of Early Jurassic sediments, mostly the mudstones
and thin limestones collectively known as the Lias. These do not usually produce dis-
tinctive scenery, except in the case of the Marlstone Rock Bed. This remarkable layer -
never more than 10 m thick - consists of iron-rich limestones and sandstones that have
been widely quarried across the western parts of Area 15, often using a network of loc-
al railways. Its unusual materials, formed in a distinctive chemical episode during the
life of the Jurassic seaway, have made it resistant to recent landscape erosion, so that it
forms a clear escarpment in the western part of Area 15 (Fig. 268).
FIG
269.
Map of slopes in the Vale of Belvoir, sub-area I, showing localities mentioned in
the text (
b1, b2
etc.), Marlstone Rock Bed of the Early Jurassic (
mr
) and Middle Jurassic
bedrock (
mj
). Located on Figure 268.
The Marlstone Rock Bed (Fig. 269,
mr
) underlies the eastern edge of the Vale
of Belvoir, the largest and most distinctive scenic feature in the northwest of Area 15.
In contrast, the less well-defined southern and western edges of the Vale are carved in
Anglian ice-deposited material.
Above the Marlstone Rock Bed, the Jurassic succession includes more of the
Early Jurassic (Lias) mudstones, carved into hilly terrain. However, the next major
slopes in the scenery are the result of resistant bedrock materials, particularly lime-
stones, of the Middle Jurassic. Because of the generally uniform tilt of the bedrock
layers at less than 1 degree to the east-southeast, the slopes in the scenery due to the
Middle Jurassic units (Fig. 269,
mj
) generally lie to the east of those due to Late Tri-
