Geoscience Reference
In-Depth Information
The easternmost (Warwickshire) part of this Landscape (
A2
) has not created dis-
tinctive present-day scenery, probably because rather weak Carboniferous mudstones
dominate the near-surface materials. Unlike Landscape
A1
, this area corresponds to a
gentle downfold in the Carboniferous layers, although its subsequent movement his-
tory has tended to raise the central area of Carboniferous bedrock while lowering its
margins, giving something of an upfolded appearance in Figure 170. This movement
pattern has included the formation of a southeastern boundary where the earlier bed-
rock has been eroded and then covered by gently tilted Triassic and Jurassic bedrock.
In contrast, the western boundary consists of the steep Warwick Fault (shown in Fig.
170).
Landscape B: New Red Sandstone bedrock
The two areas of Landscape
A
described above are surrounded by areas of New Red
Sandstone, which constitute Landscape
B
. It is convenient to divide this Landscape
geographically into
B1
, the Worcester Basin;
B2
, the Knowle Basin, and
B3
, the Avon
Valley Basin.
The New Red Sandstone provides the red (iron oxide) pigment that colours many
of the soils and cliffs of the West Midlands. Detailed mapping shows that it consists of
a lower (earlier) division called the Sherwood Sandstone Group, made largely of sand-
stones but also containing coarser-grained conglomerates and breccias (with rounded
and angular pebbles, respectively). This is overlain by an upper (later) division called
the Mercia Mudstone Group, made up largely of mudstones. In older maps and reports
these two divisions are often labelled as
Bunter
and
Keuper
, using the terminology first
invented for the excellent outcrops of the New Red Sandstone in Germany.
The red (oxidised) colour of the New Red Sandstone, together with the absence
of fossils that are typical of seawater deposits, suggests that it was originally deposited
from fresh water, particularly in rivers on land. The lower Sandstone Group appears
to be largely Triassic in age but may contain some Permian material. The coarse grain
size of these deposits suggests they were deposited by relatively fast-flowing, vigor-
ous rivers. The upper Mudstone Group is entirely Triassic in age, and its relatively fine
grain size suggests that it formed on the floodplains of more gentle rivers, although
distinct sandstone horizons (often referred to as Arden Sandstone) may mark episodes
of more vigorous flow. The minerals gypsum and rock salt occur in distinct layers in
the Mudstone Group and have been extracted commercially in some localities. These
sulphate and chloride minerals formed by precipitation from the waters of saline lakes
or coastal lagoons and reflect arid climatic conditions, rather like those of the present-
day Dead Sea.
