The Severn Valley Region - Southern England - page 177

(1) The Rhaetic (labelled R on Figs 151 and 168)
Although most of the Early Jurassic rocks (historically known as the Lias) consist of
soft mudstones that have not generally produced features in the scenery, they are un-
derlain by a group of distinctive beds that, together, have produced steep slopes where
the landscape has been carved out by erosion. We use the label Rhaetic , borrowed from
central European geological usage, to refer to this layer (also called the Penarth Group).
This group of beds is now regarded as partly latest Triassic and partly earliest Jurassic
in age. It contains at least one surface representing a relatively short time gap in the
deposition and includes mudstones, limestones and fossil beds, up to and including ma-
terial called the Blue Lias. These beds represent the arrival of coastal and lagoonal con-
ditions after a long period of New Red Sandstone river and lake sedimentation. There
are numerous small scarps, particularly in the lowlands of the Avon valley around Eve-
sham and Leamington Spa, which mark the resistance to erosion of this Rhaetic mater-
ial.
(2) The Marlstone Rock Bed (labelled M on Figs 151 and 168)
In the middle part of the Early Jurassic succession, the Marlstone Rock Bed tends to
form distinct slopes because it contains limestones and iron-rich materials, making it
a valuable resource that has been extensively quarried. At Edge Hill ( c3 ; see also Fig.
173) the Rock Bed caps a 100 m escarpment, straddling the A422 between Stratford-
upon-Avon and Banbury, some 5 km southwest of the M40 motorway. This escarpment
results from the resistance to local river erosion of the Marlstone Rock Bed, but is more
famous as the setting for the Battle of Edge Hill (Fig. 171). The area below the scarp
of Edge Hill is still often called the Vale of the Red Horse, although the carvings that
once gave it this name are no longer visible. The Vale gained fame because the colour
of its rocks was so spectacular, due to the red iron oxide of the Marlstone Rock Bed.