The South Coast Region - Southern England - page 91

CHAPTER 5
The South Coast Region
GENERAL INTRODUCTION
A S DESCRIBED IN the previous chapter, the Southwest Region records the his-
tory of part of the Variscan mountain belt, resulting from movements before 300 million
years ago. In contrast, the South Coast Region records a younger history of episodes
largely involving downward movements and the accumulation of sediment in a South-
east England Basin.
In Areas 2 and 3 of the Southwest Region (Chapter 4), the New Red Sandstone
rests on and against a landscape that had been eroded in the earlier Devonian and Car-
boniferous bedrock. This contact is an unconformity , created when an area of hills
formed by erosion along the eastern fringe of the folded Variscan mountains was
covered by New Red Sandstone sediments.
The East Devon and West Dorset Area (Area 4) provides valuable information
about the movement between the subsiding Southeast England Basin and the uplifting
crust of the Southwest. Careful tracing of layers within the basin reveals another, young-
er unconformity, often called the Western Unconformity. This has resulted from the up-
ward movement of the New Red Sandstone, Jurassic and Early Cretaceous layers in the
west, followed by an episode of erosion before they were submerged again and covered
by the Upper Greensand deposits. The younger Western Unconformity therefore repres-
ents a time gap in the depositional record of the western part of the basin, equal to the
time represented by the Wealden and Lower Greensand deposits that are missing here
but visible further east (Fig. 94). Note how the Upper Greensand lies parallel to ( con-
formable with ) the top of the Lower Greensand and earlier layers in the east, but cross-
cuts (is unconformableon ) the tilted Jurassic and New Red Sandstone layers in the west.
Note also that the Jurassic, Wealden and Upper Greensand layers are thicker in the east
than in the west, providing further support for the idea that eastern areas were moving
downwards more than those in the west.
The oldest bedrock visible in this region occurs in the Brendon and Quantock Hills
of Somerset (northwest of Taunton), which are the eastward continuation of the Exmoor
hills of north Devon (Fig. 95). The bedrock here consists of Devonian sandstones and
mudstones, formed during an episode when sands, muds and thin limestones accumu-
lated in environments that varied from fresh water (rivers and alluvial flats) to brack-