Geoscience Reference
In-Depth Information
GENERAL INTRODUCTION
M
OST OF THE BEDROCK
near the surface in the Southwest Region (Fig. 35) is
distinctly older than the near-surface bedrock in the rest of Southern England. It there-
fore provides us with information about earlier episodes, and this is all the more inter-
esting because these episodes involved movements of the crust that created a mountain
belt, the only one fully represented in the bedrock story of Southern England. Not only
does this add greatly to the interest of the Southwest, but it has resulted in the presence
of valuable minerals that have strongly influenced the human history in the Region.
Bedrock foundations and early history
Sedimentation and surface movement before the mountain building
The Southwest Region consists predominantly of bedrock formed between about 415
and 300 million years ago, during Devonian and Carboniferous times. This bedrock re-
cords an episode during which some areas of the Earth's crust rose while others sank,
as part of a general buckling of the crust that is the first indication of compression and
mountain building (Figs 36 and 37). As the rising areas became significantly elevated
they were eroded, shedding sediment into the neighbouring sinking areas that became
sedimentary basins. It is these basins that preserve most of the evidence of these events
(Fig. 38).
In material that has been further and later deformed, it is difficult to work out the
shape of the sinking areas, but many of them were probably elongated or trough-shaped,
with the troughs separated by rising ridges that ran roughly east-west, parallel to the
general trend of the later mountain belt. The troughs and ridges were caused in the early
stages of mountain building by the compression and buckling of the crust. The troughs
were generally flooded by the sea, or on the margins of relatively narrow seaways. Muds
were the commonest materials to accumulate, although sands were also in plentiful sup-
ply. Lime-rich sediments, sometimes with corals and other shelly marine animals, were
locally important. There were also periodic episodes of igneous activity that contributed
volcanic lavas to the sedimentary successions.
