The Southwest Region - Southern England - page 42

FIG 35. The Southwest Region, showing Areas 1 to 3.
During these episodes of Devonian and Carboniferous basin and ridge activity,
the Southwest Region was just one small part of a larger belt of similar activity that
extended to the west into Ireland and Canada. Canada was then very much closer, be-
cause the Atlantic Ocean is a younger feature that only started to grow (at this latitude)
about 100 million years ago, as divergence and spreading occurred along the mid-At-
lantic plate boundary. To the south and east, the same belt of activity continued across
northern France and into Germany. This broadly east-west trending belt later became
the Variscan mountain belt.
Crustal convergence that created the mountain belt
The subsidence and sedimentation were sometimes interrupted by, and generally fol-
lowed by, episodes of convergence of the Earth's crust. This was caused by compres-
sion or squeezing, broadly in a north-south direction, so that areas of bedrock were
folded and pushed closer together, making the east-west trending Variscan belt nar-
rower, as if between the jaws of a vice. The map (Fig. 39) and cross-section (Fig. 40)
show how the folds and faults of the region vary locally in their pattern, but can be ex-
plained generally by convergent movements in this north-south direction. These con-
tinued over at least 100 million years, and occurred along thousands of kilometres of
the belt. Mountain-building events such as this occur when tectonic plates collide (as
described in Chapter 3) and always have a profound effect upon the scenery in the vi-
cinity of the collision. The Variscan mountain belt is just one of the great mountain-
building episodes that have occurred periodically, throughout the Earth's history.