Geoscience Reference
In-Depth Information
causing deep movements in the 'solid' Earth that produced regional-scale uplift (Fig.
178).
This remarkable effect has already been commented on in Chapter 3. It is now
realised that loading (for example, by the arrival of an ice sheet) or unloading (for ex-
ample, by erosion) of the Earth's surface can cause Earth movements that result in the
surface moving upwards or downwards, like the response of a wooden boat floating in
a bath. The 'floating' in this geological situation is of the upper, relatively light lay-
ers of the crust floating on deeper, denser material in the Earth. This sort of raising
or lowering of the Earth's crust can happen over timescales that mean northwest Eng-
land and western Scotland are still rising today, some 20,000 years after the thickest
ice sheets there started to melt.
Theoretical models suggest that the margins of the Severn-Avon valley (the Welsh
Borders to the northwest and the Cotswolds to the southeast) might be expected to have
risen approximately 50 m due to the erosional unloading of the Severn-Avon river sys-
tem. This raising of the Cotswolds could have caused the diversion of the upper former
Thames to become the Welsh headwaters of the Severn, leaving the middle section
of the former Thames to become the new headwaters, draining only the southeasterly
flanks of the Cotswolds.
FIG
178.
After valley erosion (1), the unloading of the crust resulted in deep movements
within the 'solid' Earth that generated a bulge of movement near the surface of the crust
(2). The two effects combined (3) produced scarp edges that may have contributed to
the height of the Cotswold and Welsh Border hills.
