Movement of the Earth’s Surface from Within - Southern England - page 38

FIG 34. Example of a cross-section through the crust, showing how a divergent move-
ment pattern (A) may be modified by later convergent movements (B and C).
Vertical crustal movements resulting from loading or unloading
In addition to the direct raising or lowering of the surface by erosion or deposition,
there is a secondary effect due to the unloading or loading of the crust that may take
some thousands of years to produce significant effects. As mentioned above, we can
visualise the lithosphere as 'floating' on the asthenosphere like a boat floating in water.
Loading or unloading the surface of the Earth by deposition or erosion will therefore
lower or raise the scenery, just as a boat will sit lower or higher in the water depending
on its load.
An example of this is the lowering of the area around the Mississippi Delta,
loaded by sediment eroded from much of the area of the USA. The Delta region, in-
cluding New Orleans, is doomed to sink continually as the Mississippi river deposits
sediment around its mouth, increasing the crustal load there.
A second example of such loading is provided by the build-up of ice sheets during
the Ice Age. The weight of these build-ups depressed the Earth's surface in the areas
involved, and raised beaches in western Scotland provide evidence of the high local
sea-levels due partly to this lowering of the crustal surface.
Unloading of the Earth's surface will cause it to rise. Recent theoretical work on
the River Severn suggests that unloading of the crust by erosion may have played a role
in raising the Cotswold Hills to the east and an equivalent range of hills in the Welsh
Borders (see Chapter 6, Area 9). In western Scotland, as the ice has melted the Earth's
surface has been rising again.