Geoscience Reference
In-Depth Information
FIG
192.
Looking northwards across the City of Oxford, with the Thames (or Isis) flowing
from left to right across the foreground. Beyond that is the ancient centre of the city, with
Christ Church Great Quad acting as a landmark, with grass in the quad and Tom Tower
on its left side. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
The Vale of Pewsey (
b7
) separates the Marlborough and Lambourn Chalk Downs
from the Chalk of Salisbury Plain to the south. The Vale is characterised by lush orch-
ards and meadows and gently undulating lowlands, in contrast to the Chalk scarps and
uplands to north and south. The Vale is aligned roughly parallel to a broad upfold or an-
ticline running from Trowbridge (
b6
) eastwards, formed during the mid Tertiary. Fol-
lowing this crustal movement, the Chalk at the crest of the anticline has been eroded
away, exposing the Early Cretaceous Upper Greensand. Subsequent erosion has fo-
cused on the relatively softer Greensand, carving a broad, gentle valley with fertile,
sandy soils.
.Landscape C: The Downs of Late Cretaceous Chalk
The Marlborough and Lambourn Downs, stretching across north Wiltshire and
Berkshire, are a region of high, rolling hills underlain by Late Cretaceous Chalk. The
Chalk dips gently to the south and southeast and has a dramatic northern edge over-
looking the Vale of White Horse. In most cases along this northern edge the upper
Chalk appears to have provided more resistance to erosion than the lower Chalk mater-
ials, so that it forms the lip of the steep slopes. However, particularly in the Marlbor-
ough Downs and just south of Harwell (
b5
), hill features of intermediate height have
also been eroded into the lower Chalk levels.
