Geoscience Reference
In-Depth Information
The northeastern edge of the Hampshire Downs is an undulating plateau covered
by Clay-with-flints, a surface blanket of material that appears to have formed by
weathering of the Chalk under very variable climates, leaving a residue of flints and
clay. This surface blanket has formed on different layers within the upper part of the
Late Cretaceous Chalk, but has distinct margins with slopes facing eastwards over the
Weald uplift in Landscape
G
(
g1
) and westwards over the upstream branches of the
River Itchen (
f2
). These Clay-with-flints deposits often support small patches of wood-
land, unlike the thin, dry Chalk soils elsewhere that are covered by grassland and pas-
tureland.
The high ridge of the South Downs (
f1
) rises out of the Hampshire Downs south
of Winchester, gaining definition as it runs east towards Sussex. In Area 5, the South
Downs occupy only a thin sliver of country about 10 km wide, extending from the
Itchen valley near Winchester in the west to a few kilometres east of the A3 trunk-road
between Havant and Petersfield. Along the whole of this stretch, a remarkable plan-
view pattern of arrowhead-like Vs can be seen on the south side of the ridge pointing
northwards (see Fig. 123, for example). Both this pattern and the narrowness of the
Chalk belt are the direct result of the regional tilting of the Chalk layers, which steadily
slope southwards at between 2 and 6 degrees.
Landscape G: The western outcrop area of the Wealden Greensand
In the northeastern corner of Area 5, the Chalk is replaced by older (Early Cretaceous)
bedrock that has been brought to the surface by erosion of numerous gentle, open folds.
Together they produce the Weald uplift, a regional-scale structure that dominates the
bedrock geology of southeastern England south of the Thames, described more fully in
Area 6.
The youngest of the Early Cretaceous layers is the Upper Greensand. This 5 m
thick sandstone and siltstone, often richly cemented with iron, forms a clear series of
ridges round the western end of the Weald uplift, particularly near Selborne (
g1
). The
Upper Greensand ridges change sharply in direction around the town of Petersfield,
from a broadly southwesterly direction to an easterly direction, running approximately
parallel to the escarpment of the South Downs.
Below the Upper Greensand is the mudstone-rich Gault and, below that, the al-
ternating sandstones and mudstones that make up the thick Lower Greensand. In Area
5, the Gault lies east of the Upper Greensand, and has generally been eroded into low-
lying, featureless country, whereas the Lower Greensand sandstones further east have
resisted erosion. To the northeast of Petersfield the topography reflects the way that
river erosion has picked out fault displacements of these Lower Greensand layers.
