Geoscience Reference
In-Depth Information
BEDROCK AND SURFACE BLANKET
W
ALK AROUND THE COUNTRY IN SOUTHERN ENGLAND
and the ground be-
neath your feet is very rarely solid rock. You are walking over soil made of weathered
mineral grains and organic debris, along with other relatively soft and granular materials
that make up the surface blanket. Beneath the surface blanket lies solid rock, the bedrock
of the landscape.
Bedrock forms the bones of the land. From the colour of the soil, to the elevation
of the hills, to the types of vegetation present, the landscape is profoundly influenced by
the bedrock underlying it. For example, in Southern England the Lower Greensand (a
distinctive layer of bedrock of Early Cretaceous age, see page 26) produces soil water
with acidic chemical properties. The Lower Greensand was originally deposited as sand
over a period of a few million years, more than 100 million years ago. This layer repres-
ents a different environment of deposition from the older sediments on which it lies, and
was followed by another change of environment which produced the deposits that lie on
top of it. Both the preceding and the following bedrock deposits have alkaline chemical
properties. In certain regions the bedrock layers have now been brought to the surface of
the landscape by erosion and movements within the Earth. The Greensand is harder than
the layers above and below it (largely mudstones) and so is generally more resistant to
weathering. In some areas the Lower Greensand lies just below the surface blanket and
has resisted the general landscape erosion to form a distinct Greensand ridge running
across the countryside, characterised by special vegetation adapted to the acidity of the
soils.
It is only in cliffs or at man-made excavations such as quarries that we can see bed-
rock at the surface in most low-lying areas. By using those areas where the bedrock does
outcrop at the surface, and the results of drillings (e.g. for wells), we can discover the
types and arrangements of rock below any landscape.
