of a line from Portland Bill to Flamborough Head, there are no significant
areas of reasonably hard rock outcrop in England. In this area, sand and gravel are the only local source of aggregate,
but although they can often make good concrete, are not generally
as versatile as rock. However, south-east England has the highest demand for quality aggregate, not only because it
is one of the most densely populated part of Britain, but also because it contains
fast growing areas around the Thames estuary, Ashford, Milton Keynes
By far the largest areas of hard rock are the Carboniferous Limestone
outcrops of Mendip and the Bristol region. The darker, less pure
Carboniferous limestones, such as the Black Rock Limestone with 6 to 7%
silica, are best suited for use as aggregates. In contrast, some of
the lighter oolitic limestones, such as the Burrington Oolite, are
extremely chemically pure, with 99% calcium carbonate, and are used
in the chemical and steel industries.
Click map to enlarge view.
These Carboniferous limestones make up the majority of the Mendip
Hills. Today, aggregate extraction is concentrated at the eastern
end of Mendips in part because this is closest
to the large markets of south-east England. In addition, the quarries
can also be readily hidden in the low 'plateau' topography, and avoid
both the highly valued landscapes and water supply areas to the west.
For this reason, two of the largest limestone quarries in Europe
are located here. Smaller quarries occur further west around Cheddar,
but these supply mostly local markets.
Other hard rocks occur, particularly the Silurian andesite lavas
around Beacon Hill. These have similar properties to some Leicestershire
igneous rocks and can be used for skid-resistant road surfacing.
However, their outcrop is restricted to a small area at Moon's Hill,
Stoke St Michael. The Devonian Portishead Formation is generally
too friable and inconsistent to be used as aggregates. The younger
Jurassic limestones are highly variable in terms of chemical and
physical quality and usually occur as very thin beds, interleaved
with sandy or clayey material and as such are not well suited to
modern quarrying or aggregate and industrial requirements. However
when worked selectively in certain areas, the limestones of the Inferior
Oolite (Middle Jurassic), notably at Doulting, are sufficiently consistent
and attractive to have been used as a fine building or decorative