rocks of the Penistone area are Upper Carboniferous
(Kinderscoutian and Marsdenian) in age, so they are about
320 million years old.
These rocks were laid down in deltas on the edge
of a large continent, with mountains to the north and
south. Sands and muds were deposited by rivers in shallow
water. Because the continent was close to the equator,
the climate was warm and wet so that tropical rain forest
flourished. Dead plant material became trapped in stagnant
swamps between river channels. Over geological time it
was buried by muds and sands as the rivers in the delta
changed position and built up more deposits. The water,
oxygen and hydrogen were driven out of the plant remains,
leaving only the carbon in coal seams.
After the sediments were formed close to sea-level, they
were buried by hundreds of metres of sediment and compressed.
As the sea water was squeezed out, it carried minerals
which cemented the sand and mud grains together
to make rocks called sandstones and mudstones
rocks were tilted into a large fold, called the Pennine
anticline, shortly after they were formed. The rocks
of the Haworth area are close to the top of the anticline
so they are nearly horizontal, which gives the flat plateaux
surfaces you can see on the horizon. However, at Penistone
Hill, the rocks dip at a few degrees to the south, so
different sandstones outcrop near the top of the Hill,
as shown on the cross-section. The mudstones are less
resistant and can be weathered easily unless they are
protected by a layer of sandstone lying above, so they
form the steep slopes between each bench of sandstone.
The sandstones in this area have different names. The
Woodhouse Flags form the south side of Penistone
Hill and have been quarried widely. The High Moor Sandstone
(also called the Doubler Stones Sandstone) is older and
is found on the north and west sides of the hill and has
been quarried at Dimples Quarry. Both sandstone beds are
not continuous and probably represented flood deposits
in the delta area.
Keighley Bluestone is an unusual dark grey, fine-grained
sandstone which was deposited in shallow lakes on the
delta top. It is only a few metres thick but was quarried
widely for roadstone. This area also has a thin coal seam,
only a few centimetres thick, found in Dimples Quarry.
Another coal, the Stanbury Coal, usually about
30cm thick, is slightly younger and lies under the Woodhouse
During the Carboniferous period the sea-level changed
regularly, because of glaciations in the southern hemisphere.
Therefore the delta was sometimes drowned by sea-water
which meant that dark mudstones with marine animals were
deposited above the delta sediments. These marine bands
contain marine goniatite and bivalve fossils which
can be found in some local mudstones.
Cross section below shows the geology of Penistone Hill,
Sandstones are shown in yellow
Mudstones are found between the layers of sandstone
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