history of the Shepton Mallet area stretches back over 300 million
years. Located at the boundary between the young Jurassic rocks and
the older, harder Carboniferous strata, there is a huge variety of
rocks to be seen within a small area.
The oldest rocks, including the Devonian Portishead Formation, the Avon
Group mudstones and the Carboniferous Limestone outcrop on the southern
side of the Beacon Hill pericline. The Black Rock Limestone is exposed
in several old quarries at Windsor Hill. These quarries were opened
by John Wainwright and Co. Ltd. in about 1902. In this quarry the
German palaeontologist Walter Kunhe obtained a specimen of Oligokyphus,
a mammal-like reptile that resembled a weasel, from a fissure infilled
with Jurassic Lias Group sediments. Triassic rocks outcrop around
Most of Shepton Mallet is built on a shelf of Jurassic Downside Stone,
a near-shore (littoral) variation of the Blue Lias limestone up to
30 m thick. On the southern side of the town, the Downside Stone
rests on Triassic Mercia Mudstone and Dolomitic Conglomerate. However,
to the north, it oversteps the Triassic rocks and rests directly
on the underlying Carboniferous Limestone between Doulting and Downside.
In places erosion has cut through the Downside Stone cover to reveal
the older rocks beneath, for example in the Ham Woods valley between
Windsor Hill and Croscombe.
Aerial view of Shepton Mallet and Maesbury (click to enlarge view).
Cross-section cartoon N–S showing relationship of Carboniferous Limestone
and Mesozoic sediments (click to enlarge view).
Both the Downside and the Doulting Stone have been quarried for building
stone. The Downside Stone, and its lateral variant, the Chilcote
Stone can be seen in several disused quarries around Downside, most
of which are on private land. The largest lies just north-east of
the old railway viaduct on the B3136 south of Downside and shows
several metres of pale white bioclastic limestone. Another quarry
in Downside, now used as a builders' yard displays the striking angular
unconformity between the Downside Stone and the underlying Carboniferous
Black Rock Limestone.
The rocks are also exposed in Ham Woods and in a small roadside cutting
in Bowlish. Here, the interbedded pale grey limestone and thin mudstone
indicate a change from a near-shore (littoral) to a more offshore,
but still shallow-water setting. Farther south, Downside Stone merges
with the more typical deeper water Lias Group sediments.
To the east of Shepton Mallet, the Inferior Oolite or 'Doulting Stone'
is still being actively quarried around Doulting. It has been used
in Wells Cathedral, Glastonbury Abbey and many local buildings. It
is a working quarry so permission is needed for a site visit. However,
spectacular examples of the stone can be seen on Chelynch Road by
the quarry entrance. The quarries here were connected by a tramway
to the East Somerset Railway.
Several small streams draining the sandstone on Beacon Hill sink
underground on meeting the Carboniferous Limestone. At Thrupe Lane
Swallet, a small stream sinks in an almost vertical cave system
120 m deep. The water from this and the other stream sinks resurges
at the St Andrew's Risings in Wells.
Around Doulting are a group of springs that emerge from the base
of the permeable Inferior Oolite where it overlies the impermeable
Charmouth Mudstone. One of the springs is a holy well, named after
St Aldhelm, the Bishop of Sherborne, who died here in 709.