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South Penquite Farm

A spoil heap of quarried granite.


Geology
"...exposing the granite..."

South Penquite Farm lies on a plateau between 200 and 220 m above mean sea level, on Bodmin Moor, with steep slopes down to the De Lank River on the south-west side. There is very little rock definitely exposed in situ but what there is is granite and rhyolite (called 'elvan' in Cornwall), and thus the visible geology matches closely what is well seen in the adjacent active De Lank Quarry which produces dimension stone, and has in the past also produced crushed rock ('aggregate') of both granite and rhyolite. Two 'slot' quarries in rhyolite dykes and a broader quarry in granite, and minor workings, have been cut in this SW facing slope

The slope also has a good spread of clitter - loose blocks of rock - and most of these are probably part of a fossil landscape dating from the Ice Age (in summer in glacial periods the top surface of the permafrost would have melted and blocks slid under gravity). Some may be the result of small scale quarrying

De Lank granite with well-formed feldspar crystals.Some of the fields too have a cover of loose boulders, but most of the fields have been cleared. These boulders are the remains of granite that was more resistant to weathering, and are thus analogous to tors. Some could be called core-stones, and in a few places good granite can be seen at surface immediately adjacent. As many large boulders have been moved by machinery it is impossible to state which are still in place

In places too the boulders, and granite at surface, have been worked using feathers and tares to split off pieces - presumably for gateposts and lintels. Indeed, one granite gatepost is very obviously an up-ended piece of cheesewring tor, with characteristic curved upper surface

Coarse and fine grained versions of granite, formed at the same time as the main solidification of the magma, can be seen mostly in loose blocks: again this matches what can be seen in situ and in cut blocks in De Lank Quarry. A good example is also exposed in a gateway on the north-west side of the farm. In addition to these pegmatites (very coarse granitic rock) and aplites (finer, light-coloured granitic rock) a large range of rocks seen associated with granites across Cornwall can also be found, but only as loose blocks and predominantly used to make Cornish hedges

he latter are an impressive feature of the farm, and are being maintained. Such rocks as quartz-tourmaline rock ('schorl'), greisen (late stage altered granite), and reddish haematised rock (usually associated elsewhere with ore veins), as well as barren quartz veinstone. One block of the latter was not totally barren but had a crystal of cassiterite less than 1 mm across. Much of the flood plain of the De Lank River near Delford Bridge has been worked for alluvial tin, and just outside the farm entrance the area has been heavily worked, but this produced a heavy mineral concentrate with little panning effort

The flatness of the landscape at the farm probably implies a sea-level remaining for some considerable time at around 220m above present day level, but as with most of these features in the south-west there is very little evidence for dating. The low ground to the west of the farm is developed on Devonian slate, well seen in nearby road cuttings

The older farm buildings are mostly made of granite, with slate roofs. The slate probably came from the Delabole area which was a major supplier of slate for the SW, and Delabole Quarry still produces slates (and provides tours of its impressive 'hole' and workshops). Granite on the farm has also been used for prehistoric features such as the numerous and well preserved roundhouses and field systems, as well as for a standing stone. Thus the 270-290 million year old granite is linked not only with geomorphology as an upland but also with periglacial processes and also with prehistoric archaeology and more recent events such as alluvial mining and medieval and later habitations. No doubt ecological surveys will show additional links

While only the SW-facing slope which includes the two elvan quarries and the granite quarry has a geological conservation designation, as part of the De Lank Quarries SSSI, there is much on the farm to interest a casual observer, and much that could be developed for educational use. There is a growing rock collection at the education centre which illustrates the diversity of the geology on the farm



Dominic & Cathy Fairman
South Penquite Farm, Blisland, Bodmin, Cornwall

thefarm@bodminmoor.co.uk
Tel: 01208 850491