CLASSIC CORNISH FIELDWORK LOCATIONS

MEGILIGGAR ROCKS

Access & location:

Megiliggar Rocks (SW610266) is situated on the coast just west of Porthleven. It can most easily be reached by taking the coast road west out of Porthleven (passing the Ship Inn on your left) to Tremearne (about a mile and a half). Parking opposite the entrance to Tremearne Farm, take the footpath across the fields down to the coast. Alternative access is from Rinsey Cove car park (SW591270) and requires a 40 minute walk to reach Megilligar. Parking at Tremearne is restricted to a single car, larger parties will need to be dropped off & met or start off from Rinsey Cove.

Pros & cons:

The exposure at Megiliggar Rocks is excellent and displays a number of tectonic, mineralogical and petrological features that are not equalled elsewhere. Access is limited by the tide which turns very rapidly here; aim to arrive an hour before low water and leave not long after the tide turns. Some sections of the cliffs are crumbling badly and should be avoided. The rock platform below the high water mark is polished, weed-covered and very slippery and should be negotiated with care. Westwards along the foreshore, towards Legereath Zawn, the area is covered with large (slime covered) boulders which make passage difficult.

Note: This is a SSSI, no hammering of rock surfaces is allowed, nor should material be removed.

Geology:

The engine houses of Wheal Trewavas on Trewavas Head. The mine is situated on the lithium-rich Tregonning Granite. In the background can be seen the granite sheets penetrating the Mylor Slate Formation, close to the eastern contact, at Megiliggar Rocks.

The area around Megiliggar Rocks exposes the eastern contact of the Tregonning Granite with the Upper Devonian Mylor Slate Formation. Running out from the contact are a series of sills and dykes that extend hundreds of metres into the country rock. These sills and dykes display a range of rock types from microgranite, to aplite, to line rock and spectacular schorl-orthoclase-quartz pegmatites. The contact itself appears almost vertical and lies along a fault. This outcrop in Trequean Zawn is impossible to reach, without swimming, even at the lowest tides; with a party it is advisible to only go as far as Legereath Zawn and walk back to Tremearne, this will give the best range of available exposures.

Megiliggar Rocks from the west of Legereath Zawn; the aplite/microgranite/pegmatite sills can clearly be seen in the cliffs.

A closer view of two of the sills in the cliffs. Note the (relatively) freshly fallen angular blocks - parts of the cliff line here are quite unstable and need to viewed with caution.

The major sills dip fairly gently to the east; they maintain fairly constant thicknesses and contain a number of large xenoliths, some of which have been arrested very close to their points of origin. Occasionally the xenoliths may have acted as 'dams' to ascending fluids and will have areas of pegmatite or masses of schorl tourmaline developed across their lower sides. The sills were emplaced by penetrating along zones of weakness, in this case cleavage planes in the slates, and subsequent inflation. In many respects this reflects current thinking on the emplacement of the batholith as a whole, only at a much smaller scale. The sills post-date the D3 (extensional) deformation in this area and like the rest of the granite as a whole appear to have been emplaced by a combination of passive (fault-controlled) methods and stoping.

Stoping at Megilligar Rocks, from an original drawing by Simon Camm.

The slates show a strongly developed (S1) cleavage which is largely bedding-parallel. This cleavage is folded around SE-verging D3 folds which record post-orogenic extension. The folds are generally small in scale with upper (top sense of shear to the right) limbs of the order of ~1 metre in length and with axial traces that are close to horizontal, or dipping gently. The (S3) cleavage associated with these folds is difficult to make out in most outcrops. The slates are also faulted by a number of, largely, normal (extensional) faults, as well as later mineralised crosscourses. The mineralisation (banded chalcedony) in these fractures dates from the Lower Triassic, and marks the final stages of the polymetallic mineralisation cycle in SW England, though the fractures themselves may have been active for a considerable time before this.

Several well-exposed pegmatite sheets (crystal sizes may reach in excess of 20 cm) can be seen, particularly on the wave-cut platform; these consist largely of quartz and pink/white feldspar with a series of accessory minerals. The most common of these is schorl, which can often be seen as long euhedral crystals arranged (long axis of crystals) at right angles to the upper surfaces of the sheets. Occasionally the schorl crystals can be seen to have quartz-infilled cores and have acted as nucleation sites for muscovite mica which has crystallised in almost perfect spheres (with a radiating internal structure) up to 1 cm across. Other minerals known from the pegmatites include apatite, lollingite (iron arsenide) and the rare mineral triplite (an iron phosphate). Rare vugs in the pegmatites have produced many spectacular specimens, among the best known are the deep blue-purple apatite crystals for which this locality is famous.

Looking west along the foreshore towards the granite contact. Pegmatite sheets outcrop on the platform to the left. Note the fallen block of aplite in the foreground with a xenolith.

As you progress eastwards back to Tremearne the rock in the sills becomes progressively more apltic and quartz-rich, before finally passing into almost pure quartz at the extremeties of the sheets. During their progress some sills can be seen to transgress from one plane to another, while others send off dykes into the surrounding rock; it is easy to envisage this process isolating blocks of country rock and causing them to founder into larger bodies of magma. Megiliggar Rocks shows the stoping process at a variety of scales and it is this, in conjunction with the variety of rock types, mineralogy and structural/tectonic features that make this particular locality unique.

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This page last updated 28/02/2013