CLASSIC CORNISH FIELDWORK LOCATIONS
RINSEY COVE (PORTHCEW)
Access & location:
Rinsey Cove (SW592269) is situated on the coast just east of Praa Sands. It can most easily be reached by taking the A394 from Penzance or Helston and taking the turn-off for Rinsey at Ashton (SW603285). Travel along Rinsey Lane until you reach Rinsey Cove car park (SW591270) and then descend to the beach via the cliff path below the engine house of Wheal Prosper.
Pros & cons:
The exposure at Rinsey Cove is excellent and the granite contacts, folds, cleavages and metamorphic 'spotting' are displayed very well. 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. The pathway down is 'user unfriendly' where it reaches the beach (it can be very slimey and slippery), as is a path that turns sharp right down the cliff before the beach is reached. This opens out onto a sloping shelf some 20 feet above the beach. This may be coated in transparent slime and is particularly dangerous and best avoided. The beach area is OK on sand and dry rock, but avoid weed-coated areas and watch your footing at all times. Keep an eye on the tide - it can turn very quickly and it is possible to be cut off.
Note: This is a SSSI, no hammering of rock surfaces is allowed, nor should material be removed.
Geology:
Rinsey Cove lies within the Tregonning-Godolphin Granite and is eroded along a line of weakness where a large roof pendant, ~130 metres across, extends from the beach up into the cliffs, and then passes inland. The western contact [SW59242692] dips beneath the slate at 25°-30° for most of its exposed length, but in the northwest corner of the cove it runs along a vertical fault trending 320°, which cuts the S1 cleavage cleanly. Adjacent to the dipping contact, the granite has a well-developed roof complex consisting of banded leucogranite and aplite, with irregular pegmatite pods, which carry coarse schorl crystals with feldspar and quartz. Several xenoliths occur in the granite, at all stages of assimilation, from those which are sharply defined, with strong relict structures, to those which appear as nebulous dark patches; one large xenolith has acted as a volatile trap and has a layer of pegmatite developed on its underside. The slates above the contact are hornfelsed and show a spectacular development of white, acicular, andalusite porphyroblasts, up to 1 mm wide and 15 mm long. These crowd the rock and cover cleavage surfaces, in some instances showing a preferred mineral lineation.
A geological sketch map of Rinsey Cove (modified after an original drawing by Simon Camm, based on Bromley (1989), with added personal field data).
The eastern contact [SW59352682] lies along a vertical fault trending 050°. Adjacent to the contact the granite displays a narrow (~1 cm) chilled margin (the granite is also lightly kaolinised) and the slates are heavily tourmalinised for a distance of 2-3 cm. Approximately 3 metres to the left of the contact, a parallel vertical granite sheet, 1 metre thick, cuts the slates. As the contact is followed across the foreshore it can be seen to change attitude (apparently sharply) from vertical to a westerly dip of ~45°, possibly following a moderately-dipping joint set in the slates. Pegmatites are developed in the granite, parallel to the contact in this dipping section, and numerous small veins of granite invade the slates along joints (vertical sets trend NE-SW, NNW-SSE and E-W).
A large slate raft on the foreshore to the east of the contact appears to be an isolated section of the roof pendant; its S1 cleavage dipping SW, in common with the slates in several sections of the main outcrop. S1 dips SSE to WSW (164°-258°) at angles of 5°-30°, the variation possibly due to brittle rotation in response to fault movements. The slates carry numerous SE-verging F3 folds; these range from isolated structures with relatively small short-limb lengths (20-30 cm), to larger nested groups of folds, where short-limb lengths may reach 0.50 m. The folds typically occur within bedding-parallel detachments (bound on the upper and lower surfaces by S1) which range from 0.50 m to ~2.0 m in thickness and occur in a stacked sequence in the cliffs. The folds range in vergence from SE to SSE and their fold axes plunge NE to NNE (042°-056°) at 2°-6°. Several folds can be seen in the cliffs in close proximity, and adjacent, to the vertical granite dyke and the main contact itself; neither they nor the S1 cleavage is deformed or deflected by these contacts, both are cleanly cut and indicate that emplacement took place under brittle conditions.
At the top of the cliff, adjacent to the eastern contact, the granite is in contact with slates, that run along the cliff line to the east (the cliff top closely follows the granite roof at this point. The upper contact is underlain by a pegmatite/aplite roof complex in the granite. To the east of the contact, well-jointed granite forms the foreshore (vertical sets trend ENE-WSW, NE-SW, E-W, NW-SE, NNW-SSE and N-S), with slate rafts, xenoliths and occasional pegmatite pods carrying tourmaline, feldspar, quartz, arsenopyrite and molybdenite.
At the tops of the cliffs, within the roof pendant, a small slate quarry [SW59432695] is veined by three narrow tourmaline-rich pegmatite sheets, emplaced along the S1 cleavage (dipping here at ~10° to 282°), which unite into a single sheet on the east face. Eastwards from Rinsey cove, a number of isolated slate rafts outcrop along the line of the coast path (how many more are hidden on the slopes is unknown). At [SW5945726769] a small 1 × 1 m slate raft outcrops; S1 dips 18° to 298°. At [SW5946626748] a 2 × 1 metre raft outcrops. The slate raft, ~2 cm thick, is underlain by a thin (~1 cm) layer of quartz and coarse tourmaline, above equigranular granite. The contact is remarkably planar and lies parallel to S1, which dips 4° to 032°. At [SW5949226707] a third raft, 2 × 2 metres in size, outcrops, 46 metres above sea level. The raft is ~ 5 cm thick and is thoroughly tourmalinised at the base; the top is heavily spotted with cordierite. The base, parallel to S1, is underlain by 3 cm of pegmatite which passes down into equigranular granite (somewhat finer grained than usual). S1 dips 15° to 260° and 10° to 332°. The raft lies on a small shelving section of the cliff line and is overlooked by a steep slope, up to the 75 metre contour, which contains numerous granite outcrops. This suggests that the southern face of the Tregonning Granite is stepped and that those steps (defined by faults or master joints) trend NNW-SSE, parallel with the coast line.
This page last updated 22/09/2003
