CLASSIC CORNISH FIELDWORK LOCATIONS
CAPE CORNWALL
Access & location:
Cape Cornwall [SW349318] extends out from the coastline to the west of St Just. It can be reached by taking the sign-posted road west out of St Just (passing Cape Cornwall School on your right); this narrows and takes a number of sharp turns once out of the village and passes a number of isolated terraces and hotels before reaching the golf club, from where the roads slopes steeply to the sea. Parking in the National Trust car park above the cape, Porth Ledden, Priest's Cove and Cape Cornwall are all within easy reach via footpaths.
Pros & cons:
The exposure around Cape Cornwall are excellent, particularly in the coves to the north and south of the cape. Many features can be seen in the low cliffs of Priest's Cove and on the foreshore, depending on the tide. Porth Ledden is also an excellent locality, but is occasionally inundated with seaweed, blanketing the foreshore exposures and making footings treacherous. Care should also be exercised on the steep slopes of the cape itself, though good exposures of the metasediments can be seen from the safety of the paths skirting its base. With effort entrance can be made into the mine workings in the cliffs, but this is not recommended to anyone without experience of Cornish mine workings and the proper equipment - they are dangerous, so if in doubt, stay out.
Note: This is a SSSI, no hammering of rock surfaces is allowed, nor should material be removed.
Geology:
The Promontory of Cape Cornwall from Priests Cove. The Chimney stack and buildings on the flank of the cape were built for Cape Cornwall Mine, which closed in the 1870's.
Cape Cornwall forms an irregular headland extending 450 metres from the cliff line at Porth Ledden out into the Atlantic. The headland is formed exclusively of Mylor Slate, which is veined by tin-bearing tourmaline lodes and later quartz-rich veins.The Mylor Slates are well-exposed on the summit and flanks of the cape where they are variably hornfelsed (with some areas particularly rich in cordierite porphyroblasts); the S1 cleavage dips NW-NNW (312°-332°) at 15°-28°. Occasionally cleavage surfaces show the development of quartz rodding, which pitches NNW to N within the plane of S1 and the S3 cleavage (dipping at 40° to 310°) is also sporadically visible. At [SW35063188] on the north flank of the cape, a number of neutral-vergence to SW-verging, open folds occur, with wavelengths of 15 cm to ~2 metres. These folds have axes that plunge gently to the SE at ~5°. These folds appear to represent minor flexures and sections of F3 folds along curvilinear hinges; they occur in close proximity to strongly developed SE-verging folds, but the two types are not seen directly along strike from each other. At several points around the cape SE-verging F3 folds can be seen. These are often complex and associated with multiple S1-parallel detachments or minor, NW-dipping, thrust faults, although the apparent displacement on these structures is only a few tens of centimetres.
The geology of Porth Ledden, Cape Cornwall and Priests Cove (modified after Floyd et al., 1993 and Halls et al., 2001; with added personal field data and interpretation).
The slates are well-jointed, with sub-vertical sets trending NNW-SSE, ENE-WSW and ESE-WNW. On the southern side of the cape, on the foreshore at [SW35133171] a 4 metre thick, poorly-megacrystic, FGG sheet trends ENE-WSW (065°-082°), running for part of its length along the flank of a swimming pool in Priests Cove. This sheet dips towards 150° at 30°-70° and has flow-banded margins, which in some areas are sharply truncated. The sheet, which appears composite, also contains irregular granite enclaves and feldspar crystals which have nucleated and grown normal to internal contacts within the dyke. There are also thin veins of tourmaline-bearing pegmatite that cut across the earlier structures. The sheet is well-jointed, with a sub-vertical set trending NNW-SSE and a set dipping at ~30° to the NW.
The cordierite-bearing hornfelsed slates are well-exposed in Priests Cove on low cliffs at [SW3523631704], where a series of spectacular, angular, SE-verging folds can be seen (see Plate below). These folds have short limb lengths of ~1 metre or more, and pronounced angular hinges. A number of smaller folds occur on the limbs of these structures, all of which have the same general trend. The fold axes plunge gently NNE -ENE (030°-052°) or SW (235°) at angles of 2°-10°. The associated S3 cleavage dips WNW (296°) at 28°-30°, crenulating the dominant S1 cleavage, which on the foreshore also dips WNW-NW (282°-305°) at angles of 33°-35°, before turning to the NNW, on the southern side of the cove, close to the granite contact. Several granite sheets and veins cut the cliffs and foreshore around Priests Cove; these range up to 0.20 metres in thickness and dip moderately steeply to sub-vertically (40°-90°), trending between 070° and 082. They cut all earlier structures, including the F3 folds, and, unusually, cut across the major joints in the slates, rather than following them, although they do follow, and lie parallel to, a major joint set in the granite.
The F3 folds exposed in the cliffs on the NE side of Priests Cove at [SW3523631704]. The folds are part of a nested set of SE-verging angular folds that lie close to the granite contact (similar, smaller scale, folds are cut by this contact). The folded S1 cleavage is cut by an axial planar cleavage (S3) associated with the folds. This, and the folded slates, is in turn cut by granite sheets and veins emanating from the area close to the granite contact, which lies ~15 m to the SSE. Hammer (30 cm) for scale.
The granite contact [SW3526331622] is marked by a sub-vertical (86°) fault trending 095°; the fault plane itself now hosts the 060 m wide quartz-tourmaline-haematite-cassiterite Saveall's Lode (see Plate below) of St Just United Mine (situated on the cliffs above the cove).
The granite contact in Priests Cove. On the left, Mylor slates abut the contact. The S1 cleavage (marked) dips 15°/320° and is not deflected at the contact, which is sharp and dipping at 86° to the NNE. The faulted contact is now host to Saveall's Lode, on which the adit (0.90 m wide) is driven. On the right of the contact lies the Carn Gloose FGG which is heavily kaolinised and host to a large number of schorl/quartz/haematite veins (marked in red).
At the contact the slates are slightly bleached, but the S1 cleavage (dipping at 15° towards 320°) is cut cleanly along the fault plane without any visible deflection. The granite around the contact is heavily veined by E-W and NW-SE trending quartz-tourmaline-haematite veins. Goode et al. (1987) state that the fault has a dextral throw of more than 150 m, but no conclusive slickenline data could be found at the outcrop. The granite on the south side of the fault is a porphyritic, fine-grained, granite, termed the Carn Gloose Granite; this granite forms part of the St Just Wedge that lies between the Zennor and St Buryan Lobes of the Lands End Granite. Adjacent to the contact, and a considerable distance from it, the granite is heavily kaolinised, it is also extensively veined by a series of quartz-tourmaline-haematite veins related to the economic tin mineralisation of the district. The granite is well-jointed, with sets trending NW-SE, ENE-WSW and E-W. South from the fault it also contains irregular pegmatitic patches, which may reach over 1 metre across, with large quartz, feldspar and tourmaline crystals. The granite also contains a number of sheets and irregular enclaves of fine- and coarse-grained granite. The contacts, which are undulose and lobate, range from sharp to slightly diffuse. The enclaves are evidence of co-existing magma batches of differing sources or degrees of differentiation. That the two co-exist as identifiable units suggests a fairly short time between the introduction of these magmas, but that they were already sufficiently viscous not to mix completely, forming two immiscible (crystal-rich) liquids at the time of emplacement.
The granite contact is also exposed on the north side of Cape Cornwall, at Porth Ledden. The granite exposed on the foreshore occupies an outcrop some 150 metres by 50 metres in area. A variety of granite types are present and the contact is well exposed. The granite has a largely angular outline, with sections, particularly at the southern end, which are controlled predominantly by sub-vertical NW-trending joints, interspersed with segments which show a dipping (20°-60°) contact to the NW. The host slates are well-jointed, with a major set oriented NW-SE; it appears that the granite utilised this joint set in particular during emplacement, forming a series of 'roof and gable' blocks during inflation and tilting at the pluton margin. The granites exposed at Porth Ledden comprise three major types:- a coarse-grained megacrystic granite, with flow-aligned megacrysts, which forms the carapace of the intrusion; a megacryst-poor coarse-grained tourmaline granite, intruding the CGMG; and a quartz-tourmaline rock which displays a range of contact relationships with the tourmaline granite.
A panorama NW (left) to SE (right) across Porth Ledden. Mylor Slates form the lower cliffs below Kenidjack Castle and the wave-cut platform of Porth Ledden. The granite contact is marked, close to the mouth of the stream that runs down the Kenidjack Valley. The line of light-coloured boulders on the platform marks the CGMG outcrop and the dark quartz-tourmaline rock outcrop can be seen in the centre of the picture.
The coarse-grained megacrystic granite (CGMG) that forms the outer carapace of the outcrop is a composite intrusion that contains a number of granite sheets, particularly along the contact. These 'external' sheets appear to represent precursor intrusions rather than a marginal facies. Contacts with the CGMG may be sharp and planar, but others are more diffuse, which suggests a short time interval between the two with some sheets still above the solidus (though sufficiently viscous to maintain their integrity) when the CGMG was emplaced. The predominant rock type is a coarse-grained biotite granite with abundant euhedral orthoclase megacrysts that may reach 10 cm in length. Within this rock are occasional megacryst-poor enclaves with diffuse contacts and coarse-grained granite (CGG) and medium-grained granite (MGG) non-megacrystic sheets with, generally, sharp contacts. The CGMG (and enclosed tourmaline-bearing rocks) is well-jointed, with the most prominent sets trending NW-SE (in common with the host slates) and ENE-WSW. Close to the contact a number of angular slate xenoliths can be seen, with relict laminations still visible (and occasional small folds). On some of the shelving outcrops back from the contact the feldspar megacrysts can be seen to be lying horizontally on their 010 faces (but randomly oriented with respect to their long axes). Garnett (1962) also recognised flow alignments of feldspar megacrysts in the granite close to the granite/killas boundary in Geevor Mine, with crystals aligned sub-parallel or parallel to the contact. Booth (1966) and Reid et al. (1907) working on the Lands End Granite noted similar alignments close to the contact and that horizontal alignments of feldspars often indicated proximity to the granite roof. Closer to the contact the megacrysts, still aligned on their 010 faces, become inclined parallel to the contact itself, which dips NW (325°-336°) at 20°-30°. The contact with the overlying slates is everywhere very sharp. The granite may show a narrow chilled margin, but this is often missing. The slates immediately adjacent to the contact are baked to hard, green, massive hornfelses which have lost all traces of cleavage, but the original sedimentary laminations can still be recognised. These laminations, and the S1 cleavage further from the contact, usually lie parallel or sub-parallel to the contact itself, so that cleavage, laminations, contact and feldspars form a series of parallel structures over large parts of the outcrop in the dipping 'roof' segments of the intrusion.
The granite/killas contact at Porth Ledden. In this example, the contact is dipping at 20° to the NW. Below the contact, the granite has a thin chilled margin (1-2 cm) consisting of aplitic line rock. Above the contact the slates are contact metamorphosed to massive, poorly-fissile, green-grey hornfels. Although the bedding-parallel S1 cleavage is lost adjacent to the contact, laminations in the metasediments can still be observed, which lie parallel to the contact. Hammer (30 cm) for scale.
The inference from this is that the granite, during emplacement, tilted the slates in a NW direction during the in-situ ballooning of this section of the St Just Wedge. The feldspar megacrysts in the still-plastic (above the critical melt fraction) granite became aligned in response to the flow conditions, rotating until their 010 faces were normal to the dominant flow direction. At the contact, the transfer of heat into the cover rocks induced the dewatering of phyllosilicates leading to decrepitation in the slates, chiefly along cleavage planes, passing cleavage- and joint-bounded xenoliths into the magma, where continued decrepitation would have lead to the eventual assimilation of most of this material. With the continuation of this process, the granite/killas contact would eventually lie concordantly with the dominant (S1) cleavage, as it does in the dipping ('roof') segments at Porth Ledden and elsewhere. This process of piecemeal 'decrepitative stoping' would only draw to a halt when the granite at the contact solidified. Stoping of the metasediments around master joints would eventually lead to the development of separated blocks in the cover rocks, which could then move laterally, vertically and rotationally (within limits) with respect to each other (fully separated blocks would sink into the magma as a consequence of cauldron subsidence), forming the 'gable end' segments of the contact, and transmitting regional and localised stresses down into the underlying magma as it crystallised.
The tourmaline granite is a pinkish coarse-grained rock with abundant tourmaline. It is rarely megacrystic and those that do occur are no more than 2-3 cm in length. This granite, broadly elliptical in outcrop, has been interpreted both as a late intrusive and as a product of magma unmixing. The contacts between the tourmaline granite and the CGMG carapace are sharp and planar, demonstrating that the CGMG had crystallised fully before the latter was emplaced. In places the two granites show very close parallelism along their contacts indicating that the CGMG had inherited some of the structural weaknesses inherent in the cover rocks which were exploited by the tourmaline granite during passive emplacement. Over large areas of its outcrop the tourmaline granite has been metasomatically replaced by a coarse-grained quartz-tourmaline rock (tourmalinite), which appears to represent the boron-rich residuum of the magma, which has collected in the apical parts of the intrusion. Composed of large glomerocrysts of quartz and euhedral aggregates of schorl, with minor apatite, the tourmalinite displays a gradational contact with its host granite. In the northern part of the outcrop, close to the northern boundary fault the tourmaline granite can be seen to be selectively replaced along shallow-dipping roof-parallel joints, or zones parallel to them (low pressure zones, formed by decompression during unloading). The gradational boundary between the two may span up to 3 metres and isolated patches, metres across, of tourmaline granite occur sporadically within the quartz-tourmaline rock. In the southern part of the tourmaline granite outcrop it is fully replaced and in this area the actual contact between the CGMG and tourmalinite is marked by a metre wide band of quartz (with occasional feldspar-rich patches) which appears to be some form of stockschieder. Further outcrops of tourmalinite are also reported from the southern end of Porth Ledden.
At [SW3544132059] a granite sheet, approaching 1 metre in thickness, varies between megacrystic and poorly-megacrystic CGG. It carries irregular enclaves of non-porphyritic FGG up to 30 cm across, which have sharp contacts with the host rock. Other nearby sheets show evidence of being built up in a series of pulses, or of having differentiated in-situ with the development of aplite/line rock/pegmatite complexes. This complexity of structure and origin mirrors that of the main pluton and indicates that many of the processes that operated in the formation of the St Just Wedge Granite (and Land's End Granite as a whole) are scale-invariant.
The S1 cleavage in the Mylor Slates above the contact dips predominantly to the NW and NNW at an angle of 25°-30°. The rotation of the cleavage to the WNW seems confined to isolated blocks that have undergone minor rotational adjustment close to the 'gable ends' of the granite intrusion.Within the slates the S3 cleavage can be discerned at a number of locations(where it can be traced as a crenulation, cutting the S1 cleavage); this dips to the WNW (288°-290°) at 45°, which is broadly similar to the orientation of the cleavage further north at Enys Zawn. The associated SE-verging F3 folds are also in evidence, particularly at the southern end of Porth Ledden on the flanks of Cape Cornwall. Their fold axes plunge to the NE (and occasionally to the SW) at angles of 2°-8°. As elsewhere they are usually small-scale structures, with short limb lengths averaging <0.50 m.
At the southern end of Porth Ledden [SW35363192] granite can be observed in the upper part of the steep cliffs, trending south as the coastline turns sharply west onto the promontory of Cape Cornwall. The position of the outcrop and the slope of the cliffs suggests that this segment of the contact itself is very steep and may be faulted. On the foreshore a number of narrow granite veins extend out from the cliffs for several metres along joint planes, across the wave-cut platform.
This page last updated 22/01/2003
