Kirkcudbright - an excursion
By P Stone, R F Cheeney and D E White. Excursion 5. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.
- 1 Kirkcudbright: a volcanic vent, Hawick Group turbidites, graptolites
Kirkcudbright: a volcanic vent, Hawick Group turbidites, graptolites
1:50 000 Sheet 83 Newton Stewart & Kirkcudbright
BGS 1.50 000 Sheet 5W Kirkcudbright
Main points of interest
A Siluro-Devonian volcanic vent, Silurian turbidite greywackes and Wenlock graptolites, complex Caledonian folding.
All the localities lie SW from Kirkcudbright and involve relatively easy coastal walking: about 1.5 km at Locality 1, about 3.5 km at Locality 2, and about 7 km for Localities 3 and 4 if the coastal path is walked in both directions. A shorter return route from Locality 4 requires prior permission from Ross Farm. A low tide is advantageous but much can be seen under all but the highest tidal conditions. Vehicle access is good for Locality 1, where there is ample car or coach parking. However, as access for the other localities is via minor roads and parking space is limited, the full excursion should not be attempted in any vehicle larger than a minibus. Total driving distance from and back to Kirkcudbright is about 25 km.
Kirkcudbright is an attractive small town with a picturesque harbour situated on the estuary of the River Dee. Maclellan's Castle, a ruined tower house dating from about 1580, may be seen in the town centre. Close by the Tollbooth dates from 1627 and the old Merkat Cross still features the jougs (a form of pillory) 'for the public humiliation of offenders'. About 3 km north of the town, on the A711, the Tongland dam and hydro-electric power station may be visited and a guided tour could form an appropriate adjunct to the geological excursion.
The Kirkcudbright area is underlain by Carghidown Formation (Hawick Group) greywackes, and siltstones. These were deposited during the Silurian period about 430 million years ago. The coastal sections to the SW of the town provide splendid outcrops, illustrating turbidite sedimentology and complex Caledonian structure, and it is these that provide the focus for the excursion. Examples of igneous intrusive rocks will also be seen and graptolites of Wenlock age may be found in the Ross Formation beds, a Hawick Group component slightly younger than the Carghidown Formation and exposed farther south. Still younger strata of the Riccarton Group crop out to the SE of Kirkcudbright and are examined in detail by Excursion 11. It would be possible to include some elements of that excursion as an extension of the itinerary described here. The complex fold structure may be further examined in the excellent coastal exposures slightly farther NW at Barlocco (NX 585 486) which are described in detail by Treagus (1992).
1 Shoulder O'Craig: volcanic vent
The excursion is best begun at the car park and picnic area adjacent to The Doon and Gull Craig beside Nun Mill Bay (NX 658 487). This is situated about 5 km from Kirkcudbright and is reached via the A755 and B727. About 600 m NE from the parking area an agglomerate-filled volcanic vent (Figure 23) cuts Silurian greywacke and siltstone (Carghidown Formation). The sedimentary rocks are exposed on the foreshore in Clinking Haven as steeply inclined beds striking NE and locally folded into tight, upright structures. A good array of turbidite features can be seen on the wave-smoothed surfaces and includes graded bedding and loaded bed bases. A penetrative cleavage is developed subparallel to bedding in the finer-grained lithologies but does not continue into the vent agglomerate which was therefore a post-tectonic intrusion. The agglomerate, believed to have been intruded in latest Silurian or early Devonian times, is one of a number of such vent features scattered across SW Scotland. A fresh kersantite (biotite-plagioclase lamprophyre) phase of the vent intrusion has given a K-Ar age of 410 ± 10 Ma (Rock et al., 1986a). The vent occupies the northern side of Clinking Haven forming the Shoulder O'Craig cliffs (663 491) and probably extends for a short distance inland. Lamprophyre dykes cut both the vent agglomerate and the turbidite country rock. Detailed petrographical and geochemical data for the intrusive rocks are given by Rock et al. (1986a).
The texture of the vent agglomerate is best seen on the wave-polished surfaces on the NW side of Clinking Haven. The cliff sections provide more extensive outcrop in three dimensions and confirm that the agglomerate consists principally of variably rounded greywacke, siltstone and sporadic microdiorite or basaltic clasts set in a fine-grained matrix; the latter is largely altered to carbonate and chlorite. Clast size is very variable and ranges up to rafts of country rock a few metres in length. The preponderance of sedimentary clasts in the vent suggests that initially it emitted steam and gases for the most part and did not directly tap a source of magma. Thus the vent agglomerate should more accurately but less descriptively be termed an intrusion breccia. It is cut by a number of basalt bodies and lamprophyre dykes the larger of which are shown in Figure 23. Note the irregular and fractured biotite-olivine basalt mass which intrudes the agglomerate in the western end of the vent. It is generally clast-free and its contact with the surrounding agglomerate varies from sharp to diffuse and gradational. Oval, pillow-like textures and possible flow fractures may suggest that the intrusion was emplaced in a semi-solid state (Rock et al., 1986a). However, it does imply that the vent developed from a steam and gas escape route to a conduit for magma.
Other dykes cut the greywacke country rock and a noteworthy example occurs about 20 m beyond the NE extremity of the vent. This has been dubbed the 'Loch Ness Monster' dyke on account of its bizarre outcrop pattern. It is a kersantitic lamprophyre consisting of biotite phenocrysts set in a dark grey feldspathic matrix. The highly irregular form is thought to reflect high volatile pressure during emplacement.
2 Brighouse Bay: Carghidown Formation greywackes and structure
3 Meikle Ross: Ross Formation greywackes and siltstones
This small peninsula forms the southern extremity of the west side of Kirkcudbright Bay. It is reached by means of unclassified roads which link Brighouse Bay with Ross Farm (NX646 447). Manor Point forms the headland on the south side of Ross Bay (about 1.5 km SE along the footpath from Ross Farm) and from the Point about a kilometre of well-exposed coastal section extends south. It is most readily accessible if the path is followed to the southernmost point of Meikle Ross and the coastal section then traversed northwards.
The strata are well-bedded greywackes and siltstones with sporadic interbedded grey-green shale and belong to the Ross Formation (Hawick Group, Figure 2). Restricted graptolite faunas (Figure 26) of the M. riccartonensis Biozone (early Wenlock) have been found in rare hemipelagite horizons (localities 52-55 of White et al., 1992).
4 Fauldbog Bay: graptolitic hemipelagites
Well-developed fold structures can be seen on the west side of Meikle Ross at Fauldbog Bay (NX 642 444), a locality which is also notable for the abundance of graptolites in the interbedded hemipelagite horizons. The west-facing coast of the bay exposes strata lithologically similar to the Ross Formation but containing some thin red mudstones characteristic of the Carghidown Formation. The boundary between the two formations is taken at a fault in the northern extremity of the bay, the strata on the east side are assigned to the Ross Formation and those on the west to the Carghidown Formation. Variable and complex folding may be seen in many parts of the shore section. Sheared zones separate the folded areas from units of more uniformly bedded strata.
To reach Fauldbog Bay continue along the coastal path around the headland and up the west coast of Meikle Ross. Good exposures of graptolitic hemipelagites of the Cyrtograptus centrifugus Biozone (basal Wenlock) can be examined in intertidal reefs at the northern end of Fauldbog Bay (NX 642 445 to NX 644 443) (localities 61-63 of White et al., 1992). Graptolites collected here include Barrandeograptus? bornholmensis (Laursen), C. cf. centrifugus Boucek, cf. C. grayi Lapworth, Monoclimacis vomerina basilica (Lapworth) , Mcl. vomerina vomerina (Nicholson), Mcl. vomerina c.l., Monograptus priodon (Bronn), M. aff priodon, M. remotus Elles & Wood, Retiolites geinitzianus angustidens Elles & Wood and R. geinitzianus geinitzianus Barrande. Approximately 750 m to the south, (around NX 644 437) (localities 57-60 of White et al., 1992) hemipelagites in the intertidal reefs contain a restricted fauna of the Monograptus riccartonenis Biozone, mainly M. Riccartonensis. A selection of graptolites recovered from these localities is shown in Figure 26. They are of Wenlock age (Figures 2 and 5).
Both sides of Fauldbog Bay expose abundant minor folding but the NW side is of particular interest. Many of the folds there are inverted (downward-facing) and enclosed within shear zones reminiscent of the Brighouse Bay section; a similar origin seems likely.
From Fauldbog Bay it is possible to cross the fields eastwards towards Ross Farm if prior permission has been obtained. Otherwise the coastal route should be retraced.
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