Excursion 7 Loch Doon And Carsphairn: Ordovician turbidites, mineralisation and the Loch Doon Granite

By J.D. Floyd and D.J. Fettes

OS 1:50 000 Sheet 77 Dalmellington to New Galloway

BGS 1:50 000 sheets 8E Loch Doon, 14E Cumnock

Route map: (Figure 29)

Main points of interest Contact between the Loch Doon Granite and surrounding greywackes; lead-zinc mineralisation and abandoned lead mines; haematite vein; Ordovician greywacke, conglomerate, black shale and chert; porphyrite (porphyritic microdiorite) dykes.

Logistics The excursion begins at Loch Doon Dam, about 3 km along the minor road (signposted to Loch Doon Castle) which leaves the A713 about 2 km south of Dalmellington. At some localities parking is at the side of single-track roads; participants should show consideration for other road users by not parking in designated passing places. Localities 1, 2, 4 and 7 are only a short distance from the roadside, Locality 3 involves a longer walk mostly along paths, while Localities 5 and 6 require several kilometres of cross-country walking.

Introduction

This excursion examines the contact between the Loch Doon Granite and the surrounding greywackes (Figure 29) at Locality 2. Localities 1, 4, 5 and 7 deal with the Ordovician sedimentary succession and associated minor intrusions in the Carsphairn area. The general tectonostratigraphical relationships of the sedimentary formations are illustrated in (Figure 2) (Leadhills Group). The lead mines at Woodhead (Locality 3) and the haematite vein at Coran of Portmark (Locality 6) are of interest both to mineral collectors and to students of industrial archaeology.

The broad valley of the Glenkens, between Dalmellington and New Galloway, is also notable in containing several reservoirs for the original Galloway Water Power Scheme. This was built in the 1930s andremains the only substantial hydroelectric scheme in Scotland outwith the Highlands.

1 Loch Doon Dam: Marchburn Formation greywacke

Park beside the tourist office/toilet block beside the dam at the north end of Loch Doon [NS 477 012]. Walk south for about 600 m along the wave-washed western shore of the reservoir to examine excellent roches moutonnées with good examples of glacial striae gouged into well-polished rock surfaces. The latter reveal good sections through medium- to thick-bedded, coarse- grained greywackes and laminated siltstones of the Marchburn Formation. Beds strike between 060° and 090° and dip south at 60–80°. However, excellent graded bedding and other sedimentary structures in the greywackes clearly demonstrate that the succession youngs towards the north and that the beds are therefore overturned. This bedding attitude is common across much of the Northern Belt, especially in the area immediately south of the Southern Upland Fault.

2 Loch Doon Castle: granite/greywacke contact

From the dam, continue south on the loch-side road for a further 7 km to Loch Doon Castle [NX 485 950]. Park beside the castle and walk north along the loch shore to the rocky headland about 450 m NE of the castle and immediately below a roadside quarry. On the north side of the headland a sequence of dark hornfelsed greywackes exhibits well-developed sedimentary structures and strikes between NE and ENE.

Traversing south across the headland, north–south-trending fracture zones about 20 cm wide can be seen. Each is characterised by a series of individual fractures lying at a slight anticlockwise angle to the zone; individual bedding layers show sinistral displacement across the fractures. Locally these zones are crossed by a series of thin acidic veinlets which trend roughly ESE. The veinlets may have a stepped course caused by local diversions along the fracture planes.

On the south side of the headland, granodiorite veins become abundant as the margin of the Loch Doon pluton is approached. These veins cut the acidic veinlets described above and have a random orientation. The terminations of some of the larger veinssuggest that bedding has been forced apart on small brittle faults, though elsewhere the granodiorite veins contain xenoliths of greywacke which suggest some stoping during intrusion. The relationships between north—south fractures, acidic veinlets and granodiorite veins indicate an initial phase of brittle deformation with a sinistral displacement followed by a period of extension at right angles to the margin of the pluton.

To the south, the outcrop deteriorates but intermittent exposures of granodiorite indicate that the main contact lies immediately south of the headland. The overall trend of the contact in this area is ESE.

Proceed along the foreshore to the next headland immediately south of the castle. Here the granodiorite contains abundant xenoliths of greywacke as well as autoliths of igneous material. The xenoliths are generally aligned parallel to the contact. Xenoliths may also be seen on a second headland about 100 m farther south. A particularly interesting example is a strip of greywacke several metres long which has been intruded by relatively fine-grained granitic material prior to incorporation in the main granodiorite.

The numerous ruins of brick buildings beside the road along the loch, and the concrete blocks in the loch itself, are the remains of a seaplane base which was established at Loch Doon during the First World War. Some curious embankments on the east shore at the head of the loch were also part of this project, which swallowed up large amounts of money and caused great controversy at the time.

Loch Doon Castle itself originally stood on an adjacent small island in the loch, the top of which can still be seen when the water is low. During the 1930s, when the water level in the loch was raised as part of the hydroelectric scheme, the castle was dismantled stone by stone and re-erected at its present site.

3 Woodhead Lead Mines: lead and zinc mineralization

Health and safety note: As with any former mining area, there may be hidden dangers involved from the presence of old shafts, adits, unsafe buildings etc. and special care is required, especially for younger (and elderly) participants. Visitors should also be aware of the insidious risk of heavy-metal poisoning (lead, zinc, copper etc.) both from mineral specimens and from the widespread contamination on the site. Precautions should therefore be taken with hygiene, especially during the consumption of packed lunches etc., both on site and afterwards.

From Loch Doon, drive back to the A713 and continue south towards Carsphairn.

About 1 km north of the village, turn west on to a farm road (gated) to Garryhorn Farm. With permission, park at the farm and proceed on foot, or drive west for about 1.5 km along the rough road to the mines.

The long-abandoned lead mines at Woodhead are situated on the north bank of the Garryhorn Burn [NX 530 938]. The veins of galena, zinc-blende and minor chalcopyrite were discovered in 1838. Initially they were worked open cast before shafts were sunk and levels driven at depths of 16, 33 and 57 m. Lead production rose rapidly in the first few years, reaching a peak of 905 tons in 1842, before gradually dwindling to only 12 tons in 1873 (Wilson, 1921). However, even during its most productive period of 1840–1852, the Woodhead mines were never so important as the Leadhills/Wanlockhead operations.

There are several ruined cottages on the site together with the remains of the buildings and other works where the lead ore was dressed and smelted. As at Leadhills/Wanlockhead, there is evidence (in the headwaters of the Garryhorn Burn) of an elaborate system of contour-following channels which led water to the mines for powering pumps and other machinery. Again as at Wanlockhead, the highly poisonous fumes from the smelting hearths were led along flues in roofed-over trenches and eventually up the large stone chimneys which still dominate the site. The trench flues are now largely grassed over and form dangerous pitfalls for the unwary. Fumes from these primitive hearths contained a high proportion of lead and sulphur dioxide and, even in early times, lead poisoning was a well-known hazard for lead miners and smelters, as well as for local residents. This was often accepted as an unavoidable fact of life and diagnosed with the quaint term 'Lead Distemper'.

From the extensive physical evidence still remaining it is obvious that, in its heyday, Woodhead must have been a very busy little mining community. The isolation of the locality has contributed to its relatively well-preserved condition and it represents an important industrial archaeology site in south-west Scotland.

There are two main veins, named Woodhead and Garryhorn, both of which trend about 150°, hade NE at 62° and probably follow part of the extensive NW trending plexus of faults which delineates the valley of the Glenkens between New Galloway and Loch Doon. The vein infill consists mainly of broken country rock (greywacke of the Kirkcolm Formation) with strings and patches of calcite, dolomite and quartz in addition to the ore minerals. There is little sign of the halo of pervasive hydrothermal alteration of the country rock which forms such a notable feature at Leadhills/ Wanlockhead. Good samples of ore minerals (particularly galena) and vein breccia can still be found on the various spoil dumps throughout the site. The locations of several shafts are now marked by circular dry-stone dyke enclosures and should be treated with caution.

In a small quarry at the north end of the site, beside an infilled shaft or collapsed stope, thick greywacke beds strike 090° and dip north at 85°. Gently pitching flute casts visible on the bases of several greywacke beds indicate currents from the NE and prove that the succession youngs northwards. The west wall of the quarry is defined by a slickensided fault plane, trending 150° and hading NE at 55°. This is probably a continuation, or a subparallel branch, of the fault which controls the Woodhead Vein. A vertical breccia vein trending 170° can also be seen on the north side of the quarry.

4 Garryhorn Burn: black shale and chert

While in the area, the opportunity should be taken to examine the thick hemipelagic shales exposed in the Garryhorn Burn [NX 548 933] adjacent to and just upstream from Garryhorn Farm. A prominent microdiorite dyke forms the north bank of the burn for over 200 m whereas the south bank is formed by cherry black shales and grey bedded cherts, probably Moffat Shale Group strata brought up along a fault associated with the Leadhills Fault imbricate zone.

5 Craighit: Kirkcolm Formation conglomerates

The prominent hill of Craighit [NX 534 926], is reached by a rough walk of about 1 km south across country from Garryhorn Farm. The summit is composed of thick beds of greywacke and boulder conglomerate. The most abundant clasts in the conglomerate are fairly angular pebbles of vein quartz up to 6 cm across but most of the larger blocks are intrabasinal clasts of dark calcareous siltstone and sandstone. These are more easily eroded than the greywacke matrix and therefore weather to form hollows in the conglomerate. There is also an abundance of well-rounded pebbles and boulders of fine-grained acid igneous rocks, including the largest block seen (38 cm x 20 cm).

Looking NE from Craighit, the rounded Craig of Knockgrey, 4 km distant, is also formed by greywackes and conglomerates of the Kirkcolm Formation, but the strata there are intruded and baked by a small granite cupola. Beyond and towering in the background is the large rounded mass of the Cairnsmore of Carsphairn Granite with its associated aureole of hornfelsed greywackes.

6 Coran of Portmark: haematite vein

While still in the Garryhorn area, the more energetic visitor may wish to visit a vein of haematite [NX 505 911] to [NX 503 938] situated in the hills west of the Woodhead Lead Mines and reached by a rough walk of 3 km and climb of 250 m from the mines. The vein was extensively prospected in 1869 and again in 1876 by the Dalmellington Iron Company via a series of cross-cuts and small shafts which reached a depth of 21 m. It is reported that about 400 tons of poor quality haematite were raised but abandoned on the ground as uneconomic to transport to Dalmellington (Macgregor et al., 1920). In 1901, Messrs Colvilles reexamined the vein by a series of trenches and bores but the prospect was again found to be uneconomic.

The 3 km-long haematite vein runs almost N–S and cuts the contact between the Loch Doon Granite and the Ordovician country rocks (greywackes of the Kirkcolm Formation). Extensive investigations proved the vein to consist of a fissure between 0.6 and 3 m wide, filled with brecciated and haematite-stained country rock and with only small masses of haematite ore.

The site of the vein is close to the north end of the Rhinns of Kells, a long ridge of hornfelsed greywackes forming an eastern wall to the Loch Doon Granite. A visit to the vein could thus provide additional geological interest to enhance the spectacular views on this very pleasant ridge-walk.

7 Liggat Bridge, Carsphairn: greywackes and dykes

From Garryhorn Farm, return to the A713 and drive south for 2 km through Carsphairn. Park at the War Memorial near the junction with the B729 at the south end of the village and walk down to the Water of Deugh below the nearby Liggat Bridge [NX 569 929] on the A713 to examine medium-bedded coarse-grained quartzose greywackes of the Glenwhargen Formation. Bedding strikes 070° and dips 55° to the south though graded bedding in the greywackes clearly demonstrates that the succession youngs towards the north and is therefore overturned. Two prominent porphyrite (porphyritic microdiorite) dykes, of late Silurian to early Devonian age, are also visible in the Water of Deugh at this locality. The northernmost dyke is grey, 3 m wide and trends 065° whereas the other is pale pink, 1 m wide, trends 078° and displays obvious internal banding parallel to its margins. The colour variation presumably reflects slight differences in composition and/or grain size between the two dykes.

About 200 m downstream from the bridge, medium- and thick-bedded greywackes of the andesite-rich Portpatrick Formation are exposed in the bed of the river close to the north bank, though the contact with the Glenwhargen Formation cannot be seen at this locality. Beds here strike 060° and dip south at 70°though a paucity of way-up evidence precludes determination of younging direction. Abundant evidence from other parts of the Southern Uplands shows that the Portpatrick and Glenwhargen formations are interbedded.

References