5 Road logs

For the benefit and guidance of those who may wish, or even prefer, to become acquainted with Arran's geology by visiting and examining such rocks as are to be seen by the roadside, or within easy walking distance therefrom, the following "road logs" are offered. Drivers who do not know the island must be warned that parking, even for short periods, is often difficult or even not really feasible without creating dangerous conditions. Where appropriate, references to localities mentioned in Part 4 (the Descriptive Itineraries) are given. Parking space is usually available at the numbered localities given (see (Figure 1)).

Road Log No. 1: Brodick — The String — Blackwaterfoot — Pirnmill — Lochranza — Corrie — Brodick (A841 — B880 — A841)

1. The shore west of the ferry terminal provides good exposures of the Brodick Beds, these being cut by several dolerite dykes showing the two small areas of brecciation described by Tyrrell as "cryptovolcanic" (Excursion 1, locality 1). The larger of the latter lies near H.W.M. close to a roadside shelter.

2. With permission the pitchstone at the old school [NS 010 366] may be examined. Please do not hammer here (Excursion 1, locality 2). Proceed to the A841–B880 junction and follow the later road to the west towards Blackwaterfoot.

3. On the south side of the road at [NR 983 360] is an old quarry in steeply dipping sandstone, much shattered and cut by a NE–SW dyke. The sedimentary rocks here belong to the Lower Old Red Sandstone.

Overlooking the summit of the road on its south side is a prominent crag formed by crushed, sheared and partly amphibolitised gabbro which locally is seen to rest on baked Old Red Sandstone and to be cut by veins and dykes of granophyre and aplite.

4. At the bridge over the Allt nan Calaman [NR 969 354] fine-grained doleritic rocks are seen, both in the stream and in a roadside excavation. The little black prisms in these rocks are mainly hornblende, the white matrix being largely composed of feldspar.

5. At Glenloig farm [NR 946 351] the stream which issues from Glen Craigag exposes fine-grained diorites (Excursion 14b, locality 40). The hillside to the south provides exposures of several of the rock types found in the Central Ring Complex (Excursion 14b, localities 38–40). The imposing crags which overlook Glen Craigag from the east are formed by volcanic agglomerate.

From the end of the road to Machrie, at a remarkable old pillar box [NR 946 351], the fine scar of Craig Mhor is well seen to the east. The rock is the outer granite of the complex.

6. The above granite can be examined on the banks of the Allt nan Dris (NR 931 336; Excursion 14a, locality 1).

Southwards from Ballymichael the road runs just to the east of the largest area of alluvial deposits on the island. These are the marine alluvia of the High Lateglacial Raised Beach (the 100 Foot Raised Beach of the I. G.S. maps) covered in part by more recent fresh water alluvium.

7. At Blackwaterfoot the felsite, and other rocks mentioned in Excursion 13 (locality 9) can be examined conveniently close to the point at which the road turns north. En route to Machrie the granite hills of the northern part of the island come into view with Beinn Bharrain (721 m) in the west followed eastwards by the isolated and lower summit of Sail Chalmadale (481 m).

8. On the shore and to the north of the road junction at Machrie steeply dipping sandstones and conglomerates are cut by dykes and sill-like intrusions of dolerite, felsite, porphyry and pitch-stone, some of which are exposed only at low water. The steep dip of the sediments is particularly well seen in the raised beach cliff.

The New Red Sandstone can be distinguished from the Upper Old Red Sandstone by the presence of small pieces of agate.

9. Northwards, towards Dougrie Lodge, conglomerate and sandstone of the Lower Old Red Sandstone are well seen both on the shore and in the raised beach cliff.

The shore south of Ballykine farm [NR 870 397] provides exposures of fine-grained dark schists which occupy the core of the Loch Ranza antiform (p. 23). To the south of this locality the more gritty and, locally, pebbly schists can be proved from graded bedding to have normal dips, while to the north of it the beds are inverted.

Note: A graded bed is a sedimentary unit relatively coarse in grain size at its base and becoming progressively finer towards its top. Such a unit, or better, a succession of such units is of great value in determining the stratigraphical order in folded metamorphic rocks.

10. Just south of Imachar Point a large dyke of quartz-dolerite (of late Carboniferous age) cuts the Dalradian schists on the shore and in the raised beach. This intrusion is the most southerly known representative in Scotland of thick, generally east-west trending, tholeiitic dykes which extend in a zone northwards as far as Aberdeenshire. Similar dykes are associated with the Whin Sill of the north of England.

Rather less than 1 km north of this dyke and at the remains of a fence [NR 862 412] the schists are cut by a narrow NW—SE trending composite dyke on the NE side of which the schists have been melted locally. The blackish glass which formed when the rocks cooled can be traced for up to 20 cm from the contact (Holgate 1978, 172–4). No hammering here, please!

11. At Pirnmill gritty schists, dipping rather steeply westwards, are readily examined. For details of the rock types see Excursion 9 (locality 17) and Excursion 10 (localities 19, 22 and 23). Although gritty schists (formed from coarse sandstones and fine conglomerates which have suffered mechanical deformation and some chemical reconstitution) predominate, the shore section includes beds of finer grained material, probably originally shales, in which metamorphism is more marked. In these beds white mica gives the foliation planes a glossy appearance, while chlorite imparts a greenish colour to some of the rocks.

12. The Catacol Burn [NR 910 489], upstream from the main road, exhibits a fine section through the remains of what was probably a glacial outwash fan deposited by meltwater from the glacier which occupied Glen Catacol in Lateglacial times. This is followed by a little gorge in steeply dipping, highly baked schists cut by dykes. Rather less than 1 km from the road the granite/schist contact crosses the burn: although the actual contact is not exposed there it can be located closely (Excursion 9, localities 3–5).

13. As the road is followed from Catacol to Lochranza it can be seen that the dip of the schists, here mainly gritty, changes from northerly to southeasterly as the axis of the fold, known as the Catacol Synform, is crossed (Excursion 10, locality 29; fig. 13).

14. The car park at North Sannox Bridge [NR 994 468] is a suitable location from which to examine (a) upstream from the bridge, the Dalradian schists and the sedimentary features exhibited by them (Excursion 6, locality 11); (b) approximately 1 km west of the bridge a well exposed granite/schist contact (Excursion 6, locality 10); and (c) downstream from the bridge, the pillow lavas and associated black shales and cherts of the Ordovician succession (Excursion 6, localities 14–16).

From Sannox southwards to Brodick Old Quay, the shore, and in places the adjacent raised beach cliffs, allow ready examination of, successively, the upper part of the old Red Sandstone (Excursion 6, locality 22; Excursion 4, localities 1–3); the Carboniferous succession in this part of the island (Excursion 4, localities 4–12); the Corrie Sandstone facies of the Brodick beds intruded by dykes which are especially numerous opposite Corrie Terraces [NS 026 424] and southwards from Rudha (or Rubha) Salach to the Old Quay. Parking varies from easy to impossible.

It is suggested that the following parts of the Corrie shore section are likely to prove of most interest to the visitor who is short of time.

1. The conglomerates, cornstones. volcanic agglomerate etc.. at the Upper Old Red Sandstone/Lower Carboniferous junction (NS 023 441; Excursion 4, localities 3 and 4).
2. The olivine basalts, with phenocrysts of augite and olivine, north of the school. Weathering into spherical masses is shown in these rocks.
3. The Corrie Limestone, seen in mines west of the road opposite the harbour. For details and a warning which should be heeded see Excursion 4 (locality 8).
4. The sandstones, thin limestones and associated strata a little north of the Corrie Hotel. These rocks are of Carboniferous Limestone age.
5. In the little bay just south of the hotel, sandstones, fireclays and shales belonging to the Upper Carboniferous outcrop. Fossil plants and fresh-water "mussels" are locally present.
6. The unconformable junction of the dune-bedded Corrie Sandstone (brick red) on Upper Carboniferous sandstone (whitish, with contorted bedding) occurs a little south of the Corrie Hotel [NS 026 431]. A thin sheet of basalt cuts the Carboniferous a little below the unconformity.

Road Log No. 2: Brodick — Lamlash — Whiting Bay — Kildonan — Bennecarrigan (A841) returning to Lamlash via the Ross Road (unclassified)

En route to the summit 3 km south of the ferry terminal, red sandstones and boulder clay are commonly exposed on the east side of the road. Dykes can also be seen, e.g. just south of the Corrygills road (Excursion 3, locality 1).

15. At the summit [NS 018 334] where a Forestry Commission road joins, the analcime-dolerite of the Clauchlands intrusion is exposed rather poorly (Excursion 3, locality 11). From this point there is a good view of the Northern Granite mountains (Plate 1), and to the southeast, Holy Island and Lamlash Bay. Holy Island is again well seen from the road to Whiting Bay. That it is formed of columnar igneous rock (trachyte) overlying red sandstone can readily be appreciated.

16. Between Kingscross Bridge [NS 043 280] and the road to Auchencairn [NS 044 278] the following section can be examined on the west side of the road. Just south of the bridge, and near road level, coarse analcime-dolerite is seen. This is followed southwards by drift beyond which is an interesting section comprising a composite felsite-quartz-dolerite sill overlying and locally in transgressive relationship to sandstones which are intruded by an altered dolerite. Another stretch showing drift only is followed to the south by an extensive exposure of analcime-olivine-dolerite. Just north of the Auchencairn road junction this intrusion includes a few small rafts of sandstone.

17. Especially at low water the shore at the north end of Whiting Bay [NS 047 270] shows a fine example of a plexus of intersecting dykes, crinanitic and tholeiitic types being represented. The sediments are soft red sandstones with ripple marks and other sedimentary structures.

18. (with care) Creag Dhubh [NS 049 250] is formed by a neat example of a composite sill. Just under 10m thick it, and the sandstone into which it has been intruded, dip at 20° to the WSW. The body is composed of a central layer of quartz-dolerite flanked by felsitic layers, with merging contacts.

19. A short distance south of the houses at Largybeg variegated manly sediments (which have yielded microfossils of Triassic age) are cut by a rather irregular sill of dark, fine-grained quartz-dolerite. Above lies the rudely columnar Dippin sheet of coarse analcime-olivine-dolerite. This rock is exposed in two small roadside quarries near the 12 mile stone. In both quarries the sheet is cut by dolerite dykes.

By taking the unclassified road to Kildonan part of the Arran dyke swarm is readily seen, especially at low tide. In this area the dykes run mainly NNW—SSE and they vary greatly in width.

20. A particularly massive dyke (of crinanite) occurs near the point [NS 019 210] at which the road turns inland (see Excursion 12, locality 9). A short distance offshore lies Pladda island, mainly occupied by a sill of quartz-dolerite; the low cliff on this island is the result of marine erosion at the time of the Main Post glacial Shoreline when sea level was higher than at present (see pp. 63–66). In the distance, to the southeast, lies Ailsa Craig, an island formed entirely of a plug of alkali microgranite with petrographic affinities to the Holy Island trachyte.

En route past Auchenhew look out for Auchenhew Hill and Levencorroch Hill, lying to the north of the road. They are capped by a dolerite sill which has been cut through by a deeply incised stream, the Levencorroch Burn.

21. Close to the junction of the private road to Craigdhu farm with the main road [NS 000 213] is an old quarry in the central, acid member of the Bennan composite intrusion. This is a quartz-feldspar-porphyry composed of phenocrysts of feldspar (usually dull white) and quartz (dark, glassy) in a finer-grained pale-coloured matrix. Nearby, the western doleritic marginal member outcrops by the farm road (Excursion 12, locality 12), dipping steeply westwards.

The main road is now followed via Shannochie and Lagg to the south end of the Ross Road at the church [NR 943 220] near Bennecarrigan farm.

Rather more than a mile north of the church in a quarry [NR 947 239] is a badly preserved felsite. In the vicinity of the bridge over the Sliddery Water south of Glenree farm [NR 946 252], or Burrican as it is named on older maps, the stream provides sections in two felsites, one of which is associated with pitchstone.

Almost on the summit of the road and on its northwest side is a small quarry in which an "igneous breccia" composed of angular fragments of basic rocks are included in a matrix of pale-weathering silicic rock. Tyrrell (1928, p. 140) has sketched a specimen from this locality. Please do not hammer here: loose material can be readily had from the adjacent hillside.

In the vicinity of the 8 mile stone [NR 994 300] there are good exposures of cross-bedded, sometimes pebbly, sandstone cut by dykes of dolerite and felsite. The old quarry just east of the milestone shows sandstone cut by four dykes one of which is a porphyritic dolerite and the other, felsite. The view looking eastwards from a little farther down the road shows that, on both sides of the glen, the base of the Monamore intrusion slopes evenly downwards towards the east. The most westerly crag on the north side of the road has a banded appearance due to the presence of rafts of baked sandstone in the igneous rock.

22. The picnic area within Forestry Commission ground a short distance west of Croc is close to the eastern margin of the Monamore analcime-olivine-dolerite intrusion. A traverse of the Monamore Burn shows highly-baked conglomerate followed upstream by the intrusion which is cut by several dolerite dykes and includes screens of sandstone. At the remains of an old footbridge there is a small body of quartz-felsite. Farther upstream a much larger body of granophyric felsite occupies the banks and bed of the burn. The rock of the main intrusion has smooth, rounded surfaces in which pot holes have been formed.

Intrusions of felsite and pitchstone are to be seen cutting sandstone in the Monamore Burn almost opposite the road to Glenkiln (Glenkill on older maps).

References