MacDonald, J. G. and Heriott, A. (Eds.) 1983. Macgregor’s Guide to the Geology of Arran. Geological Society of Glasgow.
2024 note: Most grid references have been recently estimated. They do not occur in the original publication.
Excursion 1: Glen Rosa and Glen Shurig
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The main purpose of this excursion is to provide the visitor with an introduction to the geology of North Arran. It covers a wide range of formations and the localities selected for examination include sections of Dalradian, Lower Old Red Sandstone, Upper Old Red Sandstone, Carboniferous and Permian rocks, as well as sections in the Northern Granite and representative examples of some of the minor intrusions (pitchstones, felsites and dolerites) of Tertiary age. The excursion also furnishes numerous illustrations of glacial phenomena and striking views of the scenery of the North Arran mountains.
In following out this excursion from locality 1 to locality 14 the visitor is descending in the geological succession from Permian through Carboniferous and Old Red Sandstone to Dalradian, whereas in the latter part of the itinerary the sequence is an upward one from Dalradian to Permian.
1.
The general inclination of these beds is to the south at angles of about 15°. At two localities on the shore there are oval-shaped areas, 15.5 and 46m in diameter respectively, within which the conglomerates are much broken up and shattered so as to simulate the appearance of a volcanic agglomerate. There is no evidence, however, of the presence of any igneous material. Tyrrell (1928, p. 81) suggested that the phenomena may be due to the explosive effects of gases emitted from small, underlying intrusions of Tertiary age, but a more recent explanation (Astin and Macdonald 1983, pp. 47–58) attributes the breccias to a sedimentary origin associated with local faulting.
2.
The schoolhouse pitchstone belongs to the same group of porphyritic Arran pitchstones which includes the Glen Shurig dyke of locality 23 and the Tormore dykes of Excursion 13.
3.
The standing stone at the roadside at Stronach (almost opposite the school) is a slab of red Permian sandstone measuring about 3 m in height, 1.5m in breadth and about 0.6m in thickness. There are a number of such solitary monoliths in Arran, the most notable being at Auchencar farm on the west-central side of the island, which rises above ground to a height of 4.8m and may have a total length of 6.4m. Some of the standing stones, like that at Auchencar, may have originally formed part of a stone circle similar to the well-known circles on Machrie Moor and Tormore.
4.
- Red sandstone.
- Basalt—about 1.2 m.
- Felsite, spherulitic and banded, with a sharp junction against (2), about 2.1 m.
- Felsite with fragments of basalt, about 1.2m.
The basalt fragments in the lower part of the felsite have probably been derived from a second intrusion complementary to the first but not exposed (Tyrrell 1928, pp. 137–138 and fig. 19).
Continue along the footpath to the junction with the Brodick—Corrie road and note on the way the old channel of the Rosa Water to the west of Strabane. The mouth of the Rosa has shifted its position several times and its present course out to sea (like that of the lower part of the Cnocan Burn) is in part at least artificial.
5.
Red sandstones belonging to the Permian are exposed 110 m or so downstream from the bridge, so that the junction between the Carboniferous and Permian rocks must be drawn in the gap between the two exposures.
6.
Note that the outcrops are cut off to the west by a fault which shifts the Carboniferous rocks southwards to the hillside south of the String Road.
Continue along the String Road to Glen Rosa Cottages at the entrance to Glen Rosa and note how the solid rocks are nearly everywhere concealed by alluvial and glacial deposits. The pebbly red sandstones, cut by a few thin dykes, which are seen in the Shurig Water about 230m slightly north of west of the old churchyard, belong to the Upper Red Sandstone and a small outcrop of rocks referred to the same series occurs at the roadside 310 m south of the cottages. The boundaries between the Carboniferous and the Upper Old Red Sandstone and between the latter and the Lower Old Red Sandstone can only be approximately indicated on the map.
7.
On passing into Glen Rosa note the wide U-shaped valley with the steep sides and hanging valleys characteristic of glaciated regions. The floor has been levelled by the deposition of alluvial sediments through which the Rosa follows a winding course. Good sections showing river gravels and sands can be examined at a number of places where the stream has cut into former flood-plains.
Note the roughly defined higher terrace which can be traced up the glen as far as the bend in the stream just above locality 8, a distance of about 2.4 km from the mouth of the Rosa Water. This most likely was formed as outwash deposits laid down at a time when the ice stood at the moraines near locality 10.
8.
9.
10.
The Lower Old Red Sandstone sediments here have been considerably altered and are grey or greenish-grey in colour in contrast to the usual dark-red or purple hues of the series (see for example locality 7). Epidote has developed in the gritty sandstones and imparts a greenish tinge to them, while some secondary silica seems to have formed. The purple mudstones and shales have been altered to hard, blue-grey massive rocks in which the bedding is obscured. These changes are due to contact alteration in the vicintiy of the great mass of the Northern Granite. The induration and alteration of Lower Old Red Sandstone and Dalradian rocks alike have served to obscure the nature of the junction line. This was taken by Anderson (loc. cit.)to represent a continuation of the Highland Boundary Fault. Tomkeieff (1961, p. 16) referring to the discordant boundary on the hillside above Glen Rosa says that "this discordance may be due either to an unconformity or a fault". Certainly the former cannot on the local evidence be ruled out (see also p. 27).
A fine series of moraines crosses Glen Rosa except where breached and cut through by the stream. They represent stages in the retreat of the glacier which occupied the glen in Lateglacial times. Good sections of the materials composing the moraines (earthy sands and gravels with boulders and stones of all sizes) can be seen in the banks of the Rosa Water a little above locality 10.
11.
A short distance upstream and a few metres below the junction of the Garbh Allt with the Rosa Water is the schist—granite boundary. The actual contact can be closely located. The coarse granite near the contact is cut by veins of finer-grained granite and shows in places irregularly shaped drusy cavities in which good crystals of amethyst and smoky quartz (cairngorm) have been obtained. Stop at the footbridge over the Garbh Allt for a view looking northwards up the U-shaped valley of upper Glen Rosa with Beinn a'Chliabhain rising on the left and on the right the steep western face of Glenshant Hill, and the Goatfell massif beyond. Cir Mhòr (798m) dominates the head of the glen and is connected to the Goatfell ridge by the low col of The Saddle (431m) which separates Glen Rosa from Glen Sannox. Behind Cir Mhòr can be seen Ceum na Caillich, the Carlin's or Witch's Step: this deep-cut rift in the jagged outline is due to a dyke, 7.3m wide, which has weathered away more rapidly than the granite it cuts.
Ascend the Garbh Allt, keeping to its northern bank, and note the coarse granite with its characteristic jointing, well seen where the burn rushes over the falls to join the Rosa Water.
Differing views have been expressed regarding the origin of the "hanging" junction of the Garbh Allt with the Rosa. The former flows at first southwards from Coire a'Bhradain across the 300m ("Thousand Foot") Platform, but at about the 245m level, where it is joined by the burn from Coire na Cuiseig, it turns east-northeastwards and cascades steeply down to the Rosa, a fall of at least 150 metres. One interpretation attributes the discordance to widening of Glen Rosa by glacial erosion. Another explanation suggests modification of the drainage pattern. On the latter interpretation the Garbh Allt was at one time the headwaters of Gleann Easbuig which joins the Machrie Water near Glenloig, but was later captured and diverted eastwards by a small tributary flowing down the west slopes of Glen Rosa. This diversion is of recent origin and has not yet been able to cut a deep channel or gorge (for references see p. 58).
12.
13.
14.
15.
16.
A little farther downstream are good exposures of conglomerate composed mainly of pebbles of quartzite and quartz, with intercalated bands of fine-grained, deep-red sandstone and some purplish shales. These give way downstream to breccias in which large pebbles and fragments of andesitic lavas are numerous. This occurrence may be compared with that noted on Excursion 6 (locality 3). No extrusive igneous rocks are known in the Lower Old Red Sandstone succession on the east side of the island. On the west side, however, an interbedded volcanic series (hornblende- and olivine-andesites) is present in a belt stretching from near Auchencar to near Garbh Thorr. The occurrence of the breccias with igneous rock fragments in the east suggests that there, too, there was a volcanic series the denudation of which provided materials to the conglomerates.
17.
18.
The dip arrows at locality 18 (fig. 6) indicate sections where fragments of fossil plants were obtained in 1897. There are two horizons at which these occur:
- in a 0.75 m band of shale best exposed in the right bank of the stream, about 180m from its junction with the Allt Mòr;
- about halfway between (a) and the mouth of the Allt Mòr.
These plant remains are referred to Psilophyton, an early type of vascular plant characteristic of the Lower Old Red Sandstone of Angus, etc. It belongs to the Psilophytales, a subdivision of the Pteridophyta, one of the three non-seed-bearing divisions of the Plant Kingdom which includes the ferns, horsetails and clubmosses of the present day as well as many of the old Carboniferous swamp plants and trees. Psilophyton itself was a small plant possessing upright, branching, cylindrical stems some of which carried spines; the diameter of the stems varies from 1 to 7 mm. Of the Arran specimens only one could be determined specifically as Psilophyton princeps. This fossil plant was first discovered in Arran in the Allt Mòr, the tributary of the Shurig already referred to, some distance up from its mouth, by Sir A. Geikie and his students in 1882.
Follow the Allt Mòr upwards to the String Road, noting the stream exposures of hard, micaceous, reddish-purple and sometimes pebbly sandstones cut by thin basalt dykes. The dip steepens to about 80° near the road at the line where the upper limit of the Lower Old Red Sandstone has been drawn. Cross the road and ascend the hillside to locality 19; in doing so the outcrop of the Upper Old Red Sandstone, here concealed by drift, has been traversed.
19–22. At locality 19
Before leaving the hillside a traverse should be made to locality 21 to see the red, cross-bedded Permian sandstones which form the crest of the ridge between Glen Shurig and Glen Cloy. The sandstones are inclined at high angles in a more southerly direction than the underlying Carboniferous. The junction between the two cannot be closely fixed.
23.