Excursion 2: Brodick to Lamlash Bay by the coast
((Figure 6), localities 1a–3a)
((Figure 7), localities 4a–19a)
The objects of this excursion are: (1) to study the rock-types present in the two sub-divisions of the New Red Sandstone known as the Brodick Beds and the Lamlash Beds (table on p. 37); and (2) to examine some of the dykes and sills traversing the Permian along this part of the coast. Magnificent, almost continuous exposures are available in the cliffs at the back of the low raised beach platform and on the beach itself.
The great majority of the dykes along the coast here occur as vertical or nearly vertical walls of rock. In some cases, however, they are steeply inclined, usually to the northeast (see for example locality 10a). The dykes sometimes weather more rapidly than the rocks they cut and in such cases form hollow, trench-like features; sometimes they have proved more resistant to erosion than the strata around them and stand up as walls projecting above the surface; sometimes again it is the baked and hardened sediments immediately adjoining a dyke that are left projecting as two parallel ridges (see locality 5a).
1a. [NS 0234 3591] For some 300m eastwards from the pier the shore has been adversely affected by industrial development of the raised beach; at high tide some of the rocks mentioned below cannot now be examined. East of the pier the shore is occupied by massive, red, cross-bedded sandstones. Note the resemblance of these sandstones, in their characteristic large-scale dune-bedding and rounded ("millet seed") quartz grains, to the Corrie Sandstone of the Corrie—Brodick shore (Excursion 4). Gunn (1903, p. 69) regarded the masses of sandstone on the shore here (see also locality 4a) as portions of the Corrie Sandstone faulted in among the breccias, but Tyrrell and Barrett (Tyrrell 1928, p. 81) interpret them as "fossil dunes, or groups of dunes, against and around which the breccias were banked up and interbedded". Note the gashes in the conglomerates of the raised beach cliffs caused by differential erosion of the dykes.
2a. [NS 0238 3588] The red sandstones of locality la end abruptly against a north-northwesterly fault along which a dyke has been intruded: beyond this conglomerate and breccias appear in force, occupying the foreshore continuously for a distance of some 460 m. Intercalations of sandstone showing good examples of rounded quartz grains occur. The 6m NW—SE trending dyke cutting the breccias should be examined; it is an olivine-dolerite containing laborador-ite feldspar, purplish augite, olivine, some grains of iron ore and a little analcime. Note how the darker minerals (olivine, augite) are often segregated in patches giving rise, on weathered surfaces, to a characteristic spotted appearance.
3a. [NS 0308 3580] The shore section here shows conglomerates with many lenticular intercalations of red, "millet seed", dune-bedded sandstone. Some of the pebbles found in the sandstones exhibit wind-faceting. The main interest of this locality, however, is the occurrence in the cliffs of a considerable thickness of breccia containing large numbers of sub-angular or partly rounded boulders and pebbles of a vesicular, red-speckled, olivine-basalt lava. This basaltic horizon, with which sandstones carrying ashy material are associated, has been recorded elsewhere at the same position in the Brodick Beds and its occurrence suggests that contemporaneous volcanoes were active nearby. Some of the basalt lava pebbles resemble types common among the Permian lavas of Central Ayrshire.
The most conspicuous of the dykes on the shore is an 8.5 m wide intrusion of olivine-dolerite (crinanite type) running in a west-northwesterly direction.
This part of the shore section ends against a low-angled NE–SW directed fault which crosses it a short distance west of the little lagoon on the raised beach.
4a. [NS 0373 3551] It is along this part of the coast that the highest cliffs occur. Both cliffs and foreshore are formed of massive, red, cross-bedded sandstones which, it has been suggested, represent the remains of a group of dunes (c.f. locality la). The general inclination of the sandstones is about 15° to the south-southeast. Note the 11 m wide, NW trending dyke of olivine-dolerite cutting the sandstones about 270m east of the little lagoon; it is inclined to the NE at about 70°.
5a. [NS 0386 3543] Some of the dykes here show indurated sandstones on their margins standing up as ridges. Good examples of dune-bedding can be examined in the cliffs a little to the east. Note the large erratic of coarse granite on the shore.
6a. [NS 0414 3524] Dune-bedded red sandstones with some intercalated bands of breccia occur along the shore, dipping to the south-southeast at angles of between 25° and 30°. Examine the broad intrusion of felsite traversing shore and cliffs in a general east–west direction. The felsite, some 24 m thick, is a light-coloured, faintly yellowish rock with a characteristic fissile appearance due to the development of joints parallel to its surface. It is inclined at about 60° to the SSE. When examined under the microscope it shows scattered microphenocrysts of quartz set in a fine-grained groundmass of intergrown quartz and orthoclase feldspar. A thin pitchstone is developed at the base of the felsite, where indurated breccia separates it from a thin tholeiitic dolerite sill.
7a. [NS 0421 3518] Corrygills Point. Here thin-bedded, red sandstones with pebbly bands, dipping south-southeast at from 20° to 25°, occupy the coast and are cut by a number of basalt dykes. Note the different orientations of the latter.
8a. [NS 0426 3487] Here, where the path from North Corrygills leads down to the shore at Dunan, the raised beach feature widens out and there is a break in the long line of old sea-cliffs. Note the large granite erratic on the low raised beach. Portions of two higher terraces can be made out, the upper corresponding to the High Lateglacial Raised Beach (see pp. 63–65).
9a. [NS 0444 3463] Red sandstones with bands of breccia continue to occupy the shore. Note how at a point 125 m southeast of the mouth of the Corrygills Burn a 1.2 m dyke running NNE has cut across and shifted a 30 cm dyke directed ENE. Splendid examples of dune-bedding can be seen along this part of the coast.
10a. [NS 0448 3455] Note here the beginning of the "Mile Dyke" which runs along the foreshore in a southeasterly direction for about 1.3 km. Along the greater part of its course it forms a trench-like feature and can be seen in places to be steeply inclined to the northeast. It is an olivine-dolerite.
11a. [NS 0462 3432] About 450m southeast of the mouth of the Corrygills Burn a felsite sheet, associated with pitchstone on its lower (northern) margin, crosses the shore in a west-northwesterly direction. Where best exposed on the shore it is some 6 to 7.5 m thick but it decreases rapidly in thickness seawards to a metre or so. It has been intruded along the bedding of the red sandstones, here dipping to the SSW at between 25° and 35°. Its upper margin is highly irregular and appears to be much more steeply inclined than the sediments. Its sill-like form is seen when traced inland, where it cuts the raised beach cliff and appears in both branches of the Corrygills Burn about 1.6 km to the west-northwest (Excursion 3, localities 2 and 3). It is a tough dull greenish-grey rock crowded with spherical bodies up to 3mm in diameter, which show a dark centre and often a dark border. A polished face shows the spherulites standing out as light-coloured spots against the dull groundmass of the rock. Each spherulite has a radial structure, due to the individual crystal fibres being set round a common centre. It should be noted that there is a good deal of variation in the extent to which the spherulitic structure is developed; sometimes the spherulites are closely packed and sometimes there is a considerable development of irregular granular aggregates which give the rock in such cases a spotted appearance. The lower part of the sill is banded and fissile.
This spherultic felsite is a well-known rock in geological literature and has often been described (see for example, McCulloch 1819, vol. ii, pp. 404–406; Teal 1888, plate 39, fig. 1; Tyrrell 1928, pp. 212–230).
The pitchstone which accompanies the felsite on its northern or lower margin has been described as the "small pitchstone" of the Corrygills shore as opposed to the "great pitchstone" of locality 15a. Where the sill traverses the raised beach cliff the pitchstone is about 60 to 90 cm thick; it appears again in two pockets in the upper surface of the sandstone underlying the sill, near high-water mark. Tyrrell (1928, p. 212) describes the pitchstone in one of these pockets as having "a wrinkled appearance like ropy lava". The pitchstone is also spherulitic and presents a strikingly beautiful appearance under the microscope, the spherulites and indeed their individual fibres standing out against the dark glassy matrix which forms the bulk of the rock.
12a. [NS 0483 3402] Quartz-breccias and coarse sandstones containing numerous but generally small agate pebbles occupy the coast between localities 1 la and 12a (overenthusiastic collecting over the years has resulted in the removal of a great deal of the agate; visitors are requested not to collect here, merely to note the presence of the agate pebbles, if they can find any, and pass on). These rocks may be regarded as forming the highest member of the Brodick Beds. In addition to the agates they contain pebbles of acid igneous rocks (granite, felsite, etc.) of Old Red Sandstone types, as well as the usual assortment of fragments of quartzite, vein-quartz and schist. Barrett (1925, p. 268) has recorded the occurrence of pebbles of andesitic lava with the agates. The assemblage of pebbles suggests derivation from a land area composed mainly of Dalradian schists but also in part of Old Red Sandstone and possibly Carboniferous rocks. Agates are common infillings of the vesicles (cavities in lavas produced by escape of gases) in the lava flows of Lower Old Red Sandstone age in Ayrshire and elsewhere on the mainland.
13a. [NS 0489 3393] Examine sections in red sandstones dipping southwest ter south-southwest at an angle of about 15°. Note the relatively small size of the included pebbles as compared with those found farther north along the coast. The beds from here onwards to Lamlash are regarded as falling into the Lamlash Beds (see table p. 37). A pale-coloured dyke of quartz-porphyry traverses the raised beach cliff but appears to end against the "Mile Dyke". It runs in a general easterly direction and may be connected with the intrusion of quartz-porphyry forming Dun Dubh (Excursion 3, locality 5).
Fine views may be obtained from this part of the shore of the northern escarpment of the great Clauchland Hills sheet.
14a. [NS 0499 3382] Note here the yellowish-weathering dyke of quartz-porphyry 1.5 to 5.5 m broad, running in an east-northeasterly direction.
15a. [NS 0510 3360] This is the locality of the well-known pitchstone sheet of the Clauchlands shore. It is sometimes referred to as the Corrygills pitchstone or as the "great pitchstone" on the Corrygills shore in contra-distinction to the thinner pitchstone intrusion accompanying the spherulitic felsite of locality 11a. It has frequently been mentioned or described (eg. Tyrrell 1928, pp. 213, 230, and references therein). The pitchstone is exposed at the base of the crags, where it is inclined to the SSW at much the same angle as the sandstones associated with it (30°). Note should be taken of its slightly transgressive character, of its rudely columnar structure, and of the somewhat decomposed bands present near its margins, especially the lower one. Its thickness is generally given as 3.7 to 4.3m but it may reach as much as 6m. It can be followed along the foot of the cliff for some 135 m and reappears on the foreshore, 185 m or so farther south, for a distance of 15 m, approaching within a metre or so of the "Mile Dyke". It is a dark, bottle-green, glassy rock, with a characteristic resinous lustre. Under the microscope it shows a pale yellowish, glassy base through which are scattered needles or microlites and feathery or arborescent growths composed of very small crystallites. A few well-shaped crystals of quartz are also present. There are thus four aspects to this rock; the crystals, the needles or microlites, the much smaller crystallites, and the glassy base.
Pitchstone flakes have turned up at a number of prehistoric sites in Scotland as far afield as the Tweed Valley and Caithness. Many of these closely resemble Arran pitchstone, in particular the "great pitchstone". Rock from this locality, and probably from some of the other pitchstone localities in Arran, was evidently traded extensively by man as far back as the third millenium B.C.
16a. [NS 0539 3336] The rocks here are all red sandstones belonging to the Lamlash Beds. As they approach the margin of the Clauchlands sheet their dip increases to 50°, they lose their red colour and at the contact they are indurated. Note the large granite erratic on the shore.
17a-18a. Between these localities the Clauchlands sheet occupies the shore continuously. At 17a [NS 0562 3309] the contact is seen to be partly vertical and partly to dip to the SSW. Against the country rock, as well as around included blocks, basaltic and doleritic "contact rocks" (see p. 46) are developed. Traced upwards, these give way to much coarser types: some 6m above the contact the sheet is formed of a gabbro carrying very coarse grained patches with large crystals of black augite and cut by pale-coloured veins. At locality 18a [NS 0568 3276] the upper margin of the intrusion is concordant with the overlying baked sandstones which dip at 45° to the south-southwest.
The Clauchlands sheet may be described as a coarse-grained analcime-olivine-dolerite (crinanite), carrying, in addition to the two minerals named, labradorite feldspar and augite, together with apatite and iron ores. There is much variation in the proportion of mafic minerals (olivine and augite) and the lighter constituents (feldspar and analcime), a variation correlated with changes in the colour of the rock. In some exposures dark-green olivine is extremely abundant. The analcime is often replaced in part or completely by zeolitic minerals due to secondary changes which took place after the main consolidation of the rock.
19a. [NS 0504 3276] The sections along the shore at or near this locality are red, generally flaggy and cross-bedded sandstones with pebbly bands. Note how the sandstones become bleached along their contacts with the northwesterly basalt dykes.
References