Chapter 12 Old Red Sandstone of Foula

Introduction

Foula is a pear-shaped island, 3.5 miles (5.6 km) long from north to south and 2.5 miles (4 km) wide from west to east (Figure 21). It is situated in the Atlantic Ocean 14 miles (22 km) WSW of Wats Ness in the Walls Peninsula and has a prominent skyline, which, when viewed from the mainland, presents three steep north-facing escarpments with relatively gentle south-facing dip slopes. The most northerly escarpment forms Soberlie Hill, 721 ft (220 m) high; the central one has three peaks which are, from west to east, The Kame (1220 ft, 372 m), The Sneug (1373 ft, 406 m) and Hamnafjeld (1126 ft, 343 m). To the south of these the island is traversed by a west-north-west trending glacially moulded valley, known as The Daal. South of this lies The Noup (803 ft, 245 m), a hill which has a less prominent north-facing escarpment than the others.

The greater part of Foula is formed of relatively soft sandstone with subordinate shale and siltstone bands. This sedimentary series appears to be of Middle or Upper Old Red Sandstone age and has a total exposed thickness of about 6000 ft (1800 m). The sandstone forms prominent vertical cliffs all along the west coast of the island. These have a maximum height of 1220 ft (372 m) at The Kame, which forms one of the highest sea-cliffs in the British Isles.

The Old Red Sandstone sediments are bounded in the east by a 0.5 mile (800-m) wide strip of metamorphic rocks and the two formations are separated from each other by a slightly curving north-north-west trending fault. At its outcrop on the north coast this fault is inclined at 60° to the west, but at its southern coastal outcrop it is inclined at only 35° to the south-west.

Owing to its unique position and prominent topography Foula attracted many of the early geological investigators who visited the northern isles. There are accounts of the topography and geology of Foula by Jameson (1798), Hibbert (1822), Nicol (1844) and Gibson (1877). The first comprehensive geological account of Foula is by Heddle (1878, pp. 124–30). More recent accounts dealing with the geology of Foula are by Peach and Horne (in Tudor 1883, p. 395), Finlay (1926, pp. 564–5), and Wilson (in Summ. Prog. geol. Surv. 1934, pp. 71–3).

Field relationships

The Old Red Sandstone of Foula consists of approximately 6000 ft (1800 m) of relatively soft, predominantly medium-grained, grey to buff sandstone, with small isolated pebbles of quartz and granite and with a number of partly or predominantly argillaceous or silty horizons. Many of the sandstones are cross-bedded with a consistent dip of foresets towards the east to south-east sector. The structure is simple, consisting of a very open, southward plunging syncline. Close to the fault which separates the sediments from the metamorphic rocks to the east, the dip of the strata is locally much steeper and the strike is roughly parallel to the fault.

Though the sediments of Foula display a remarkable uniformity throughout their thickness, it is possible to recognize the following nine more or less distinct members (Figure 21):

These nine groups have been mapped along the west coast and through the centre of the island. Though small isolated rounded pebbles are present in the cross-bedded sandstones throughout the sequence lenses of extraformational conglomerate with abundant large pebbles are absent. The pebbles appear to be of uniform composition throughout and consist of white, yellow and amber-coloured quartz, quartzite and, rarer, red granite. The smallest pebbles are, in many instances, microcline.

On the east coast of Foula just south of the junction with the metamorphic rocks at Shoabill, there is a banded group of sediments, which presents some features not found in any of the argillaceous or banded groups enumerated above. The lower 60 ft (18 m) of this group consist of alternate sets of siltstone, up to 4 ft (1.2 m) thick, and medium-grained sandstone. The siltstones are reddish brown or pale green in colour and contain thin laminae and pseudo-nodules of fine-grained sandstone. Desiccation cracks and polygons are abundant and individual polygons are up to 2 ft (60 cm) in diameter. The sandstone infillings are up to 3 in (7.6 cm) thick. Several of the sandstone posts contain mudstone 'chips' which are the cracked and dried-up remains of thin layers of mud. A bed of sandy siltstone at the top of this alternating sequence contains a 3-ft (90-cm) thick band, which exhibits intensely convolute and slumped lamination. This contains recumbent 'slump balls' which are up to 1 ft 6 in (45 cm) thick, and have their convex side facing south-east, suggesting that their shape was to some extent determined by lateral slipping of unconsolidated material towards the north-west, which is contrary to the prevailing direction of current movement elsewhere on the island. The slump-bedded horizon is overlain by up to 40 ft (12 m) of buff-weathering medium-grained sandstone with partings, up to 18 in (45 cm) thick, of greenish grey siltstone. This is, in turn, succeeded by 20 ft (6 m) of massive greyish green unlaminated poorly graded clayey sandstone full of angular clasts of sediment up to 1 in (2.5 cm) long, but in some instances reaching a length of 8 in (20 cm). The clasts consist principally of siltstone and mudstone and the entire bed has the characteristics of a mudflow conglomerate (see Bluck 1967, p. 144).

The junction between the sedimentary and metamorphic rocks at Shoabill has been shown as a fault on the one-inch and six-inch geological maps. The actual junction is a well-defined shear-plane inclined at 35° to the south-west and underlain by up to 6 ft (1.8 m) of sheared metamorphic rock. It is, however, overlain by up to 10 ft (3 m) of sheared 'conglomerate' which contains sub-rounded pebbles and blocks of gneiss. The latter are up to 2 ft 6 in (75 cm) in diameter and are set in a matrix of sheared gneissose material. It is not certain if this rock is a true conglomerate or if the partial rounding of its clasts is entirely due to shearing. The author favours the latter interpretation. The upper junction of the 'conglomerate' is undulating, but sharply defined and cut by a number of small faults. It is overlain by hard, slightly reddish medium-grained sandstone, which contains, just above its base, a 1 ft 6 in to 3 ft (45–90 cm) thick concordant sill-like mass of brittle, fine-grained yellowish rock, which in the field resembles a felsite, but is in thin section seen to be an intensely sheared sandstone. The junction between metamorphic rocks and sediments in this area is without doubt a faulted one, but if the 'conglomerate' described above is of sedimentary origin and is not a true fault breccia as the author believes the present junction must be close to the original unconformity between the two groups. If that were the case the subsequent fault movement would here appear to have been more or less along the unconformable junction.

Conditions of deposition

The greater part of the Foula succession appears to be composed of fluvial sandstones laid down by relatively fast-flowing, possibly braided, rivers. The lower part of the sequence was probably deposited by currents from the west to north-west sector, and the upper part by currents from the west or south-west. The presence of red and purple siltstones and mudstones with sand-filled desiccation cracks in the lower half of the sequence, well seen on the north and east coasts of the island, suggests that there were some periods with a quieter fluvial regime, possibly dominated by meandering rivers with flood plains on which top stratum deposits were preserved. The uniform composition of the sandstones (p. 176) and their scattered pebbles (p. 174) throughout the sequence suggests that the source area remained much the same during the entire period.

Petrography

The medium- to coarse-grained pebbly sandstones of Foula are generally bi-modal with scattered well-rounded pebbles, 3 to 1 cm in diameter, set in a well-graded arkose or feldspathic sandstone (cf. Pettijohn 1957, p. 291). The latter is composed of subrounded to subangular grains commonly ranging in diameter from about 0.75 to 0.1 mm ((Plate 8), fig. 6). The quartz-feldspar ratio varies from 50:50 in the coarsest sandstones to 70:30 in the fine-grained sandstones. Quartz grains are generally more rounded than feldspars, which consist of both untwinned potash feldspar and rarer microcline, as well as sodic plagioclase. Though the feldspar grains are, in some instances, patchily seticitised or kaolinitized, they are commonly fresh. Other grains consist of felsite, and more rarely, fine-grained sediments such as siltstone and clayey limestone. There are also small grains of apatite, zircon and tourmaline and small rounded grains of opaque ore minerals. Both muscovite and biotite, in some cases mantled by chlorite or partially altered to chlorite, are present in all sections and form up to 20 per cent of the total volume in the flaggy partings. The detrital matrix composed of an aggregate of clay minerals and chlorite normally forms less than 5 per cent of the total volume. In some sandstones of all grain sizes, however, a calcareous cement forms up to 15 per cent of the rock and has patchily replaced the feldspar grains.

The fine-grained flaggy sandstones normally have grains ranging from 0.5 to 0.08 mm in diameter and have a considerably higher quartz-feldspar ratio (70:30 to 80:20) than the medium-grained sandstones, as well as a higher proportion of partially chloritized mica flakes. The dark grey ungraded sandstone with scattered pebbles of sediment, exposed on the shore south of Shoabill, close to the junction with the metamorphic rocks (p. 174), is unique in Foula. It consists of widely scattered subangular to subrounded grains of quartz and feldspar (quartz-feldspar ratio 9:1) together with subrounded fragments of siltstone and clayey limestone which range in size from over 5 to 0.03 mm, as well as some grains of felsite and tourmaline. The matrix forms over 25 per cent of total volume of the rock and is composed of a matted mass of mica flakes set in an indeterminate brownish aggregate. This highly ungraded immature sediment contrasts strongly with the generally well-graded sandstones of Foula.

Structure

The sedimentary rocks of Foula form an open syncline plunging at 20° to 30° to the south. The eastern limb of this syncline steepens appreciably close to the eastern boundary fault and the strike of the beds in a 109 yd (100 m) wide zone bounding the junction is roughly parallel to this fault. In the exposures along the north shore where the fault is inclined at 70° to W10°S, the adjoining sediments are intensely shattered and cut by a number of small faults which trend N60°W and range in inclination from 60° to 85° to the south-west, as well as by several faults which are sub-parallel to the main fault.

Apart from the faults associated with the eastern boundary fault the Foula sediments are relatively free from major faulting. Two north–south trending faults with small throws and westward inclinations cut the north coast at Trolli Geo and a small geo 150 yd (135 m) farther east. These have small downthrows to the west and both appear to die out southwards.

The sediments throughout the island are cut by several systems of closely spaced faults with very small displacements. These are well seen on the large expanses of bare bedding planes such as those forming the south-east side of Logat Stacks. On the latter three intersecting sets of minor faults and joints are developed. These are :

1. Strike faults, trending E25°S, spaced 10 to 15 yd (9–14 m) apart with small downthrows (1 to 3 ft, 0.3–0.9 m) to the south-south-west.
2. Very small faults trending E30°N with very small downslip to south-south-east.
3. Small faults trending N22°E, about 25 yd (23 m) apart with small displacement to east-south-east.

Intense jointing and possible small-scale faulting is also well seen on the shores of South Ness, where the dominant trend is N10°W. On the hill slope north-east of the Wick of Mucklaberg the intersection of the south-south-west dipping bedding planes in the Daal Flaggy Sandstone, which are parallel to the steep undercut hillside, with west-north-west trending joints provides conditions ideal for landslides. The Smallie, a deep cleft over 250 yd (230 m) long and 6 ft (1.8 m) wide and as much as 100 ft (30 m) deep (Heddle 1878, p. 128) is the most prominent of a series of joints which have been opened by the down-dip slipping of the sandstone to the south.

References

BLUCK, B. J. 1967. Deposition of some Upper Old Red Sandstone conglomerates in the Clyde area; A study of the significance of bedding. Scott. Jnl Geol., 3, 139–67.

FINLAY, T. M. 1926. The Old Red Sandstone of Shetland. Part I: South-eastern Area. Trans. R. Soc. Edinb., 54, 553–72.

GIBSON, G. A. 1877. On the Physical Geology and Geological Structure of Foula. Rep. Br. Ass. Advmt. Sci, 90.

HEDDLE, M. F. 1878. The County Geognosy and Mineralogy of Scotland, Orkney and Shetland. Truro.

HIBBERT, S. 1822. A Description of the Shetland Islands. Edinburgh.

JAMESON, R. 1798. An Outline of the Mineralogy of the Shetland Islands, and the Island of Arran. Edinburgh.

NICOL, J. 1844. Guide to the Geology of Scotland. Edinburgh.

PETTIJOHN, F. J. 1957. Sedimentary Rocks. New York.

TUDOR, J. R. 1883. The Orkneys and Shetland: Their Past and Present State. London.

SUMMARY OF PROGRESS 1934. Mem. geol. Surv. Gt Br. Summ. Prog. for 1933.