The Old Red Sandstone of Shetland.

By B. N. Peach, A.R.S.M., F.G.S., and John Horne, F.G.S.

(Read before the Royal Physical. Society, 19th February 1879.)

The Old. Red Sandstone of Shetland, though inferior in development to that of Caithness or Orkney, claims special attention on account of the interesting proofs which it affords of the previous extension of that formation, as well as the remarkable history of the volcanic phenomena which characterised that period. Though the areas now occupied by the sedimentary rocks are limited in extent, there can be little doubt that they convey but a faint impression of the original extension of this formation in the Shetland Isles. The fine mural precipices of Old Red Sandstone which are visible in some of the islands, notably in Bressay and Foula, furnish a striking proof of the importance of the relics which have escaped denudation.

As far back as the year 1811, Dr Fleming pointed out the occurrence of vegetable impressions in the sandstones of Bressay, in a paper published in the Memoirs of the Wernerian Society, vol. i., entitled, "A Mineralogical Account of Papa Stour." Since that time numerous plant remains have been found in the members of this formation at different localities in Shetland.

In 1853 Dr Hooker referred some plant remains collected from the Lerwick sandstones, by the Bight Hon. Henry Tuffnell, to calamites; while, in 1858, Sir Roderick Murchison intimated the discovery of Esthcria in the Lerwick beds, which linked these strata with those of Caithness and Orkney.

Dr Hibbert, in his admirable work on the Shetland Isles, page 2 laid down approximately the limits of the different Old Red Sandstone areas. In 1877 Dr George Gibson published a thesis descriptive of these rocks; and in 1878 Professor Geikie, in his exhaustive monograph on the Old Red Sandstone of the North of Scotland, described the relations of the Shetland representatives to the other members of this formation in Orkney and Caithness. He refers specially to the proofs of volcanic activity in Papa Stour, the geological structure of which is given in detail.

During the summer of 1878 we made some traverses in the islands for the purposes of determining the disputed question of their glaciation, and in the course of these traverses we felt it to be necessary to map out with as much minuteness as time would permit the boundaries of the various Old Red Sandstone areas, on account of the important evidence which they furnish regarding the movements of the ice in the glacial period. We were induced to work out the order of succession on the eastern side of the Mainland as well as the relations of the associated contemporaneous and intrusive igneous rocks in the western districts.

While pursuing this object we were fortunate enough in discovering in the Walls district a rich series of plant remains in rocks which have been hitherto considered as forming part of the metamorphic series. Mr C. W. Peach has kindly named the plant remains for us, and from his description it is evident that they are identical with the plants found in the Old Red Sandstone formation of Caithness and Orkney. The rocks in which they are embedded must therefore be relegated to that period, though they seem to have undergone a considerable amount of metamorphism.

In this paper we propose to give a brief sketch of the different areas occupied by these rocks in Shetland, indicating as far as possible the succession of events and the relations of the contemporaneous and intrusive igneous rocks. We shall endeavour to show that during the early phases of that period, the Mainland, which is the largest of the Shetland group, must have formed an island somewhat smaller in size than now, round whose coast-line the basement breccias accumulated; but eventually as the land slowly sank beneath page 3 the sea-level, the higher deposits overlapped on to the gneissose rocks, and ultimately buried them. The long process of denudation to which the Shetland archipelago has been subjected has removed in a great measure the greater portion of these deposits; those which now remain being protected in part by the hard gneissose rocks against which they have been brought by dislocations of the strata.

Owing to a series of faults which form the boundary line over a great part of Lerwick, Quarff, Conningsburgh, and Dunrossness, it so happens that different zones in the fore-going vertical section are brought into conjunction with the gneissose rocks. The true base of the series, however, is exposed in the neighbourhood of East Quarff on the north side of the bay, and again on the south side towards Flaoabister, while still another locality in which the basement breccias occur, is to be seen near Loch Spiggie in Dunrosness. In each of these localities the breccia varies in chancier according to the nature of the underlying rock. Perhaps the finest exposures of this breccia are to be seen on the hils to the north of East Quarff, and round the shore towads Brenista Ness. Here it forms well marked cliffs, the beds being inclined to the east at an angle of 25°, and resting on a highly eroded platform of the metamorphic schists. The prominent ingredient in this deposit round the bay of Est Quarff is the underlying rock, which consists mainly of gray schists. Blocks of this material sometimes measure time feet across, retaining their angular edges and showing little trace of aqueous action.

Near Loch Spiggie, in Dunrossness, the fragments mainty consist of pink syenite and serpentine derived from the under- page 4 lying rocks which form the floor on which the breccia rests in that neighbourhood. The occurrence of fragments of these rocks in the basement breccia is of great moment, as it helps us to fix the age of the pink syenite and serpentine between Quendale Bay and Loch Spiggie.

In the bay west of Brenista, the overlying series of the Brenista flags is thrown against the breccias and underlying schists by a fault which is traceable inland in a N.N.W. direction. Between East Quarff and Fladabister, however, the relation between the two is seen in several fine exposures which show the gradual passage from the breccia into the overlying chocolate flags. But farther, about half-way between these two localities, the basement breccia, which is upwards of 200 feet thick on the shore, thins out inland to a few feet, and in some places disappears altogether, so that the Brenista flags rest directly on the underlying rocks. This interesting phenomenon evidently points to a gradual sinking of the area during the deposition of the successive members of this formation.

Returning to the shore section north of East Quarff, we find a gradually ascending series from the Brenista flags to certain coarse conglomerates seen in a small stream at the head of the bay of Gulberwick, which are totally different in character from the breccias already described. The included pebbles are well rounded and are to a large extent composed of different materials from the basement beds. These beds are traceable up the slope of the Gulberwick hollow to the road between Lerwick and Scalloway, where they form crags on the hill face. They are also traceable across the hills northwards to Rovey Head, about two miles north of Lerwick, where they are thrown against the metamorphic rocks by a fault which is well seen on the shore. From Rovey Head southwards to the ridge overlooking the head of Fitch Dale, this fault forms the boundary line between the metamorphic rocks and the conglomerates. It follows, therefore, that the underlying Brenista flags and the basement breccia have been thrown out along this line.

Again, on the shore south of Rovey Head, and to the east of Gulberwick, the Rovey Head conglomerates are succeeded page 5 by a thick series of coarse sandstones, passing into pebbly grits, with occasional conglomeratic layers. These have been termed by us the Lerwick sandstones, because they are most strikingly exhibited in the neighbourhood of the capital of Shetland.

The patches of Old Red Sandstone rocks which occur between Ocraquay and Aith's Voe, and between Sandlodge and Hoswick, are faulted against the metamorphic rocks, as described by Professor Geikie and Dr Gibson. The strata in these isolated areas, as well as in the island of Mousa, belong to the series of the Brenista flags. Near the fault they are highly inclined, but at some distance from it they dip towards the south-east, at angles varying from 15° to 20°. The well-known veins of copper and iron ore at Sandlodge, which we had ample opportunities of examining through the kindness of Mr Walker, occur in these rocks.

Again, at Levenwick, the fault is seen on the shore, which brings this flaggy scries into conjunction with the metamorphic rocks. It may be followed southwards along the base of the hills towards the Dunrossness Manse, where it probably dies out, as the flags succeed the basal breccias of Loch Spiggie without any apparent dislocation. Round Boddam, and southwards towards Lambhoga Head, the characteristic features of the Lerwick grits and sandstones are displayed, the underlying Rovey Head conglomerates being represented on the shore west of Sumburgh Head, and west of Boddam.

On the eastern side of the Mainland, therefore, the highest beds are represented by the Lerwick sandstones, and it is only when we pass to the east side of Bressay that the overlying series is to be met with. Along the eastern shores of Bressay, and in the island of Noss, the beds consist of grey, blue, and red flags, with occasional bands of breccia and reddish grey sandstones, which remind one forcibly of the flaggy series in Caithness and Orkney. The appearand of these beds encourages the hope that ichthyolites will yet be found in them, though a careful search failed to bring any to light. At the base of Noss Head we discovered a zone of dark shale, with limestone nodules, which strongly resenbles the well-known fish-bed on both sides of the Moray Firth.

The flaggy strata of Bressay and Noss are pierced by a remarkable series of volcanic pipes, which we shall refer to presently when we come to discuss the igneous rocks associated with this formation.

In the peninsular tract of country which lies between Weesdale and the western shores of Walls and Sandness there is a great series of rocks, which have hitherto been considered as forming part of the metamorphic series. At the north-west corner of this area a small strip of ground, bordering the coast at Melby House, about a mile and a half in length, is occupied by Old lied Sandstone rocks, which have been referred to by previous observers. The strata in this small patch consist of reddish sandstones, with dark blue flags and shales, which are faulted against the quartzites and shales of Sandness Hill.

With this exception, however, the strata in the tract now referred to consist of red and grey quartzites, with red and pale shales. The quartzites are traversed by joints in all directions, which are abundantly coated with peroxide of iron, and in many places they have a marked schistose character. We were fortunate enough in discovering an abundant series of plant remains in these altered rocks, some of which are tolerably well preserved. Mr C. W. Peach has referred the plants to Psilophyton and Lepidodendron nothum, and regards them as identical with the plants occurring in the Old lied Sandstone rocks of Caithness and Orkney. It follows, therefore, that the rocks in which they are embedded, altered though they be, must be relegated to this formation.

Contemporaneous Igneous Rocks of Old Red Sandstone Age. —In the western district of North Mavine, between Stennis and Ockren Head, there is an important development of lavas and ashes, associated at certain localities with ashy sandstones and red flags, which belong to this period. These porphyrites and tuffs resemble in every respect the volcanic rocks of the same age in the Ochils. Excellent sections of these rocks are exposed in the coast-line from Braewick to Stennis, and thence to Ockren Head, where they have been tunnelled in a wonderful manner by the action of the sea. The structure of the area is comparatively simple, as the beds lie in a synclinal fold, the dip near Braewick being to the north of page 7 west, while along the western shore the porphyrites and tuffs dip to the south of east. On the west bank of Roeness Voe, about a mile from the mouth of the sea loch, the porphyrites are thrown against the intrusive quartz-felsite by a fault, and in Braewick Bay it is highly probable that the same relation exists between the two, though the evidence is obscured by the sandy beach.

In the Holm of Melby a bed of slaggy porphyrite occurs, dipping to the west; and again in Papa Stour Professor Geikie^* has described a similar series of volcanic rocks, exposed here and there along the base of the cliffs underneath the sheet of pink porphyry. These are likewise associated with beds of sandstone and conglomerates, and are doubtless on the same horizon as the volcanic rocks of North Mavine.

On the eastern shore of Bressay, opposite the north end of the island of Noss, we discovered a bed of tuff, interbedded with the flags, which is probably connected with the volcanic pipes in that neighbourhood.

Intrusive Igneous Roclcs of Old lied Sandstone Age—In North Mavine, as well as in the districts of Delting, fondness, and Sandsting, on the Mainland, there is a seres of intrusive masses, which doubtless belong to this period. These intrusive rocks vary considerably in lithohgical character, but they all agree in possessing a large proportion of silica, while the felspar is almost invariably orthœlase. The dome-shaped mass of Roeness Hill is formed o this material, while northwards it extends to the shores of the Mainland, opposite the island of Uya. It likewise crosses the peninsular tract west of Hillswick to the Heals of Grocken, reappearing in the Drongs, and on the west site of Meikle Rooe. From the marked columnar structure which characterises these rocks on the banks of Roeness Voe and between the Heads of Grocken and Braewick Bay, as will as from the manner in which the Roeness mass spreads over the edges of the metamorphic rocks, we are inclined to believe that the granite and quartz-felsite in that district 3 an intrusive sheet, which was injected between the underying metamorphic rocks and the overlying Old Red Sandtone page 8 strata, which have been long since removed by denudation. The quartz porphyry of Papa Stour, which covers nearly the whole of the island, is an intrusive sheet injected along the lines of bedding of the Old Red Sandstone rocks, as numerous sections clearly show. Indeed, a small patch of red sandstone is still to be met with resting on the pink porphyry at the Horn of Papa. Though on trace of the once superincumbent strata is now visible on the Roeness mass, this is not to be wondered at, when we consider the great denudation which has taken place since Old Red Sandstone times.

In the district of Sandsting there is satisfactory evidence to prove that the granitic mass between Gruting and Skelda Voes has been injected along the lines of bedding of the altered Old Red Sandstone rocks. While the inclination of the granite mass is nearly the same as that of the quartzites and shales, it may frequently be observed cutting across the sedimentary rocks, and sending veins of pink felsite across the lines of bedding of the quartzites. We are inclined to believe that the metamorphism which the Old Red Sandstone rocks have undergone between Weesdale and Sandness may be due to the existence of masses of granite not far from the surface, for it is highly probable that the isolated masses of highly siliceous intrusive rocks in the west and north of the Mainland are connected underneath, though this cannot be proved to be the case on the surface.

On both sides of Noss Sound, in Bressay and Noss, we discovered a series of volcanic pipes tilled with a coarse agglomerate made up of fragments of the stratified rocks pierced by these vents. There is a singular absence of blocks of porphyrite in the agglomerate, but a thin vein of this rock is traceable for a short distance along the side of the old orifice.

Farther on the shores of Roeness Voe, as well as in Meikle Rooe, the quartz-felsites are traversed by a series of porphyrite dykes running in a north and south direction, which probably represent the last indications of volcanic activity during the Old Red Sandstone period in Shetland.