The Old Red Sandstone of Orkney.
The Old Red Sandstone of Orkney.
While engaged in working out the glacial geology of Orkney, during our leave of absence from official work, in the autumn of 1879, we had occasion to pay some attention to the geological structure of the Old Bed Sandstone, which is so largely represented in that group of islands. In the course of our traverses we detected certain points regarding the physical relations of the strata which have not as yet been described; and we likewise noted a new and interesting feature in the history of this formation in Orkney, viz., the proofs of contemporaneous volcanic action in Lower Old Red Sandstone times. In the paper now laid before the Society we purpose to describe briefly the general results of these observations.
The abundance of ichthyolites in the flagstones of Orkney was made known through the descriptions of Agassiz, and more recently by Hugh Miller in his well-known volume "The Footprints of the Creator." In the opening chapters of that work he makes the following statement:
"It is not too much to affirm that in the comparatively small portion which this cluster of islands contains of a system regarded only a few years ago as the least fossiliferous page 2 in the geologic scale, there are more fossil fossil inclosed than in every other geologic system in England, Scotland, and Wales, from the coal measures to the chalk inclusive."
In spite of the inducement herein contained, the ichthyology of Orkney has never been so vigorously or exhaustively worked out as that of Caithness and the Moray Firth basin.
The paper published by Sir E. Murchison in the Quart. Jour, of the Geol. Soc.* contains a brief description of the geological character of the deposits, and an attempt to correlate the strata with the representatives of the same formation in Caithness. He refers to the axis of ancient crystalline rocks near Stromness on which the Lower Conglomerate rests unconformably, to the large development of the Flagstone series, which is analogous to that of Caithness and to the great succession of red and yellow sandstones of Hoy, which graduate downwards into the flagstones.
* Quart. Jour. Geol. Soc., XV., 410.
† Trans. Hoy. Soc. Edin., XXVIII., 345. This memoir contains references to other papers than those we have quoted on "The Old lied Sandstone of Orkney." We have chiefly referred to those publications which treat of the geological structure of the islands.
In the Mineralogical Magazine for December 1879, Professor Heddle published a paper on the Orkney Islands, in which he describes a well-marked trough which runs through the centre of the group of islands. The strata which occupy the centre of the trough he describes as "loose arenaceous freestones, with silicious granules sometimes so coarse as almost to entitle them to the designation of grits." Moreover, he notes the important fact that these arenaceous strata repose conformably on the ordinary blue flags of the islands.
The results of our observations confirm the statements we have just quoted from the papers of Professor Geikie and Professor Heddle. In the course of our recent traverses we examined nearly the whole of the coast line of Westray, Sanday, Eday, Stronsay, Shapinshay, South Ronaldshay, and the Mainland, a portion of Hoy, and some of the small islands of the group. The succession of the strata is more clearly defined in the northern islands, and we shall therefore begin by describing the relations of the Flagstone series as they are exposed on the coast sections of Westray, Eday, and Sanday.
Along the western shores of Westray there are admirable sections of grey and rusty-coloured flags, dipping in a westerly direction at a gentle angle. The bluff cliff of the Noup Head (about 240 feet) in the north-west corner of the island, consists throughout of finely-bedded rusty-coloured flags; and similar strata are met with on the slopes of the Fitty and Gallow Hills, to the south-west of Pierowall. The remarkable terraced appearance which these hills present when seen from Pierowall or Cleat, is characteristic of the flagstones. This feature is due to the denudation of softer members of the series, which must have been mainly accomplished in preglacial times. This is evident from the occurrence of polished surfaces and ice markings in many of the successive ledges on the hill slopes.
At Nethergarth, in Tuquoy Bay, the flagstones roll over to the east, and this easterly dip continues, with some gentle page 4 undulations, along the south-western shore to the promontory of Rapness. And so also along the eastern shore from Newark by Rackwick, Stangar Head to Weatherness, the grey and rusty flags are inclined to the east and south of east. The same easterly dip is observable on the southern promontories of Papa Westray. It follows, therefore, that we have a low anticlinal arch with several minor foldings in the island of Westray, the axis of which crosses the island from Tuquoy Bay northwards in the direction of the western shore of Papa Westray. The flagstones exposed in the south-eastern part of the island are merely the repetitions of those met with in the western portions.
As we approach Weatherness, which forms the southeastern promontory of Westray, the flagstones are more highly inclined to the east. On the islands of Fara Holm and Fara, the same high angle is observable with a similar easterly dip; and there can be little doubt that the grey flags in the latter islets are higher in the series than those at Weatherness. The flagstones exposed on the western shore of Eday between Fara's Ness and Seal Skerry are merely the southern prolongations of the flaggy beds in Fara and Fara Holm. On the whole, then, the succession of the strata between Westray and Eday is tolerably clear, notwithstanding some short gaps in the sections.
The structure of the island of Eday is comparatively simple. The strata form a well-marked syncline, the axis of which lies to the west of the Flighty and Fara's Ness Hills. The centre of this trough is occupied by a great series of yellow and red sandstones, which rest conformably on the flagstones already described. The shore sections on the east and west sides of the island are so clear and convincing, that no one can possibly dispute the conformable passage of the flagstones into the overlying arenaceous series. So strongly do the sandstones of Eday resemble the Upper Old Red Sandstones of Hoy, that Sir Roderick Murchison placed them on the same horizon. But a minute examination of the coast sections proves that they really belong to the Flagstone series, and are therefore of much older date.
A traverse along the shore from Fara's Ness to the sandy page 5 bay about a mile to the east, shows the gradual alternation of sandstones and flags at the base of the arenaceous series. At the promontory now referred to, the grey flagstones are seen dipping to the east at an angle of 30°; but not far to the east they are interstratified with bands of flaggy sandstone. These beds are overlaid by false-bedded yellow sandstones which contain numerous brecciated bands made up of angular fragments of crystalline rocks. These false-bedded sandstones likewise contain two thin zones of grey flagstones, which resemble in every respect those at Fara's Ness. It is apparent, therefore, that the change of physical conditions indicated by the respective groups of strata must have been gradual.
The same conformable passage between the flagstone and the overlying arenaceous series is observable on the east coast near the Kirk of Skail, and to the north of Warness, which forms the south-western promontory. Owing to the synclinal fold in the strata, the flagstones at Fara's Ness are brought to the surface again at Warness, and these beds are prolonged in a north-north-easterly direction towards the Kirk of Skail. At both of these localities grey and white sandstones are interbedded with the flags, and these pass upwards into conglomeratic red and yellow false-bedded sandstones.
|Red and yellow sandstones.|
|Reddish shales,||15 feet.|
|Hard white sandstone,||20 feet.|
|Grey calcareous flagstones.|
The sandstones at the top of this section are flaggy at their base, but become more massive and conglomeratic upwards. The included pebbles consist of fragments of mica schist, quartzite, gneiss, granite, and other metamorphic rocks all stained of a reddish colour. The occurrence of these pebbles tends to confirm Professor Geikie's expressed opinion that the Lower Old Red Sandstone strata of Orkney were laid down on a very uneven surface of the older crystalline rocks; for an exposure of these latter must have still existed at no page 6 great distance, when the highest beds of the Lower Old Red Sandstone series now preserved in Orkney were being deposited.
In the bay to the north-east of Stenniehill a zone of grey flagstones with fish remains is interleaved with the sandstones, as we found to be the case on the west side of the island in the bay of Fara's Ness. This zone can be traced at intervals across the island in a south-westerly direction to the coast line west of the Wart of Eday.
About half a mile to the south of the entrance to the Calf Sound, the base of the arenaceous series is again exposed on the coast line. The grey and rusty flags form a low arch on which the coarse-grained sandstones rest conformably. The flags are truncated on the north side by a small fault which brings down the overlying sandstones. The greater portion of Eday and the whole of the Calf of Eday are occupied by these sandstones. Perhaps the finest exposure of the series is to be seen on the Red Head of Eday (209 feet), which forms the northern promontory of the island. These strata form prominent hills in the centre of the island, whose features are totally different from those characteristic of the Flagstone series in Westray. As a rule the beds are extremely coarsegrained, and frequently conglomeratic, with much false bedding; indeed, as we have already remarked, they have a striking lithological resemblance to the Upper Old Red Sandstones of Hoy.
Southwards from the Kirk of Skail, along the shore, there is a steadily descending section of the flagstones for nearly a mile and a half. There is no great thickness of strata exposed however, as the coast line forms only a very small angle with the line of strike for some distance. North of Veness the Flagstone series is abruptly terminated by a fault which brings down the overlying sandstones to the west. This dislocation runs in a north and south direction, and passes out to sea to the west of the Veness promontory. There is therefore a small detached area of the arenaceous series in the south-east corner of the island.
In the island of Sanday the thick sandstone series of Eday and the underlying flagstones are repeated partly by foldings page 7 and dislocations of the strata. Along the western shore between Spurness and Stranquoy, the grey flagstones are exposed interbedded with red and grey sandstones which are conglomeratic in places. These beds are inclined to the west and north of west at angles varying from 50° to 70°. The conglomeratic sandstones and red shales interbedded with the flags are seen on the western shore, about a mile to the north of Spurness. It is highly probable, therefore, that the strip of the Flagstone series, extending from Spurness to Stranquoy, is on the same horizon with the flags, which immediately underlie the Eday sandstones.
The strip of the Flagstone series now referred to is bounded on the east by a dislocation which brings in the Eday sandstones. This fault is well seen on the shore, a short distance to the north-east of Spurness, where the red and yellow sandstones which dip to the west at an angle of 40° are brought into conjunction with the grey flags. The effect of this dislocation is also well exposed on the shore in Stranquoy Bay. The chocolate-coloured sandstones and shales are seen on the east side of the fault dipping in a south-westerly direction; while to the west of the fault the grey flags are bent round in the form of an arch. The effect of this dislocation is shown on the accompanying sheet of horizontal sections.
To the east of Stranquoy the arenaceous series, which is brought in by the fault just described, is traceable for nearly a mile. The red and yellow sandstones are admirably seen in Pool Bay and on the shore at Hack Ness, having a persistent dip to the west at angles varying from 15° to 25°. A thin zone of interbedded flagstones is exposed in the bay to the west of Hack Ness, which reminds the observer of similar zones on the same horizon in Eday.
The gradual passage of this arenaceous series downward: into the flagstones is presented on the shore at Moy Ness where the same alternations of flags and sandstones at the bast is observable, which obtains in Eday. Northwards along the shore towards Bacaskeal Bay there is a steady descending series of grey and purple flags.
In the northern promontories of the island, and specially along the shore from Hermaness Bay to the Holms of Eyre page 8 the grey and purple flags are repeated by gentle foldings. There is, therefore, no great thickness of strata exposed in the central portion of Sanday. The time at our disposal did not permit us to visit the north-eastern promontories of Sanday, nor the island of North Ronaldshay; but from the observations of previous observers only the Flagstone series seems to be represented at these localities.
The greater portion of the island of Stronsay is occupied by the flagstones, but at one or two localities there are small detached areas of the arenaceous series. On the northern headlands, the grey flags are exposed dipping in a northwesterly direction, and the same inclination is observable in Odin Bay and Linga Sound. About a mile to the north of Holland, on the western shore, the yellow sandstones are thrown against the flags by a fault which is admirably seen. The sandstones dip to the north of west, and, on following the coast line southwards, they graduate downwards into the grey flags.
In the south side of the island, at Housbay, and in the bay west of Lamb Head, the flagstones roll over to the south-east, and a similar passage upwards into the yellow sandstones may be noted. The small area occupied by this series at Lamb Head is bounded on the north side by a fault.
A small patch of yellow sandstone, which is quarried for building purposes, occurs between Odin Ness and Burgh Head, but as it is bounded by faults, its relation to the flagstones is not apparent.
The greater portion of the island of Shapinshay is likewise occupied by the Flagstone series. The sections exposed on the coast line prove, beyond all doubt, that the same beds are constantly repeated by gentle undulations. Along the western shore, between Stromberry Ness and the Gait, the general dip is to the north-north-west, but as the observer traverses the shore of Veantro Bay and the coast line between Balfour Castle and How, he cannot fail to note the frequent changes of dip which bring the same beds to the surface again and again. In the south-eastern corner of the island, however, there is a small patch of red and yellow sandstones inter-stratified with grey flags. Though the gradual passage be- page 9 tween the two groups, which is so clear in the northern islands, cannot be made out in Shapinshay, there can be little doubt, from the character of the strata, that the patch of sandstones, between Haco's Ness and Kirkton, are near the base of the arenaceous series. A small fault separates the two groups to the south of Kirkton, which obscures the relations between them, and it is highly probable that this dislocation may be the northern prolongation of the great fault to be described presently, on the Mainland. This conjecture is strengthened by the fact that, though the flagstones on Shapinshay roll about in every direction, yet the preponderance of dips is towards the south-east, and this is especially .he case on the eastern shore of the island. Bearing this in mind it is easy to account for the Eday sandstones being brought in by the aid of a comparatively small fault. It is important to note this dislocation, for these beds are continued across the channel to Inganess on the Mainland, whence they strech across to Scapa, and are extended on to near Orphir Krk, being cut off from the flagstones along their whole northern boundary by a large fault with a downthrow to the south.
An interesting feature connected with the patch of said-stones and the associated flags at Haco's Ness, is the occurence of interbedded volcanic rocks, clearly proving he existence of volcanic action in Lower Old lied Sandstone times in Orkney. The lithological character of these rocks, as well as their appearance under the microscope, will be described under a separate heading.
On the Mainland, the arenaceous series just describedas occurring in Shapinshay, and which is likewise represened in Stronsay, Sanday, and Eday, is well developed. But before describing the relations of the two groups as repesented on the Mainland, we shall refer briefly to the develpment of the Flagstone series in the western part of the island. The unconformity between the flagstones and the axis of crystalline rocks at Stromness has been frequently described. Our observations tend to confirm the conclusions alredy arrived at by Professor Geikie, that the conglomeratic strata which repose on the gneissic rocks merely indicate a leal base. It is quite true that the brecciated flagstones re page 10 mainly derived from the underlying crystalline rocks, but the conglomeratic character disappears within a short distance of the gneiss. Professor Geikie has alluded to the fact that the general dip of the flagstones in Hoy is to the north and northwest, and consequently the flaggy strata, which rest unconformably on the gneissic ridge, are probably higher in the series than those in Hoy. This evidently points to a gradual subsidence of the area during the deposition of the Flagstone series.
The strata represented in the north-western portion of the Mainland are evidently the southern prolongations of the flaggy series which we have already described as occurring in Westray. They are admirably exposed on the shore between Burness and the Brough of Birsay, and along the western coast line. They are likewise well developed in the island of Rowsay, where they form the characteristic terrace-shaped hills. The lithological characters of the flaggy series in Westray, Rowsay, and the north-west of Pomona, are precisely similar.
On the coast line also, between Irland Bay and Houton Head, similar strata are met with, rolling about in gentle folds.
Again the flaggy series of Shapinshay reappears on the headlands of Carness and Work Head, north-east of Kirkwall, where the general inclination is to the north-west.
From Ingauess Head south-westwards to Scapa Bay, and along the shore to Smoogra Bay, and thence to Orphir Kirk, a strip of red and yellow sandstones with red marls is traceable. These red and yellow sandstones are the southern prolongations of the Eday sandstones. They are bounded on both sides by faults which bring them against the underlying flagstones. The dislocation which forms the northern boundary line has been traced by us for a distance of nearly ten miles from Orphir Kirk north-eastwards by Scapa Bay to the bay east of Inganess Head. The fault is admirably seen at various localities, but perhaps one of the most interesting of these is on the west coast of Scapa Bay, where the main fault as well as a minor dislocation are seen.
On the high cliff which bounds the west side of Inganess Bay, friable red clays are associated with the red sandstones. page 11 They decompose readily, and break up into small cubical fragments.
Along the shore from Scapa southwards to Howquoy Head, the same red and yellow sandstones are brought into conjunction with the flags by a fault which runs almost parallel with the coast. At Scapa the rocks consist of red mottled sandstones, underlaid by coarse honey-combed yellow and white sandstones, which alternate with calcareous flags and dark bituminous schists. Owing to numerous foldings these beds are often repeated.
To the east of this arenaceous series, the flagstones reappear in Inganess Bay. They are well developed on the shore near Tankerness, and along the coast line between Mull Head and Air Point. Flagstones only, are exposed on the shore to the east of St Mary's, and also along the road from Græmeshall to Kirkwall, save where the narrow patch of red sandstones already mentioned crosses from Inganess to Scapa.
In South Ronaldshay the lowest beds exposed are to be met with at its southern extremity, where they form the Old Head overlooking the Pentland Firth. Here the rocks consist of grey calcareous flags, charged with abundant remains of Coccosteus and other ordinary Caithness fishes, which are well preserved as in Caithness, and are not represented by a black lead-like smudge characteristic of those from Skail. Following the eastern coast line, these beds continue for a considerable distance with a northerly dip. There are several faults, some of which are occupied by veins of barytes and iron pyrites, but they seem not to be of any great magnitude. A little to the north of Halcro Head, the flags pass under a series of yellow and red sandstones and red shales, which at Windwick are suddenly truncated by a large east and west fault, the effect of which is to bring up the underlying flagstones. Near the fault flagstones are inclined towards it, but they soon recover their northerly dip, and at Stow Head once more dip below the sandstones. Owing to the sandstones being arranged in a small trough, the flags soon reappear, and after rolling about for some miles along the coast line, they page 12 finally plunge under the sandstones near the mouth of Watersound, never to reappear on South Ronaldshay; for the reel sandstones, with occasional thin intercalations of red flags and massive bands of red marly clay, extend to Crow Point in the extreme north-west. These beds are continued along their strike into Burra, where the passage from the flags into the red sandstones is well shown at the eastern entrance to Watersound. The islands of Flota, Fara, and Cava, show the same alternation of flagstones with red and yellow sandstones and red marls. In Flota the dip is almost north at Pan Hope, where the passage from the flagstones to the sandstones is well seen; while in Fara and Cava, the inclination is more to the north-east.
In the district of South Walls and the promontory of Brim's Ness, in Hoy, the rocky cliffs exhibit the ordinary grey flagstones. On the shore of the Longhope, between Melsetter and the Inn, they pass under the red and yellow sandstones; and the same relation is observable on the coast facing the Atlantic, a little to the south of Melsetter. On a former visit of one of the authors with Professor Geikie considerable difficulty was experienced in tracing the fault which divides the Upper Old Red Sandstones from the arenaceous series associated with the flagstones, till the intercalation of the sandstones with the Lower series was realised.
From what has been said it will be seen that Scapa Flow occupies the site of a geological basin, towards which the rocks dip on every side, and along the shores of which the highest beds of the Lower Old red Sandstone exposed in Orkney are to be met with as well as in Eday and Sanday.
The beds in South Ronaldshay are exceedingly like those exposed along the shores of Gills Bay on the opposite Caithness Coast and in the intermediate island of Stroma, and are, in all probability, their prolongations to the north-east, which is the direction of their strike. We have thus a definite horizon to start from, for the highest beds in Orkney are also the highest in the Caithness series.
It is worthy of note that the sandstones become coarser as they are traced northwards. In South Ronaldshay there are great masses of friable marly clays, intercalated with page 13 the sandstones. In Eday and Sanday there are only about ten feet of such strata, while the sandstones are very coarse and even conglomeratic, and approach much more to the type of the Lower Old Red strata of Shetland.
Igneous Hocks in the Lower Old Red Sandstone.—The only" contemporaneous rocks of this nature occur at the south-east corner of Shapinshay, between Haco's Ness and Foe, where they form the coast line for about half a mile. They are perfectly conformable with the flagstones. The upper surface of the diabase is highly vesicular and amygdaloidd, and exhibits all the characters of a regular lava flow. The flagstones overlying it are not altered in the slightest along the line of contact. The base of the volcanic series is not seen, though the sea has cut trenches in the rock at least thirty feet in depth. Where the dip is visible, it is sea wares. In the cliff they are covered by the flags, but they crop out inland not far from the coast line. These rocks are dark green in colour, weathering olive green. They may be considered as varieties of diabase, which have undergone considerable alteration. Some of the specimens contain much calcre, which fills drawn-out vesicles, indicating the flow of tie molten lava. We have had some of these rocks sliced, and examined under the microscope, which confirms the opinion regarding the extreme alteration which they have undergone.
One of the sections is found to be largely constituted of a plagioclase felspar, with a small amount of intervening chlorite, but with much altered olivine. The felspar is much decomposed, and forms the bulk" of the rock. The olivine, which is now changed into a pale greenish yellow serpentine, is distributed in large crystalline grains, and is abundant. In places the chlorite is represented by masses of radial aid vermicular groups of crystals which appear to have under-gone a change to the same serpentinous mineral as that which replaces the olivine. Magnetite is irregularly distributed as grains, and also frequently, either wholly or party, envelopes the crystals of olivine.
Another section shows that the rock consists of closdy crystallised plagioclase and much interstitial augite, with a page 14 considerable amount of olivine. The augite and the olivine have been converted into serpentine, although a few crystals, as well as portions of crystals, still remain unaltered. The felspar in many places is permeated with the same mineral. Much magnetite is present, together with quartz, some calcite, and serpentinised chlorite.*
Intrusive Igneous Rocks.—Among the few examples of this class met with are the two necks filled with volcanic agglomerate already described by Professor Geikie as occurring on Hoy, and which he has shown, in all likelihood, helped to supply the volcanic platform which underlies the main mass of the Upper. Old lied Sandstone of that island, and are therefore to be considered of that age.
Several dykes of basalt were observed among the islands. They are most numerous and conspicuous on the west coast of the Mainland from Brackness to Skail, but as they have been so often described, it is unnecessary to refer to them in detail. They have the same lithological characters, and behave exactly in the same manner as the dykes in other parts of Scotland, which have been regarded as the product of volcanic energy in Miocene times. A noticeable feature about the Orcadian representatives is, that they are usually divided up the centre of the dyke by a line of vesicles. This is not an uncommon feature elsewhere.
Summary.—The descriptions we have given of the Lower Old lied Sandstone strata as represented in the Orkney Islands, tend to confirm the conclusion previously arrived at by Professor Geikie, that these flagstones, with the associated arenaceous series, must be correlated with the higher subdivisions of the Caithness series. It is highly probable, therefore, when the ichthyology of Orkney is worked out in detail, that the fossils will be identical with those derived from the higher portions of the Caithness series. The great development of the sandstone series in the northern isles is of special interest, as it shows that the strata gradually assume the arenaceous character which is so prevalent in Shetland.
* In the examination of the microscopic sections we have been kindly aided by Mr T. Davies.
Moreover, it is of importance to note that the coarse silicious sandstones and marls, which are the highest representatives of the Lower Old lied Sandstone in Orkney, must not he confounded with the massive red sandstones which form the noble cliffs on the west side of Hoy. The latter rest unconformably on the flagstones. It is evident, therefore, that after the deposition of the Flagstone series, with its associated sandstones and marls, the bed of this inland sea was elevated so as to form a land surface. These strata were subjected to a considerable amount of denudation ere they were again carried below the water in Upper Old lied Sandstone times
M'Farlane & Erskine, Printers, Edinburgh.