Chapter 22 General account and distribution of the system in the North-West Highlands, with a discussion of the conditions of deposit indicated by the strata

By J. Horne [discussion by] By B. N. Peach.

The detailed examination of the region between Durness and Skye has furnished ample evidence of the unconformability between the Torridon Sandstone and the overlying Cambrian formations, originally established by Professor Nicol and independently by Sir Henry James. From the relations of the strata it is obvious that the conditions of sedimentation which prevailed in Torridonian time were interrupted throughout the North-West Highlands, and that thereafter important movements of the terrestrial crust here took place, followed by extensive denudation of the pre-Cambrian rocks. As already indicated, the denuded surface thus produced differs widely from the deeply-eroded surface of Lewisian gneiss beneath the Torridon Sandstone. Instead of an undulating plateau carved into hills and valleys, the platform on which the Cambrian strata were deposited, composed in certain areas of Lewisian gneiss, in others of Torridon Sandstone, and in some cases partly of both, was reduced to a great plane probably of marine denudation. (See (Plate 33))

The persistence of this plane is, indeed, one of the prominent structural features of the region. That it represents the sear floor on which the Cambrian sediments were laid down may be confidently inferred. Where the Cambrian basal beds rest on the gneiss the latter has undergone a peculiar phase of decomposition; in particular the felspar has been changed into agalmatolite, as noted by Professor Heddle. Where they lie on Torridonian strata the latter are bleached and the felspars are kaolinised, thus contrasting in a marked degree with the normal development of the red sandstone. Lastly, a fine conglomerate or pebbly grit, from a few inches to two feet in thickness, occurs at their base.

In certain sections in Ross-shire, where the red sandstones and overlying Cambrian strata dip in the same general direction, and wherethere is no great difference in the angle of inclination of the respective beds, the discordance is not readily detected. But such instances are exceptional. As a rule, the red sandstones are either horizontal or inclined to the E.S.E. at lower angles than the quartzites, and hence they are transgressed, bed after bed, by the basal quartzites, which eventually come to lie directly on the Lewisian gneiss. This double unconformability is admirably displayed on the slopes of Ben Garbh, along the southern shore of Loch Assynt, and on Canisp. (See (Figure 36), which gives a section from Canisp by Breabag to River Oykel).

In the rock-masses displaced by the post-Cambrian movements, confirmatory evidence is obtained of the unconformable relation between the Torridonian and Cambrian strata. The detailed mapping of the region has proved that the original eastern limit of the red sandstones lay far to the east of the areas not affected by these movements. In Assynt, Sutherlandshire, the disrupted materials contain representatives of the Applecross and Diabaig groups of the Torridon Sandstone with the conglomerate at their base. These are overlain by the Cambrian quartzites, which pass transgressively across the red sandstones to the Lewisian gneiss, thus furnishing excellent examples of the double unconformability. Similar relations, though on a less conspicuous scale, have been recorded in Ross-shire. Again, in some of the thrust masses outliers of red sandstone occur, indicating marked erosion in pre-Torridonian time.

It is obvious, therefore, that during the interval which elapsed between the deposition of the Torridon Sandstones and Cambrian quartzites, the former were thrown into a series of gentle folds; a vast thickness of them was then removed, so as to expose the Lewisian rocks over wide areas, and the surface of the region was reduced to a fairly uniform plane, as shown in the accompanying diagram. (Figure 16)

A remarkable feature of the Cambrian series of strata in the North-West Highlands is the uniformity of their order of succession along the narrow belt extending for 100 miles from Loch Eireboll to Skye. All the important zones, and even the minor sub-divisions, have been traced with little variation in their respective thicknesses throughout that region.

The Cambrian sediments form three well-marked divisions: A, a lower arenaceous series composed wholly of quartzites, the upper portion being crowded with worm burrows; B, a middle series, partly calcareous and partly arenaceous, comprising the fucoid beds and Salterella grit; C, an upper calcareous division, including dolomites and limestones. The discovery of the Olenellus fauna in members of the middle division in 1891 furnished a definite horizon from which to work out the stratigraphical succession above and below. Fragments of Olenellus have been found in the fucoid beds at various localities along the line between Loch Eireboll and Sleat in Skye, thus proving beyond doubt the continuity of the zone.

In certain areas in the counties of Sutherland and Ross the pipe-rock zone of the quartzite has been divided into five horizons, in accordance with the nature of the worm burrows and the lithological character of the strata. At the top of the third sub-zone (see Part 3., Section IV. — vertical section showing horizons of intrusive rocks in Cambrian strata), a massive band of quartzite has yielded Salterella similar to that found in the Salterella grit. This discovery links the quartzites with the Olenellus zone, and hence the base line of the Lower Cambrian division has been drawn at the bottom of the arenaceous series.

The succession of dolomites and limestones, with an aggregate thickness of about 1500 feet, has been arranged in seven groups. The lowest of these (Ghrudaidh group) has been further subdivided into several zones, of which the most important are the two bands of dolomite charged with Salterella. The upper limit of the Lower Cambrian formation has, therefore, been placed in this sub-division above the horizon of these fossiliferous bands.

The total thickness of Cambrian strata in the North-West Highlands is about 21.00 feet. The various sub-divisions into which the whole series has been grouped are given in descending order in the subjoined tabular statement:-

Vertical section of Cambrian strata

In the continuous belt of Cambrian strata that stretches throughout the counties of Sutherland and Ross from Loch Eireboll to Loch Kishorn, a well-defined line separates the undisturbed strata to the west from those which have been affected by the post-Cambrian movements to the east. Owing to these displacements, the width of this belt varies considerably. On referring to the geological map accompanying this memoir, it will be seen that, for many miles to the north and south of Loch Broom, it is less than a mile across, while in the mountainous region of Assynt, where Cambrian, Torridonian, and Lewisian rocks have been repeated by successive thrusts, it is about eight miles broad.

The prominent members throughout this belt are the quartzites, which, in the area unaffected by the great lines of thrust, give rise to conspicuous features in the landscape, sometimes appearing as outliers capping lofty mountains of Torridon Sandstone or Lewisian gneiss, and again forming gently-inclined slopes dipping towards the E.S.E. They have likewise a remarkable development in the displaced masses in Assynt and throughout the tract of deer forest between Kinlochewe and Strathcarron.

The so-called "Fucoid-beds", 40 to 50 feet thick, are frequently concealed by a covering of vegetation, and hence good sections, particularly of the higher bands that yield the Olenellus fauna, are rare. Nevertheless, owing to the composition and structure of the dolomitic shales and their mode of weathering, their outcrops are readily traced alike in the undisturbed and disrupted areas. The exposures that have furnished the best preserved specimens of Olenellus and its associates occur in the unthrust area in the Dundonnell and Kinlochewe deer forests, but the same zones are frequently to be met with in the displaced masses on the mainland, though organic remains in such cases are not readily found.

In like manner the Serpulite (Salterella) grit, about 30 feet thick, forms a well-marked horizon from Loch Eireboll to Loch Kishorn, where it usually gives rise to a small escarpment between the underlying fucoid beds and overlying basal limestone.

The distribution of the dolomite and limestone in the counties of Sutherland and Ross is of special interest, as it illustrates the complicated geological structure of that region. In the Durness basin on the north coast of Sutherland, where the Cambrian strata have been disconnected from those at Eireboll by faulting and denudation, the dolomite and limestone form the predominant members. Indeed, nowhere else in the North-West Highlands do we find the complete sequence of the calcareous series given in the foregoing vertical section. In that isolated area there are excellent exposures of the two richly fossiliferous sub-divisions. (Group V., Balnakeil, and Group VI., Croisaphuill) But along the line of complicated structure between Loch Eireboll and Loch Kishorn the dolomite and limestone usually appear as lenticular strips or patches, save at Eireboll House, at Inchnadamff, and at Kishorn, where they occupy considerable areas. The outcrops are interrupted by reversed faults or powerful thrusts, which bring forward lower beds or repeat the basal zones of the .calcareous series. The highest beds along the belt between Eireboll and Kishorn belong to the Sailmhor dolomites (Group III., vertical section, p, 366), which appears on the limestone plateau of Inchnadamff. No member of the highly fossiliferous zones of Durness has yet been detected along the line of complication in the counties of Sutherland and Ross. The Orthoceras found by Mr. Charles Peach in Assynt must have come from one or other of the bands of dolomite charged with Salterella at the base of the Ghrudaidh group.

That the Cambrian rocks originally extended further west on the mainland is clearly proved by the occurrence of a patch of the basal quartzites at Achiltibuie, beyond the Coigach Mountains, Ross-shire — about nine miles to the west of the existing main belt — where they have been let down by normal faults in the midst of the Torridon Sandstone. In the sequel (Part 4) it will be shown that before they were disrupted and piled up by the post-Cambrian movements, the Cambrian formations must have spread over the territory for miles to the east of their present limits.

In Skye the members of this system have only a limited distribution, being restricted to certain areas near the Ord in Sleat and to the tract between Broadford and Loch Slapin. All these exposures have probably been affected by the great post-Cambrian displacements. There is perhaps one exception, to which attention will be directed in the sequel (Part 4., Chapter 39) Among the various thrust masses in this island, evidence has been obtained of the unconformability between the Torridon Sandstone and Cambrian strata. Near the Ord, all the main Cambrian zones, ranging from the basal quartzites to the Sangomore dolomites and limestones (Group IV., vertical section, p. 366) have been met with, while to the west of Broadford, at Ben Suardal, there are representatives of the Balnakiel and Croisaphuill Groups, as first pointed out by Sir A. Geikie.<ref>Quart. Journ. Geol. Soc., vol. xliv., p. 62.</ref> Indeed, it is worthy of note that all the sub-zones of the pipe-rock zone of the quartzite have been identified in Sleat, that fragments of Olenelloides have been found in the fucoid beds in the Tokavaig burn near the Ord, and that a suite of fossils characteristic of the Balnakeil and Croisaphuill zones has been obtained from the dolomites and limestones at Suardal.

Conditions of deposit indicated by the Cambrian strata of the North-West Highlands

From the distinctive ethological characters of the various Cambrian sediments, and from the nature of the enclosed organic remains, some probable conclusions may be drawn as to the conditions under which the Cambrian series of the North-West Highlands was accumulated. The extensive denudation which produced the wide submarine plane on which this series of strata was laid down must have required a prolonged period of time for its accomplishment. During this protracted interval the ancient land, composed of Lewisian and Torridonian rocks, was planed down to the sea-level or perhaps just beneath it. Upon a portion of this plane which was being subjected to earth-stresses, applied in a north-west and south-east direction, and thus at right angles to those which plicated the Lewisian rocks, the Cambrian coast line was determined with a north-east and south-west trend. South-east from that shore the Cambrian strata were deposited in a continuously deepening ocean.

The conglomerate, never exceeding a few feet in thickness, which lies at the base of the Cambrian series, is made up chiefly of quartz pebbles, which represent the residue of the hardest materials of the older rocks beneath. It is followed by the lower quartzite, which over the whole area presents a remarkably uniform thickness of about 200 feet, pointing to the deposit of sandy and gravelly sediment parallel to and not far from the shore line. From its frequently pebbly character and its persistent more or less false-bedded structure, this quartzite appears to have been accumulated rapidly in comparatively shallow water subject to considerable current-action. In the northern part of the region it contains few or no intercalations of shale, but towards the extreme south and south-west, beds of micaceous shale, sometimes 10 feet thick, make their appearance towards the base of the group, showing that the "mud-line" was occasionally reached in that direction. Few or no organisms have left their traces in this quartzite zone.

During the accumulation of the upper quartzite quieter conditions of deposit seem to have prevailed in slightly deeper water further from the shore. The sea-floor was probably subsiding more rapidly than could be compensated by the deposition of sediment, though a reversion to former conditions may have prevailed for a short time while the members of the sub-zone of the "trumpet pipes" was being laid down, perhaps during a pause in the downward movement. After this episode, however, the depression appears to have been renewed at a more rapid rate than ever, for the topmost bed of the "pipe-rock" is in places studded with glauconitic grains, which are known to accumulate in comparatively clear water. Throughout the whole period of time represented by the upper quartzite the fine white siliceous sand probably settled slowly on the sea-bottom amidst much organic matter, with which it became so commingled as to be swallowed by millions of such worm-like creatures as have left their burrows and casts in vast abundance in the rock. To these cylinders of indurated sand the name of "pipes", given by Nicol, from their fancied resemblance to the stems of tobacco-pipes, has generally been adopted by succeeding geologists.

Following upon the "pipe-rock", the so-called "fucoid-beds" indicate a still further subsidence and greater distance from shore. The "mud-line" had now been reached, although the sandy dolomitic beds in the lower portion of the zone may represent occasional reversions to the conditions under which the upper quartzite was formed. Nevertheless, the succession of sediments shows that before the uppermost portions of this zone were laid down the "mud-line" was not only reached, but even at times was overstepped, so that pure organic deposits were allowed to accumulate. On this clear and tranquil sea-bottom the dead organisms were subjected to the solution of their calcareous portions and the partial replacement of their lime by magnesian salts from the sea water, whereby dolomitisation was produced. The same strata also contain a considerable amount of phosphate of lime, evidently derived from the decomposed tests of such organisms as trilobites and hingeless brachiopods, which appear in considerable numbers in some of the bands. As a further indication of the depth and clearness of the water, it may be added that glauconite is common in these sediments.

The succeeding zone of the "Serpulite Grit" points either to a pause in the subsidence or more probably to a slight oscillation which allowed sandy sediment to encroach for a time beyond the "mud-line", and thus to restore the conditions in which the "pipe-rock" was formed. But after this temporary interruption the slow subsidence was once more resumed, as may be inferred from the abundance of dolomitic material towards the top of the zone.

After the deposition of the Serpulite Grit, the stratigraphical chronicle shows that the depression of the sea-floor became more persistent and quicker in pace, and thus that subsidence was considerably more pronounced than deposition. It is true that the few feet of shale which lie at the base of the great "Durness Dolomite" indicate that at that stage in the history the "mud-line" was once more reached, and that a few scattered sand grains are still to be found, even in the lowest beds of the dolomite itself. But the overlying great mass of calcareous and dolomitic sediments, more than 1500 feet in thickness, marks a continuous sinking of the sea-bottom, on which there gathered organic and chemical deposits almost free from terrigenous material. It further shows that this condition of things continued for a prodigiously long period, which, in palaeontological chronology, as indicated by the contained organisms, lasted from Lower Cambrian to Lower Silurian (Ordovician) time. The main part of this thick series of deposits appears to be derived from the calcareous and siliceous organisms of the plankton and of the animals that fed upon the rain of these organisms that fell upon the bottom.

The condition of the remains of the larger organisms, as will be described in the next chapter, shows the rate of deposition to have been so slow that both the calcareous and siliceous organisms lay uncovered long enough to be in large measure dissolved before they were entombed in the tardy accumulation of ooze. If this has been the case with the more massive and therefore more resisting organisms, it is tolerably certain that the minuter calcareous forms must have suffered the same fate in a still more marked degree, and thus that the finely-divided calcareous mud which formed the food of most of these creatures would undergo continual dissolution. During this process magnesian salts from the sea-water seem to have been substituted for some of the abstracted calcareous salts, so that the sediments were dolomitised upon the sea-bed itself, or, at least, in such superficial layer's as were still accessible to the sea-water. The Cambrian plankton may have contained unicellular calcareous plants, as that of the present ocean does, in which case the small amount of magnesian salts contained in these plants may have been concentrated by the abstraction of the more readily soluble calcareous matter, and thus may have added to the percentage of magnesium carbonate in the dolomites. In some of the zones the alternation of limestones with dolomites may be explained by a more rapid deposition of the former than of the latter. As further evidence of the great amount of solution that took place upon the old sea-floor, numerous layers of detached nodules, and even continuous sheets of chert, can be seen to have been formed in the calcareous muds before their compression and consolidation. Some of these cherts have been found by Dr. George Jennings Hinde to contain sponge spicules, but probably most of the silica was obtained from mare simply organised minute animals and plants of the plankton. The oolitic structures which occur in some of the limestones are at times replaced by chert.