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The Pamphlet Collection of Sir Robert Stout: Volume 22

[Boulder Clay]

Within the county this deposit is of two distinct types, corresponding with the two opposing streams of ice just described. We have (1.) a stiff, dark grey, shelly boulder page 17 clay, containing an admixture of rocks of local origin, with a large proportion of blocks which are foreign to the district; (2.) a reddish or brown boulder clay, containing no shells, and including stones derived from the area traversed by the local ice-flow. A line drawn from Shebster midway between Keay and the Forss Water, south-eastwards by Scotscalder Station and Strathmore Lodge to Dunbeath, marks approximately the inland limit of the dark grey boulder clay charged with marine shells. The coast-line from Dunbeath to Duncansbay Head, and thence to near Keay, forms the boundary line on the east and north. Over the whole of this triangular-shaped area, measuring about 300 square miles, the shelly boulder clay is distributed in irregular sheets; and when we consider the remarkable features presented by this deposit and the extent of ground which it covers, there is little wonder that its origin has given rise to considerable discussion. Throughout the area occupied by the shelly drift, the striations have a general north-westerly trend, as already indicated; and in a subsequent paragraph we shall describe certain facts connected with the dispersal of the stones in this deposit, which indicate that the ice-flow must have been from the North Sea towards the Atlantic. The reddish or brown boulder clay, containing no marine shells, lies to the west of the line which marks the inland limit of the shelly drift. We shall first describe the moraine profonde produced by the local ice-flow.

On the shore, between the Ord and Berriedale, the deposit consists of a reddish-brown gritty clay, with well-striated stones composed of rocks belonging to the immediate neighbourhood. Amongst these may be mentioned pink granite, gneiss, quartzite, granitic breccia, red flags, grits, and shales, all of which occur in the vicinity. No shells were observed in this deposit, nor any of the secondary rocks which are so prevalent in the shelly drift. As already described, the stria; along this part of the coast point seawards, and the deposit has evidently been produced by ice which radiated from the adjoining high ground. But to the north of Berriedale and onwards to Dunbeath, where the striae begin to bend inland, the boulder clay completely changes its character. The colour page 18 becomes dark grey, blocks of the secondary rocks make their appearance, and organic remains are abundantly met with. Indeed this latter deposit occurs in great force in the Dun-beath Water, as described by Dick and Jamieson. In the Berriedale Water, however, which drains the northern slopes of Morven and the Scarabens, there are high banks of the reddish-brown ground-moraine, resembling in every respect the sections between Langwell and the Ord. Mr Jamieson states that he observed in the Berriedale Water sections some of the dark blue-grey stuff commingled with the red boulder clay, in which, after some search, he found "nine or ten small pieces of shell and a bit of a Balanus."* But this commingling of the separate deposits occurs near the margin of the shelly drift, where the conflicting ice-streams must have shifted their ground, according to the relative pressure, which need not necessarily have been constant. Such an admixture of the ground-moraine of the respective ice-streams is just what might be expected under these conditions.

We can now explain the overlap of the dark-grey shelly drift on the red conglomerates, grits, and flags north of Berriedale towards Dunbeath, referred to by Mr Jamieson. A similar overlap occurs at the Sarclet, five miles south of Wick, where the same conglomerates and red flags are brought to the surface by means of an anticlinal fold. It is evident that this overlap is due to the forcible invasion of that area by the ice from the North Sea, which pushed along underneath the mass the pebbly silt and sand charged with marine shells lying in its path.

Again, in Strathmore, on the banks of the Thurso river, this reddish-brown boulder clay is exposed above Strathmore Lodge. At the bend above the lodge, close by the footbridge, there is an excellent section of this deposit on the right bank of the stream. It consists of red gritty boulder clay, with well-striated stones, which have been derived from the metamorphic rocks to the west. But not far below the lodge the dark-grey shelly boulder clay makes its appearance simultaneously with the change in the trend of the ice-markings,

* Quart. Jour. Geol. Soc., vol. xxii., p. 270.

page 19 and from thence it forms a series of bluff cliffs, from 20 to 25 feet in height, as far as the rocky ravine below Dirlot Castle. In these sections we detected numerous blocks of secondary rocks, with fragments of marine shells.

In the burn of Isauld, and by the roadside leading to Shebster, sections of red boulder clay, free from shells and secondary rocks, are found resting on the grey flagstones. Associated with pieces of the grey flags are numerous striated blocks of red sandy flags and grits. A slight knowledge of the geological structure of the district satisfactorily explains the overlap of the red boulder clay on the grey flagstones east of Reay. On the hills round Ben Rah, and southwards to Loch Scye, there is a coarse granitic breccia, largely composed of fragments of orthoclase felspar. In the upper reaches of the Forss Water this granitic breccia passes underneath a great series of chocolate-coloured flags, which cover a strip of ground a mile in breadth between Achsteenalate and the east bank of Loch Shurrery, and these beds are overlaid in turn by the grey flagstones of Ben Dorrery. Now, the local ice, which streamed north-east and north from the hilly ground between Ben Rah and Ben Shurrery, must have crossed these zones at the base of the Old Bed Sandstone; and hence numerous blocks of the chocolate flags would be mingled with the ground moraine, and the colour of the deposit would naturally become red. In its northward march this local ice must have invaded the area occupied by the grey flagstones between the burn of Isauld and Shebster. It is not necessary, therefore, to invoke a movement from the north-west to explain this overlap. In the light of the foregoing facts, all difficulty regarding its occurrence disappears.

To the west of Sandside Lodge, by the roadside, there is a section of similar reddish-brown boulder clay of local origin, containing blocks of grey micaceous gneiss, granite, pink felstone, grey and red flags.

To the east of the line already described, as marking the inland limit of the shelly drift, the boulder clay differs widely in character from that just described. The shelly drift is not distributed uniformly over the whole area. It reaches page 20 its greatest depth along the stream courses and in the various bays which indent the rocky coast-line. Excellent sections are exposed in the bed of the Forss Water, the Thurso river, at Scrabster Harbour, in Wick Bay, and Dunbeath Water. In many places it reaches a depth of 40 feet, and at Scrabster Harbour its thickness is upwards of 100 feet. In the undulating dome-shaped tracts it thins out to a foot or so in thickness, and in many places it disappears altogether.

We have little to add to Mr Jamieson's accurate account of the physical characters of this deposit. Again and again we had occasion to confirm many of the observations recorded in his paper. We shall therefore have to repeat the description of some of the well-known features dwelt on by him in order to show that they cannot be satisfactorily explained by the theory of floating ice.

The shelly boulder clay is of a dark-grey slate colour when moist, but frequently, in the upper part of the sections, it assumes a brown or ferruginous tint, which may be due to oxidation. It is evident that this brown tint is a mere surface discoloration, because when the deposit is dug into for a little way the slate colour appears. Throughout it consists of an extremely stiff gritty clay, charged with stones of various sizes. In all the sections the stones are scattered irregularly through the matrix. Occasionally lenticular seams of sand occur in the midst of the deposit, but their presence does not impart a stratified arrangement to it. Indeed, all those who have carefully examined the sections agree in stating that this shelly drift has no trace of stratification, and that in physical character it resembles ordinary Scotch till. Marine shells in a more or less fragmentary form are scattered irregularly through the gritty matrix as well as the seams of sand. There is one section described by Jamieson, on the south side of Wick Harbour, which is of importance, as it shows a slight change in the character of the deposit in the upper parts of the cliff. The section is about 50 feet high, the lower part of which consists of a dark-coloured and very tough gritty clay, with very small stones and numerous remains of marine shells. Thin seams of sand are mixed with this deposit, but there is no trace of stratification. In page 21 the upper part of the section the deposit is of a brownish tint, and less compact. A few feet from the top of the section there are some large blocks of granite, sandstone, and various metamorphic rocks, which are distinctly ice-worn. The occurrence of these blocks, however, is quite exceptional. Along the cliffs to the south of Wick the same change in colour is observable, but fragments of shells are also met with in this material, and the stuff is quite homogeneous from top to bottom.

The nature of the stones embedded in the deposit deserve special attention, as they furnish important evidence regarding the ice movement. Throughout the area the prevalent ingredients are, of course, blocks derived from the underlying Caithness flags. It is particularly observable that in the sections along the eastern seaboard there is a comparative absence of ordinary-sized blocks of this material. Indeed, with certain exceptions, the deposit is not stony, as the matrix contains only small well-rounded pebbles. When the sections are followed inland, however, the stones derived from the Flagstone series increase both in number and size. This feature is satisfactorily explained on the supposition that the shelly boulder clay of the eastern seaboard was mainly composed of the pebbly silt and sand lying on the bed of the North Sea, which was gradually mingled with materials obtained from the flagstones as the ice advanced inland towards Thurso and Reay. The presence of the large blocks of flagstone in the upper part of the sections exposed round Wick Bay may be accounted for in the same manner. It is probable that the ice took some time to remove the silt from the sea bottom, and it was not till it had done so that it was enabled to quarry the underlying rocks out of which to manufacture boulders. Dr Fenck has explained similar phenomena met with in the Danish drifts in the same way. We frequently noted that the larger blocks of the flagstones lay with their long axes parallel to the direction of the ice-flow, while they are invariably striated in the same direction. This feature was observed by Jamieson in the Milton and Haster burns, and it is capitally displayed in the sections in the Thurso river, between Dirlot and Strathmore Lodge. page 22 Here there are occasional blocks of grey and chocolate flags, upwards of 3 feet in length, arranged as described.

Owing to the remarkable uniformity in the character of the Caithness Flagstone series there is some difficulty in determining the direction of the ice-carry from the dispersal of the local rocks in the boulder clay. There is one striking instance, however, to which we paid special attention, which confirms the opinion that the ice must have come from the south-east. The peninsular tract of ground, which is situated between Brough Bay and Dunnet Bay, extending northwards to Dunnet Head, measuring about five square miles in extent, is occupied by coarse yellow and red sandstones, which are brought into conjunction with the Flagstone series by a fault. This dislocation runs from Brough Bay southwards by St John's Loch and the church of Dunnet to Dunnet Bay. Now, in the boulder clay sections to the east of the fault, no trace of these characteristic sandstones is to be seen on the shore or inland, whereas the Caithness flagstones have been carried on to the surface of these Upper Old Bed Sandstone rocks. Had the ice-flow been from the north-west, the phenomena would have been precisely the reverse of what we have stated, as blocks of these massive sandstones would certainly have been mingled with the moraine profonde to the south-east of the fault.

Again, on the shore about four miles to the south of Wick, at the Sarclet, massive beds of conglomerate, attaining a thickness of nearly 300 feet, are brought to the surface by means of an anticline.* Blocks of this conglomerate can be traced inland from this locality, both in the boulder clay and on the surface in the direction of Thurso.

But in addition to these local rocks there is a large percentage of blocks which are foreign to the Caithness plain. Amongst these may be mentioned granite, porphyritic felsite, diorite, gneiss, mica schist, quartzite, oolitic limestone, oolitic brecciated conglomerate, grey sandstone belonging to the

* Trans. Roy. Soc., Edin., vol. xxviii., p. 376.

Dick mistook the boulders of this rock, which he found between Thurso and Dunnet, for fragments derived from the conglomerates of Port Skerry in Sutherlandshire, from which they differ considerably.

page 23 Oolite formation, septarian nodules, along with chalk and chalk-flints. Indeed, over all the tract occupied by the shelly drift chalk-flints are occasionally met with on the surface, having escaped denudation while the matrix which enclosed them has been worn away. Some pieces of jet were also obtained by Mr C. W. Peach in the sections at Wick, and several specimens of belemnites were found by him both at Wick and in the Thurso river. It ought to be borne in mind that the secondary rocks in the dark-grey clay are co-extensive with the shells, and where these blocks occur shells are common. These foreign blocks are hardly ever found in those places where the deposit is only a foot or two thick, and the same remark applies to the organic remains. In that case the blocks are almost invariably composed of the underlying rocks.

We believe that Mr C. W. Peach was the first to recognise the close resemblance between the blocks of the secondary rocks in the shelly boulder clay and their representatives on the Sutherlandshire coast. Many of the included blocks contain the same fossils as those chronicled from the latter locality. Indeed, nearly all the blocks of secondary rocks, save the chalk and chalk-flints, might quite well have been derived from the Sutherlandshire coast, or the outliers which occur in the basin of the Moray Firth. But though cretaceous rocks do not occur in place on the shores of the basin now referred to, they are believed to exist on the bed of the Firth. In addition to these, several blocks of fossil wood are met with in the shelly drift which are identical with those found by us in the Odin Bay section, in Stronsa, Orkney. Sections of this rock show distinct cell structure under the microscope, and they have been determined by Mr Kidston of this Society as specimens of Pence Lindleyana of Oolitic age. The same rock is embedded in the oolitic shales in Sutherlandshire, where it is burned for lime.

The occurrence in the shelly boulder clay of these blocks of secondary rocks which are known to exist in the basin of the Moray Firth, is an additional argument in favour of the theory that the ice-flow across the Caithness plain was towards the north-west.

page 24

The distribution of the shells in this deposit seems to favour the same conclusion, for along the eastern seaboard the shells are abundant, but they are more difficult to obtain as the sections are followed inland towards Thurso and Reay. The appearances presented by these organic remains indicate glacial abrasion precisely in the same manner as the stones in the same deposit. This fact has been noticed by various observers, and has been repeatedly referred to in connection with the question of the origin of the till. They are broken, smoothed, and striated like the stones associated with them; indeed they resemble the shell fragments we found in the Orkney boulder clay, though in the latter case they are more fragmentary and not so well preserved. The most common forms met with are Cyprina Islandica, Mactra solida, M. truncata, Turritella ungulina, Astarte clliptica, A. borealis. But though the shells as a rule are more or less broken and in many instances striated, yet in some cases entire valves have been dislodged. Mr Jamieson found "one entire valve of Astarte borealis, another of A. elliptica, and two small ones of A. compressa, likewise a specimen of Natica nitida, and another of N. Islandica, both almost perfect." The striking example of the complete bivalve Anomia, referred to by Mr Jamieson and Dr James Geikie, was found by Mr C. W. Peach protected in the hollow of a stone. Indeed it is highly probable from the appearances presented by many of the fragile shells that they were frozen in the ground moraine, and in this way escaped complete destruction.

A glance at the accompanying list conclusively shows that the sections have yielded but a small number of forms characteristic of the littoral zone. After years of vigilant searching, Mr C. W. Peach obtained only a few species which lived along the shore, among which may be mentioned two specimens of Purpura lapillus, a few specimens of Patella vulgaris, Mytilus rarely, and Tapes pullastra also rarely. We shall point out presently that the scarcity of these shore forms, and the great abundance of species whose habitat lay in deeper water, have an important bearing on the question of the origin of the shelly boulder clay.