An interesting feature connected with the glaciation of Caithness is the development of moraines and morainic deposits in several parts of the county. Hitherto they have not been described. In Strathmore they are well developed, and they stretch far down the valley to the edge of the great plain. The most easterly limit of the moraines and gravels is at Dalemore, about a mile to the East of Dirlot Castle. This point is situated about fourteen miles from the county boundary at the head of the strath. Near Westerdale, about a mile to the north of Dirlot, the grey shelly boulder clay forms a great plain, through which the Thurso river has cut a channel, and formed an alluvial terrace. This platform of boulder clay is dotted over with conical heaps and ridges of sand and gravel, at a height of 200 feet above the sea. Sections of these heaps are exposed by the roadside on the way to Dirlot, which show that the material consists of sand and gravel, more or less stratified, with occasional blocks of conglomerate and sandstone, measuring 3 feet across. Indeed, the sections closely resemble the kame series of the midland counties of Scotland. One of these ridges is specially noteworthy on account of its length, extending from the farm- page 31 house of Dirlot to Dalemore, a distance of nearly a mile. Its height varies from 20 to 30 feet.^*

From Dirlot westwards to Strathmore Lodge conical mounds and ridges rest on the plain of grey shelly boulder clay. They are not abundant, however, occurring only at intervals, and chiefly on the left bank of the stream. From this point to Dalnawillan Lodge, which is about eight miles up the strath from Dirlot, similar heaps can be traced. Towards the latter locality they become more numerous, and on the left side of the valley at Dalnawillan they are well developed. Here the moraines form huge mounds and ridges, excellent sections of which are exposed by the roadside and in the burn courses.

It is observable that the material gradually changes its character as we ascend the valley, for while towards the eastern limit it is sandy and gravelly, with distinct stratification, it becomes more compact, and the stones are not so well rounded near the head of the strath. Indeed, it approaches the type of moraine matter which is commonly met with in upland valleys. We are inclined to believe that the kamiform ridges near Dirlot mark the easterly extension of the later glaciers, for it is highly improbable that they are of marine origin, when no trace of similar deposits has been observed between this locality and the eastern seaboard. On the other hand, the fact that the mounds can be traced at intervals from Dirlot to Dalnawillan, where the material resembles ordinary moraine matter, indicates a probable connection between the different deposits.

The evidence supplied by these later accumulations is important, because they rest both on the reddish-brown boulder clay and the grey shelly drift. They steal across the surface of the shelly drift for a distance of three miles between Strathmore Lodge and Dalemore, so that there can be no doubt that the shelly drift is of older date than the deposits under consideration. This is the only locality where we found the shelly boulder clay overlaid by gravel ridges and moraines; indeed, so far as our observations went, page 32 there is a marked absence of such accumulations throughout the area occupied by this deposit, as noted by Mr Jamieson.

Between Dalnawillan Lodge and Altnabreac Station, we observed moraine heaps composed of the same material as the mounds at the former locality. Over much of the moor also there is an irregular covering of gravelly material exposed in pits, which may belong to the same series. In Strathmore we observed the same material in places where no mounds could be seen, which leads us to believe that this covering may have been deposited by flood waters from the melting ice.

Again, on the moor to the west of Loch Shurrery, moraine heaps occur, and by the roadside leading to Loch Scye pits have been dug in coarse gravelly and rubbishy material, which evidently belongs to the same formation.

In the Braxside burn, which drains the western slopes of Ben Rah, south of Reay, moraines may be seen extending across the valley, and they occur at intervals on the moor northwards to Sandside. But to the west of Reay, in the direction of the county boundary, similar deposits are irregularly distributed over the slope. They increase in number and in size on the col and along the slope towards Strath Halladale, in the county of Sutherland. Indeed, the deposits of the later glaciation in this strath are grandly developed. The bottom and sides of the valley are covered with groups of moraines, displaying at some points a marked concentric arrangement. Numerous blocs perchés are strewn on these mounds, composed of granite and granitic breccia. The material consists of a compact stony and rubbishy matter, gravelly in some places and clayey in others, with sub-angular and rounded stones, few of them being striated.

Now, it is interesting to note that while the traces of the later glaciation overlap on to the grey shelly boulder clay at Dirlot, they do not reach the outer limit of the red boulder clay at Reay. Taking Shebster as the boundary line between the two boulder clays, the later morainic deposits "tail off" about three miles from this limit. But when we think of the large tract of country between Reay and Strathmore over page 33 which these deposits are spread, it will be readily admitted that they form an important feature in the history of the glacial phenomena of Caithness. Moreover, if we take into consideration the physical features of the north-west part of Caithness, the absence of deep valleys, and the limited elevation of the hills, we can hardly escape the conclusion that these later accumulations were deposited by a more or less continuous sheet of ice.

It is rather remarkable that, while these traces of the later glaciation are so abundant in the north-western part of the county, they should not have been observed in the large valleys in the south-east. We traversed the course of the Berriedale Water from the slopes of Maidenpap (2313 feet) to the sea, and observed no indications of moraines on the boulder clay slopes. It is possible, however, that small moraine heaps may yet be met with in the higher reaches of the Langwell, Berriedale, and Glut Waters.

Erratics.

Over the Caithness plain occasional boulders have been observed resting on the boulder clay, or partly buried in that deposit, which bear unmistakably the impress of glacial action on their smoothed and striated sides. They cannot be said to be numerous; still a few have been chronicled by Mr Dick and Mr C. W. Peach in their rambles, while we met with several examples during our traverses. The smaller boulders have been removed from the fields in the course of the reclamation of the land, and have been used for building dykes. At Greenvale a boulder of the Sarclet conglomerate was noted by us, and erratics of hornblendic granite. East of St John's Loch boulders of granite were also observed. Along the road from Greenvale to Ham various blocks of foreign rocks occur, which have been borne off the fields, amongst which may be mentioned grey and pink granite, quartzite, grey micaceous gneiss, red sandstone like the beds at Ham, and conglomerate. No boulders of the Upper Old Bed Sandstones, which form the tract already referred to between Brough and Dunnet Bay, were observed to the page 34 south-east of the fault, which is quite in keeping with the rest of the evidence in favour of an ice movement from the south-east throughout the Caithness plain.

Mr Dick noted a large granite boulder on the hill-side above East Murkle, near Castletown, a similar one at the head of Weston Loch, and two of the same material round the same loch. He has also recorded the occurrence of a conglomerate boulder near the Slater's Obelisk at Holborn Head. Mr C. W. Teach observed blocks of the Sarclet conglomerate near Weydale, south-east of Thurso.

West of Eeay numerous blocs pcrchés occur on the moraine heaps, consisting of granite and granitic breccia, and at Dalnawillan, in Strathmore, blocks of metamorphic rocks also occur on the mounds.

Conclusion.

We must now consider the evidence which has been adduced in the foregoing pages with the view of determining the probable physical conditions which prevailed during the formation of the various superficial deposits in Caithness. We have endeavoured to show that across the plain there is one prevalent system of ice-markings running south-east and north-west, which, from the appearances presented by the striated surfaces near Latheronwheel, the Old Man of Wick, and Brough, seem to have been produced by ice moving from the south-east. This conclusion receives additional support from the fact that, as we proceed from the Ord to Reay along the tract lying between the county boundary and the inland limit of the shelly drift, the strife point E., E.N.E., N.N.E., N., and eventually swing round to the N.W. The traverses we made across this tract place beyond doubt that the local ice, radiating from the hilly ground to the west, moved outwards towards the Caithness plain, but having there met a powerful opposing ice-current, it was compelled to change its course and turn round in the direction of the Atlantic.

That such was really the case is confirmed by an analysis of the evidence supplied by the boulder clay. There are two page 35 deposits of this nature, the one comprising local rocks and produced by local ice; while the other is richly charged with marine shells, and contains blocks which are foreign to the county. The areas occupied by the two boulder clays correspond with the limits of the respective ice-streams, as indicated by the striations on the rock surfaces. Moreover, in spite of the lithological uniformity which prevails throughout the tract occupied by the Caithness flagstones, there are certain data connected with the dispersal of the stones in the shelly boulder clay which are only explicable on the supposition that the ice came from the south-east. Blocks of the Sarclet conglomerate can be traced inland in the boulder clay, while striated blocks of the grey flagstones occur in the moraine profonde west of the fault at Brough. Had the movement been from the north-west, then assuredly we would have found material derived from the massive yellow sandstones at Dunnet Head in the ground-moraine to the south-east of the fault. But this is not the case. In addition to this, there are blocks of oolitic limestone, oolitic breccia, septarian nodules, fossil wood, belemnites, chalk, chalk-flints, etc., in the shelly boulder clay, some of which are identical in lithological character and fossil contents with the representatives of these rocks in the basin of the Moray Firth and adjoining tracts. The occurrence of these foreign blocks in the grey drift is not explained by a movement from the north-west, while it is quite in keeping with the theory that the ice which filled the basin of the Moray Firth was deflected and forced to overflow the Caithness plain. In view of all these lines of evidence it is impossible to resist this conclusion.

When we consider the physical character of the reddish-brown boulder clay, it so completely resembles the ordinary lower till of Scotland, that no one who believes in the land-ice origin of boulder clay would hesitate to ascribe it to the action of that agent. The features presented by the shelly drift are somewhat different as we have shown, and for this reason the question of its origin has given rise to some diversity of opinion. But a careful consideration of the various phenomena connected with it shows that there is page 36 really no valid argument against the land-ice origin of this deposit.

It might be argued that the shelly drift is a product of coast-ice driven along the shore by currents; but the evidence derived from the organic remains is quite at variance with such a hypothesis. It has already been stated that the most careful searching has only brought to light a few specimens characteristic of the littoral zone, while the great majority of the shells belong to deeper water. Moreover, such a supposition leaves quite unexplained the gradual deflection of the local ice in its eastward course, neither does it account for the actual inland limit of the grey shelly boulder clay. Another formidable objection to this hypothesis, which is also applicable to icebergs or floe-ice, is the entire absence of stratification throughout the wide area occupied by this deposit. Dr Croll long ago pointed out that if the grey shelly drift were really due to floating ice, it would undoubtedly have shown signs of stratification. We know that the finely laminated shelly briclc-clays round the coast of Scotland, which occasionally contain striated blocks, point to aqueous disposition. But those who have examined the numerous sections of the grey drift in Caithness unite in saying that in physical character it is indistinguishable from ordinary boulder clay. Mr Jamieson states that it "resembles the Old Boulder Clay of the middle of Scotland in regard to its physical arrangement, but differs therefrom in the prevalence of marine organisms scattered through it." And in order to account for the occurrence of these organic remains, he imagines that "a set of marine beds containing Arctic shells were probably deposited over the low part of Caithness; and much drifting ice seems to have passed over the district from the north-west, which crushed and destroyed these marine beds, broke the shells, and mixed them up with the superficial débris into that mass of rough pebbly mud which now overspreads the surface."

Now, there is nothing improbable in the supposition that such marine beds were deposited in pre-glacial or inter-glacial times on the low ground of Caithness, though none has been chronicled by Mr C. W. Peach, Mr Jamieson, nor page 37 by ourselves. The only record of stratified beds underneath the boulder clay rests on the authority of Mr Dick.^* He describes a section seen in a small stream running into Gill's Bay, which has cut a channel down to the solid rock through a deposit of grey boulder clay, containing chalk, chalk-flints, and oolitic rocks, and yielding remains of Mactra, Cyprina, Turritella, and Dentalium. Below the boulder clay he observed a bed of gravel with broken shells resting on red sandstone. Again, on the south side of the Moray Firth, one of us found, in the summer of 1880, while prosecuting the geological survey of Banffshire, a series of stratified sands, with marine shells, which are covered in part with boulder clay. These shelly sands indicate a marine depression to the extent of 500 feet in inter-glacial times. It would seem, then, that there is evidence in favour of the existence of stratified beds with Arctic shells below the boulder clay in the north of Scotland. But even admitting the existence of such deposits, it is difficult to see how floating ice could so act on them as to produce the phenomena presented by the shelly drift. In such a case there would have been signs of stratification in the deeper sections, as, for instance, in the Scrabster Harbour where the deposit is upwards of 100 feet thick. Nay, more, such a theory does not account for the greater abundance of marine shells along the eastern seaboard, and the gradual increase of blocks derived from the Caithness flagstones as we move inland from the east coast. Neither does it explain the deflection of the local ice.

It is perfectly evident, therefore, that the phenomena of the grey shelly boulder clay cannot be satisfactorily explained on the hypothesis of floating ice, and we are therefore forced to accept the only remaining solution, that it is really a product of land ice. Indeed, when we view the evidence supplied by the striated surfaces and the boulder clay in the light of our previous work in Orkney and Shetland, it will readily be admitted that the glacial phenomena of these widely separated areas have a close relation to each other. They point to the union of the Scotch and Scandinavian ice-sheets on the floor of the North Sea. The ice which flowed page 38 into the basin of the Moray Firth, as well as the local ice which streamed outwards in the direction of the Caithness plain, was deflected towards the north-west by reason of the greater force of the Scandinavian mer de glace. The pebbly mud and marine shells would be borne inland from the bed of the North Sea across the low-lying part of Caithness, where they would be commingled with the débris of the flagstones, and any marine deposits which might have been deposited in pre-glacial or inter-glacial times. Blocks of the various secondary formations derived from the areas crossed by the Scotch ice would also be mingled with the ground moraine.

It is no doubt true, as Mr Jamieson pointed out, that the mollusca are of a less Arctic type than those obtained from the stratified shelly clays of Elie, Errol, and other localities. But this may quite well be explained by supposing that they belong to a pre-glacial or mild inter-glacial period. The evidence in favour of alternations of climate in glacial times is steadily accumulating, during which there were constant migrations of northern and southern fauna. It does not follow, therefore, that because the fauna of the Caithness boulder clay is of a less Arctic type that the deposit does not belong to the boulder clay period. There can be little doubt, from the evidence we have adduced, that the reddish-brown boulder clay of local origin is of the same age as the grey shelly drift.

The widespread traces of moraines and gravels prove that long after the Scandinavian mer de glace had retreated, and the climatic conditions had become less severe, local glaciers moved outwards from the hilly ground to the west, depositing their materials alike on the red and the shelly boulder clay.

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