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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