IV. Boulder-clay.

This deposit is not spread over the general surface of the low undulating tablelands in the form of a more or less continuous covering. It occurs mainly round the bays, whore it frequently attains a considerable depth, while the inland districts are covered with a thin clayey soil, duo to the decomposition of the underlying flagstones. We shall have occasion to describe one or two sections of Boulder-clay which may be traced continuously along the shore for half a mile, and which are quite undistinguishable from the Lower Boulder-clay of Scotland. Occasionally thin patches of this deposit are to be found on the cliff-tops, containing well-striated stones and foreign rocks, clearly indicating that the islands must have been overflowed by the ice.

In the island of Westra the Boulder-clay is sparingly distributed, but some excellent sections are to be met with round the bays in page 655 the southern districts. At Rackwik, near Stangar Head, it consists of a tough tenacious gritty clay, which is chiefly made up of red sandstone fragments, about 80 per cent, of the larger blocks being composed of sandstones which are foreign to the island. Some of these blocks, which arc finely smoothed and striated, measure six feet across. The deposit rests on the grey flagstones, and some small subangular fragments derived from the underlying rocks are likewise included; but the great majority of the stones consist of sandstones which we identified as belonging to the island of Eda. Crossing the peninsula to the shores of Tuquoy Bay, similar sections arc presented, resting on grey flagstones, the included blocks being composed of the underlying rocks, red sandstones, quartzites, and chalk-flints. It is important to note that the red sandstone blocks do not form such a large percentage at this locality, but that they gradually diminish in number as wo recede towards the north-west. On the slopes of Cleat hill fragments of granite, quartzite, diorite, and dolerite are associated with the flagstones in this deposit; while still further north, near the church, blocks of red and white honeycombed sandstone and small pink granite stones were observed in the Boulder-clay in addition to the local rocks.

Along the west coast hardly any Boulder-clay is to be met with; but some thin patches arc to be seen on the cliffs at Nonp Head, containing well-striated stones derived from the flagstones of the island. Occasional smooth blocks of Red Sandstone occur in the hollows amongst the debris of the underlying rocks, which arc, in all likelihood, the relics of the onco existing Boulder-clay.

Now it is evident, on a moment's consideration, that the gradual decrease in number of the red sandstones in the Boulder-clay, as we traverse the island from the south-eastern headlands towards the west coast, indicates that the ice-flow must have been towards the Atlantic; and when wo consider that these sandstones nowhere occur in situ in Westra, and that they could only have been derived from the adjacent islands of Eda and Sanda, we are forced to conclude that the ice-movement must have been altogether independent of the islands.

Along the east coast of Eda the Boulder-clay is not so abundant as in some of the more sheltered bays on the opposite side of the island, which is satisfactorily accounted for by supposing that the rocky slopes facing Eda Sound were exposed to the full sweep of the mer de glace. Here and there, however, patches do occur, as on the north shore of Lonton Bay, where the deposit contains smoothed and striated chalk-stones, along with blocks of red and white sandstones and grey flagstones. Its most noteworthy feature is the presence of worn fragments of marine shells, which are scattered irregularly through the stony clay. Similar sections occur in the bay of Calf Sound near the pier, and also along the west coast near the Wart of Eda, where shell-fragments were likewise observed.

Perhaps the finest section of this deposit in Eda occurs along the 'banks of a small stream which flows into the bay about a mile east of Fara's Ness on the west coast. The stream has cut down through page 656 the stony clay to the finely grooved pavement of sandstone, so that the glacialist can examine thoroughly the nature of the deposit. It consists of tough red clay, packed with smooth and striated stones scattered irregularly through the section. There is no trace of stratification in the deposit, as it retains the same tumultuous character throughout. The stones are beautifully striated along the major axis, and arc mainly composed of the underlying red and yellow sandstones, varying in size from an inch to several feet across. In addition to these wo noted smooth chalk-stones, chalk-flints, and subangular blocks of the grey flagstones. The most interesting feature, however, is the occurrence of small worn fragments of marine shells which are scattered indiscriminately through the deposit; they are smoothed and striated precisely in the same way as the stones in the Boulder-clay, as if they had been subjected to the same abrasion.

Shelly Boulder-clay was also observed on the west coast of Sanda, between Spur Ness and Stranquoy: and sections of the same deposit are to be found in Bacaskeal Bay. In the Burness peninsula, near the Holms of Eyre, the shore is bounded by low cliffs of purple shales and flags, with a coating of Boulder-clay, which is just sufficient to cover the surface of the rocks. It is chiefly composed of fragments of the underlying rocks, but likewise contains fragments of sandstone, granite blocks, and smoothed stones of gneiss and schist, all of which, except the sandstone, are foreign to the island.

In Stronsa several important sections were met with both on the east and west sides of the island. On the shores of Linga Sound, not far from the narrow isthmus of Aith, a soction of shelly Boulder-clay occurs resting on grey sandstone, the deposit being upwards of 25 foot thick, and comprising chalk, chalk-flints, and white quartz, in addition to the blocks derived from the flags and sandstones of the island. Further, on the north-east corner of Rousholm Bay a thin coating of this deposit rests on the flagstones, which are bent over to the north-west in the direction of the ice-flow.

One of the best exposures of Boulder-clay in Orkney occurs on the eastern shores of Odin Bay, in Stronsa, where it forms a continuous cliff for nearly half a mile. At intervals the section is obscured by a grassy covering, but every succeeding storm washes anew the face of the cliff, and exposes a fresh surface for examination. The deposit, which varies from 20 to 30 feet in depth, consists of a tough gritty clay of a reddish colour, full of well-smoothed and striated stones, which are mostly of small size. There are few largo boulders to be seen, the largest rarely exceeding a foot in diameter. There is not the slightest trace of stratification from one end of the section to the other, as the stones are disposed irregularly through the clayey matrix. By far the greater number of the included blocks have been derived from the flagstones and the sandstones which occur in the neighbourhood; but the following rocks are likewise represented:—granite, pink porphyritic felstone, gneiss, schist, quartzite, white quartz, dark limestone, with abundant plant-remains, which is probably of Calciferous-Sandstone age, oolitic page 657 limestone, oolitic calcareous breccia, fossil wood (probably oolitic), chalk, and chalk-flints, all of which are foreign to the island. When we come to collate the evidence regarding the primary glaciation, we shall discuss the probable localities from which these blocks were derived. At present it is sufficient to state that the evidence is clearly in favour of their having been carried from the mainland of Scotland.

Equally important is the presence of numerous fragments of marine shells throughout the deposit. Though we examined the section with the utmost care, we did not succeed in dislodging a complete shell; indeed so worn are the fragments that it was with the utmost difficulty that we obtained specimens sufficiently well preserved for determination. Nearly all the fragments are smoothed and striated, like the stones in the Boulder-clay; and there can be little doubt that these characteristics are due to the very same cause in both cases. Amongst the broken shells we detected fragments of Cyprina islandica, Mytilus, and Mya truncata; but a careful search, after severe storms, by some local collector would certainly increase this list considerably.

In the island of Shapinshay shelly Boulder-clay occurs at various localities on the east coast, as at Kirkton, where it contains finely striated chalk-stones. The best sections, however, occur along the western shore, and especially in the bay south of Galtness, where it forms a bluff cliff washed by high tides. This cliff furnishes valuable evidence regarding the ice-cam', inasmuch as we noted amongst the included stones blocks of the slaggy diabase which occurs in situ in the south-east corner of the island, along with striated fragments of the sandstones which are associated with the volcanic rocks. In this section smooth blocks of chalk and oolitic limestone, with numerous fragments of marine shells, were also observed.

If we traverse the Mainland from Scapa and Kirkwall westwards, by the Loch of Stennis, to the crystalline axis north of Stromress, similar conclusive evidence regarding the north-westerly movement of the ice is obtained from the Boulder-clay. On referring to the map of Orkney, it will bo seen that the narrow zone of red and yellow sandstones which crosses the Mainland from Inganess to Scapa extends south-westwards along the shore as far as Orphir Kirk. Now, in the shelly Boulder-clay in Kirkwall Bay, to the east of the pier, striated blocks of red sandstone are commingled with the flagstones in the clayey matrix. The latter are by far the most numerous, and arc likewise beautifully scratched along the major axis; but the sandstone blocks constitute a fair percentage of the included stones. From the lithological charactcr of these blocks, we had no hesitation in concluding that they had been derived from the sandstones to the east of Kirkwall.

Again, in the sections occurring on the coast between Houton Head and Irland Bay, the observer cannot fail to note the gradual increase in the number of the sandstone blocks in this deposit as he approaches Houton Head, a phenomenon which is quite intelligible when he remembers that the striations along the shore point W. page 658 12°–42° N., the latter being the prevalent direction. Indeed at Houton Head the ice-markings are nearly parallel with the coastline, so that the sandstone blocks could not possibly have come from Hoy. Blocks of the same rock are strewn on the hill-slopes above Gorsness, to the north-east of Maes Howe. It is a significant fact that not a single block of the granite or gneiss which occurs in situ to the north of Stromness and in the island of Graemsa is to be found in the Boulder-clay between Irland Bay and Houton Head, or anywhere to the east of the axis of crystalline rocks; but as soon as the western limit of these rocks is crossed, numerous blocks of granite and gneiss are strewn on the slopes and along the cliff-tops between Brak Ness and Inganess. Had the ice-movement been from the north-west, the phenomena would have been precisely the opposite of those we have described.

In the southern islands this deposit is not abundant; but in South Ronaldshay, on the shores of Water Sound, cast of St. Margaret's Hope, we observed patches of it containing blocks of sandstone, flags, and chalk, with comminuted shells. In this instance the shells, when being dislodged, crumble readily to a white powder.