LITMIR.BIZ

      FIG. 15

      —Eigg showing the Sgurr and the terraced appearance of the island. (After A. Harker)

      Sills of igneous material, if on a big scale, may be sufficient to form islands by themselves. In the Shiant group, there are two main islands, Garbh Eilean and Eilean an Tighe, tied by a shingle beach (Fig. 16). The former is over 500 feet high and on its northern side is bounded by sheer cliffs. Farther to the east the lower part of the cliffs is scree-covered and partly overgrown. Eilean an Tighe, just over 400 feet high, is also bounded by high cliffs. Both islands are essentially formed of a single sill of dolerite, some 500 feet thick, and inclined at angles of 10°–15° to the south-west. Consequently the western sides of both islands have comparatively gentle slopes, whereas the scarp slopes to north and east give almost vertical cliffs, except that the great boulders fallen from them collect at their foot. The sill also shows columnar structure, both the diameter and length of the columns being far greater than in Staffa. The columns are nearly straight, but curve slightly near their summits on Eilean an Tighe and on parts of the north face of Garbh Eilean, where some individual columns are 350 feet high, and have an average diameter of five feet. Whilst the columns are generally hexagonal, other patterns also occur.

      Apart from the columnar jointing there is also another set of major joints running approximately north-north-east and south-south-west. On the cliff face on the north of Garbh Eilean near Glaic na Crotha, they have a marked effect, and the rock is broken into great and roughly parallel slabs, instead of into individual columns. On the south-west of the same island, where the coast is very steep, there is a similar phenomenon, and there, too, the columns overhang and fall into the water. Hence, some of the big joints have opened out and great fissures running in a northerly direction into the rock have been made by wave action working along lines of weakness formed along these major joints.

      The adjacent island, Eilean Mhuire, is also cliff-girt, but is made up of nearly horizontal sills of dolerite separated by three beds of Jurassic strata. To the west of Garbh Eilean is a chain of small islands, called collectively Galtachean. They too are composed of dolerite, probably the remnant of a great single sill. All show well marked columnar structure. The inclination is very variable and the curvature of the columns often conspicuous. All the Galtachean have steep and rocky north-facing cliffs. The Fladda isles are also doleritic and show good columnar structure, caves, and other similar features. They are interesting, but not as spectacular as the Shiants.¹

      The little Isle of May in the Firth of Forth is another example of a single sill, in this case inclined to the north-east at a low angle. Here again the dip slope is gentle, and there is a steep slope to the south and west. There are many east-west faults cutting through the island, which, while they have caused little if any displacement, have nevertheless broken the dolerite so that the sea has been able to eat out gullies or geos along these lines of weakness. In the northern, and flatter, part of the island, the geos allow water at high tide to pass through the island.

      FIG. 16—The Shiant Islands. (After F. Walker)

      Another interesting feature is that although columnar jointing is often conspicuous in the western cliffs, it is effectively masked by jointing parallel to the cooling surface of the sill. Other major joints have also played a great part in cliff structure; they have led to the development of rock stacks and arches, the best examples being the Mill Dorr just north of Pilgrim’s Haven. On the gentle north-east side the dip and jointing are approximately parallel, and a group of skerries, called the Middens, has been produced a short distance offshore.

      The cliffs reach a maximum height of 150 feet on the western side, and there are clear traces of raised beaches at 50 feet, and probably at 25 feet and 100 feet. Marine erosion has therefore made great use of the lines of weakness in the sill to give it its present features. It is not, however, clear how far the features are the product of present-day conditions of sea level.

      There are many other examples of sills and lava flows that might be given from the western isles of Scotland. As another instance, however, consider the Great Whin Sill which provides such striking and characteristic features on the Northumberland coast. There are various outcrops on the coast, but the main one is that between Dunstanburgh Castle and Cullernose Point. At both these headlands good columnar structure is seen in the high cliffs facing north at Dunstanburgh and south at Cullernose. Between the two places the sill has a seaward inclination, and between tide marks the water is washing a clean surface, much broken by rough and irregular jointing, although pentagonal outlines are commonest. There are also enlarged joints, crush-lines, and small faults, nearly all of which trend east or north-east. There is a major break at Craster harbour, Hole o’ the Dyke, which appears to be due to a crush belt. In it is a mass of Whin showing joints running in all directions. Since the inclination is seaward, the ground rises inland, and a few hundred yards from the shore the Whin forms a fine escarpment facing westwards. Thus, most of the shore exposure corresponds to the gentle slopes we have met in the Shiant Islands and in the Isle of May. (See Pl. XXI).

      In strong contrast to the dark cliffs of basalt and dolerite are the pink and grey cliffs of granite in the Land’s End Peninsula, the Isles of Scilly, Peterhead, and a few other places. Granite varies a good deal within itself, but it is characteristic of it that it often weathers into great cuboidal masses. The form of cliffs will, however, depend a great deal upon the nature and inclination of the joints and of other lines of weakness. Often granite cliffs form fine castellated masses, since both marine and subaerial erosion work quicker along joints and other planes of weakness, thus causing the solid rock in between to stand salient as buttresses and walls. This is excellently developed between Land’s End and Penberth, where the cliffs are steep and subject to the full onslaught of the Atlantic. There are many coves, and the cliff top, over which salt spray is blown, is a fine open heather-covered moorland reaching as far as St. Levan. Nearer Lamorna, the cliffs have a gentler slope and are less spectacular; much the same is true near Land’s End.

      The bizarre features of granite weathering, partly marine, partly subaerial, are remarkably displayed in the Isles of Scilly, which are wholly granite islands, probably at one time part of a larger mass like Bodmin Moor, or any of the other granite masses of Cornwall and Devon. They are now partly drowned, and subjected to powerful marine erosion. The sea has eaten into joints and other planes of weakness and together with subaerial weathering has produced curious features resembling great blocks roughly piled upon one another. Their surfaces are clean, but rough in detail owing to the differential rates of erosion of the various minerals of which the granite is composed. Sometimes the surface of the large blocks may be hollowed out into kettles and pans; in other places weathering and marine erosion may produce spiky forms. Often large slabs are left projecting or overhanging. In the lower parts where the cliffs, in the ordinary sense of the word, are insignificant or even absent, the outlines may be most irregular. The granite readily breaks down into sand, and bayhead beaches and tombolos are common. There are also cracks, running generally a little west of north. These are most conspicuous on the northern shores, where the Atlantic waves eat out the crushed granite along them. There is reason for thinking that these cracks are a result of the latest movements which have affected the granite, but they definitely have a great influence on the configuration of the islands as a whole, and of the waterways leading to the Interior sea.

      In some islands, especially in Great Ganilly, there is a series of vertical cracks along which there has been some alteration of the granite. These cracks are often referred to as Greisen¹

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