The Trap Dykes of Stromness: Part 4

By Rev James Ritchie from Orcadian Papers (pub 1905)

(Continuing our articles on The Trap Dykes of Stromness)

There are two considerations that seem to show that the trap was deposited, not earlier at least than the close of the Old Red period.

  1. There is no superficial deposit of trap, so far as I can learn. If the intrusion had taken place at the commencement of the period, we should have found an overflow.
  2. A more decisive fact is the compact structure of the trap itself. All irruptions of molten matter from beneath contain particles of water in their composition. These particles, on reaching the surface, expand into innumerable little globes of steam, turning the mass into pumice stone. But if the molten trap is so far beneath the surface that the weight of rock above is at least as great as the expansive force of steam below, no steam at that depth is formed, the trap does not become porous, but remains compact in structure. Now all these dykes are very compact and hard, and we may therefore infer, that when they were intruded into the strata, a vast depth of rock lay over them, probably as great as the Hoy masses at present, before the greater part of it was removed, in process of time, by denudation.

If the trap dykes were deposited at the close of the Old Red era, we may reasonably expect them to furnish us with some indication of the processes by which the land attained its present configuration. Comparing the strata at Dunnet Head, Hoy, Black Craig, and other points, we may assume that the whole of what is now Orkney formed, at one period, along with Caithness, one continuous range of strata under water, and possibly thereafter once continuous tract of land above it, though now broken up into a large number of separate islands.


Relief map of Orkney

The present arrangement of land and water is somewhat complex; and I think it might help one to remember it, if we were to take some point in the sea, not far from Hoy Head, and from that point draw three concentric semicircles along the leading waterlines, the first including the West Mainland, Flotta, and Walls, the second embracing Rousay, Shapinsay, Deerness and South Ronaldsay, and the outer semicircle including Westray, Eday and Stronsay. Then from the same centre let radii be drawn across these semicircles; one out through Water Sound, a second to Auskerry, south of Stronsay, a third to the south-east side of Sanday, and a fourth to the north of Rousay and North Ronaldsay. Both the circles and the radii pursue the great waterways. Only, it is not with the circles we have now to do, but merely with the four radii intersecting them.

I may here observe that the upheaval of granite is always accompanied by the elevation of the land, while the upheaval of trap, in the form of dykes, seems to have been concurrent with the depression of the land. Where there have been many active volcanoes (as in South America at present), there may be found whole mountain ranges of trap and basalt, like the Pentland range in the Lothians, or the Sidlaw hills in Strathmore. But where the movements of the land have been more leisurely and gradual, while the granite strikes along the land ridges, the trap seems to strike along the water lines, in a locality such as ours.

We can understand how this might be brought about if we think what might happened when our lower Old Red was partly deposited, and was still under water. The water, percolating the soft or the sandy strata till it reached the surface of the intensely hot granite underneath, would expand into steam were it not for the weight overhead, the strata, buoyed up, would begin to rise, and this process of elevation would go on till the soft and overstrained strata gave way, either at the summit of elevation, or along the weaker lines from the summit outwards, like the four or five rents, or volcanic vents as they are called, on our own shores, already alluded to.

Through these vents thus opened the steam would escape and the water in turn would rush in, causing when expanded into steam, a further upheaval and this process would continue until the surface of the granite below became comparatively cool, the strata settled down to their former position, the rents closed hard, twisting the strata, or were slowly filled up by crystallised materials of various sorts, additional strata were deposited above, and a long period of quiet followed.

More next Sunday on ‘The Trap Dykes of Stromness’

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