The Trap Dykes of Stromness: Part 5

By Rev James Ritchie from Orcadian Papers (pub 1905)

(Continuing our articles on The Trap Dykes of Stromness we conclude with part 5)

At the close of the Old Red period, when the strata had been completed though still under water, a much more serious disturbance took place. By the re-awakened volcanic forces, large areas were raised high above the water. The strata at length broke down everywhere in long irregular rents, falling in, on both sides of the rent, upon the molten matter below, and by their immense weight, forcing the lava up to the surface, filling every rent and crevice, and cooling and hardening before it had time to crystallize into granite. The crushed and shattered upper rocks after much unquiet were gradually removed by denudation, and after the lapse of ages, the Old Red Sandstone , in our quarter, took the form we are now so familiar with that of the Orkney Islands

Part of what I have thus written is ascertained fact, but the greater part is of course, mere conjecture, and given for purposes of illustration. Let us now, however, return to the facts of the case. We have in connection with this parish three separate trap dykes. It would be absurd to suppose that the vast volcanic forces that produced them, were restricted to our parish in their action. These dykes are the result of far-reaching rents in the strata, rents that traversed the whole district in olden times, and gave shape and form to our islands. And it is surely a very remarkable and significant fact that, if we were to pursue the lines which these several trap dykes indicate we would be led along the great leading water ways that separate our northern islands.

Thus the dyke at Breckness would bring us to the dyke at Rinniebister, and thence to the south point of Shapinsay where a geological map I have seen, shews trap of the same kind and running the same way, and thence to Auskerry, thus traversing the greater part of the water line from Finstown to Auskerry. The Netherton trap dyke leads us first to Cairston and thence north east to the south-east side of Sanday traversing the water line from Finstown to Start Point.

The Evie dyke points through the north end of Rousay to the north of North Ronaldsay another prominent water line while the dyke at Yesnaby points to Hoy Head, determining the western boundary of the islands. The map I have referred to shews a line of trap from Hoy Head running the direction of Water Sound, another great water line. It shows also deposits of the same trap at the Old Man of Hoy, and at a point on the coast near Melsetter, which may have determined the line of the south west boundary of the islands, a line pointing across the Pentland Firth to Stroma and Duncansbay Head.

Should these views be corroborated by more extended researches all over the islands what increased interest would gather around the trap deposits in our neighbourhood! We should see in them the memorials and relics of a period in the dim, remote past, when the waters, which had held the field for unknown ages, were at last violently agitated as the land looked up from beneath their surface, and rose at least fifteen hundred feet into the sky, in all directions rending and sinking or rising and settling until it finally took the form and outlines it still retains. Of the vents of the epoch of terrific convulsion it is surely something to be able to say that we have still beside us in the Trap Dykes of Netherton and Cairston, of Breckness and Yesnaby not merely the contemporaries and witnesses of these vents but remains of one of the actual though passive agents which brought them about.

Next Sunday The Ruins of Breckness by W.G.T. Watt of Skaill

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