By Rev James Ritchie from Orcadian Papers (pub 1905)
(Continuing our articles on The Trap Dykes of Stromness)
The other trap dykes do not seem to require more than a bare notice, because we see only a small section of each of them, and they all present the same general characteristics. At the point of Breckness there is a line of upbreaking of the rocks, without a trap, one of four or five such lines of upbreaking betwixt Ness and Breckness, all pointing toward Hoy.
Rounding the point of Breckness thirty yards or thereby, we come upon a trap dyke proper, of no great thickness, level with the rocks, and running up into the land in the direction of the farm of Crafen. I hesitate to say that I have seen the same dyke many miles to the eastward. In driving to Kirkwall, I observed a trap dyke crossing obliquely a burn close to the public road at a bridge near the farm house of Rinniebister, in the neighbourhood of Wideford Hill.
Trap dykes often extend for miles in the same direction, some of them being more than fifty miles long. Can this one, so far east, be the same dyke as at Breckness? It is of the same thickness, and appears to come from the same direction. It may not be in the same mathematical line, but we know that they do not observe an absolutely straight line, but an irregular one, with frequent lateral dislocations, as may be seen both at Cairston and at Netherton.
At Yesnaby there is another dyke, running up the face of the steep rocks near Yesnaby Castle and thence stretching close inshore on the level of the ordinary rocks below, in the direction of Hoy Head. The only other dyke I have seen is at Costa Head, in Evie. Level with the Old Red Sandstone rocks, with the remains of the Coccosteus lying exposed on their surface, the trap dyke appears, three or four feet in thickness, and pointing in the direction of the northern end of the opposite island of Rousay.
I may add, that in Cairston, a little beyond the trap dyke, there is deposit of very hard structure and greyish white colour on the surface, which does not seem to take the form of a dyke, but appears to be a deposit of porphyry, a rock of somewhat similar origin to trap, and at Deerness, just opposite Copinsay, there is a considerable deposit of porphyry of a dark brown colour, with crystals of felspar, in large white quadrangular patches on the dark surface.
Returning to the trap dykes, we may enquire into their probable age, though here our calculations cannot lead us to any very definite conclusions. The Old Red strata were undoubtedly subject to great alternate elevations and depressions during their accumulation, and therefore trap may have been intruded among them at any stage. But it is understood that the era of the Old Red was one of comparative tranquility so far as the volcanic forces were concerned, and that these forces were extremely active, both at the beginning and at the close of that era, as we have only the Old Red deposits in this quarter, the question of age is virtually narrowed to the alternative enquiry whether the trap rocks originated at the beginning or at the close of that system.
More next Sunday on ‘The Trap Dykes of Stromness’