The Trap Dykes of Stromness: Part 2

By Rev James Ritchie from Orcadian Papers (pub 1905)

Proceeding to the west side of the Holm, I found the dyke there also, standing about two feet in height and stretching down into the harbour. It occupied the line of the old slip, in fact was utilised for that purpose, having been formed into a slip by stones built over and behind it. No trace of it now remains, as the new slip, which is broader and higher than the old, has been built right over it, a fact which may prove rather tantalising to future enquirers, and which forms a very good illustration of the difficulties that sometimes attend investigations of this kind.

But a more interesting question now arose as to the opposite side of the harbour – Will the trap be found traversing the granite, or, will the granite have obliterated the trap? I had observed that the dyke ran towards the south end of the harbour, where the shore is not so fully built over. I proceeded thither, and just beyond the last house on the seaward side of the street, I found a very singular arrangement of the granite.

There is here a deep depression in the rock, and as seen from the street there are two very distinct rents in the granite running along the bottom of the depression. Raised granite is often rent and cracked, but not commonly in this fashion. The two rents are very marked; they run in straight lines, parallel to each other, about six feet apart, and pointing to the inner Holm. The space betwixt these two rents is not occupied by solid granite, nor is there any appearance of trap; but it is filled by angular masses of various rocks, jammed in together, some of them granite, some resembling granite imperfectly crystallised, and along with these, a goodly proportion of a rock which I have retained a specimen.

At first I was inclined to think that this rock might be altered or baked trap, trap changed in structure and colour from being brought into contact with the granite while the latter was still intensely hot. Having no certain means of determining this point here, I sent specimens of it and of the trap to a friend in Edinburgh, who was fortunate enough to get the opinion of Professor Geikie of the Geological chair in the University. He only examined them with a hand lens, but he was satisfied that the rock was quartzite or baked sandstone, and that it had no connection with the trap dyke.

That point being settled, the question still remains as to the character of this gap in the granite filled in with altered sandstone and other rocks. Does it indicate the line which the trap dyke once took though now no longer present? I will not enter upon this question now, but may notice four points that must be taken into account in discussing it.

  1. According to trustworthy geologists, granite may be of any age up to the time of the New Red Sandstone or even later.
  2. The trap dyke traverses our lower Old Red Sandstone from its base upwards.
  3. The granite is in immediate contact with the Old Red at the Life Boat House and elsewhere, and incloses in its upper surface, as in a matrix, a vast number of water-worn stones and blocks, just where the Old Red Conglomerate ought to be.
  4. The gap in the granite referred to is of the very width, and occupies the very position in which the trap dyke would be expected to be found.

Apart from this question altogether, it is perfectly plain that the dyke at Cairston is an extension of that at Netherton, but I have not been able to trace it eastward beyond the point already referred to.`


The Old Man of Hoy stands on granite and here you can see the many layers of sandstone (photo F Grahame)

More next Sunday on ‘The Trap Dykes of Stromness’

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