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ON EGG-SHAPED STONES DREDGED FROM WICK HARBOUR. 135

On Egg-shaped Stones Dredged from Wick Harbour. By D. TAIT.

(Read 20th March 1907.)

FROM time to time, rounded, mammillated, and egg-shaped stones have been dredged from the bottom of Wick Harbour, many of which are remarkably symmetrical in form. They vary in diameter from four or five inches to about a foot and a half. These stones have attracted the attention of many persons in Wick interested in natural science, and were brought to my notice by Bailie D. R. Simpson, of Wick, who has a large collection of them in his garden (Plate XII.). Bailie Simpson has suggested that their occurrence in the boulder-clay in the bottom of the harbour is due to transporta­ tion eastwards by the pr'eglacial Wick River. He attributes their peculiar shape to grinding in potholes in the preglacial river bed. He also notes that the sandstone of which they are composed is not seen in situ near Wick, so that they must have been brought from a distance. After a close inspection of the stones in Bailie Simpson's garden, I have come to a different conclusion for the following reasons. First—all the stones without exception are composed of the same rock, a yellowish or greyish micaceous sandstone. If they had been a collection of pothole stones carried by a river from a distance, we would have expected some variation in their composition. Secondly—though some are egg-shaped and smooth, others are not so, but show peculiar projections in­ dicating that their form cannot have been acquired by grinding in potholes. One specimen in particular shows a number of concentric sunken lines, and one at least is mammillated in the fashion often seen in calcareous sandstones. These facts in my opinion indicate that their outward form is due rather to concretionary action than to grinding in potholes. These stones are composed of a light greyish or yellow sand­ stone, unlike any of the Old Red Sandstone rocks seen in situ in the neighbourhood, or in the very perfect coast sections north and south of Wick; but it does resemble a sandstone containing Jurassic fossils found in the boulder-clay all over the east coast of Caithness. Attention to the glacial phenomena of the district shows clearly that these Jurassic rocks have been Downloaded from http://trned.lyellcollection.org/ at West Virginia University on July 16, 2015

136 EDINBURGH GEOLOGICAL SOCIETY. carried northwards from the neighbourhood of Brora, where Jurassic rocks are in situ.1 Shortly after having seen Bailie Simpson's collection of fcC egg- stones " I visited Leovad Sand Quarry, 12 miles W.S.W. of Wick, and about 20 miles N.N.E. of the nearest Jurassic rocks. Some of the concretions to be seen there are remarkable for their great size, the largest being ten or twelve feet in diameter. They are composed of a hard, durable rock, while the material enclosing them is weathered to a sand, soft enough to be dug with a spade. There is little doubt that this rock has undergone decomposition in the place where it is now found. One of the concretions which I obtained is egg-shaped, and in all particulars resembles some of those collected by Bailie Simpson from Wick Harbour. The sandstone of Leovad Quarry is unlike any other sandstone in its own neighbourhood. It is separated from the surrounding Old Red Sandstone rocks by covered ground, and its relation to them concealed. It may, therefore, be a small outlier of Jurassic rocks lying uncomformably on the Old Red Sandstone. The concretions of Leovad Quarry appear to be made of the same rock as the egg-stones dredged out of Wick Harbour, so there is some reason for thinking that both have the same origin and are from the same strata. Both have also a closer litho- logical resemblance to the fragments of Jurassic sandstone common in the boulder-clay of Caithness than to any sandstone of Old Red age in the neighbourhood. They also have a strong likeness to certain Jurassic sandstones that occur in Skye and Eigg, which contain similar concretionary structures. It seems not impossible, therefore, that the egg-shaped concre­ tions of Wick Harbour and the concretionary sandstone of Leovad Quarry may both be of Jurassic age, those of Leovad Quarry being in a very small outlier of Jurassic strata lying unconformably on the Old Red Sandstone, and the " egg-stones " from the harbour derived from a bed on about the same geological horizon near Brora, and carried northwards as boulders to Wick by the ice of the Glacial Period.

1 See Messrs Peach and Home, " On the Glaciation of Caithness," in Trans. Roy. Phys. Soc. Edin., vol. vi. p. 316. Downloaded from http://trned.lyellcollection.org/ at West Virginia University on July 16, 2015

COLLECTION OF EGG-SHAPED STONES DREDGED FROM WICK HAEBOUR.