Stone Polygons beside Loch Lomond

Abstract

STONE polygons are polygonally arranged bands of flat pebbles, usually surrounding a core of mud, common in Arctic regions in soils composed of mud and stones. They may be associated with curved arcs of stones, each of which is also standing on end. These structures are well known in Spitsbergen, Norway, Iceland*, and Antarctica, and have been found in the Alps. Their formation has been discussed in a long and scattered literature. The chief theories of their formation have been clearly summarized in a paper by Professor J. S. Huxley and Dr. N. E. Odell, in this Journal (vol. 63,1924, pp. 207-229). The most widely accepted explanation attributes these struc? tures to the repeated outward push of the pebbles whenever the ground is frozen, and the flow back of the mud during thaw. That theory was advanced by Professor Hogbom (Bull. Geol. Inst. Upsala, ix, 1908, pp. 41-59) and has been widely adopted, as by Professor Hawkes for a case on a raised beach in Iceland (GeoL Mag., 1924, pp. 509-513, pl 28)and by Dr. C. S. Elton (Q.J.G.S., vol. 83,1927, pp. 163-193, pl. 10-12), Nansen (En Ferd til Spits? bergen, 1920 ; Spitsbergen, 1921) on the other hand regarded the polygons as due not to the migration of the pebbles but to the reduction to powder of those in the centre of a group by repeated freezing. This comminution of the stones no doubt takes place to some extent when they consist of soft shale; and both processes are regarded by Huxley and Odell as having shared in the formation of these structures. A modification of the frost-push theory has been advanced by Thoroddsen for the examples in Iceland, where they are known as rudemarks. His account of the phenomenon, although one of the earliest, has been often overlooked (Thoroddsen, 'The Botany of Iceland,' Pt. I. Physical Geography, 1914, pp. 259-261). He attributed these structures to the combined effect of irregular evaporation from sodden stony clay which has been cracked by frost into clay prisms, and of the pebbles being pushed by repeated frost and thaw into the shrinkage cracks. My interest in these formations was arowsed the day Sir Martin Conway's expedition landed at Green Harbour, Spitsbergen, on 18 June 1896. From the observations made there and at other places in Spitsbergen Professor Garwood and I concluded that these polygons were due to the repeated freezing and thawing of the ground. The stone hexagons appear due to the water freezing into large ice crystals charged with mud and stones, like the sand-charged calcite of the Fontainebleau Sandstone. We did not describe the structures as we expected to find examples of them in the English boulder clays; but to my repeated surprise I have failed to find them in the British glacial drifts, and I am not aware of any definite case of them having been described in glacial deposits, though vertical stones in such are not nixconftnion. Embryonic frost-formed stone polygons I have frequently observed in recent deposits in Scottish valleys, and I have occasionally seen them well dbveJoped, as on the summit of the Merrick (2764 feet), the highest summit in Scotland south of the Highknds (as remarked Q.J.GJS., vol. 83,1927, p. 193), and on Ben Lawers.