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Thirl Stane Natural Arch

Kirkbean, Borron Point, Dumfriesshire.

From the south the sea has eroded an almost vadose passage along a fissure in a narrow ridge of Thirlstane sandstone. The ridge is parallel to the coast so the fissure links a sandy inland area with the shore.

Described by the surveyors of the Ordnance Survey as: "a large rock about 100 feet square having a natural gothic arch thro' it, and through which runs the flow and ebb of the tide. The arch will admit of a common sized man to walk upright under it. It is situated on the sea coast of the farm of Powillimount, and is considered to be one of the greatest curiosities along this Coast." [Scotland's Places]

"The ridge is formed by the 25 m-thick Thirlstane Sandstone Member. At the base of the Thirlstane Sandstone the contact with the underlying strata is irregular, and intraformational fragments and large plant remains are present in the lowest beds. The fine natural arch at Thirlstane and nearby exposures to seaward display various sedimentary structures in a pinkish grey, medium-grained, well-sorted, quartzose sandstone. The rocks are characterised by large-scale trough cross-bedding and a spectacular development of liquefaction structures which increase in frequency and magnitude from south to north along the outcrop. It is worthwhile examining the extraordinary degree to which the original bedding has been disrupted. In a detailed study of these structures Ord et al. (1988) recognised different types of structure including oversteepened and recumbently folded cross-stratification, domes, sand volcanoes and zones of anastomosing, vertical cracks. They attributed the magnitude and frequency of the liquefaction structures to causes such as local seismicity, and deduced the presence of a syndepositionally active fault lying north of the present outcrop.

Deegan (1970) proposed that the Thirlstane Sandstone formed as an offshore sand barrier which initially enabled the inshore lagoonal environment of the Powillimount Sandstone Formation to develop. The evidence is equivocal, however and, as Ord et al. suggest, an alluvial origin is also a possibility." [McMillan, 1996]

Alternative Names: Thirlstane Arch