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Dubh Artach (; ) is a remote skerry of basalt rock off the west coast of Scotland lying west of Colonsay and south-west of the Ross of Mull. A lighthouse designed by Thomas Stevenson with a tower height of was erected between 1867 and 1872 with a shore station constructed on the isle of Erraid. The rock is subject to extraordinary sea conditions with waves of or more being encountered by the keepers. Despite these adverse conditions several men served the light for lengthy periods until it was automated in 1971. ''Dubh Artach'' is the official name of the lighthouse, although the skerry itself is also known as Dhu Heartach. Various interpretations have been provided for the original meaning of the Gaelic name, of which "The Black Rock" is the most likely. == Geology == In pre-historic times Dhu Heartach was covered by the ice sheets that spread from Scotland out into the Atlantic Ocean beyond the Outer Hebrides. After the last retreat of the ice around 20,000 years ago, sea levels were up to lower than at present.〔McKirdy ''et al.'' (2007) p. 207.〕 Although the isostatic rise of land makes estimating post-glacial coastlines a complex task, circa 14,000 BP it is likely that Dubh Heartach was at the western edge of a large land bridge linking what is now the island of Ireland to Scotland. This land mass included the islands of Jura and Islay and was probably connected to the mainland of Scotland by an isthmus near Loch Craignish, south of Oban.〔Murray (1973) pp. 67–69.〕 Steadily rising sea levels would then have slowly isolated and finally all but submerged Dhu Heartach, which today is a rounded, dark green mass of basaltic rock called augite, which is long and wide, and which rises to above sea level.〔Nicholson (1995) pp. 146-57.〕 Submarine surveys indicate that the rock is at the eastern end of a valley stretching into the Atlantic, which may "account for the seemingly abnormal seas to which the tower is subjected".〔Stevenson, D.A. (25 April 1876) ''The Dhu Heartach Lighthouse''. Proc. Inst. Civ. Eng.〕 It is an isolated outpost of the Inner Hebrides archipelago. The author Robert Louis Stevenson wrote:
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