Geology
In pre-historic times Dhu Heartach was covered by the ice sheets which 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 425 feet (130 m) lower than at present. 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.
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 240 feet (73 m) long and 130 feet (40 m) wide, and which rises to 35 feet (11 m) above sea level. Submarine surveys indicate that the rock is at the eastern end of a valley stretching 80 miles (130 km) into the Atlantic, which may “account for the seemingly abnormal seas to which the tower is subjected". It is an isolated outpost of the Inner Hebrides archipelago. The author Robert Louis Stevenson wrote:
An ugly reef is this of the Dhu Heartach; no pleasant assemblage of shelves, and pools, and creeks, about which a child might play for a whole summer without weariness, like the Bell Rock or the Skerryvore, but one oval nodule of black-trap, sparsely bedabbled with an inconspicuous fucus, and alive in every crevice with a dingy insect between a slater and a bug. No other life was there but of sea-birds, and of the sea itself, that here ran like a mill-race, and growled about the outer reef for ever, and ever and again, in the calmest weather, roared and spouted on the rock itself.
Read more about this topic: Dubh Artach