[2] The southern group of rocks and Ruadh Sgeir are formed from potassium-feldspar-phyric monzogranite intruded as part of the Caledonian Igneous Supersuite towards the end of the Caledonian orogeny (late Silurian to early Devonian period) and form an outlying part of the Ross of Mull pluton.

[5] Between 1867 and 1872 a lighthouse was built on the isolated reef of Dubh Artach, some 16 km (10 mi) southwest, in response to the hazards these rocks jointly presented to shipping.

[1] Nicholson (1995) calls them "4+1⁄2 miles [7 km] of jumbled granite teeth" and that "the extent and confused nature of this reef claimed untold numbers of vessels plying between America or the Baltic ports and Oban".

Stevenson's father, Thomas was the designer of Dubh Artach lighthouse,[12] and the young Robert Louis knew the area well.

The tourist on this trip can see upwards of 3 miles [5 kilometres] of ocean thickly sown with these fatal rocks, and the sea breaking white and heavy over some and others showing their dark heads threateningly above water".

[14] This passage begs comparison with Kidnapped itself: Altogether it was no such ill night to keep the seas in; and I had begun to wonder what it was that sat so heavily upon the captain, when the brig rising suddenly on the top of a high swell, he pointed and cried to us to look.

Away on the lee bow, a thing like a fountain rose out of the moonlit sea, and immediately after we heard a low sound of roaring.

If I had kent of these reefs, if I had had a chart, or if Shuan had been spared, it's not sixty guineas, no, nor six hundred, would have made me risk my brig in sic a stoneyard!

Haswell-Smith (2004) states that the name is derived from the Gaelic for "loud murmering or thunder"[1] although in a different context Mac an Tàilleir describes Torrain as meaning "hillocks".