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. 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". It is an isolated outpost of the Inner Hebrides archipelago. The author Robert Louis Stevenson wrote: