Hydropunctaria maura

Hydropunctaria maura, still often called by the older name Verrucaria maura and commonly known as tar lichen, is a species of saxicolous (rock-dwelling), crustose lichen belonging to the family Verrucariaceae. A perennial species that does not experience seasonal variations, it is the type species of the genus Hydropunctaria. The medulla is a black basal layer that forms columns (Latin: punctae) to the upper surface and isolates the algae into pockets near the upper surface. The black band formed by H. maura can often be seen at a distance as a marker of the high water point....read more on Wikipedia.