Tarry confusion

Just below the upper tidal limit, the rocks often are slippery, covered with a blackish, tarry, oily-looking crust, treacherous to step on, unappealing to touch. It's worth stopping to look at it, though: it had scientists stumped for years, not knowing what it was.

Turkish washcloth, Petrocelis phase.

More Turkish washcloth, Mastocarpus papillatus

The crusts are about 1/4 inch thick, well glued down; they don't scrape off or stick to curious fingers. Dry, they're almost black, but otherwise they are usually a dark, bluish red. They grow around, not over barnacles. (Something in the barnacle chemistry that they don't like, maybe?)

In older books, they're assigned to the genus, Petrocelis. Eventually, they were discovered to be a phase in the life cycle of Turkish washcloth seaweeds.

The most confusing stage is the tetrapsorophyte, once thought to be a different alga altogether.  It is encrusting and looks like a patch of soft tar on a rock.  Once the life cycle was figured out the species became known as Mastocarpus papillatus.  However, common synonyms still exist such as Gigartina papillata.(*) (Joe Lutz)

Either Turkish towel or Turkish washcloth. The towel is much larger, but otherwise similar. The bumps are reproductive structures, which give rise to the crustal phase.

I've been calling all the Turkish variants "towels", but they are distinct species. I Googled them for a bit of clarity, and got more confused; not only are the names changing, but even the number of species is in the process of being sorted out. And there are other seaweeds that have a crust phase, as well, difficult to distinguish on the beach. So for now, I'll stick to towels (big) and washcloths (small).

Another crust, with small patch of another seaweed, possibly Endocladia muricata. (I could be wrong. Easily.)

It commonly forms the top-most conspicuous band of seaweed along that coast. (Central California) E. muricata often grows with Pelvetiopsis limitata (dwarf rockweed) and Mastocarpus papillatus (Turkish washcloth), on rocks in the high intertidal. (Wikipedia)

There are more things in heaven and earth, Horatio, 

Than are dreamt of in your philosophy. 

(Shakespeare -Hamlet (1.5.167-8)

*Kozloff, 1973, was still calling the washcloth Gigartina, but recognizes the crust as a stage in its life cycle.