Can you see the orange coloration along the lower part of the wall and forming rings around the boulders?
Here's a closer view (below). The boulder is about 2.5 feet across (~76 cm). Any guess about what's causing the orange color?
There are orange sponges in this area that could look like this from a distance, but it didn't seem quite right, so we got down for an even closer view:
Spirorbid tubeworms! Wow, neither of us had seen such dense and expansive aggregations before.
The next photo is helpful because some of the tubeworms are underwater with their bright orange tentacles expanded, and others are out of the water with their tentacles withdrawn so it's easier to see their white tubes (with just a hint of orange at the entrance).
Most spirorbid tubeworms are brooders (brooding their larvae) and once released their larvae have very short planktonic durations — swimming in the water column for a very short time before they settle onto a rock (or other suitable surface).
Perhaps the surge channel we walked by is a good setting for high retention, where many larvae are released and then settle in close proximity to each other instead of being swept away by waves and currents?
If you're interested in seeing microscope views of these fascinating tubeworms, review the post from 6 February 2012.