Volcanic Sediment?

I noticed something odd on the Cruz Bay, St. John webcam earlier when I was adding it to my links. There seems to be black sediment being brought on shore by the waves.



I've seen something similar in my own back yard when coal barges would kick up small waves on the Kanawha River. Coal sediment would splash on shore in much the same way. Since the Virgin Islands are volcanic, I'm pretty sure this is volcanic sediment. Pretty neat.