Hmm, a bunch of what, pea or slightly smaller size pebbles. Vaguely similar in size (unlike scattered impact fragments), lots of vessicles, slightly rounded, probably dark grey vs the nearly black soil?

Maybe I spent too much time growing up near volcanoes but it'd be very easy for me to believe they were tephra of some sort -- small cinders. Not as fine grained as pumice, more like the crap you trudge through up a relatively fresh basaltic cinder cone.

Looks more like that than seashells at least (and remember, this stuff is dark dark grey), but I certainly agree it's an odd looking soil!