[Giving a thoughtful noise, the selkie leans over the rocks as his eyes search the water until he finds where the sand bar starts again as it stretches out past the opening to the little cave they're in. He continues laying on his belly as he pulls himself over to the edge of the rock, using his hands to scoop the sand toward himself into piles mimicking what he saw on the shore.

It's not quite the same- his are pretty lumpy and a lot wetter than the ones he saw- but it's close enough.]

I think I heard one of the smaller ones call them..."sand castles". But they don't really look like castles at all, do they?