Pill

A pill seems to be forming in the west side of the river bank north of Wark bridge. 
I ran this term past a couple of friends out with their dogs on the bridge this morning and they'd never heard of it.
I think it's because it is a name given locally by communities around the mouth of the River Avon near Bristol for these small indentations made in the river bank by the flow of the river. There is also a village called Pill there with an ancient maritime history of river men and pilots.