Rock Art at Martha's Vinyard

I went to Martha's Vinyard some years ago. The beach I went to was beautiful, and the cliffs were striking, made of multi-colored clay. But most amazing was that along the boulders that lined the beach, someone balanced scores of rocks in the most unlikely positions. The scene had tremendous primitive power. I picked several rocks up, hoping to discover the secret, but there was no secret. And I was unable to re-balance them!