Please post a picture. Beach has many different meanings. Small lake, Great Lake, ocean. All three have different geological significance. I found this rock on a beach by a moutain lake in Montana, is more helpful. The facts are helpful to identifying a rock. Geological activity in the great lake region caused deep rifts in the earth's crust leaving many characteristic formations and geological anomalies. The picture and a better description of the area it was found in will help the community to identify your rock.