Rock piles at the end of outcrops

By an "outcrop" I mean a ridge of bedrock with little soil so the rock shows through, and not more than 20 or so feet wide - an exposed ridge of bedrock at an elevation drop-off. We have seen different types of enhancement to outcrops including filled cracks and little alignments running along the outcrop. Outcrops often appear over water and they always give some kind of view outward. Narrow, there is a sense of place, especially at the end.

I saw this in Groton last weekend a the end of an outcrop, looking southwest over water:There is a ciruclar opening formed by the pile that is about 6 or 8 inches across the inner diameter. Any ideas what it might be?

Another pile at the end of an outcrop, a short distance from the above: