Someone on another stone site recommended I look up the book, The Archaeology of Natural Places by Richard Bradley. I hope eventually to own and read it, but in the meantime I browsed what I could on Amazon and found this bit about natural rock formations in Finland to which offerings were made. It mentions rocks shaped like humans, animals, or birds, as well as rocks of an unusual size or color, or rocks "split open during cold weather to provide fissures or natural portals leading into the surface of the rock." It even says that in some parts of Scandinavia low walls were built around some of these. I liked the quote, "[The Lapps] let the gods choose their own shape."
All this sounded very familiar, and the topic of the book, the relationship of culture with natural rather than built features, relates closely to sites I've been to. Frequently although not always, it appears that no boulders have been altered at these sites. They're more likely to be singled out by connection with walls in a way similar to what they mention here. I don't think it is necessary for the cultures to be connected in any way in order for studies like this to be relevant to what we're looking at. This book appears to be one that belongs in the rock piles enthusiast's library. And, hey, if you find a two-fer, send me one.
Here's a review, written for canoers, but it gives an idea of what the book involves.