Here are three examples but there are many more on the Spring Hill Conservation Land.
My thinking about these things [see here] has changed slightly. Most people go along with the narrative that split rocks are doorways to the underworld. So what is that wedge doing in there?
I used to think the wedge was in order to keep the door from slamming shut - leaving a spirit on the outside. Later I thought maybe the wedge was to keep the spirit in there. At some point FFC suggested the wedge was a way for a user to claim ownership of that particular split. But now, these day, my favorite guess is that the wedge is like the stone placed in the middle of a prayer seat after the seat is used - as the proper way to end the ceremony. So for now anyway I think that the wedge was inserted after someone did a ceremony involving speaking down into the crack.
Maybe someday there will be some actual information rather than guesswork on the topic. For example is there any information about ending the vision quest ceremony?