Split wedged rocks from Harvard

On Saturday, after visiting the Harvard Cave, we saw a few rock pile sites. Here is a split wedged rock (a "Spirit Door") from near the Cave. These simple structures are appealing.
Here is another from one of the other sites we found.
I wrote about these structures more extensively: [Click here]

In the first picture, you may be able to see the remains of a drill hole, to the right of the wedge. This indicates that the rock was drilled and -probably- blown apart. Yet after that the rock was wedged. This is the sort of puzzle that intrigues me and asks for some sort of explanation. My best guess is that a someone blew up the rock maliciously and someone else repaired it. Another possibility is that it was split deliberately to create a door to the underworld, and then it was stoppered for later use. Of course these are fanciful thoughts, but what is your explanation for the phenomenon?