As I review my vacation photos, it becomes striking how often propped boulders are found in the Southwest as well as the Northeast. Manitous are common in RI, where the granite breaks into slabs, but are rare in the Southwest, possibly because the stone there does not break into slabs. In the Southwest, there are also very few rock piles or cairns, and few stone rows. Since there is no apparent agricultural use for propped boulders, it is safe to say they were constructed by Natives and not settlers. The location of the propped boulders may give a hint as to their significance. In RI, hilltops are often crowned with boulders left by glaciers, but only certain boulders are propped. Here, for an example, is a hilltop with two large propped boulders.
Here are two views of the first: