The first propped boulder is this one, about 10 feet long, and propped three feet off the ground.
Here is a diagram of this hill, which is surrounded by wetlands. The boulders are drawn larger than scale for clarity.
These mysterious boulders may make great art subjects, but it is hard to explain their purpose. Maybe the space under the propped boulder was a portal to the spirit world, which would explain donation piles as offerings or mementos. Or maybe tribal shamans obtained medicine from these boulders. Or maybe they were shelters for natives on vision quests. The number of propped boulders in New England and beyond suggests some important function in native spirituality.