How to prop boulders

Here in the Northeast we often see  a massive propped boulder with lots of rocks under it.  It would have been an amazing engineering feat to lift this boulder and push a supporting rock under it.

I always assumed the small rocks were donations, but close examination shows squarish shapes and wide variation in size.  Notice how the lower surface of the boulder is flat, and that there is a sharply angled edge between it and the upper plane.

  Maybe the propped boulders were created by quarrying away rock from the base of a boulder, and then pushing in a supporting stone.  The stones visible under the boulder would be  fragments of the boulder.
     Here is one of my favorite propped boulders.  At this angle, it is easy to see how the rock was chipped away to create the "cave". The gouges on the lower surface are visible in the late afternoon sun.

The boulder below may have been a propped boulder in progress.  Note how there is a large horizontal crack in the bottom, as if chunks were to be removed.  This stone has the smooth, oblong  shape that propped boulders often have.

Propped boulders could also  be created by chipping away stone from a huge boulder leaning against an outcrop.

Here the lower face of a huge boulder has been chipped away, creating a straight edge and a small opening next to bedrock.  The stones have been piled next to it.  Someone may interpret this as donations next to a sacred place, another as quarrying in progress.
     The previous two pictures came from a hilltop containing several propped boulders, many platform cairns and farm walls. Some of the quarrying probably  produced stone to build the farm walls. I will show more of this site in a later  post.
     This hilltop site first attracted the natives with its huge boulders, some resting on bedrock.  Perhaps  the original purpose of native quarrying was to remove stone from boulders and create a "cave".  Unfortunately, the reason behind this laborious task is unknown.  The removed stone, along with other broken stone,  was stacked  into  cairns. Maybe the act of creating propped boulders and cairns was a form of sacrifice and devotion.  After the arrival of Europeans, native quarrying skills were put to use creating stone walls.

The  hilltop site guardian.