The Sioux, like most other Native American people, were polytheistic.  They believed fervently that powerful spirits inhabited everything in their world.  Large boulders were given a special distinction because, on the plains, they were very rare.  When they came upon a boulder, the Sioux warriors painted sacred symbols on it's rough surface and prayed to the Spirit inhabiting the boulder.  They considered the rocks to be the oldest things in the world because the rocks were very hard and dense.  Without them even having a keen knowledge of geology I think there is actually a certain scientific truth to that concept.  Worked for them.