Supposedly they were originally encased in limestone blocks shaped to give a smooth outside surface, but those blocks have been hauled off over the years to mak

Supposedly they were originally encased in limestone blocks shaped to give a smooth outside surface, but those blocks have been hauled off over the years to make other things. So, a pile of rocks is only stable over long time periods in the presence of humans to the extent that the materials are economically worthless. Piles of dirt are also surprisingly stable. Around Stonehenge are mounds that are presumably 4 or 5 thousand years old.