[kiryandil]

Those were not ‘carved’ they came from tumbling in a stream, and someone just kept them because they were interesting.

My friends have a house on one of the Great Lakes that is nicknamed "Boulder Beach". The lake tumbles the rocks there and many of them are roundish.

These ancients may have figured out how to duplicate the process with a "stream tumbler", as you posted.

[Mr. K]

Or they just found some that were already tumbled