UPDATED - Latest Finds - Artifacts in Denver

The next pics are 2 separate pieces.  Both appear to show very similar patterns, colors and shape- a horse. Both also have similar surface symbols resembling a horse. 

The dark horizontal symbol on the surface of this piece is the most common and numerous I have found. It appears to be a serpent with mouth slightly open. 

Similar serpent symbol on this piece below

Some interesting surface symbols

The Great Plains was once a vast intra-coastal waterway