Click on the Tanacoye mounds link and the shell gorget with the etching on it...I recall a post where someone said " I wonder what it means".
After being to the site twice..and to the camp across the highway here is my perception of the etching:
It is a simple drawing of the valley in which Tanacoye mounds resides. Just across the street..the East and West Frio rivers join...each with its own valley. The little squiqqley showing smoke rising from one of those valleys as it would be with a large campsite.