Archaeologists aren't sure why Mississippian Indians engraved small sandstone tablets with birdman images and crosshatching 1,000 years ago. Maybe the tablets were used as visual aids for spiritual storytelling. Maybe they were dipped in dye and stamped on deerskin to create patterns. "Maybe (a tablet) was displayed when you were traveling from one place to another," said Bill Iseminger, assistant site manager at Cahokia Mounds State Historic Site in Collinsville. "It was a passport to show your rank or status or authority." Whatever their purpose, the tablets are considered archaeologically significant because they provide rare pictures from an ancient culture....