Archaeological Site (IMAGE) University of Edinburgh Caption Archaeological site in Arizona, U.S., with a distinctive black layer, indicating substantial environmental changes beginning about 10,800 BC, with impact debris at its base. Credit Comet Research Group Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.