Fieldwork at Gran Barranca, Argentina (image) University of Washington Caption University of Washington researchers collect samples from Gran Barranca, Argentina, that offers access to layers of soils, plant remains, volcanic ash and sand going back millions of years. The section pictured represents 800,000 years of layering and is mainly composed of volcanic ash deposited by wind and rivers. Credit R. Madden/U of Chicago Usage Restrictions Credit required 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.