Core Work (IMAGE) Columbia Climate School Caption James Lea (left) and Francesco Muschitiello drill into the ground to sample the sedimentary deposits of an ancient glacial lake in Sweden. The layered deposits, called varves, document the ice sheet's response to changing atmospheric conditions. Credit Francesco Muschitiello Usage Restrictions Credit must be given 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.