MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY MICHAEL SZOENYI / SCIENCE PHOTO LIBRARY
Glacial groove marks. These grooves in the rock were made as the weight of a glacier slowly moved material over limestone rock. They are the longest known glacial groove marks in the world (as of 2005), and are part of the Glacial Grooves State Park, on Kelley's Island, Ohio, USA. From the orientation of the grooves it can be deduced that the glaciers passed over moving southwards. Some were up to 3 kilometres thick. The most recent retreat of the glaciers was some 10,000 years ago.
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