Glacial erratics are stones and rocks that were transported by a glacier, and then left behind after the glacier melted. Erratics can be carried for hundreds of kilometers, and can range in size from pebbles to large boulders. Scientists sometimes use erratics to help determine ancient glacier movement.

These glacial erratics were left behind on the fractured bedrock at Olmstead Point in Yosemite National Park. —Credit: Roger H. Goun

Glacial erratics dot the landscape near the Beartooth Mountains in Montana. —Credit: Brian Washburn

Big Rock, also known as Okotoks Erratic, is located near the Canadian town of Okotoks, south of Calgary, Alberta. Weighing in at 15,000 metric tons (16,500 tons), it is the world’s largest glacial erratic.  —Credit: Jon Noad

In this photograph from Pyramid Mountain in New Jersey, a huge glacial erratic remains exactly where the glacier left it—perched on top of three other boulders to create a formation known as Tripod Rock. —Credit: Wally Gobetz