The Glacial Grooves on the north side of Kelleys Island are the largest easily accessible such grooves in the world. They were scoured into solid limestone bedrock about 18,000 years ago by the great ice sheet which covered part of North America. A trough 400-feet long, 35-feet wide, and up to 10-feet deep remains today. The ice, probably hundreds of feet thick, flowed from the north in what is now the Lake Erie basin. The Devonian limestone containing the grooves contains marine fossils that are 350 to 400 million years old. Many other grooves were quarried away during the last century.

Fencing now protects this National Natural Landmark. A walkway and stairs permit visitors a good view of the grooves.