Castle Rock, which rises 195.8 feet (59 m) over the waters of nearby Lake Huron, was created by erosion of surrounding land. After the Wisconsinan Glaciation, post-glacial Lake Algonquin formed. The Ice Age melt off caused the waters of Lake Algonquin to be much higher than the water level of Lake Huron is today. Over time, the declining water eroded much of the land. Castle Rock, which resisted this erosion, is made of limestone breccia; it is a sea stack or sea chimney, geologically similar to several features on nearby Mackinac Island, such as Arch Rock or Sugar Loaf.