Great Falls is a striking stretch of the Potomac River about 14 miles above Washington, D.C., where the river drops 76 feet in less than a mile through rapids, falls, and narrow Mather Gorge. Though often associated with both Maryland and Virginia, the river and falls lie on the Maryland side, with protected parkland on both shores. The bedrock is about 750 million years old, while the falls formed around 35,000 years ago during the last glacial period. Long an obstacle to travel, the site prompted construction of the Patowmack Canal and later the Chesapeake and Ohio Canal, and it remains a major natural landmark and recreation destination.