French explorers used this bold bluff as a landmark and named it Mt. La Grange because it resembled a large barn. About 10,000 years ago, meltwater from glaciers carved a deep channel here, and Barn Bluff became an island in the five-mile-wide river that filled the valley. A nearly vertical fault line at the bluff's southwest edge marks a crack that developed millions of years ago; as a result, most of the bluff on the river side dropped 150 feet, leaving bluff dolestone next to green sandstone at the fault line. The bluff measures 3100 feet long, 800 feet wide, 334 feet above the river, and 1001 feet above sea level. Limestone was quarried from the Oneota stratum for about 40 years until citizen protest stopped it in 1908, and in 1910 Barn Bluff was donated to the city for a public park.