About 200,000 years ago, a lake formed in the Hemp Canyonlands with its upper end near the meeting of Trout and Hemp creeks. The lake bottom was about above the present bottom of the Hemp Creek Valley. The surface of the lake was the rim of the Hemp cliffs a few kilometres south of The Flatiron. The lake was created by a blockage downstream on the Clearwater River, but its actual location is unknown, and this could have been an ice dam, a landslide or a lava flow. The Flatiron erupted into this lake and was once much bigger than today. This action is called a ponded lava flow. The lava used to extend right across the Hemp Creek Valley, but floods from melting glaciers carried away the lava and left The Flatiron as a remnant. The water flow might have been 100 times the size of Hemp Creek today. The lava that is exposed today cooled very slowly and formed into columns, one of the park's finest examples of columnar basalt. The columns that make up the Flatiron have been falling away regularly and piling up at the base, so the Flatiron keeps gets thinner.