The Grand Canyon of the Yellowstone runs about 20 miles and is more than 1,000 feet deep. Lower Falls drops 308 feet into it, though from the rim the waterfall can look small beside the walls. Hydrothermal activity altered the rhyolite, softening it for the Yellowstone River to erode; iron and sulfur compounds help create the yellow, orange, and brown colors. One estimate puts an earlier canyon here around 160,000 to 140,000 years old. Other research says much of today's canyon formed during and after the last ice age, probably through floods. Exactly how those floods cut it is still debated.