The Yellowstone River is cutting downward through the solidified rhyolitic lava flows from an eruption 275, 000 years ago to form the Grand Canyon of the Yellowstone. The Canyon currently extends 20 miles from the Canyon area north toward Tower Junction. Through time the downcutting of the river has resulted in the falls moving upstream, increasing the length of the canyon. The canyon contains both upper and lower falls. The set of falls formed because resistant rhyolite lava rock did not erode as much as the lava that had been weakened by heat and chemical in ancient hot springs basins. Excellent views of the Canyon and Falls can be seen from Artist Point and Uncle Tom's Trail on the south rim.