Black Canyon of the Gunnison Hike to Green Mountain Professional Review and Guide
"The Black Canyon of the Gunnison River is surely one of the most awesome spectacles in Colorado. The canyon’s average depth is 2,000 feet, and in many places it is much deeper. But the characteristic that makes the canyon so unusual is the sheerness of its walls. At the Painted Wall, for example, the gorge is 2,300 feet deep, yet the horizontal distance from the river to the edge of the north rim is scarcely 500 feet.
Adding to the visual effect of the seemingly bottomless chasm is the dark color of the metamorphic gneiss and schist that make up most of the inner canyon walls. This ancient stone was formed during Precambrian time and is well over a billion years old. It is also extremely hard, and to a large extent that is why the walls are so sheer. The hard, crystalline rock has great compressive strength, and it does not erode away as fast as a softer material might."