At Black Canyon, the Gunnison River carves through rock that is 1.7 billion years old, some of the oldest rock on the planet. Called basement rock, it forms the foundation of Earth’s crust and is only exposed occasionally, such as at the lower levels of the Grand Canyon. The block of basement rock that Black Canyon is carved through is called the Gunnison Uplift. Once exposed, the upper portions of the uplift had eroded away before the Rocky Mountains formed. Sixty million years ago, the same massive, continental rising that birthed the Rockies also lifted the Gunnison Uplift, priming it for another round of erosion.

Thirty million years ago, the West Elk Mountains to the north of the Gunnison Uplift and the San Juan Mountains to the south of it were volcanic, spewing ash that buried the uplift once again. The rise of the West Elks forced the ancestral Gunnison River to flow to the south of these new mountains. The river cut easily through the volcanic tuff and sedimentary rock between the West Elks and the San Juans.

But farther down, the river hit the much harder basement rock of the Gunnison Uplift. By now it was too late, the river’s course was set and it continued to carve, slowly, through this much harder rock where it was now exposed on the surface for sixty-five miles.