Over a billion years ago, molten rock was squeezed into fissures, forming the light-colored bands that thread Black Canyon's otherwise dark walls. As the hot fluid slowly cooled and hardened, minerals formed into crystals, including mica, feldspar, quartz, and sometimes garnet. These lighter colored igneous bands, called pegmatite dikes, are more resistant to erosion than the dark gneisses into which they intrude, so they weather more slowly. Their vertical position here, combined with that resistance to weathering, has produced the fin-like appearance of parts of the canyon walls. Pegmatite appears at nearly every overlook, and the dike visible across the canyon nearly half a mile away is the same one present here, revealing the relentless cutting power of the Gunnison River.