Gunnison River

We visit Black Canyon, where a sign post tells us:

"Over a billion years ago, molten rock was squeezed into fissures forming the light-colored bands which form Black Canyon's otherwise dark walls. You are standing on one of these bands. As the hot fluid slowly cooled and hardened, minerals formed into crystals. Look closely; you may recognise flecks of mica, feldspar, quartz, and perhaps even garnet.

"These bands of lighter colored igneous rock - called pagmatite dikes - are more resistant to erosion than the dark gneissis (sounds like 'nice-es') into which they introdude. Consequently the pegmatite weathers more slowly then the gneiss. The vertical position of the dikes here, combined with their resistance to weathering, has produced the fin-like appearance of parts of the canyon's walls. You will see pegmatite at almost every overlook.

"The pegmatite dike you see across the canyon nearly half a mile away is the same one you are standing on! Imagine the cutting power of the Gunnison River."