A wall of rock exposed in the Black Canyon of the Gunnison in western Colorado. The wall consists of dark grey-ish, Precambrian-age gneiss and schist dissected by bands of pink, granitic, also-Precambrian-age pegmatites (igneous intrusions that were injected into fractures in the gneiss long after it formed and while it was all still buried deep underground). The metamorphic rocks formed around 1.8 billion years ago, but existed as sedimentary rocks of undetermined age before being metamorphosed. The pegmatites, which include crystals upward of 2 m (6 ft.) in length, are a bit younger at around 1.35 billion years old. The green plant life at the top of the wall grows on mudstone exposures of the Upper Jurassic (~150 million year old) Morrison Formation, which has produced a few dinosaur fossils in the park.