Okay, admit it. You had something else in mind when you saw the title, "Pegmatite
Dikes." These dikes are over a billion years old and were formed
when molten rock was squeezed into fissures forming the light-colored bands that
vertically thread Colorado's Black Canyon of the Gunnison. Being
igneous rock, they are more resistant to erosion and thus end up standing out
from the walls.