Thursday, July 3, 2008
Time Beyond Imagining - An Intro to the Colorado Plateau
The principle of superposition tells us that in a sedimentary sequence, the oldest layers will be exposed deeper within the surface, presuming that no overturning has taken place. It is when we find the base rocks upon which the deepest sedimentary layers are laid that we find the truly ancient rocks. On the Colorado Plateau, this happens in a couple of places: the Inner Gorge of the Grand Canyon, at Colorado National Monument, and especially at Black Canyon of the Gunnison National Park. If you wish to pick up a few pieces of the ancient crust, you might check some of the quarries in Unaweep Canyon south of Grand Junction, Colorado and other locales on the Uncompahgre Highlands; collecting is generally allowed on Bureau of Land Management and National Forest lands.
Black Canyon of the Gunnison is extraordinary. Over time, the deep crust in this region has been lifted and pushed upwards into mountain ranges thousands of feet high, in Proterozoic time, in late Paleozoic time, and now in Cenozoic time. Mountains rose, and mountains were eroded away. Eroded so completely that shallow seas washed over the remains of the mountains and covered them with hundreds and thousands of feet of additional sediment.
Today, the Gunnison River is misplaced, in a sense. Standing on the rim, one is struck by the way the land slopes away from the canyon walls; the deep valley more or less crosses a mountain ridge. The formation of the present-day canyon, which at 2,000 feet is often deeper than it is wide, is an entire post in itself, to be saved for another time. I want to concentrate on the ancient rocks that form the canyon walls.
The Proterozoic is what we call the time period from 2.5 billion years ago to about 545 million years. It is a vast expanse of time; all the story of complex multicelled life on planet Earth took place in the most recent 545 million years. The Proterozoic was four times as long!
The rocks exposed in the canyon walls at Black Canyon are not sedimentary; they have been changed by heat and pressure into metamorphic rock: dark biotite mica schist, gneiss, and hornfels. On the Painted Wall of Black Canyon, seen in the photo above, one can also see light colored dikes of granitic rock that melted and intruded into the older rocks about 1.4 billion years ago.
The story of these most ancient rocks on the Plateau begins with tomorrow's post!