Thursday, November 17, 2011

Memorable Places Here and There on the Colorado Plateau: The Solitude of Nankoweap

Fifty-three miles downriver from Lees Ferry, the put-in for all trips heading into the Grand Canyon, the Colorado River makes a dramatic, sweeping S-turn where its gorge widens into an area called Nankoweap.

A thousand years ago, give or take, a large, flat delta built by numerous debris flows and flash floods, similar to what we see today, was an open invitation for Ancestral Puebloans to grow crops such as corn, one of their staples.

These Native Americans called Anasazi, which is actually a Navajo term meaning "enemy ancestors" or "ancient people who are not us," stored their grain high above the river in granaries etched into the cliffs, where this photo was taken. For scale, notice (above) the hikers descending a trail on the talus slope toward their raft. A few windows of the granary (below) can be seen from the trail.

Why are some regions of the Grand Canyon wide and open with a tranquil river such as Nankoweap and others narrow with towering rock walls and a river that's fast and furious? We know the Grand Canyon was carved by the action of the running water (or more appropriately its carried burden). Perhaps this is an overly simplistic statement, but true nonetheless. But, we must look for other variables to explain the differences in canyon architecture.

As the river downcuts into its bed, it encounters rock layers of variable resistance. Less resistant rock erodes more readily and laterally undercuts more resistant rock. This causes the overlying rock to collapse which widens the canyon. A direct relationship exists between canyon geometry and hardness of the rock strata. Thus, the canyon in the region of Nankoweap widens at the expense of the erodable Bright Angel Shale at its base that undermines and weakens the rock overburden. As the canyon widens, so follows its river bed. That slows the river's rate of flow and encourages the formation of those big deltas as the water releases its sediment. Perfect for farming! Fertile, irrigated and flat. 

Below the shale lies the Tapeats Sandstone which will come into view in another six miles, when the river dissects deeper into its bed. Above lies the Muav Limestone, the cliffs just above river level. These formations comprise the classic, transgressive triad of the Cambrian known as the Tonto Group, formed when the rising Panthalassic Ocean (or ancestral Pacific Ocean) began to lap across the region of the future Grand Canyon around 525 million years ago. The South Rim looms in the distance with the Middle Permian Kaibab Limestone at the top which means we’re viewing the near full extent of the Grand Canyon’s Paleozoic column of deposits.

Suggested Reading: Carving Grand Canyon by Wayne Ranney, 2005. 

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