Friday, May 29, 2009

Time Beyond Imagining - Land of Giant Lakes

The Cenozoic Era was a time of great change in the Colorado Plateau area. Mountains rose to great heights around the margins, especially in the Rockies, and rivers carried vast amounts of sediments across the region. But the rapid warping of the crust disrupted the flow patterns of the rivers, and some ended in vast lakes that covered large parts of Utah and Wyoming. In many areas all the Cenozoic sedimentary rocks were stripped away, but in some choice locations the lake sediments were preserved. These rocks form the bedrock of two truly unique parks on the plateau: the world-renowned Bryce Canyon National Park, and the much less-known Cedar Breaks National Monument (Ah-ha! You thought the top picture was Bryce Canyon, didn't you? It's not).

Lake sediments are usually clay-rich and very soft and unresistant to erosion, so they rarely form cliffs unless they are protected by some kind of hard caprock. The Claron or Wasatch formations that make up the cliffs of the two parks include freshwater limestone which is a great deal more resistant to erosion. The rocks have been fractured by pressure from nearby faults, and vertical cracks (joints) have allowed erosion to form the unique towers of stone called hoodoos. Oxidation of iron bearing minerals in the rock led to the intense red color that dominates the attention of the observer.

Fossils are rare in these rocks, but we know from other related rocks on the plateau that a few tens of millions of years after the great extinction event that ended the reign of the dinosaurs, the survivors had evolved into a rich ecosystem that included birds, mammals, reptiles, amphibians and fish. Insects are even occasionally found as fossils in the rocks. The mammals, which emerged from the Cretaceous extinction as small rat-sized insectivores diverged into all manner of carnivores, large grazing animals, and smaller forest browsers. The biggest plant-eaters were almost dinosaurian in bulk, and some of the carnivores as intimidating as any raptor. And yet, this was a world that was coming to resemble something more familiar to us in the present day. We might not have any more of the giant browsers on this continent, but similar animals survive (barely) on the African continent. And the tigers and lions and hyenas, although a bit smaller than their ancestors, are still as capable as any of these carnivores of the geologically recent past.

Bryce Canyon National Park is well known, making part of a recreational triangle that includes Zion and the North Rim of the Grand Canyon (and which now includes Grand Staircase - Escalante National Monument). Roads traverse the tops of the cliffs from one end of the park to the other, and a series of rather spectacular trails wind among the hoodoos. Elevations range up to 8,000 feet or so.

Cedar Breaks lies to the northwest, at the edge of the Wasatch Mountains overlooking the town of Cedar City and the barren mountains and desert flats of the Basin and Range province. It sits several thousand feet higher than Bryce Canyon, so roads may be closed by snow well into May. If the hoodoos at the Breaks are maybe less in stature than those at Bryce, they also tend to be more colorful, and the forest and snow banks provide beautiful contrasts that are not often visible to the casual tourists who visit Bryce in droves.

The end of the story approaches! A project I thought would last a month or so has stretched into an entire year. Still to come: an explosion of volcanism sweeps across the region!

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