Friday, February 20, 2009

The Cretaceous Parks of the Colorado Plateau: A City in the Wilderness

I started a brief history of the Colorado Plateau eight months ago, never imagining what an extensive project it would become. I have been working my way through time, from two billion years ago to the present day. Several months ago, thinking I was approaching the end of the narrative, I started to describe the parks displaying Cretaceous rocks, the last period of the dinosaurs (the Mesozoic era), and I find I am beginning the twelfth entry just for that period. But what a fascinating place we are exploring today! Chaco Culture National Historical Park is also just a bit eerie...

The New Mexico plateau country south of Farmington is a barren looking landscape dissected by the occasional shallow arroyo. The treeless terrain is populated by a few Navajo families who graze animals on the arid grasslands. At the end of a 13 mile gravel road, in one of the most isolated and lonely parts of the reservation, one can witness the ruins of Chaco Canyon. For 300 years, beginning about 800 A.D., this was the center of the Ancestral Puebloan universe. The park preserves a collection of some of the most ambitious architectural developments of the Puebloan peoples, with numerous villages of great kivas, plazas and great houses that are unrivaled in their complexity. Some were 4-5 stories high. The villages lay along carefully measured lineations and alignments. Thousands may have lived or traded or worshiped in this shallow canyon. The dwellings included some 200,000 ponderosa beams that had to be hauled from Mt. Taylor or the Chuska Mountains, some 30 miles away.

The Great Houses were constructed at the base of cliffs of the Mesa Verde Group, specifically the shales of the Menefee Formation, and the cliffs of the Cliffhouse Sandstone. The canyon was deepened as the soft Menefee was eroded, undercutting the cliffs, which then collapsed into the gorge. This process was recognized by the Puebloans. At Pueblo Bonita, the builders had tried to buttress a huge rock that threatened to break away from the cliff, and one could say they were successful, in that the rock did not fall until 1941, when it destroyed 60 rooms in the pueblo. The debris can be seen on the left side of the top photo.

The inhabitants of the region were also cognizant of their skies; the cliffs of the canyon contain petroglyphs and and pictographs that may include pictures of the 1054 A.D. supernova that produced the Crab Nebula, and Halley's Comet. The Sun Dagger on Fajada Butte marks the location of a ray of morning sun on the morning of the Winter Solstice.

One other remarkable feature of the park are the roads that radiate out from the canyon, pointing in all directions. The roads run in exceedingly straight lines, sometimes for many tens of miles, and sometimes straight up cliffs (see the bottom photo). Rocks were cleared for a width of fifteen feet and more. They are hard to see from ground level today, but are apparent in aerial photos and satellite images.

Chaco Canyon is a special place to visit; it is difficult to get there, but is well worth the effort. There is a modest campground, a visitor center, and an interesting network of trails. Check it out!

We have reached the end of the Cretaceous Period! This was the beginning of huge changes on the Colorado Plateau, both in the landscape and for the ecosystems. The seas retreated for the final time, and the land began to be twisted and deformed. The entire region began to rise, and erosion began to strip away the thousands of feet sediments that had accumulated during the preceding 500 million years or so. The dinosaurs, and something like two thirds of all the species in existence at the time were obliterated. The world had changed...

More later!

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