Wednesday, September 8, 2010

Mesa Verde- geology and archeology

(after Wanek 1959 and Griffitts 1990)

A while ago we decided that we would spend the last of the summer explorations looking at the back country of Mesa Verde National Park. In the past few years, the park has allowed ranger led treks to look at some of the less explored regions of the park. It was one of these we decided to join.

Mesa Verde is made of basically four different formations, all of Cretaceous age. The Mancos shale forms the lower slopes and valley bottom. The Mancos is a marine shale and is ubiquitous in Western Colorado. Above the Mancos shale is the Mesa Verde Group composed of the Point Lookout sandstone, a beach front sandstone; the Menefee formation, a near sea-level marshy shale and the Cliffhouse sandstone, another shoreline sandstone. The regional dip is a shallow 2-3 degrees towards the south away from the La Plata mountains.

Many of the cliff dwelling are found in alcoves created where the permeable Cliffhouse sandstone meets the impermeable Menefee shales. The waters from rain and snow slowly percolated through the Cliffhouse sandstone until they reach an impermeable layer. The slight dip of the rock units channeled the moving water to the south creating many springs on the north facing slopes. These springs over time undercut the overlaying sandstone creating alcoves. The Anasazi (or Ancestral Puebloans) found these ready made alcoves complete with water sources a natural place to build some dwellings.

Looking up from the canyon bottom. The Menefee coal beds can be seen in the center. The cliffhouse sandstone at the top.
A large dwelling area built in an alcove created by water running through a horizontal crack. A large spring is just visible in the lower right at the bottom of the Cliffhouse sandstone.
While crossing a large expanse of sandstone we were treated to an amazing collection of hematite concretion shapes.
Another ruin showing how a spring created the alcove. The floors of the dwellings are built directly on the Menefee.
An alcove in the making right at the interface between the permeable Cliffhouse sandstone and the impermeable Menefee shales.

1 comment:

Ron Schott said...

What? No photos of the lovely Mancos Shale?!? Unbelievable! ;-)