NPS Photo by Tom Lyttle
The Lewis Shale is an olive-gray marine shale thinly interbedded with claystone, siltstone and sandstone as well as scattered beds of limestone. It overlies the Cliff House Sandstone and is exposed only near the northern boundary of the park. This unit was deposited at water depths greater than the Cliff House Sandstone and represents the last significant transgression of the Western Interior Seaway into northwestern New Mexico. Few fossils have been found in the Lewis Shale unit within the park.
Did You Know?
The climate of Chaco Canyon 1,000 years ago was very similar to the climate in the park today, with annual precipitation of only eight to nine inches. The Puebloans constructed water control features like dams, canals, and headgates for farming in such a dry environment.