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?
Many of the animal mounds you see at Chaco were made by Ord's kangaroo rats. They often dig in the soft midden (trash) areas in sites because they are elevated, easy to dig in, and don’t flood. In their colonies there are often more burrow openings than there are rats! (Drawing courtesy: ICWDM.org)