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 park considered a number of ideas for mitigating the damage that would be done if Threatening Rock fell. Proposed solutions included using dynamite to destroy it, anchoring it to the canyon wall with cables, or clearing out the rock debris behind it. It fell on January 22, 1941.