The Lewis Shale is a geologic formation in the Western United States.

It preserves fossils dating back to the Campanian to Maastrichtian stages of the late Cretaceous period.

[4] The upper part of the formation contains the Huerfanito Bentonite Bed, long recognized in the subsurface but recently discovered to crop out near Regina, New Mexico.

The bed lies in the upper Baculites scottii biostratigraphic zone and has a radiometric age of 75.76 ± 0.34 million years.

[10] The Lewis Shale in the San Juan Basin has historically been a minor source of natural gas.