Inscription:

Kaibab Squirrel Area
Has been designated a
National Natural Landmark
This site possesses exceptional value as an illustration of the Nation’s natural heritage and contributes to a better understanding of the environment.

1965
National Park Service United States Department of the Interior

https://www.hmdb.org/m.asp?m=94912

To understand why it’s interesting, refer to the Kaibab squirrel page on Wikipedia:

The Kaibab squirrel (Sciurus aberti kaibabensis) is a tassel-eared squirrel that lives in the Kaibab Plateau in the Southwest United States, in an area of 20 by 40 miles (32 by 64 km). The squirrel’s habitat is confined entirely to the ponderosa pine forests of the North Rim of Grand Canyon National Park and the northern section of Kaibab National Forest[1] around the town of Jacob Lake, Arizona. This squirrel is not found anywhere else in the world.