From the National Forest website:

This 11,550 acre Wilderness boasts what has been described as the most diverse riparian area in Arizona. Over thirty species of trees and shrubs and over a hundred species of birds have been observed in this unique habitat. The stream seems to appear out of nowhere, gushing 20,000 gallons a minute out of a series of springs at the bottom of a 1,600 foot deep Canyon. Over the years these calcium laden waters have laid down huge deposits of a material called travertine. That rock-like substance encases whatever happens to fall into the streambed - forming the fossils for which the area is named.

Most people come to Fossil Creek to sunbathe, wade, hike and birdwatch. It's also a great place to take photographs. The lushness of the riparian area strikes a sharp contrast to the brittle desert that surrounds it. While you're here, keep an eye out for javelina. These collie dog-sized wild pigs are plentiful in the area.
For more information contact:
Mogollon Rim Ranger District, HC 31, Box 300, Happy Jack AZ 86024, (928) 477-2255 FAX (928) 527-8282
or
Verde Ranger District, 928-567-4121 or
Red Rock Ranger District, P.O. Box 20249, Sedona AZ 86341, (928) 282-4119