. . . the monument contains significant geological and paleontological objects. The late-Miocene to late-Pliocene Ringold Formation, known as the White Bluffs, was formed from river and lake sediments deposited by the ancestral Columbia River and its tributaries. These striking cliffs form the eastern bank of the Columbia for nearly half of the length of the Reach, and are significant for the mammalian fossils that they contain. Fossil remains from rhinoceros, camel, and mastodon, among others, have been found within these bluffs.

— President William Clinton, June 9, 2000, in the Presidential Proclamation creating the Hanford Reach National Monument.

One of the major landmarks within the Monument, the White Bluffs, is the upper component of the Ringold Formation, which dates to between three and eight million years ago. The formation is composed of a 1,000-foot thick deposit of interbedded lacustrine and fluvial silts, sands and conglomerate, with some paleosol remnants. The source of the sediments is unknown, although ideas about their origination include the Clearwater/Salmon drainage system from Idaho, the Pend Oreille River in northeastern Washington, and an ancestral Columbia River.

Regional uplifting about three million years ago resulted in the present upper Columbia River down cutting through about 600 feet of the Ringold Formation to its present elevation of 300 feet. This last erosional event has exposed a multitude of vertebrate and some invertebrate fossils in the Ringold Formation. Of particular note are rhinoceros and anadromous salmonid fossils from the late Miocene.

The subsequent White Bluffs component of the formation contains even more fossils, including 27 species of mammals alone. Among the fauna found are rodents, lizards, frogs, turtles, fish, rabbits, bears, canids, cats, ground sloths, peccaries, deer, mastodons, camels, horses and zebras. Of particular interest is the nature of the fish species found (primarily warm-water species, such as catfish and sunfish) and those not found (salmonids), supporting the theory of two separate river systems during the Miocene. The river system responsible for the White Bluffs deposit may not have been connected to the Pacific Ocean, hence the lack of anadromous fish remains.

In addition to the fossils found in the White Bluffs, petrified wood can be found in the Saddle Mountains, Umtanum Ridge, and Yakima Ridge. Scatterings of petrified wood can also be found in the Dry Creek and Cold Creek drainages.

It should be noted that the collection of fossils is prohibited on the Monument.