Illustration by Rob Wood based on research and artwork by Doris Tischler.

Tropical subtropical forests mantled the local terrain 54 to 37 million years ago. We know of these forests because of a splendid sample of fossil seeds, nuts, fruits, leaves, branches, and roots. The Clarno Nutbeds are among the finest fossil plant localities on the planet, with hundreds of species -- many new to science -- preserved. The conspicuous mammals were the browsing brontotheres and amynodonts -- conspicuous because of their great size and ungainly appearance. Impressive, too, were the strong-jawed scavengers, hyaenadonts, and ruggedly framed predators such as Patriofelis. Although a few of these animals lived into the early Oligocene -- 34 million years ago -- they have left no modern descendants.

However, some of their contemporaries -- the equids, rhinos, tapirs, and cats -- do have present-day descendants, most of which are no longer found in North America.

Illustration by Rob Wood based on research and artwork by Doris Tischler.

The John Day Formation spans almost 20 million years. Even a cursory examination reveals that many types of environments existed in landscape.

Deciduous forests replaced the earlier subtropical forests of the Clarno Formation. Number fossil plant localities containing a great number of different species indicate the vast biological diversity of the early Miocene Epoch. More than 100 groups of mammals have been found in this formation as well, including dogs, cats, swine, oreodonts, horses, camels, rhinoceroses, and rodents.

Multiple volcanic events during the deposition of the formation produced large amounts of volcanic ash. The resulting tuff, interspersed throughout the fossilb-bearing beds, allows determination of accurate radiometric dates. The chronology derived from the tuff helps scientists determine the rate at which plants and animals changed as they evolved.

Illustration by Rob Wood based on research and artwork by Doris Tischler.

The interval between deposition of the John Day and Mascall times was marked by intermittent flows of basaltic lava that repeatedly leveled and denuded the region. By 15 million years ago these eruptions had ceased and the basalt was weathering into soil. A moderate climate, sufficient precipitation, periodic deposits of volcanic ash, and the basaltic parent material combined to produce highly fertile soils, and from these soils arose lush, nutritious grasses and mixed hardwood forests -- much like those found today in the eastern United States.

The Mascall savanna was home to a great variety of animals that we might recognize as horses, camels, and deer, as well as bears, weasels, dogs, and cats. Some dwelt in the woodlands, while others adapted to the grasslands. At the same time large mammals made a resurgence. Among them were the gomphotheres (early elephants), as well as sizable rhinos and bear-dogs.

Named for Rattlesnake Creek, a minor tributary of the John Day River, the coarse deposits of the Rattlesnake Formation complete the paleontological story of the John Day Basin.

Tectonic pressures from the south and north folded and buckled the rock beneath and around the base of the Strawberry Volcanics, eventually thrusting the modern Strawberry-Aldrich mountains upward as much as 1.5 miles above the John Day Valley. As the mountains rose and eroded the valley was slowly filled with deposits. These strata represent the last major episode of deposition in the basin. Erosion then began and continues to sculpt the landscape of today.

The interface between the Mascall and the Rattlesnake formations is easily distinguishable to geologists and paleontologists because the nature of the materials in the two sequences are quite different. It is somewhat subtle to the untrained eye. The most spectacular clue is best seen from the geologic exhibit on Route 26, south of Picture Gorge, where a 5% angular unconformity between the formations can be clearly seen. The underlying Mascall beds are tilted at a different, steeper angle than the younger Rattlesnake deposits (formed after a major tilting of the Mascall Formation and lower layers).

Within the Rattlesnake Formation is a large, blocky, welded tuff layer. It was formed from a fiery tidal-wave of super-heated volcanic gases and particles from an explosive volcanic source far to the south (perhaps Harney Lake) -- an ignimbrite. This rimrock layer can be seen above Picture Gorge and eastward along the John Day Valley -- relics of this massive volcanic event. This tuff averages 90 feet thick, and yields no fossils. Above this ignimbrite layer are more Rattlesnake Formation erosional deposits. However, here, as deposition from the Strawberry-Aldrich mountains continued, the character of the deposits graded toward finer particles as the nearby mountains wore down.

The fossil-bearing strata in the Rattlesnake are often so poorly consolidated that extracting a fossil frequently requires artificial stabilization of the rock to keep it in one piece. The Rattlesnake Formation is of great importance as one of four correlative localities on which the Hemphillian North American Land Mammal Age is based.

Though they contain considerably fewer fossils than the Clarno, John Day, or Mascall formations, fragmented fossils have been found of horses, sloths, rhinoceroses, camels, peccaries, pronghorns, dogs, bears, and others. It is an interesting population, with some species resembling those of today and others, those of a distant past. A preponderance of grazing animals over browsers suggests a much dryer and cooler climate, dominated by grasslands.