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Geologic Setting of the John Day Country, Grant County, Oregon


The John Day Country of today covers an area of 4,000-5,000 square miles in the southwestern part of the Blue Mountain region of Oregon, and is in the borderland between two major geologic provinces. One, to the north, is the Columbia Plateau which consists of flat or gently tilted flows of basalt covering about 100,000 square miles. The other, to the south, is the Basin and Range Province which extends into Mexico, and is characterized by a wide variety of complexly folded and faulted rocks. The Strawberry-Aldrich Mountain Range, rising from 7,000 to 9,000 feet in altitude along the south side of the John Day River valley (see Figure 13 on page 21), is part of a 150-mile long, east-trending mountain chain that locally separates the two provinces.

Many features that record events in the geologic history of northeastern Oregon may be seen in the rocks along the paved highways within the John Day Country. A road log describing some of these features is included as a part of this booklet. The locations of principal points of interest, keyed in the road log to state highway mile posts, are shown on the map accompanying the road log.

The known part of the geologic history of the John Day Country began with lava flows and deposition of volcanic ash, sandstone, shale, and small lenses of limestone in a late Paleozoic sea more than 250 million years ago. Sometime between 200 and 250 million years ago, peridotite and gabbro (dark, magnesium-rich varieties of igneous rock) rose from great depths and, as molten material (magma), invaded the preexisting marine deposits. These igneous rocks now form the core of Canyon Mountain and can be seen along the precipitous walls of Canyon Creek. Masses of chromium ore (chromite) were carried upward with the molten material. After erosion had exposed the peridotite and gabbro, the area was submerged again and, during Late Triassic and Early Jurassic time (about 180 million years ago), the Aldrich Mountain area was part of a seaway into which lavas flowed. Thousands of feet of volcanic ash from active volcanoes accumulated in the sea, and between eruptions great thicknesses of mudstone and shale were deposited. The cuts along U. S. Highway 395 between Canyon Creek and Bear Valley are in these rocks. During part of this volcanic activity, Canyon Mountain stood as a high landmass, but finally it too was deeply buried. Again the region emerged and probably was dry land during the last half of Jurassic time (135 to 150 million years ago).

In Early Cretaceous time, molten material was intruded to form the granitic rocks in the Aldrich Mountains and near Dixie Butte, northeast of Prairie City. The gold veins in Canyon Mountain and in most of the Blue Mountain region probably were formed at that time by solutions from the granitic magma. Scattered patches of fossiliferous sandstone and conglomerate like that in Goose Rock (see Figure 5 on page 10) show that the sea encroached on the Blue Mountain region briefly during Cretaceous time after erosion had exposed the granites. The shoreline was not far east of the John Day area.

For the past 60 million years, eastern Oregon has been a land of volcanoes, mountain building, and erosion. After the retreat of the Cretaceous sea and an undefined period of erosion, volcanic eruptions from widely scattered centers during the Eocene Epoch buried the region under several thousand feet of volcanic rocks which now form the Clarno Formation; locally these rocks consist mostly of andesitic lava flows and coarse mud-flow breccias. The Clarno Formation was extensively folded and faulted and deeply eroded before another series of volcanoes in and somewhat east of the Cascade Mountains erupted rhyolitic ash that was blown eastward and deposited as the John Day Formation during the Oligocene Epoch. The John Day Formation appears to have been restricted to a lowland area which geologists today call the John Day basin, located between the present site of the Cascade Mountains and the ancestral Blue Mountains. The Clarno and John Day beds in this basin are world famous, having yielded thousands of bones, leaves, and pieces of petrified wood that were buried and preserved by the volcanic ash in a manner similar to the burial of Pompeii by the eruption of Mt. Vesuvius in 79 A. D. Archaic large mammals such as the titanotheres, and small ancestors of modern mammals such as the 4-toed forest horse Eohippus, which was the size of a fox terrier, roamed subtropical forests during the Eocene Epoch. Many important links in the evolutionary chain of mammals have been found in the John Day fossil beds; for example, the 3-toed horse Mesohippus, which was a forest dweller the size of a sheep, had teeth for browsing instead of grazing.

The modern day landscape began to take form in Middle Miocene time, when the basalt flows which cover most of the area and are exposed in the walls of Picture Gorge (hence their name, Picture Gorge Basalt) buried the landscape formed on the John Day Formation and older rocks. The basalt erupted from long cracks or fissures in the Earth's crust and formed floods of very fluid lava which flowed for distances up to one hundred miles. These basalts are recognized by geologists to be of a distinct type, commonly called plateau or flood basalt. Basalt, frozen in the fissures, forms dikes that commonly weather out above the adjoining softer rocks, as shown in the picture of Kimberly Dike on page 16.

While the Picture Gorge Basalt was flooding most of the John Day basin, several volcanoes in the vicinity of Strawberry Mountain erupted andesitic to rhyolitic lava and ash, and built up cones similar to Mt. Hood and the other high peaks of the Oregon Cascades. Geologists have named the remnants of these cones the Strawberry Volcanics. Because these volcanoes rose several thousand feet above the top of the Picture Gorge Basalt, ash and erosional debris from them washed out over the basalt; these materials constitute the Mascall Formation. Although the mountains were timbered, the lowlands probably were open and grassy. Bones and teeth of a pony-size three-toed horse, Merychippus, have been dug from the Mascall Formation. When the eruptions ceased in Early Pliocene time (about 10 million years ago), the entire Blue Mountain region probably resembled the eastern part of the Cascade Mountains in Oregon today, particularly the area between Mt. Hood and Crater Lake.

The present Strawberry-Aldrich Mountain range and the ridges and valleys north of it were formed when the Earth's crust buckled and broke under strong compressive forces from the north and south. Partly by bending or folding, and partly by breaking along the John Day and other faults, the Strawberry-Aldrich Mountains were gradually raised a mile and a half to two miles above the valley to the north. The floor of the main John Day River valley was filled with gravels eroded from the rising mountains and, as the folding and faulting and erosion slowed, an extensive, broad, gently sloping surface was formed on top of the valley fill. While these gravels (called the Rattlesnake Formation) were accumulating, however, a great volcanic eruption spread an ash flow more than 100 feet thick over the entire length of the John Day valley floor; remnants of this flow form prominent rim-rocks between the town of John Day and Picture Gorge. Bones of Hipparion, a horse the size of a pony which had feet and teeth like modern horses, have been found in the gravels under the ash flow.

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Last Updated: 28-Mar-2006