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

by Thomas P. Thayer

GEOLOGIC SETTING OF THE
JOHN DAY COUNTRY,
GRANT COUNTY, OREGON

UNITED STATES DEPARTMENT OF THE INTERIOR/ GEOLOGICAL SURVEY

The town of John Day, the John Day River valley, and the Strawberry Range, looking south and southeast. The tailings piles left by gold dredges have been levelled for building sites since the photograph was taken in 1946.

One of the Pacific Northwest’s most notable outdoor recreation areas, the “John Day Country” in northeastern Oregon, is named after a native Virginian who was a member of the Astor expedition to the mouth of the Columbia River in 1812.

There is little factual information about John Day except that he was born in Culpeper County, Va. about 1770. It is known also that in 1810 this tall pioneer “with an elastic step as if he trod on springs” joined John Jacob Astor’s overland expedition under Wilson Price Hunt to establish a vast fur-gathering network in the western states based on a major trading post at the mouth of the Columbia River.

The expedition arrived in the vicinity of the Grand Tetons, in what is now Wyoming, in September 1811, and with the onset of winter met disaster along the Snake River. When they ran out of food near the present site of Twin Falls, Idaho, Hunt divided the expedition into four parties to seek food and a feasible route through the canyons. As described in Washington Irving’s Astoria, the party that included John Day became widely separated from the others; experienced terrible hardships while wintering with Indians near Huntington; and was eventually reduced to just John Day and Ramsey Crooks. By mid-April of the following year, Day and Crooks reached the junction of the Columbia and Mah-hah Rivers, where a band of Indians took everything they had, including their clothes. Because of this incident, the Mah-hah River was renamed the John Day. Returning up the Columbia to seek help from friendly Indians, they were rescued by a party of trappers in canoes, and finally reached Astoria on the 11th of May 1812.

Although the first discovery of gold in Oregon reportedly was made in 1845 on one of the upper branches of the John Day River by a member of an immigrant train, the settlement of the John Day Country really began in 1862, when gold was discovered in Canyon Creek just above Canyon City. Since then, possibly $30,000,000 worth of gold has been mined, mostly from gravels in and along Canyon Creek and along a 10-mile stretch of the John Day River. Lumbering and ranching are now the principal industries of the region.

The growth of tourism in Oregon and Grant County and the accompanying increase of interest in geology have stimulated the preparation of this leaflet. The Grant County Planning Commission and State Department of Geology and Mineral Industries have cooperated most cordially in the program to better inform interested visitors about the geology of the country they are seeing.

GEOLOGIC HISTORY

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.

THE PRESENT LANDSCAPE

Differences in the rates of erosion of various rocks were as important as folding and faulting in forming the topographic features in the John Day Country. The chief agents of erosion are chemical weathering processes that eventually break down or decompose all rocks, and running water which carries away the weathered or partly weathered and broken material. Picture Gorge is narrow because the basalts in the walls are very resistant to weathering and break along vertical joints into large blocks that do not move easily. In contrast, the John Day River has cut a broad, flat-floored valley in the Mascall Formation at the south end of Picture Gorge because the ashy beds weather to fine clay which is easily washed away.

Landslides have marked the face of the John Day Country. Large rock masses commonly slide where steep cliffs form in soft rocks that are capped by hard resistant rocks. When the cliff face becomes too high and steep, the soft beds give way and large blocks or masses slide downward, usually tilting backward as they move, as shown in the diagram of Cathedral Rock on [page 15]. Cathedral Rock and two small landslides north of the river, about two miles east of John Day, show this classic form. Many landslides, however, are just jumbled, hummocky masses of slumped material. Large-scale landsliding in the John Day Formation under the Picture Gorge Basalt is colorfully displayed along the river for 8 miles north of Picture Gorge. Some landslides occur suddenly, but many move forward only as fast as the toe or lower end is eroded away, as at Sunken Mountain.

Glaciers have sculptured the principal valleys that lie above an altitude of about 5,000 feet. Moving ice, hundreds of feet thick, plucked out semi-circular amphitheaters called cirques at the heads of the valleys. The 2,000-foot cliffs above Little Strawberry Lake were formed this way. Rocks held in the ice, like the teeth of a giant rasp, ground off irregularities and widened the valley bottoms to a broad U shape as the glaciers moved down the valleys. The effectiveness of ice scour can be seen by comparing glaciated Strawberry Creek above Strawberry Lake (as shown in [figure 15] on page 22) with unglaciated Picture Gorge or with Canyon Creek just above Canyon City.