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

Part 1

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

UNITED STATES DEPARTMENT OF THE INTERIOR/ GEOLOGICAL SURVEY

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.

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