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22 chapters
GEOLOGIC SETTING OF THE JOHN DAY COUNTRY, GRANT COUNTY, OREGON
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 Columb
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GEOLOGIC HISTORY
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. Th
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THE PRESENT LANDSCAPE
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
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A. 155.4
A. 155.4
Holliday Rest Area. The main John Day fault, which here is buried under river gravel, is believed to be just south of the rest area. A parallel step fault ( Fig. 2 ) cuts from left to right across the southern slope of Mt. Vernon Butte about where the juniper trees thin out. Flows of Picture Gorge Basalt in the upper part of the butte slope down to valley level on your left. Rocks in the foreground and to the right are rudely bedded volcanic breccias of the Clarno Formation. The Clarno Formation
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B. 144.0
B. 144.0
Fields Creek Road. In the road cuts just south of the highway, the John Day fault ( Fig. 3 ) passes through the Mascall Formation where the slope or dip of the beds changes abruptly from gently southward to steeply northward. The fault continues westward under the floor of the valley. Fossil leaves and snails can be found in the beds south of the fault, and 1,000 feet farther south vertical Picture Gorge Basalt flows are exposed. To the north across the valley at the White Hills, beds of the Mas
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C. 138.5 to 135.0
C. 138.5 to 135.0
Vertical Ribs. The prominent vertical ribs, visible south of the river in steep slopes below the high bench (pediment), are flows of Picture Gorge Basalt tilted vertically in the north limb of the Aldrich Mountain anticline. The John Day fault follows the base of the steep front in which the ribs are exposed....
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D. 132.6
D. 132.6
Volcanic Ash Flow. The rimrock north of the John Day River between here and Dayville is a volcanic ash flow that erupted about five million years ago as red hot pumice highly charged with gas. A rapidly moving incandescent cloud probably filled the ancient John Day River valley and deposited ash to a depth of more than 100 feet over a distance of 60 to 70 miles. As shown in Figure 4 , the ash flow blankets about 200 feet of gravels that had been deposited in the valley. The rimrock and the grave
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E. 126.0
E. 126.0
Picture Gorge. Visible to the left of Picture Gorge, from north around to west, in order of their deposition and geologic age from oldest to youngest, are the Picture Gorge Basalt, Mascall Formation, and Rattlesnake Formation ( Fig. 5 ). The Picture Gorge Basalt flows and ashy beds of the Mascall Formation were tilted southward together and eroded before the Rattlesnake Formation was laid down horizontally across them. Picture Gorge and the present valley were then cut by the John Day River afte
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F. 122.0
F. 122.0
Thomas Condon Viewpoint, John Day Fossil Beds State Park. From Picture Gorge to the cliffs opposite, the Picture Gorge Basalts and varicolored ash beds of the John Day Formation rise together nearly 2000 feet. The basalt that caps Sheep Rock is an erosional remnant. The lower beds in the John Day Formation are colored red by clay eroded from thick soil on the Clarno Formation, which is exposed in the farthest red hill. The soil was formed by tropical weathering some 30-35 million years ago, befo
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G. 118.7
G. 118.7
Munro Area, John Day Fossil Beds State Park. The valley of the John Day River has been widened to nearly five miles by erosion in the John Day Formation. Large tilted slide blocks of the John Day Formation and basalts jumbled together show how important landsliding of soft beds under hard rocks can be in widening valleys. Fig. 6.—Sheep Rock from Thomas Condon viewpoint. Here one can appreciate the regularity and extent of basalt flows of the flood or plateau type, which form the Columbia Plateau
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H. 116.2
H. 116.2
Cathedral Rock. The bluff called Cathedral Rock is the front face of a large block of the John Day Formation that has slid from the west ( Fig. 7 ). Inside the next horseshoe bend downstream a large mass of basalt is tilted down against Cathedral Rock. From the highway 1.1 miles farther north one can see the side of the tilted block along the river, and the same two prominent red and olive-drab ash layers in the high bluff from which the block slid. The horseshoe bend was formed as the river was
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J. 107.0
J. 107.0
Kimberly Dike. The low bluff across the river is formed by a vertical dike of basalt which is about 60 feet wide. This dike cuts through the John Day Formation and, farther north, the Picture Gorge Basalt ( Fig. 8 ). It crosses the river valley diagonally and can be traced nearly four miles. The dike is formed of once-molten rock that “froze” in a fissure which was a channelway for lava that fed a flow on the earth’s surface. Many similar dikes are visible along the road east of Kimberly. The ba
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K. 5.0
K. 5.0
Basalt Dike. A basalt dike 15 feet wide cuts basalt flows in bluffs north of the road and forms a wall 20 feet high in places; it is visible also across the river....
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L. 11.2
L. 11.2
Parallel Dikes. Two prominent parallel dikes, among others, form crests of hogbacks south of the river. A small tapering dike cuts pink and white beds of the John Day Formation in the river bluff. This dike is the southwest end of an irregular intrusive mass of basalt in which the river has cut a steep-walled gorge....
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M. 13.2
M. 13.2
Basalt Intrusion. The road is cut through 300 feet of basalt intruded into the John Day Formation. Both contacts are well exposed. The basalt—when molten—baked and reddened 3 to 6 inches of the adjacent beds. (At 13.9 the highway crosses the North Fork of the John Day River and follows the Cottonwood Creek Valley.)...
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N. 18.0
N. 18.0
Irregular Dikes. Above the highway several small irregular basalt dikes cut the white beds of the John Day Formation. Parts of the largest dike are 10 to 15 feet high....
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O. 20.1
O. 20.1
Cottonwood Creek Valley. The view northwestward down Cottonwood Creek toward Monument ( Fig. 9 ) exemplifies the development of broad valleys by erosion in soft beds of the John Day Formation under the gently warped Picture Gorge Basalt. To the south, the valley ends against massive rocks of the Clarno Formation which were raised as a block by movement along the Hamilton fault. The red beds are in the lower part of the John Day Formation. Note the contrast between the irregular massive intrusion
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P. 22.0
P. 22.0
Sunken Mountain. A small landslide in the lower part of the John Day Formation is called Sunken Mountain. When the valley wall was over-steepened by normal stream erosion, the jumbled material in the lower part broke away and slid down from the steep bare slopes above. Absence of tilted trees indicates that the slide is not very active now. The bare “badland” slopes are being eroded by rain wash. The cliffs of the John Day Formation, which the road climbs half a mile farther east, are the result
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Q. 26.4
Q. 26.4
Long Creek Mountain. An uplifted block of Picture Gorge Basalt 1400-1500 feet thick forms Long Creek Mountain; Round Basin is eroded in the John Day Formation on which the basalt rests ( Fig. 11 ). The base of the basalt can be seen in road cuts on either side of Basin Creek. The Hamilton fault follows the gulch to the left just below the parking area, goes up the tree-filled gulch across Basin Creek, along the low ridge at the northeast edge of Round Basin, and then along the northern foot of t
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R. 99.5
R. 99.5
Fox Valley. Down-warped flows of Picture Gorge Basalt dip toward Fox Valley from all sides to form a basin ( Fig. 12 ). The valley is eroded out of ashy beds and gravels of the Mascall Formation which fill the center of the basin to an estimated depth of 1000-1200 feet. Faults form parts of the northern and southern borders of the basin. The straight, timbered, northward-facing steep slope less than a mile southeast of the viewpoint marks a fault....
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S. 180.3
S. 180.3
Strawberry Range. This range and the Aldrich Mountains form a mountain range 50 miles long; Strawberry Mountain, altitude 9038 feet above sea level, is its highest peak. The eastern two-thirds of the Strawberry Range ( Fig. 13 ) was raised as a great block by uplift on the John Day fault, which follows the northern base of the mountains. The rocks in Strawberry Mountain and to the east are mostly lavas which poured out over the land, whereas the Canyon Mountain part of the range consists of gabb
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Selected References To Detailed Reports
Selected References To Detailed Reports
(From material provided by Thomas P. Thayer) As the Nation’s principal conservation agency, the Department of the Interior has basic responsibilities for water, fish, wildlife, mineral, land, park, and recreational resources. Indian and Territorial affairs are other major concerns of America’s “Department of Natural Resources.” The Department works to assure the wisest choice in managing all our resources so each will make its full contribution to a better United States—now and in the future....
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