PURPOSE OF THE REPORT
A detailed office study of modern topographic and geologic maps and aerial photographs of Yampa Canyon and its environs in the Uinta Mountains has brought to my attention some striking features, one of which is very unusual or (so far as I know or can ascertain) even unique. Among the more outstanding features are:
1. A natural division of the 45-mile canyon into three partsa short upper section, and a middle and a lower section of roughly equal length.
2. A marked change in river pattern from the middle section to the lower section.
3. In the middle section, radical differences in topography and geology on the two sides of the river.
4. Also in the middle section, but only on the north side of the river, several curious scallop-shaped erosion surfaces now partly rimmed by cliffs and with moderately sloping floors; these surfaces, perhaps the most striking feature of the entire canyon, are herein called "meander-migration scars."
5. In the lower section, where the canyon is incised chiefly in the next younger formation, topography different from the two types in the middle section, and (with one significant exception) an absence of the meander-migration scars.
6. The relations (geographic and geologic) between the Yampa River and the Yampa, Red Rock, and Mitten Park faults.
The features observed in this area, apparently not widely known, should be of interest to geologists and geomorphologists, and should offer intriguing problems both in themselves and as clues to further unraveling of the geologic history of the Uinta Mountains and the origin and development of the courses and spectacular canyons of the Yampa and Green Rivers. They are therefore presented, described, and illustrated in this report. The descriptions are followed by suggestions offered in possible explanation of the cause and significance of the major features.
The term "meander" is used in this report a little more broadly than is customary. Most writers restrict the term to rather systematic smooth curves or loops of a river whose course may be called serpentine. The somewhat broader application herein is adopted partly for convenience and partly to emphasize the progressive development by which initial irregularities in a river's course tend almost immediately to begin growth, by lateral erosion, into larger and more systematic curves until they fully deserve to be called meanders. This progressive change was particularly well analyzed and described by Davis (1914, p. 4-7), who, however, carefully refrained from indicating the exact point of development at which the initial irregularities (his "bends or turns") become "meanders."
The term "incised" (with its related noun "incision") is herein used with its simple meaning of "cut into." It is intended to be noncommittal as to process of origin, cycle, and shape of cross section (symmetric or asymmetric). The word "incised" and other terms such as "intrenched" (or entrenched), "inclosed," and "ingrown," have been used by numerous writers with either general meaning or varying specialized significance; but as there has been no uniformity or full acceptance they seem confusing rather than helpful.
The geologic history of the Uinta Mountains, the origin of the anomalous courses of Green and Yampa Rivers, and the age relations and significance of the Bishop conglomerate and the Browns Park formation have been subjects for much study and many debates for nearly a hundred years. Among the pioneer students of these problems were J. W. Powell and S. F. Emmons; after the turn of the century, important contributions were made by several geologists, among them J. L. Rich and E. T. Hancock.
My own acquaintance with these problems began in the summers of 1921 and 1922 when I surveyed geology and oil and gas prospects in northwestern Colorado (Sears, 1924b). Interest was intensified and broadened in scope during the second season when, progressing westward, W. H. Bradley, James Gilluly, and I reached the Uinta Mountains and had some opportunity to examine parts of Browns Park, Cold Spring Mountain, and their environs. We realized that our observations and conclusions with regard to the mountain-and-river problems were incomplete, for we could reach only a small fraction of the pertinent region and had to study most of that fraction rather hastily as a sideline to our main assignment. Furthermore, we were handicapped by the lack of satisfactory topographic maps of our area (the one accompanying Powell's classic report on the geology of the eastern portion of the Uinta Mountains, 1876, being quite inadequate for our purpose) and of course the lack of aerial photographs, which would have been invaluable. Nevertheless, our concepts took form and were brought together in a report published 2 years later (Sears, 1924a).
During the subsequent decade Bradley spent three seasons in extending fieldwork westward on the north flank of the Uinta Mountains and far out into the basins to the north and northeast. This additional work enabled him to modify and expand our earlier concepts and to assemble his views in a comprehensive report (Bradley, 1936).
Since publication of Bradley's report in 1936, much new material bearing directly on the area here discussed has become available. In what was almost wholly an office study, I have derived information particularly from the following sources:
1. Aerial photographs of Dinosaur National Monument, scale 1:31,680; taken chiefly in 1938 for the Soil Conservation Service, U.S. Department of Agriculture.
2. Topographic map of Dinosaur National Monument, Colorado-Utah: U.S. Geol. Survey. Scale 1:62,500; contour interval 50 feet. Surveyed in 1941.
3. Untermann, G. E., and Untermann, B. R., 1954, Geology of Dinosaur National Monument and vicinity, Utah-Colorado: Utah Geol. and Mineralog. Survey Bull. 42.
4. Topographic maps of Hells Canyon and Canyon of Lodore South quadrangles, Colo.: U.S. Geol. Survey, 1954. Scale 1:24,000; contour interval 40 feet.
5. Aerial photographs for the quadrangles named in item 4, scales 1:28,400 and 1:17,000; taken in 1951 for the U.S. Geological Survey.
6. High-altitude aerial photographs of the eastern part of the Uinta Mountains and vicinity; taken in 1953 for the Army Map Service.
7. Topographic map of the Vernal 2-degree quadrangle, Utah-Colorado: Civil edition reprinted by the U.S. Geological Survey from V502 military edition compiled by the Army Map Service in 1955. Scale 1:250,000; contour interval 200 feet.
8. Finally, during a brief reconnaissance of the eastern part of the Uinta Mountains (spring of 1959) in the company of D. M. Kinney and W. R. Hansen of the Geological Survey, J. M. Good of the National Park Service, and Mr. and Mrs. G. E. Untermann of Vernal, Utah, several days spent in the immediate neighborhood of Yampa Canyon.
GEOLOGIC MAP IN PRESENT REPORT
Yampa Canyon and its environs, as discussed in the present report, occupy nearly half of Dinosaur National Monument. Hence the base for the map (p1. 1) of the area is taken from the Geological Survey's topographic map of the Monument, listed above as item 2.
Geologic datathe boundaries of the Weber sandstone and the Morgan formation, the strike-and-dip symbols, and the positions of the Yampa, Red Rock, and Mitten Park faultsare taken wholly from the map forming plate 2 in the report by G. E. and B. R. Untermann listed above as item 3. Though transfer of these data was done as carefully as possible, mechanical difficulties doubtless caused some departures from absolute precision, especially in the true altitude of formation boundaries on steep slopes where contour lines are very crowded. However, I believe that these departures are too slight to cause any injustice to the authors of the original map; certainly they are without significance to the problems herein discussed.
Special obligation and gratitude are felt to Mr. and Mrs. G. Ernest Untermann of Vernal, Utah, for their generous permission to use freely the data in their report on Dinosaur National Monument and for many courtesies and helpful information given later, both in person and through correspondence.
The National Park Service, through Mr. Jess H. Lombard, Superintendent, Dinosaur National Monument, kindly loaned negatives and authorized use of several photographs as illustrations.
Mr. Byrl D. Carey, Jr., and The California Co. furnished valued data on the thickness of the Browns Park formation obtained through drilling.
Dr. Arthur D. Howard, Stanford University, helped me with several discussions on special problems of geomorphology.
To Mr. John M. Good, geologist for the National Park Service, thanks are due for information given by correspondence and during our visit to the Uinta Mountains in the spring of 1959.
Finally, I am grateful to Wilmot H. Bradley, Douglas M. Kinney, and Wallace R. Hansen, of the U.S. Geological Survey, who shared with me their knowledge and varied interpretations of Uinta Mountain geology.
Last Updated: 09-Nov-2009