YELLOWSTONE NATURE NOTES

EARLY MOUNTAIN BUILDING

The park when viewed from an airplane or a high mountain top, presents a broad flat plateau surface with an average elevation of 7,500 feet hemmed in by mountain ranges on three sides and open to the southwest. When placed in regional perspective Yellowstone is observed to fit into the general pattern of the Rocky Mountains where range after range follows a general northwest-southeast alignment. The lofty peaks of the Teton Range abut against the southern edge of the park, while the Snowy-Beartooth-Absaroka trend delineates the northern and eastern park boundaries. To the northwest the Gallatins rise above the plateau surface.

Mountains are ephemeral features in terms of geologic time. The poetic concept of "the eternal hills" is fine for a human life span but of course does not apply to geology.

Long before these mountains formed and for countless spans of time, much of the western United States was inundated by shallow arms of the ocean and in these downwarped areas great thicknesses of marine sedimentary rocks were deposited. Toward the end of the Cretaceous period, about 70 million years ago, a change began to take place and this region that had been submerged for so long a period of time began to feel the intensified pulse of the restless interior of the earth. The downwarp that had served as the repository for thick accumulations of stratified marine rocks was slowly subjected to compressive and vertical forces which initiated the gradual uplift of broad tracts of land destined by subsequent uplift and erosion to become the chains of mountains now included in the Rockies. With continued compression the formations yielded plastically to the deforming stresses until the rupture point was reached and then failure by faulting occurred. One can only speculate on the number of earthquakes that must have accompanied these times of mountain building!

In the pioneer days of the geological sciences investigators were hampered by a belief that the earth was not very old and consequently all of the observed crustal deformation had to be accounted for in a relatively short period of time. They thought in terms of great earth "revolutions," "catastrophies" and "convulsions," to account for mountain building processes. Modern day geologists favor the idea of more continuous deformation over extremely long periods of time.

PLATE 3. Devil's Slide, five miles north of Gardiner. Montana. Southwest flank of Beartooth Uplift along the Gardiner thrust fault exposing near vertical beds of Paleozoic and Mesozoic age. ". . . there has been a terrible convulsion here in the past . . ." A. C. Peale, 1871 Hayden Report. Vol. 1, p. 174.

Effects of this time of mountain building are most conspicuous in the Beartooth Mountains north and east of the park. Wilson (1934) has mapped the Gardiner thrust fault from its exposure near Devil's Slide to a point three miles east of Mt. Everts. He suggests that it may continue in a southeasterly direction as far as Cody, Wyoming, thus delineating the margin of a large thrust mass of rock constituting the Beartooth Mountains.

It is obviously impossible to know how high these first mountains stood, but very likely they were an impressive feature of the landscape but subject to the relentless attack of erosion, they were being worn down as uplift continued.

At Devil's Slide we can view the exposed remnants of one flank of this uplifted mountain block where erosion has performed the autopsy revealing structure that at one time was deeply buried.

FIG. 4. Evolution of Beartooth Mountain Block as in illustration of Mountain Building Process.