Geological Survey Bulletin 1347 The Geologic Story of Yellowstone National Park

The many episodes of mountain building and volcanism all left their lasting and unmistakable imprints across the face of the Yellowstone country. During the latter part of the Tertiary Period, erosion, too, had begun to make its own deep marks. But only in the last 100,000 years or so have the powerful exterior forces of the earth—chiefly running water and moving ice—had a virtually free hand in shaping the Park's landscape. Nevertheless, in this short period of time they have wrought profound changes.

Glaciation

A giant boulder of Precambrian gneiss lies among the trees beside the road leading to Inspiration Point on the north rim of the Grand Canyon of the Yellowstone (fig. 34). This boulder, measuring approximately 24X20X18 feet and weighing at least 500 tons, is of considerable interest, not so much for its great size but because it is completely out-of-place in its present surroundings. The boulder rests on rhyolite lava flows of Quaternary age, at least 15 miles from the nearest outcrops of the ancient gneiss to the north and northeast. Obviously, this seemingly immovable chunk of rock was pushed or carried a long way by some very powerful transporting agent before it was finally dropped. A natural force of such magnitude could only have been exerted by moving ice; in fact, no further proof than this one boulder is needed for us to conclude beyond question that glaciers once existed in Yellowstone. There is, to be sure, much additional evidence that the Park region was extensively glaciated. Deposits of out-of-place boulders (glacial erratics), like the one mentioned above, are found nearly everywhere (fig. 35), and the mountains and high valleys still bear the vivid scars of ice sculpturing (figs. 36 and 37).

GIANT BOULDER (glacial erratic) of Precambrian gneiss near Inspiration Point on the north rim of the Grand Canyon. The boulder, measuring 24x20x18 feet and weighing more than 500 tons, was dropped at this locality by glacial ice; it now rests on the much younger Plateau Rhyolite. The distance that the boulder was carried or pushed was at least 15 miles. (Fig. 34)

GLACIATED TERRAIN along the Northeast Entrance road. The boulders, many of them measuring 10 feet across or more, were carried into the area by ice flowing down Slough Creek from mountains north of the Park during the Pinedale Glaciation. As the glaciers melted, the boulders were left stranded in hummocky, morainal deposits. Shallow depressions in the irregular topography are now commonly filled by small ponds. (Fig. 35)

CANYON PROFILES. Typical profiles of a canyon cut by a stream (A) and of a canyon gouged by a glacier (B). Glacial cirques (C) are shown at the head and high on the side of the glaciated valley. (Fig. 36)

GLACIAL CIRQUE on east face of Electric Peak, northern Gallatin Range. During several episodes of glaciation, this steep-walled amphitheaterlike valley was cut and filled by ice which fed glaciers moving downslope to the lower right. The cirque floor is now covered by a thick deposit of rock rubble underlain in part by ice, and the whole mass is still moving slowly downhill as a rock glacier. The dark rock at lower right is part of the Electric Peak stock, composed of diorite (fig. 20) and other kinds of intrusive igneous rocks. The rocks in the cirque walls are chiefly Cretaceous shales (light to moderately dark color) with thin sills of igneous rock (very dark color). (West-looking oblique aerial photograph, courtesy of William B. Hall, University of Idaho.) (Fig. 37)

The principal requirement for the formation of glaciers is simple: more snow has to accumulate during the winter than is melted during the summer. If this condition continues for a long enough period of time (measured in centuries), the snow compacts to ice, and extensive icefields grow until they finally begin to move under their own weight, thereby becoming glaciers. Records show that the average year-round temperature is 32°-33° along Yellowstone Lake, 35°F at Old Faithful, and 39°F at Mammoth. Each winter, snow accumulates to depths of 5-10 feet throughout much of the Park. If the average annual temperatures were to decrease a few degrees or the yearly snowfall were to increase a foot or so, either change could possibly herald the beginning of another ice age in the Yellowstone region.

Yellowstone was glaciated at least three times. These glaciations are, from oldest to youngest, the pre-Bull Lake, Bull Lake, and Pinedale. Their precise age and duration are imperfectly known, but estimates based on a few radiometric determinations are: (1) the oldest glaciation (pre-Bull Lake glaciation) began more than 300,000 years ago and ended between 180,000 and 200,000 years ago; (2) Bull Lake Glaciation began about 125,000 years ago and ended more than 45,000 years ago; (3) Pinedale Glaciation began about 25,000 years ago and ended about 8,500 years ago. The pre-Bull Lake and Bull Lake are known only from scattered deposits of rock debris (glacial moraines) and other features, but the distribution of these deposits indicates that glaciers were widespread throughout the region and occurred both between and during eruptions of the Plateau Rhyolite. The effects of the Pinedale glaciers, on the other hand, are obvious in many parts of the Park, and the history of this youngest glacial cycle (described below) is known in much greater detail than that of the two older ones.

In the early stages of Pinedale Glaciation, an enormous icefield built up in the high Absaroka Range southeast of the Park area. A glacier, fed by this icefield, flowed northward down the upper Yellowstone valley and into the basin now occupied by Yellowstone Lake. At about the same time, another great icefield formed in the mountains north of the Park and sent long tongues of ice southward toward the lower Yellowstone and Lamar River valleys. Smaller valley glaciers flowed westward out of the Absaroka Range along the east edge of the Park, and still others formed along the main ridges and valleys of the Gallatin Range, in the northwestern part of the Park. Thus, many huge masses of ice from the north, east, and southeast converged and met in the Park.

For the next 10,000 years, the ice thickened and spread out over more and more of the Park area. The mass centered over the Yellowstone Lake basin grew to a depth of 3,000 feet or more and dominated the entire scene; it formed a broad "mountain" of ice which became so high that it caused more snow to fall upon itself and was cold enough to prevent much of this snow from melting. Eventually the Pinedale glaciers covered about 90 percent of Yellowstone (fig. 38). At this stage, probably about 15,000 years ago, only the west edge of the Park, and perhaps a few of the highest peaks and ridges within the Park, remained free of ice. It is interesting to note that although ice moved across and buried the ancestral Grand Canyon of the Yellowstone, it did not flow down and scour the canyon (fig. 36). If it had, the canyon would look much different than it does today (fig. 41).

EXTENT OF ICE in Yellowstone National Park during the maximum spreading of the Pinedale glaciers, probably about 15,000 years ago. Long arrows indicate direction of strong flowage of ice; short arrows show direction of less vigorous ice flowage. The dark-blue area shows the main ice mass centered over the Yellowstone Lake basin in the southeast corner of the Park. Many of the high peaks and ridges such as Mount Washburn, which are here shown free of ice, were glaciated at least once during the past 250,000 years. Whether they were covered by the Pinedale glaciers, however, is still an unresolved question. (Based on information supplied by G. M. Richmond, K. L. Pierce, and H. A. Waldrop.) (Fig. 38)

After their maximum advance, the Pinedale glaciers began to melt, leaving behind the rock debris they had gouged from the landscape and had pushed or carried along with them. These glacial moraines are now found in many areas throughout the Park. In places, glacial ice and (or) rock debris formed natural dams across stream valleys, thereby impounding lakes. Parts of Hayden Valley, for example, contain layers of very fine sand, silt, and clay several tens of feet thick (fig. 39) that accumulated along the bottom of a large lake. This lake formed behind a glacial dam across the Yellowstone River near Upper Falls. Some of the glacial dams broke and released water catastrophically, causing giant floods; the occurrence of one such flood is particularly evident along the Yellowstone River valley near Gardiner, Montana.

FLAT-LYING BEDS of fine sand, silt, and clay near the mouth of Trout Creek in Hayden Valley. These beds were deposited in a glacially dammed lake that covered part of Hayden Valley when the Pinedale glaciers were melting. The height of the streambank is about 40 feet. (Fig. 39)

By about 12,000 years ago the thick Pinedale ice sheet had melted entirely from the Yellowstone Lake basin and most other areas of the Park, although valley glaciers continued to exist in the mountains until about 8,500 years ago. Then, following a short period of total disappearance, small icefields formed again in the heads of some of the higher mountain valleys. Since the melting of the Pinedale ice, however, none has descended as a glacier into the lower stretches of the valleys. Even though a few snowfields persist locally throughout the summers (except during the warmest years), no glaciers exist in the Park at the present time.

Running water—canyons and waterfalls

Yellowstone is, among its many attributes, the source of large and mighty rivers. Located across the Continental Divide, the Park feeds two of the most extensive drainage systems in the nation— (1) the Missouri River system (and ultimately the Mississippi River) on the Atlantic side, via the Yellowstone, Madison, and Gallatin Rivers, and (2) the Columbia River system on the Pacific side, via the Snake River (fig. 1). These streams are fed by an annual precipitation which averages about 17 inches at Old Faithful and Mammoth, but which is considerably greater in the mountain ranges.

Many stretches of the main river valleys in Yellowstone are broad and flat bottomed. In these, the stream gradients range from about 10 to 30 feet per mile, and there is little erosion going on at present (Hayden Valley is a good example, fig. 4). But here and there the gradients are steeper, and the valleys are narrow and rugged. In some places these streams drop 50 or even 100 feet per mile, and the fast-moving waters have carved deep V-shaped gorges (fig. 36).

Waterfalls, features for which Yellowstone is also justly famous (fig. 40), generally result from abrupt differences in rock hardness. If a stream flows over rocks that are more resistant to erosion than the rocks immediately downstream, a ledge or bench will form across the streambed at that place because the less resistant rocks are worn away faster. And, as the ledge becomes higher, the softer downstream rocks will erode even faster. A true waterfall is one in which there is a free, vertical fall of water. If the ledge or ledges form only a rough, steep runway in the streambed, then the term "rapids" or "cascades" is more appropriate.

WATERFALLS in Yellowstone National Park. A, Lewis Falls on the Lewis River. The falls cascade over the steep edge of a rhyolite lava flow. B, Upper Falls on the Yellowstone River. The brink of the falls marks the contact between dense, resistant rhyolite lava (which forms the massive cliff) and more easily eroded rhyolite lava containing a high proportion of volcanic glass immediately downstream, as shown in figure 42. C, Gibbon Falls on the Gibbon River. The river tumbles over a scarp etched in the Yellowstone Tuff. The scarp first formed along faults at the north edge of the Yellowstone caldera 600,000 years ago, at a point that now lies 1/4 to 1/2 mile downstream. Continued erosion has caused the falls to recede northward to their present position. D, Tower Falls on Tower Creek. The rocks at the brink of the falls, and in the vertical cliff beneath, are coarse breccias and conglomerates of the Absaroka volcanic rocks. The channel of Tower Creek has not been cut down rapidly enough to keep pace with the downcutting of the main channel of the Yellowstone River, which lies a short distance downstream from the base of the falls. (Fig. 40)

The existence of many waterfalls in Yellowstone today is due in large part to the fact that, because of recent volcanism and glaciation, much of the region's topography is very young in terms of geologic time. Streams, even some of the largest ones, have not had enough time to wear away all the features that may produce waterfalls, cascades, or rapids along their channels. This is particularly true along the margins of lava flows, where there are sharp dropoffs between the tops of the flows and the lower ground beyond. The Grand Canyon of the Yellowstone and the Upper and Lower Falls, well illustrate the erosive power of running water.

Grand Canyon of the Yellowstone

Except for Old Faithful, the Grand Canyon of the Yellowstone is probably the best known and most talked about and photographed feature in the Park (fig. 41). Although not so deep or wide as some of the other great canyons in America, its sheer ruggedness and beauty are breathtaking. Here the aptness of the name "Yellowstone" can be fully appreciated and understood, for the viewer is at once engulfed in a sea of yellow hues streaked and tinted with various shades of red and brown.

GRAND CANYON AND LOWER FALLS of the Yellowstone River, as viewed upstream (southwest) from Artists Point on the south rim. The yellow-hued rocks lining the canyon walls are soft, hydrothermally altered rhyolite lavas. The rocks at the brink of the falls consist of less altered and therefore more resistant rhyolites. The falls, 309 feet high, formed at the contact between the hard and soft rhyolite units. (Photograph courtesy of Sgt. James E. Jensen, U.S. Air Force.) (Fig. 41)

At first glance, the canyon may appear to be a giant crack which suddenly opened up and into which the Yellowstone River then plunged headlong over high waterfalls at its south west end. This, of course, is not the way the canyon formed. Nevertheless, it is apparent that certain unusual conditions caused the river, after winding slowly through flat-floored Hayden Valley for about 13 miles, to cut a precipitous gorge 1,000-1,500 feet deep and 20 miles long (fig. 42C). A full explanation must be based on all the many events surrounding the eruption of the Yellowstone Tuff, the collapse of the Yellowstone caldera, the outpouring of the Plateau Rhyolite, and the various episodes of glaciation. Geologic studies show that all these events took place while the canyon was being cut, and that each one played an important role in its development. Hot-water and steam activity likewise was a significant factor. However, despite its many complexities, the history of the Grand Canyon can be divided into a few major stages, as outlined below:

1. From more than 2,000,000 years ago to about 600,000 years ago, a shallow canyon was gradually being cut into the Absaroka volcanic sequence by the ancestral Yellowstone River as it eroded headward from a point near the present confluence of the Yellowstone and Lamar Rivers (fig. 33). By the time of the climactic volcanic eruption in central Yellowstone 600,000 years ago, the head of the "old" canyon probably had been eroded southward nearly to the place where the north rim of the Yellowstone caldera was to form later (fig. 42A). This point now lies about 5 miles below Lower Falls.

2. Ash-flow tuffs that were erupted 600,000 years ago filled the "old" canyon, and the river recarved its channel, chiefly along its previous course.

3. A large lake formed behind (south of) the north rim of the caldera, the damming resulting in part from lava flows of Plateau Rhyolite that poured out across the caldera floor in this area between 600,000 and 500,000 years ago. Eventually the lake rose and spilled northward into the head of the "old" canyon, causing additional downcutting in what is now the lower 15-mile stretch of the canyon.

4. As the lake emptied, the river began to erode upstream into the thick rhyolite lava flows toward the present site of Lower Falls; the process was very similar to that of a common stream gully eroding headward into a hillside. At a stage somewhat more than 300,000 years ago, the head of the canyon probably lay near the falls, and the river had cut a channel 400-600 feet deep along this upper 5-mile stretch (fig. 42B).

5. Approximately 300,000 years ago the canyon area was covered by ice during pre-Bull Lake glaciation. During and after the retreat of this ice, sediments accumulated in a lake that occupied the upper reaches of the canyon between the present site of Upper Falls and Inspiration Point. Subsequently, very little downcutting was accomplished until about 150,000-125,000 years ago, when the canyon was eroded nearly to its present depth.

6. Canyon development was further interrupted by the advance and retreat of glaciers during Bull Lake and Pinedale Glaciations. During and since the melting of the Pinedale glaciers about 12,000 years ago, the canyon has attained its present depth, and its walls have acquired much of their picturesque erosional form. The Yellowstone River now maintains a fairly uniform gradient (60-80 feet per mile) throughout the 20-mile-long gorge, even though different segments of the canyon were cut at different times and through different kinds of rocks (fig. 42C).

DEVELOPMENT OF GRAND CANYON. Profiles along the floor of the Grand Canyon of the Yellowstone as it appears today (C) and as it appeared at two older stages in its development (A and B). Note particularly the various kinds of rocks through which the canyon has been cut, and how rock differences have influenced the location of the two falls. Diagonal lines indicate unaltered rhyolite; large dots, rhyolite with much volcanic glass; small dots, hydrothermally altered rhyolite; and circles and dots, Absaroka volcanic rocks. (Based on information furnished by R. L. Christiansen and G. M. Richmond; vertical scale is exaggerated about 10 times.) (Fig. 42)

The spectacular erosional development in the upper 5-mile segment of the Grand Canyon, which is the only part seen by most Park visitors, except for the very lower end near Tower Falls (fig. 33), has taken place mostly within the past 150,000-125,000 years. One reason for such a rapid rate of erosion stems from the fact that this part of the canyon overlies one of the wide ring fracture zones of the Yellowstone caldera (fig. 22). The fracture zone extends to great depth, providing a ready avenue of travel for the upflow of hot water and steam rising in the Yellowstone thermal system, as described in the following chapter. Through many thousands of years, the upward percolation of the hot fluids has caused severe chemical and physical changes (known as hydrothermal alteration) in the rhyolite lava flows. One spectacular result of the alteration has been the change from the normal brown and gray color of the rhyolites to the bright yellow and other colorful hues now seen in the canyon walls (as well as in many other places throughout the Park). Another significant result of alteration has been the weakening of the rocks; that is, the altered rocks are softer and less resistant to erosion than unaltered rocks. Hence, the river has been able to erode these softer rocks, upstream to Lower Falls, at a very rapid rate.

The position of Lower Falls, as might be expected, coincides with a change from highly altered to less altered rhyolite; the difference in the erosion rates of the two kinds of rocks here is self-evident (figs. 41 and 42C). The position of Upper Falls is likewise closely controlled by differences in rock hardnesses. The rhyolites on the upstream side are hard and dense, whereas those on the downstream side contain a high proportion of volcanic glass which causes them to be more easily eroded (fig. 42C).