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GLACIAL PHENOMENA IN THE YELLOWSTONE

PARK.

BY WM. H. HOLMES.

IN common with very many of the more elevated districts of

the Rocky mountains, the Park district presents a variety of glacial phenomena. In exploring the deep valleys of the higher ranges, the geologist is never surprised at encountering on all hands partially rounded masses of transparent rock. These are pretty sure to be found on most of the old flood plains of the streams and often high up the sides of the valleys. They are frequently the only remaining records of ancient glaciers which have filled the valleys at different stages of their erosion. The glaciation of rocks in situ, in the narrow gorges, also bears testimony to the former existence of glaciers. Loose boulders are doubtless, in many cases, carried from their original beds by the force torrents, and not infrequently reach places very far distant from their original station by a gradual creeping or sliding movement -the result of undermining or yielding of the soil beneath. It is, therefore, far from safe to conclude that wherever erratic rocks are found, glaciers have formerly existed, especially in cases where these rocks may have had their origin in surrounding highlands, or even quite distant mountains of very considerable elevation.

In a region like this, however, there is every reason to suppose that glaciers once existed on a very extensive scale. The park, with the great continental water-shed that surrounds it, forms one of the grandest masses of highland in the United States.

In early quaternary times, as now—if there have been no important changes of level in the meantime—the general level of the park district exceeded eight thousand feet, and the broad areas

? Extracted from the unpublished Report of the U. S. Geol. Survey of the Terri. tories. Exploration of 1878.

of mountainous country on the west, north and east, represent a former general elevation of twelve thousand feet or more.

Glaciers exist now in the neighboring Wind River and Teton mountains at elevations much below twelve thousand feet, and in the midst of glacial times descended in immense sheets to four thousand and five thousand feet. It would, therefore, be a matter of surprise if traces of glaciers were not found here, not only in the high valleys, but upon the surfaces of the broad plateaus of the park. There is, however, a singular absence of well defined glacial moraines. The tens of thousands of granite boulders that occur on both sides of the Yellowstone valley, from Cinnabar mountain to the north base of Amethyst mountain, generally lie upon the smooth surface of the flood planes of the river, or upon low ridges of alluvial drift. The significance of this fact may be that the transporting glaciers existed in the earlier stages of the erosion of the valley, and that the morainal ridges have been destroyed by the river as it oscillated from side to side in the succeeding stages of its descent from the plateau level to its present bed. These great boulders would, in such a case, be the more durable masses of the moraines stranded on the various food planes for want of water power to transport them.

When we come to search for the source of the granite, we are led to observe an interesting fact. The only bodies of granite rock within the limits of this valley are found either on the north side or on the bottom at no considerable elevation above the river. But the erratic masses occur to a great extent on the south side of the valley and at all elevations. In the vicinity of Mt. Evarts they reach the upper surface of the plateau more than two thousand feet above the river bed. It is evident that these masses of granite were transported to their present resting places either before the valley existed or that the ice streams were so deep as to fill the valley to the brim and thus carry and strand them. Still it is a question whether in the latter case these boulders would ever reach their positions on the south side-supposing the glaciers to follow the course of the valley—as they would have to accomplish the feat of crossing the whole width of the glacier as a boat would cross a ferry. This could really only occur in case there should be such an increase in the masses of ice descending from the highlands to the north, as to completely fill the valley, sweep across its course and overspread the broad table-land to the south. This table-land I have named the Park plateau; it is wholly volcanic, and is separated from the base of the granite highlands on the north, by the valley of the Yellowstone proper, and by the East fork, its geologic as well as topographic continuation. It extends, with but few interruptions, one hundred miles to the south. We are here led to inquire whether or not there are evidences of former glaciers on this plateau. Such evidences do exist, but they are certainly not such as we might expect. Instead of well-defined moraines, an area dotted by erratic boulders and broad expanses of polished surfaces as in the Wind River and Teton mountains, we find only a few rocks other than those that may have been derived from the plateau itself. It should be remarked, however, in this connection, that the soft rhyolites which form the greater part of the plateau, would not retain glacial markings for any considerable length of time.

An occasional small block of granite indeed is found, and sometimes at unexpected levels, as on the slopes of the Washburn mountains many hundreds of feet above the general level of the plateau. A very few have been observed beyond Mt. Washburn, on the south side. The most remarkable example of these is a boulder resting upon the brink of the grand cañon, about a mile and a half below the great falls and nearly eighteen miles from the northern border of the plateau.

On a stormy day in December I undertook to meander the grand cañon from the falls to the base of Mt. Washburn, and during a storm of rain and sleet took shelter under the overhanging edge of a great rock in the dense timber. Considerably to my surprise I discovered it to be a very compact coarsely crystalline feldspathic granite. In shape it is somewhat rectangular, the edges for the most part sharp and unworn, the result of spawling by the heat of forest fires. In cubical dimensions it will probably exceed two thousand feet. It is within a stone's throw of the brink of the cañon and rests upon a sheet or a series of sheets of rhyolite, not less than one thousand feet in thickness, as may easily be determined by an examination of the section exposed in the cañon walls below.

In seeking the possible source of this rock we naturally turn to the south, towards the sources of the Yellowstone. The plateau along the river's course and around the lake is totally volcanic. The great ranges to the east and south of the lake are not known to contain a single exposure of crystalline rock. That there are no such formations in the whole drainage of the Upper Yellowstone is established by the fact of the almost total absence of granite pebbles on the shores of the lake or in the bed of the river. The home of this wanderer must be sought elsewhere. To the north, beyond the valley of the third cañon and the East fork, lies the granite highland previously mentioned. To the north-west, beyond the valley of Gardiner river, at the southern end of the Gallatin mountains, is another exposure of granite at an elevation sufficient to have given origin to it. The distance in either case is upwards of twenty miles. From the great falls the river descends in a northerly direction until it strikes the base of the granite highland; here it unites with the East fork and turns to the west along the south base of that highland, following the line of the great displacement until it passes the granite gateway of the second cañon (see accompanying map). To reach its present position from the northern locality, the boulder must cross the course of the great valley of the East fork and the third cañon and ascend the river, as it now exists, a distance of twenty miles, avoiding on its way, by a circuitous route, the intervening Washburn range and the opposing mass of Amethyst mountain. If from the Gallatin mountains, it must first have crossed the valley of the upper Gardiner river and afterwards a considerable spur of the Washburn mountains—a journey of twenty miles south-east. Notwithstanding the fact that this pathway would, with anything like the present topography, seem to present fewer obstacles to the advance of a glacier than that from the north, I cannot regard it as at all probable that this was its course. The mass of the Gallatin mountains is not great. Glaciers originating in its short abrupt valleys would have no great longitudinal extent, and would probably advance no farther than the deepest part of the valley that lies along their immediate base.

The great ranges to the north are of sufficient extent to give birth to ice rivers of the grandest proportions. The present distribution of the erratic fragments of granite tends to strengthen the impression that they had their origin in the north. If this be admitted, it becomes at once clear that the erosion of the grand cañon has been accomplished since the close of the giacial period, or at least that a second erosion has taken place if a cañon did exist prior to the glacial epoch.

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That a very profound erosion had taken place along the course of the cañon at a very early date is proved by the fact that during the rhyolitic period as well as the basaltic and andesitic, there

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Joyce's Process A-B, Section line. C-D, Ocean divide. E-F. Line of Great Fault. G, Third cañon. H, Grand cañon. The black dots indicate the position of granite boulders.

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Glacial Phenomena of the Yellowstone Park.

were cañons almost as deep as the present one, into which the coulées cascaded. At one spot near the northern base of Mt. Washburn the section of a fossil river is exposed, more than

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