THE BOULDER GLACIERS
General description.The mountainous area immediately north of the Yellowstone Park and bounded by the Yellowstone river on the west and north is an alpine region abounding in grandly beautiful scenery. It is characterized by a great plateau of Archean gneisses, deeply trenched by the many streams which have their sources in the snow banks and lakes of their summits, and by high peaks of volcanic rocks whose peculiar erosion adds pleasing variety to the types of mountain scenery afforded by the limestones to the north and the Archean masses on which they rest.
This block of mountains was very generally covered by ice during the period of glaciation, which sent ice streams down the principal gorges and mountain valleys. Both the plateau summits and these mountain canyons present undoubted evidences of the work of the ice in producing the present configuration of the surface.
To the northward there are two ice streams occupying the principal valleys, those of the Boulder and its tributary, the West Boulder. The path of these glaciers and the mountain area in which they originated present us with very interesting and typical examples of alpine glaciation.
The principal stream draining this alpine area is the Boulder river. Heading in the many lakelets which lie nestled in the mountain basins about Haystack peak, the stream flows rapidly through a narrow gorge which it has cut through the granites in past glacial times to the point where its volume is augmented by a considerable stream called Bridge creek. From here northward it flows in a bowlder-choked channel through the great canyon of the Boulder, whose glistening walls of polished gneiss rise 3,000 feet high on either side. For the last few miles of its course through this impressive gorge the stream meanders somewhat slowly through a network of channels bordered by dense growths of spruce and fir, with thickets of brushwood; then quietly glides through boggy meadows past the cottonwood-covered islands at the gate of the mountains. Immediately above the Natural Bridge the stream flows in a limestone bed, in which it soon cuts a rapidly deepening gorge as it flows northward in a series of rapids and cascades to disappear in a snowy white feathery fall of 25 feet into the tunnel of which the Natural Bridge forms the portal. Emerging a hundred yards to the north and 100 feet below, the stream falls into a tranquil pool at the head of a gorge that is a mile or so long and has limestone walls 100 feet high. The roof of the first short tunnel is a dry river bed, the floor of a gorge that is a continuation of the picturesque cutting above the Natural Bridge. The numerous potholes and water-worn ledges show that the river must flow over this channel in time of flood, to fall over a vertical wall into the pool 100 feet below. The beautiful canyon cut in a low limestone anticlinal north of this is, however, immediately deserted by the river, which at once enters a tunnel in the western wall, only to emerge at the northern end of the canyon. That this gorge is occasionally occupied by the river is attested by the driftwood and gravel in its bottom; but the great slopes of talus, where firs 60 feet high are growing, show that such occupancy is but temporary.
The valley of the Boulder begins at the north end of this deserted river canyon, and is cut in the Cretaceous sandstones and shales, resting upon the steeply upturned Paleozoic limestones which flank the Archean gneisses, and whose upturned ledges form the end of the canyon. This valley is about three-fourths of a mile wide, and flanked by walls of Cretaceous rocks, a few hundred feet high on either side. In this upper valley the river peacefully meanders in graceful curves and ox-bow loops, through broad alluvial flats covered by cottonwoods, until it is joined by the West Boulder. Here as it leaves the morainal drift the valley immediately broadens out to a nearly uniform width of a mile, and the bottom lands and terrace flats are of greater extent and generally occupied as farms. In this part of its course the river flows more rapidly, dashing against the many great bowlders which fill its channel, or cutting the bluffs of volcanic breccia out of which the valley has been carved farther northward.
This lower valley is extremely beautiful and picturesque; its softer contours and bright green slopes being in pleasing contrast to the wilder and grander scenery of the upper part of the river's course. The walls to the west are often picturesquely pinnacled and buttressed by the erosion of the breccia, while on the east of the stream are broad and charming alluvial terraces. A broad and distinctly cut bench scores the slopes of the east, and runs into a broad gravel covered terrace, which at the mouth of Cherry creek is 250 feet above the river. It forms the beginning of the terrace system of the Yellowstone through which the Boulder has cut its way to join that stream in the valley about Big Timber. In this lower part of its course the stream is bordered by a great terrace formed of flats of bowlders and gravel, brought down by the stream from the morainal drift of its upper course.
Like most mountain streams the river has a descent so great that it has removed and carried down the greater part of the morainal débris which choked its course, a work that is still in progress.
Névé fields.It is impossible to define the névé fields of this region because the summits of the high plateau, over 10,000 feet above the sea, show glaciated surfaces which can not be differentiated from those found at lower elevations where there is undoubted evidence of true glacial movement in the general topographic relations, and where the summits and mountain basins show morainal accumulations in sheltered nooks that have undoubtedly been left by the shrunken remnants of the greater glaciers. These summits are characterized by an abundance of lakelets occupying rock basins, or occasionally held by dams of morainal material.
The gorges which head in these basins and amphitheaters, are typically glacial in form and show good examples of ice scoring when they are cut in the gneisses. Upon the divides and ridges at the heads of the gorges, bowlders are frequently found as instanced at the head of Sixmile creek, where a gneiss erratic, a foot in diameter, rested upon volcanic breccia, at 9,400 feet in a position which it could only have reached by transportation across an intervening valley several hundred feet deep. It is significant as evidencing general movement northward of the ice sheet covering the central portion of the range as well as the more common movements down the mountain gorges.
The observations of Mr. Iddings, by whom the general geology of this mountain tract was studied, showed that glaciated surfaces prevail about the heads of all the more important drainages the slopes being characterized by glacial striae, running away from the divide and down the valleys.
My own observations show similar glaciated areas at the head of the Boulder about Haystack peak. Haystack basin, a mountain-top terrace, shows beautifully glaciated surfaces of diorite with local drift deposits and occasional erratics from the neighboring peaks. Two glaciated lakelets make this basin an attractive camp. The slopes of Haystack, crossed by the trail from the gold mines, show good examples of glaciated ledges roughened by plucking.
Canyons.From the Haystack mines northward to Bridge creek, the canyon is decidedly rugged, with a terraced bottom such as is common in the glaciated canyons of the Sierra Nevada. The adjacent slopes are rugged, and though showing well glaciated surfaces lack the general rounding so prominent in the canyon farther northward, and are definitely though irregularly benched. These surfaces are barer than those to the north, and apparently more freshly glaciated. The slopes have been bared by avalanches, whose paths are plainly marked by areas of devastation, in desolate contrast to the flower-covered opens or the picturesque pine groves of the neighboring slopes. The stream flows through a channel, very narrow, steep and rocky; there are no alluvial flats, though well-wooded benches covered with talus extend along the canyon bottom.
The first and in fact the largest open or park of the canyon, called Hicks park, is situated about ten miles below the mines. The stream leaves its rocky bed a few miles below this and flows thereafter over a bed of bowlders and morainal débris filling the valley bottom. In fact the quantity of débris left by the retreating glacier is so great that the stream, notwithstanding its steep descent, has been unable as yet to clear it out. Below Hicks park the canyon walls are much nearer together than farther north and show beautifully glaciated moutonnéed surfaces. (See Plate IV.) Considerable morainal accumulations fill the canyon bottom at the mouth of a large stream from the west, the open hills being known as Lick park. Two lakelets fill hollows in an old river channel in this moraine, and sharp ridges of débris fill the bottom. Below this point the canyon walls recede, and the glacial U shape is very prominent.
This canyon of the Boulder is extremely disappointing to the traveler. The walls below Lick park, though rising to great altitudes, 3,000 feet and more above the stream, are receding, generally wooded, and do not fulfill the promise of rugged beauty given by the entrance to this great gorge. This is no doubt largely due to the general roundness caused by glacial erosion. The bosses of rock in the canyon bottom and the upper walls alike show glacial rounding and polishing shown in Plate IV, and the resulting smoothness is a disappointment so far as scenic beauty is concerned. The limestone slopes at the mouth of the canyon and about the Natural Bridge show the result of the glacial erosion of sedimentary rocks. From here northward the ice has filled the valley trough and overflowed the walls covering the adjacent slopes. Its margin is easily defined by the rather heavy deposits of drift and débris, but it is evident that there was little, if any, erosion of these surfaces, for the ledges of limestone show no abraded surfaces, nor does the drift farther north contain fragments of these rocks. This failure to erode the sedimentary rocks is the more remarkable since the ledges dip at an angle of some 30° in the direction of the movement and the alterations of shale and sandstones have brought the ledges of harder rock into bold relief as hogback ridges.
Moraines.In this lower part of its course the river has removed most of the morainal débris from the valley bottom, and it is only upon the surfaces above the valley wall that the glacial débris forms noticeable morainal heapings. These extend northward to the mouth of the west fork of the river.
Notable morainal heapings occur on the East Boulder in its upper mountain valley, but present no claims to novelty. A high monoclinal wall prevented their extension northward into a basin filled by glacial gravel sands, an attractive area, upon which the white settlers look covetously. It is a broad, open basin, the center being a nearly level gravel terrace through which the streams heading in the mountains to the east have cut down some 100 feet without reaching bed rock. To the south the slopes are not glacial, the limit of the drift being about the edge of this alluvial basin. From here to the mouth of the east fork the drift covers the lower slopes east of the stream, but does not surmount to rocky ledges of Cretaceous sandstone outcropping above. The area between the East fork and the Boulder is a low triangular doao heavily covered by drift. A scar made by the river shows in one place a thickness of 50 feet of stratified sand and gravel, overlaid by drift, but erratics are rare.
The glacier that occupied the valley of the West Boulder did not extend as far northward as that of the main stream, and ends just north of the point where the McLeod wagon road crosses the stream.
The ice stream which filled the valley of the West Boulder overflowed the walls of this valley, and left its morainal débris to mark its limits on the adjacent slopes. This drift forms a narrow belt, in general parallel to the stream, resting upon the eroded surfaces of the upturned and flexed beds of the upper Cretaceous. Where it is crossed by the McLeod road near its northern termination, it shows the usual knob and sink topography of these moraines, the hillocks of the large drift, the sinks, often the beds of dried ponds. The bowlders average 3 to 5 feet in diameter. This part of the moraine shows water channels, a feature that is absent from the moraine on top of the bluffs upstream. On the east side of the river the moraine terminates in a considerable accumulation of débris forming a tumultuous assemblage of hummocky knobs and sinks, the material being quite coarse and but little rounded, and manifestly largely from the surface of the ice. The summit of this moraine is formed of some ten parallel bowlder ridges, with grassy, bowlderless troughs between, some 25 to 30 feet wide.
The moraine upon the west side of the river presents few points of interest. Back of the "Blue Rim" cliffs it forms a considerable lakelet by damming the natural slopes, and its margin is very sharply defined upon the Laramie sandstones. As a rule the surface of this moraine is very rough, the drift varying greatly in size, and including many large blocks principally of granite, gneiss, and schist. Near the mouth of Davis creek the drift contains abundant bowlders of limestone and of conglomerate, but these rocks do not extend more than a mile north of their outcrops. Immediately back of the "Blue Rim" cliffs there is a moraine flat 200 yards wide at the top of the bluff, and back of this the usual hummocks prevail.
The country east of the river is very generally heavily mantled by morainal débris for some distance back from the edge of the bluffs. The table-land south of the McLeod road shows a fine alluvial area that is very fertile, owing to the damming back of the drainage by the moraine. The moraine itself is here very rough, and dotted with eight lakelets. Further south this character is preserved to the mouth of the mountain canyon, corresponding to the outcrops of the Carboniferous rocks. At the mouth of this grandly picturesque mountain gorge the canyon bottom is choked with drift and the stream is yet cutting down through the drift, and has at some late date formed a lake immediately above the mouth of Davis creek, whose grassy flats form a fine hay ranch.
Terraces.Although it will be seen from this account of the morainal accumulations of the Boulder, and its branches that the amount of débris is considerable, yet it is small compared with the erosion which the ice has accomplished. In fact, the accumulations of the West Boulder, though not extending so far northward, are greater than those of the main stream. The question at once arises what has become of the eroded material?
Although these glaciers had a length of 10 to 20 miles, there are no great morainal embankments to show their former magnitude, such as abound in the Wind River mountains and the Hoback ranges south of the park. The reason is perhaps to be found in the great terraces of gravel that border the Yellowstone. The lower course of the Boulder river is marked by terraces of assorted drift, but the narrowness of the valley has not favored the preservation of the great trains of débris which undoubtedly were carried down the valley from the termination of the glacier. The larger part of this material was probably swept northward by the flooded stream, and helped to build the broad and extensive terraces of the Yellowstone. It should be noted, however, that there is a considerable terrace on the east side of the Boulder, corresponding in level to the great upper terrace of the Yellowstone, south of Big Timber. It is a very level area about a mile across, in greatest extent, sloping to the north downstream, covered by well rounded cobbles and gravel of gneiss, limestone, shale, conglomerate, and other rocks found to the south. This terrace is, however, but part of the great system that forms so prominent a feature of the piedmont country of this region and record an elevation and erosion, subsequent to the maximum of glaciation.
The Boulder glacier was the usual type of mountain ice stream, common in the Rocky Mountain region south of the limit of the great Cordillera glacier described by Dawson. It differs but slightly from those that streamed outward from the deep-cut gorges of the Crazy mountains, where the source of supply was small and local.
Further studies are in progress in other parts of the Cordillera, from the Canadian line southward, and also of the isolated groups of mountains, the Crazies, Highwood, and others that form such striking features of the plains country east of the main ranges. It is believed that the results will contribute much of value to the history of the Pleistocene in this region and throw light upon some of the little understood problems that confront the geologist.
Last Updated: 14-Jul-2009