Geological Survey 18th Annual Report (Part II) Glaciers of Mount Rainier Rocks of Mount Rainier

WINTHROP GLACIER.

The névé of Winthrop Glacier extends to the summit of Mount Rainier. A part of the snow that accumulates in the great summit crater between Crater Peak and Liberty Cap flows eastward down the precipitous slope of the central dome and contributes to the growth of the extensive névés covering all that side of the mountain. The eastern slope of the mountain is more heavily snow covered than any other portion, mainly for the reason that the prevailing westerly winds cause the snow to be deposited there in greatest abundance. The great peak rising in the path of the moist winds from the Pacific produces something like an eddy in the air currents on its eastern side, and thus favors deep snow accumulation.

All of the eastern side of the mountain above an elevation of 8,000 to 10,000 feet, except the precipitous ridges and jutting crags, is névé covered. The appearance of this side of the mountain in summer is shown on Pl. LXIX. As may be seen in the illustration to some extent, the snow is much crevassed, and is broken by faults where the cliffs are steepest. This is due to the ruggedness of the rocky slopes beneath, which yield unequally to the erosion of the descending ice and snow, but no amphitheaters or cirques have been excavated.

Near its lower limit the névé is divided by two rocky promontories, as has been previously described, known as The Wedge and Little Tahoma. These prowlike rock masses divide the névé into three primary glaciers—the Winthrop, Emmons, and Cowlitz—as shown on the accompanying map.

The névé of Winthrop Glacier descends below The Wedge, and terminates above timber line at an elevation of approximately 8,000 feet. Below the lower margin of the névé the solid blue ice of the glacier proper, in places heavily covered with débris, extends far down the valley between rugged mountains, and ends at an elevation of between 4,000 and 5,000 feet.

From the end of the glacier one branch of White River flows out as a swift turbid stream, heavily loaded with coarse débris.

One of the characteristic features of the glaciers about Mount Rainier, as already mentioned, is the occurrence of well-marked domes, the summits of which are commonly fractured so as to produce radiating crevasses. Several of these domes occur in Winthrop Glacier, both in the névé portion and in the glacier proper, and furnish abundant material for study.

The domes in the glaciers have the appearance that might be expected to result if a sheet of ice or of névé snow 100 to 200 feet thick could be lowered down vertically onto a surface on which there were haystack-like domes of rock 100 to 300 feet high. In such a case the ice over the domes would become fractured, while on the generally even surface between it would settle down with less breaking and conform to the general contour of the rocks beneath.

We know, however, that the snow and ice supplying the glaciers flow from higher regions, and that it must advance over the domes. This means that the ice has in fact an upward motion, for the glaciers rise in passing over the elevations in their beds. From the summits of the domes the snow, or ice, as the case may be, descends in all directions, but usually the slope is steepest on the downstream side.

A series of ice domes on the glaciers about Mount Rainier might easily be selected ranging from regular domes with practically equal slopes on all sides, through other similar forms with precipitous lower face, to precipices down which the ice descends, forming what are termed ice cascades. From analogy with the ice cascades I shall designate the elevations on the surfaces of the glaciers here described ice domes. Like the cascades, the domes vary in their characteristics according as they occur in a névé region or in a glacier proper.

Pl. LXXIV. RECENTLY ABANDONED MORAINE ON WEST SIDE OF CARBON GLACIER, JUST BELOW ANDESITE CLIFF.

The domes in the névé region, as on the upper portion of Winthrop Glacier below The Wedge, differ in appearance from those in the glacier proper, owing to differences in the physical properties of névé snow and ice. The domes in the névé have more or less of a network of wide fissures about their summits, with radiating fissures extending down the sides, which gradually contract and die out in the course of 100 or 200 yards. The radiating fissures on the upstream sides of the domes are commonly less extensive and much less conspicuous than those extending in other directions. The fissures in the summits of the domes in the névé at the time of our visit were widely expanded, and when the temperature was below freezing one could safely walk over the rough cavernous snow partially filling them and penetrate to the very center of the system of breaks. In some instances a nearly rectangular column of snow 30 feet on a side and rising 50 or 60 feet above the partially filled crevasses bordering it rose in the center like a huge obelisk.

Farther down Winthrop Glacier, in the glacier proper, the domes are equally conspicuous, but the crevasses in them are less regular and not nearly so wide as in the instances just cited. Just below the skirt of the névé as it existed at the time of our visit there are four domes inclosing a basin in which there was a shallow pond about an acre in area, which was conspicuous on account of its blue color. These domes rose, by estimate, about 50 or 60 feet above the ice on the upstream side, but the ones farthest downstream, when seen from below, had nearly twice this height. Their slopes on the upstream side descended at an angle of about 4° to 6°, but on the downstream side they were considerably steeper, probably 10° to 12°, and in one instance at least 20° or 25°. Just above each dome there is an absence of crevasses, and the ice seems to be compressed, but small breaks appear soon after the ascent begins. These crevasses are curved and tend to surround the dome as contours, but near their ends trend away from it in curves concave toward the center and die away. The crevasses become broader on the summit of the dome, especially on the brink of the steep downstream side, and as the ice descends are closed. The lower side of one of the larger domes was visited, and a hollow in the surface of the glacier was found below it. Standing in this hollow and facing the dome, one sees a steep descent, much like an icefall, with pinnacles of blue ice along the crest. On each side of the dome there is a curved ridge of ice, making the east and west walls of the depression in which the observer stands. These curved ridges become lower and lower as they leave the dome, and, uniting, completely inclose the depression, the lowest point on the rim being opposite the dome. A stream fed by surface-melting coursed along in the bottom of the depression, flowing in the direction of glacial motion for 5 or 6 rods, and then plunged into a crevasse. In the depression below other similar domes there were shallow ponds.

The barriers below the domes are evidently due to a strong flow of ice about their sides, which formed the lateral curved ridges that unite a short distance below.

That the domes are due to bosses of rock rising in the bed of the glacier is shown, as previously stated, by the appearances of such domes at the extremities of some of the glaciers which have been uncovered by the melting of the ice. One such dome stands near the extremity of Winthrop Glacier, and now forms a portion of its west wall, but was formerly completely surrounded and overridden by the glacier. The exposed rock domes are rounded, strongly glaciated, and more or less covered with morainal material. Perched bowlders, which a strong push would dislodge, are frequently seen on them. The rock domes are, in all observed instances, composed of dense, hard rock. One in Carbon Glacier, on the brink of the lower fall, is of granite. The others observed are of dense igneous rock, probably andesite. The rock about the bases of these domes is perhaps softer and more easily eroded, but that this is really the case has not been proved by observation, owing to the débris that occupies the depression and conceals the rock beneath. The impression that one gains on examining the rock domes exposed by the retreat of the glaciers is that they result from ice erosion and have been left in bold relief by the wearing away of softer rocks about them.

The manner in which a glacier rises over a dome is of interest in connection with the much-discussed problem of the cause of glacial motion. A detailed study of ice domes would apparently furnish evidence for deciding whether a glacier behaves as a rigid body that is thrust forward by a force acting from above, or as a plastic body moving under the influence of gravity. The manner in which the ice, in the case of a dome in the glacier proper, flows about the obstruction so as to form lateral ridges which gradually approach and inclose a depression below certainly favors the idea of plasticity. The ice appears to be forced over the summit of a rock dome and to rise higher than the adjacent surface upstream, both by the pressure of ice above and by the drag of the deeper ice current on either side.

It is difficult to conceive of the manner in which a rigid body would behave under the conditions here referred to, but there appears to be no reason for assuming that it would close in below the domes; we should expect, rather, to find an open channel below each obstruction, with precipitous and much-broken walls.

On the east side of Winthrop Glacier, below The Wedge, the rocks rise precipitously and form cliffs, from which much débris falls. The glacier is evidently sapping the cliffs that border it from The Wedge to near its termination. On the west side the limit of the névé fields is indefinite, there being many crags that rise through the snow above the timber line.

Pl. LXXV. UPPER ICE CASCADES OF THE COWLITZ GLACIER.

Below that horizon, however, the glacier's margin is sharply defined by bordering precipices. The ice has there melted back from rugged cliffs so as to leave a marginal valley some 200 feet deep, in which a stream flows. The margin of the glacier is heavily moraine-covered and much broken by crevasses. In places it is impassable. The extremity of the glacier, as already stated, flows past a bold rock dome, which was formerly covered by the ice, and at a later stage, as the glacier receded, divided into two branches, the eastern one being the broader. As the glacier continued to retreat, the tongue of ice to the west of the rock dome melted back, and a heavy lateral moraine was deposited as a free ridge, having a curved course, which extends out from the shore and joins the rock dome. The end of the moraine near the shore has been cut away by the stream following the side of the glacier, and when traced toward the border of the valley is found to end in a precipitous slope. Standing on the crest of the moraine, on which a few small spruce trees are growing, one can look down into a deep valley to the west of the rock dome, across which the moraine has been built. The slope of the moraine on that side is steep and the descent is about 400 feet. On the side overlooking the glacier the descent is still more precipitous. The ice has shrunk away from the moraine and is now fully 100 feet below its crest.

Above the rock-dome and bordered by the sharp-crested moraine just mentioned there is an embayment in the side of the glacier, occupied by heavily débris-covered ice that is fast melting away. This stagnant ice has a markedly different aspect from that on other portions of the glacier where motion is still in progress.

The topography of stagnant moraine-covered ice is difficult to describe, but easily recognized. A prominent feature is the great number of short, steep ridges or crests, rising to a broad, gable-like angle in the center and bounded on one side by a steep crescent-shaped slope of dirty ice and on the other by a much more gentle slope, which is heavily moraine-covered. The steep slopes usually face southward, but this is not an invariable rule. Below the steep slopes, and filling the spaces between adjacent crests, there is usually a deep accumulation of stones and dirt. As melting progresses these concentrated masses of morainal material are left as débris pyramids. The moraines on stagnant ice are usually darker than on other portions of a glacier, probably on account of the greater abundance of fine, earth-like débris, which retains moisture and is always wet. The steep slopes of the ice crests in the case of the Mount Rainier Glacier are dark-brown and frequently appear nearly black, as is also true of the stagnant border of Malaspina Glacier, Alaska. These dark, wet slopes are not common on moving ice.

The eastern branch of Winthrop Glacier ends in a low slope to the east of the rock-dome referred to, and has but little morainal material in it. Above the low termimus the ice rises steeply, and has been washed clean by surface streams. Above this steep rise the ice is profoundly crevassed and can be crossed only by patiently choosing a way from one narrow ice blade to another between deep gashes in the solid blue ice.

There are many other features of Winthrop Glacier that demand attention, but in the absence of a detailed map and other illustrations an attempt to describe them is probably not advisable.

One interesting feature near the end of Winthrop Glacier, reported by E. S. Ingraham, but not seen by me, is a deep, narrow cleft in the rock on the border of the valley and parallel to its longer axis, about a mile below where the glacier now terminates. This gorge is probably the result of stream erosion along the side of the glacier when it was much more extended than now. That this is the true explanation of its origin, however, remains for future travelers to determine.

EMMONS GLACIER.

The best example of the manner in which the general névé field descending the slopes of the central dome of Mount Rainier is divided by wedges of rock into primary glaciers is furnished by Emmons Glacier and its neighboring ice streams. The origin of the wedges now dividing the névé, by the erosion of the outward-flowing ice, has already been explained.

Below The Wedge and Little Tahoma, Emmons Glacier is a well-defined ice stream, about 5 miles in length, with bold, rocky cliffs on each side. The glacier becomes heavily charged with débris along its borders from the adjacent cliffs and in the lower portion of its course is completely covered with stones and dirt on each side. These lateral moraines become broader and broader toward the terminus of the glacier, leaving a tapering, lane-like tongue of clear ice between, but before the actual terminus is reached the ice over the entire surface is concealed by a continuous sheet of brown and barren débris. On the right-hand side of the glacier, for one or two miles above the terminus, abandoned lateral moraines occur in parallel ridges, marking a gradual shrinking of the ice. A similar record occurs also on the left side, but the moraines are there broader, and show by their color and by the relief of the surface that they rest on stagnant ice. On the side of the valley, above the stagnant moraine-covered ice just referred to, there are abandoned moraines which are banked against the steep cliffs forming the valley side.

The tongue of clear ice near the extremity of the glacier is some 2 or 3 miles long and, although gradually tapering downstream, is much of the way about one-third of the width of the valley. The grade is there low and the ice not much broken by crevasses. Down the central portion of this tongue there are two light medial morainal-bands, derived from rocky crests in the central part of the névé mostly above the horizon of Little Tahoma. These narrow rocky ledges may be seen in Pl. LXXIII.

Pl. LXXVI. SURFACE OF COWLITZ GLACIER.

The manner in which débris causes a decrease in the flow of the ice containing it, and final stagnation if the rock material reaches a large percentage,¹ leads to interesting suggestions in the case of the glacier here described.

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¹I. C. Russell, The influence of débris on the flow of glaciers : Jour. Geol., Vol. III, 1895, pp. 823-832.

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The excessive load of stones and earth on the sides of the glacier, the central portion being notably free from such accumulations, causes the marginal portions to become heavily charged with englacial débris as melting progresses. The flow of the ice is thus checked, and the marginal portions of the glacier become stagnant. This is equivalent to a narrowing of the valley through which the ice flows, and the clear portion is enabled to progress farther before melting than it would if the border had not become stagnant. Emmons Glacier, like all the other primary glaciers on Mount Rainier, is evidently wasting away and its terminus receding. The process just referred to, by which the channel available for the flow of the clear ice becomes contracted, might lead to an advance of the terminus in spite of the fact that a general wasting away is in progress. The stagnant ice along the sides of the valley not only causes a decrease in the width of the channel, but shields the clear ice from melting more effectually than cliffs of rock would do. The clear ice under the present conditions is also able to advance farther than it would in a broad valley where its sides would be exposed to melting; or than it would between cliffs and rocks which, by reflecting heat, would cause even greater marginal melting than would occur in a broad valley.

Emmons Glacier is deeply intrenched. Near its terminus it is bordered on each side by bold, rugged mountain ranges, left in relief by the excavation of the valley in the which glacier lies. The valley becomes narrower and its sides still more rugged below the terminus of the glacier. The stream flowing down it, a branch of White River, like other similar rivers already mentioned, is overloaded with coarse débris and is aggrading its channel.