Mount Rainier Part 17

As for the skeleton character of the dividing crests, it will be readily seen to be the outcome of the headward gnawing of opposing cirques. In some places, even, the deploying process has attenuated the ridges sufficiently to break them through. West of Crater Lake is an instance of a crest that has thus been breached.

It is a significant fact that the empty cirques about the Mother Mountains lie at elevations ranging between 4,500 and 6,000 feet; that is, on an average 5,000 feet lower than the cirques on Mount Rainier which now produce glaciers. Evidently the snow line in glacial times lay at a much lower level than it does to-day, and the ice mantle of Mount Rainier expanded not merely by the forward lengthening of its ice tongues but by the birth of numerous new glaciers about the mountain's foot. The large size of the empty cirques and canyons, moreover, leads one to infer that many of these new glaciers far exceeded in volume the ice streams descending the volcano's sides. The latter, it is true, increased considerably in thickness during glacial times, but not in proportion to the growth of the low-level glaciers.

Nor is this surprising in view of the heavy snow falls occurring on the mountain's lower slopes. There is good reason to believe, moreover, that the cool glacial climate resulted in a general lowering of the zone of heaviest snowfall. It probably was depressed to levels between 4,000 and 6,000 feet. Not only the cirque glaciers about the Mother Mountains, but all the neighboring ice streams of the glacial epoch originated within this zone, as is indicated by the alt.i.tudes of the cirques throughout the adjoining portions of the Cascade Range. By their confluence these ice bodies produced a great system of glaciers that filled all the valleys of this mountain belt and even protruded beyond its western front.

To these extensive valley glaciers the ice flows of Mount Rainier stood in the relation of mere tributaries. They descended from regions of rather scant snowfall, for the peak in those days of frigid climate rose some 10,000 feet above the zone of heaviest snowfall, into atmospheric strata of relative dryness. It may well be, indeed, that it carried then but little more snow upon its summit than it does to-day.

The North Mowich Glacier is the northernmost of the series of ice bodies on the west flank of Mount Rainier. Like the Carbon Glacier, it heads in a cirque at the base of the Liberty Cap ma.s.sif, fed by direct snow precipitation, by wind drifting, and by avalanches. The cirque is small and shallow, not as capacious even as either of the twin recesses in the Carbon Glacier's amphitheater. As a consequence the ice stream issuing from it is of only moderate volume; nevertheless it attains a length of 3-3/4 miles. This is due in part to the heavy snows that reenforce it throughout its middle course and in part to overflows from the ice fields bordering it on the south. These ice fields, almost extensive enough to be considered a distinct glacier, are separated from the North Mowich Glacier only by a row of pinnacles, the remnants evidently of a narrow rock part.i.tion or "cleaver," now demolished by the ice. The lowest and most prominent of the rock spires bears the appropriate name of "The Needle" (7,587 feet).

The debris-covered lower end of the glacier splits into two short lobes on a rounded boss in the middle of the channel. This boss, but a short time ago, was overridden by the glacier and then undoubtedly gave rise to an ice dome of the kind so numerous farther up on the North Mowich Glacier and also characteristic of the Winthrop Glacier.

Separated from the ice fields of the North Mowich Glacier by a great triangular ice field (named Edmunds Glacier) lies the South Mowich Glacier, also a cirque-born ice stream, heading against the base of the Liberty Cap ma.s.sif. It is the shortest of the western glaciers, measuring only a scant 3 miles. Aside from the snows acc.u.mulating in its ill-shaped cirque it receives strong reenforcements from its neighbor to the south--the Puyallup Glacier.

Toward its lower end it splits into two unequal lobes, the southernmost of which is by far the longer. Sharp cut rock wedges beyond its front show that when the glacier extended farther down it split again and again.

The north lobe is of interest because the stream that cascades from the Edmunds Glacier runs for a considerable distance under it. In the near future the lobe is likely to recede sufficiently to enable the torrent to pa.s.s unhindered by its front.

What especially distinguishes the Puyallup Glacier from its neighbors to the north is the great elevation of its cirque. The Carbon, North Mowich, and South Mowich Glaciers all head at levels of about 10,000 feet. The amphitheater of the Puyallup Glacier, on the contrary, opens a full 2,000 feet higher up. Encircled by a great vertical wall that cuts into the Liberty Cap platform from the south, it has evidently developed through glacial sapping from a hollow of volcanic origin.

From this great reservoir the Puyallup Glacier descends by a rather narrow chute. Then it expands again to a width of three-fourths of a mile and sends a portion of its volume to the South Mowich Glacier. In spite of this loss it continues to expand, reaching a maximum width of a mile and a total length of 4 miles. No doubt this is accounted for by the heavy snowfalls that replenish it throughout its course.

Its lower end consists of a tortuous ice lobe that describes a beautiful curve, flanked on the north by a vertical lava cliff. A lesser lobe splits off to the south on a wedge of rock.

Immediately south of the elevated amphitheater of the Puyallup Glacier the crater rim of the volcano is breached for a distance of half a mile. Through this gap tumbles a voluminous cascade from the neve fields about the summit, and this cascade, reenforced by a flow from the Puyallup cirque, forms the great Tahoma Glacier, the most impressive ice stream on the southwest side. Separated from its northern neighbor by a rock cleaver of remarkable length and straightness, it flows in a direct course for a distance of 5 miles.

Its surface, more than a mile broad in places, is diversified by countless ice falls and cataracts.

A mere row of isolated pinnacles indicates its eastern border, and across the gaps in this row its neves coalesce with those of the South Tahoma Glacier. Farther down the two ice streams abruptly part company and flow in wide detours around a cliff-girt, castellated rock ma.s.s--Glacier Island it has been named. The Tahoma Glacier, about a mile above its terminus, spits upon a low, verdant wedge and sends a lobe southward which skirts the walls of this island rock, and at its base meets again the South Tahoma Glacier. From here on the two ice streams merge and form a single densely debris-laden ma.s.s, so chaotic in appearance that one would scarcely take it for a glacier. Numerous rivulets course over its dark surface only to disappear in mysterious holes and clefts. Profound, circular kettles filled with muddy water often develop on it during the summer months, and after a brief existence empty themselves again by subglacial pa.s.sages or by a newly formed creva.s.se. So abundant is the rock debris released by melting that the wind at times whips it up into veritable dust storms.

Beautifully regular moraines accompany the ice ma.s.s on both sides, giving clear evidence of its recent shrinking.

The partner of the Tahoma Glacier, known as the South Tahoma Glacier, heads in a profound cirque sculptured in the flanks of the great b.u.t.tress that culminates in Peak Success (14,150 feet). It is interesting chiefly as an example of a cirque-born glacier, nourished almost exclusively by direct snowfalls from the clouds and by eddying winds. In spite of its position, exposed to the midday sun, it attains a length of nearly 4 miles, a fact which impressively attests the ampleness of its ice supply.

In glacial times the glacier had a much greater volume and rose high enough to override the south half of Glacier Island, as is clearly shown by the glacial grooves and the scattered ice-worn bowlders on that eminence. As the glacier shrank it continued for some time to send a lobe through the gulch in the middle of the island. Even now a portion of this lobe remains, but it no longer connects with the Tahoma Glacier.

An excellent nearby view of the lower cascades of the South Tahoma Glacier may be had from the ice-scarred rock platform west of Pyramid Rock. From that point, as well as from the other heights of [Indian]

Henrys Hunting Ground, one may enjoy a panorama of ice and rock such as is seen in only few places on this continent.

East of the South Tahoma Glacier, heading against a great cleaver that descends from Peak Success, lies a triangular ice field, or interglacier, named Pyramid Glacier. It covers a fairly smooth, gently sloping platform underlain by a heavy lava bed, and breaking off at its lower edge in precipitous, columnar cliffs. Into this platform a profound but narrow box canyon has been incised by an ice stream descending from the summit neves east of Peak Success. This is the Kautz Glacier, an ice stream peculiar for its exceeding slenderness.

On the map it presents almost a worm-like appearance, heightened perhaps by its strongly sinuous course. In spite of its meager width, which averages about 1,000 feet, the ice stream attains a length of almost 4 miles and descends to an alt.i.tude of 4,800 feet. This no doubt is to be attributed in large measure to the protecting influence of the box canyon.

It receives one tributary of importance, the Success Glacier, which heads in a cirque against the flanks of Peak Success. This ice stream supplies probably one-third of the total bulk of the Kautz Glacier, as one may infer from the position of the medial moraine that develops at the point of confluence. In the lower course of the glacier this medial moraine grows in width and height until it a.s.sumes the proportions of a ma.s.sive ridge, occupying about one-third of the breadth of the ice stream's surface.

A singularly fascinating spectacle is that which the moraine-covered lower end of the glacier presents from the heights of Van Trump Park.

A full 1,000 feet down one looks upon the ice stream as it curves around a sharp bend in its canyon.

A short distance below the glacier's terminus, the canyon contracts abruptly to a gorge only 300 feet in width. So resistant is the columnar basalt in this locality that the ice has been unable to hew out a wider pa.s.sage. Not its entire volume, however, was squeezed through the narrow portal; there is abundant evidence showing that in glacial times when the ice stream was more voluminous it overrode the rock b.u.t.tresses on the west side of the gorge.

The name of P. B. Van Trump, the hardy pioneer climber of Mount Rainier, has been attached to the interglacier situated between the Kautz and the Nisqually Glaciers. This ice body lies on the uneven surface of an extensive wedge that tapers upward to a sharp point--one of the remnants of the old crater rim. A number of small ice fields are distributed on this wedge, each ensconced in a hollow inclosed more or less completely by low ridges. By gradually deploying each of these ice bodies has enlarged its site, and thus the dividing ridges have been converted into slender rock walls or cleavers. In many places they have even been completely consumed and the ice fields coalesce. The Van Trump Glacier is the most extensive of these composite ice fields. The rapid melting which it has suffered in the last decades, however, has gone far toward dismembering it; already several small ice strips are threatening to become separated from the main body.

In glacial times the Van Trump Glacier sent forth at least six lobes, most of which converged farther down in the narrow valleys traversing the attractive alpine region now known as Van Trump Park. This upland park owes its scenic charm largely to its manifold glacial features and is diversified by cirques, canyons, lakelets, moraines, and waterfalls.

In the foregoing descriptions the endeavor has been to make clear how widely the glaciers of Mount Rainier differ in character, in situation, and in size. They are not to be conceived as mere ice tongues radiating down the slopes of the volcano from an ice cap on its crown. There is no ice cap, properly speaking, and there has perhaps never been one at any time in the mountain's history, not even during the glacial epochs.

Several of the main ice streams head in the neves gathering about the summit craters, but a larger number originate in profound amphitheaters carved in the mountain's flanks, at levels fully 4,000 feet below the summit. In the general distribution of the glaciers the low temperatures prevailing at high alt.i.tudes have, of course, been a controlling factor; nevertheless in many instances their influence has been outbalanced by topographic features favoring local snow acc.u.mulation and by the heavy snowfalls occurring on the lower slopes.

[Ill.u.s.tration: GEORGE OTIS SMITH.]

XV. THE ROCKS OF MOUNT RAINIER

BY GEORGE OTIS SMITH

Director George Otis Smith of the United States Geological Survey was born at Hodgdon, Maine, on February 22, 1871. He graduated from Colby College in 1893 and obtained his Doctor of Philosophy degree from Johns Hopkins University in 1896.

He had begun his geological work in 1893 and from 1896 to 1907 he was a.s.sistant geologist and geologist of the United States Geological Survey. Since 1907 he has been director of that important branch of the Government work.

He had been studying the rocks of Mount Rainier before he joined Professor Russell in the explorations of 1896. The record of those studies was published at the same time as Professor Russell's report in the Eighteenth Annual Report of the United States Geological Survey for 1896-1897. With his permission the record is here reproduced in full. So far as is known to the present editor it is the most complete study yet published on the rocks of Mount Rainier.

The earliest geological observations on the structure of Mount Rainier were made in 1870 by S. F. Emmons, of the Geological Exploration of the Fortieth Parallel. The rock specimens collected at this time were studied later by Messrs. Hague and Iddings, of the United States Geological Survey.[27] This petrographical study showed that "Mount Rainier is formed almost wholly of hypersthene andesite, with different conditions of groundma.s.s and accompanied by hornblende and olivine in places." The only other petrographical study of these volcanics is that of Mr. K. Oebbeke, of Munich,[28] upon a small collection made on Mount Rainier by Professor Zittel in 1883.

On the reconnaissance trips on the northern and eastern slopes of Mount Rainier, during the seasons of 1895 and 1896, the writer had opportunity to make some general observations on the rocks of this mountain, and the petrographical material then collected has since been studied. The observations and collections were of necessity limited, both by the reconnaissance character of the examination and by the mantle of snow and ice which covers so large a part of this volcanic cone.

Two cla.s.ses of rock are to be discussed as occurring on Mount Rainier: the lavas and pyroclastics which compose the volcanic cone and the granitic rocks forming the platform upon which the volcano was built up.

VOLCANIC ROCKS

GEOLOGIC RELATIONS

On Crater Peak a dark line of rock appears above the snow, and here the outer slope of the crater rim is found to be covered with blocks of lava. A black, loose-textured andesite is most abundant, and from its occurrence on the edge of this well-defined crater may be regarded as representing the later eruptions of Rainier. Lower down on the slopes of the mountain opportunities for the study of the structure of the volcanic cone are found in the bold rock ma.s.ses that mark the apexes of the interglacial areas. Examples of these are Little Tahoma, Gibraltar, Cathedral Rock, the Wedge, and the Guardian Rocks. These remnants of the old surface of the cone, together with the cliffs that bound the lower courses of the glaciers, exhibit the structural relations very well.

Even when viewed from a distance these cliffs and peaks are seen to be composed of bedded material. Projecting ledges interrupt the talus slopes and express differences of hardness in the several beds, while variations in color also indicate separate lava flows and agglomeratic deposits. Gibraltar is thus seen to be composed of interbedded lavas and pyroclastics, and on the Wedge a similar alternation is several times repeated, a pink agglomerate being exceptionally striking in appearance.

These lava flows and beds of volcanic ejectamenta thus exposed dip away from the summit at a low angle. The steepest dip observed was in the amphitheater at the head of Carbon Glacier, where in the dividing spur the dip to the northeast is about 30. Some exceptions in the inclination of the beds were noted on the southeastern slope, where in a few cases the layers are horizontal, or even dip toward the central axis of the cone. In general, however, the volcanics composing Mount Rainier may be said to dip away from the summit at an angle somewhat lower than that of the slopes of the present cone. In the outlying ridges to the north, the Mother Range, Crescent Mountain, and the Sluiskin Mountains, the structure seems to be that of interbedded volcanics approximately horizontal. The extent of the volcanics from the center of eruption has not been determined. Similar lava extends to the south, beyond the Tattoosh Range, and volcanics of similar composition occur to the north, in the Tacoma quadrangle. The latter lavas and tuffs may have originated from smaller and less important cones, now destroyed by erosion.

A radial dike was observed at only one locality, near the base of Little Tahoma. In several cases the lava ma.s.ses, as seen in cross section, are lens-shaped, and where a.s.sociated with fragmental beds have unconformable relations. This shows that some of the lava flows took the form of streams, relatively narrow, rather than of broad sheets. Such a feature is in accord with the distribution of rock types. Thus along Ptarmigan Ridge for considerable vertical and horizontal range the rock shows only slight variation. The distribution of rock types will be more fully discussed in a later paragraph.

Of how large a part of the lava flows the crater still remaining was the point of origin is a question to be answered only after more detailed observation has been made. The best section for the study of the succession of flows and ejectamenta is the amphitheater at the head of the Carbon Glacier. The 4,000 feet of rock in this bold wall would afford an excellent opportunity for this were it not that frequent avalanches preclude the possibility of geologic study except at long range.

MEGASCOPIC CHARACTERS

The volcanic rocks of Rainier are of varying color and texture. Dense black rocks with abundant phenocrysts of gla.s.sy feldspars, rough and coa.r.s.e lavas of different tints of pink, red, and purple, and compact light-gray rocks are some of the types represented upon the slopes of this volcanic cone. In color, the majority of the rocks may be grouped together as light gray to dark gray. The black and red lavas are less common. In texture, the Rainier lavas are, for the most part, compact.

Slaggy and scoriaceous phases are common, but probably represent only a small part of the different flows. Near the Guardian Rocks large ma.s.ses of ropy lava are found which suggest ejected bombs.

Agglomeratic and tuffaceous rocks are of quite common occurrence, although less important than the lavas. Vesicular lavas occur at several localities, and fragments of a light-olive pumice, many as large as a foot in diameter, wholly cover some of the long, gentle slopes southeast of Little Tahoma and in Moraine Park.

Contraction parting or jointing is often observed, being especially characteristic of the basaltic types. The platy parting is the more common, but the columnar or prismatic parting is well exhibited at several localities. The black basaltic lava east of Cowlitz Glacier shows the latter structure in a striking manner. The blocks resemble pigs of iron in size and shape, and where exposed in a vertical cliff these seem to be piled in various positions.

The rocks on the higher slopes of Mount Rainier are in general very fresh in appearance. An exception may be noted in the case of the rocks at the base of Little Tahoma, where some alteration is evident.