size. This is not a fortuitous circumstance; rocks under a certain
size, and especially fragments of little thickness, cannot produce
pedestals; in fact, far from conserving the snow under them, they
accelerate its melting and sink below the surface. This is especially
true of dark-colored rocks. Objects of dark color, as is well known to
physicists, have a faculty for absorbing heat, whereas light-colored
objects, especially white ones, reflect it best. Dark-colored
fragments of rock lying on a glacier, accordingly, warm rapidly at
their upper surface and, if thin, forthwith transmit their heat to the
snow under them, causing it to melt much faster than the surrounding
clean snow, which, because of its very whiteness, reflects a large
percentage of the heat it receives from the sun. As a consequence each
small rock fragment and even each separate dust particle on a glacier
melts out a tiny well of its own, as a rule not vertically downward
but at a slight inclination in the direction of the noonday sun. And
thus, in some localities, one may behold the apparently incongruous
spectacle of large and heavy rocks supported on snow pillars alongside
of little fragments that have sunk into the ice.
There is also a limit to the depth which the little wells may attain;
as they deepen, the rock fragment at the bottom receives the sun heat
each day for a progressively shorter period, until at last it receives
so little that its rate of sinking becomes less than that of the
melting glacier surface. Nevertheless it will be clear that the
presence of scattered rock débris on a glacier must greatly augment
the rate of melting, as it fairly honeycombs the ice and increases the
number of melting surfaces. Wherever the débris is dense, on the other
hand, and accumulates on the glacier in a heavy layer, its effect
becomes a protective one and surface melting is retarded instead of
accelerated. The dirt-covered lower ends of the glaciers of Mount
Rainier are thus to be regarded as in a measure preserved by the
débris that cloaks them; their life is greatly prolonged by the
unsightly garment.
In many ways the most interesting of all the ice streams on Mount
Rainier is the Carbon Glacier, the great ice river on the north side,
which flows between those two charming natural gardens, Moraine Park
and Spray Park. The third glacier in point of length, it heads,
curiously, not on the summit, but in a profound, walled-in
amphitheater, inset low into the mountain's flank. This amphitheater
is what is technically known as a glacial cirque, a horseshoe-shaped
basin elaborated by the ice from a deep gash that existed originally
in the volcano's side. It has the distinction of being the largest of
all the ice-sculptured cirques on Mount Rainier, and one of the
grandest in the world. It measures more than a mile and a half in
diameter, while its head wall towers a sheer 3,600 feet. So well
proportioned is the great hollow, however, and so simple are its
outlines that the eye finds difficulty in correctly estimating the
dimensions. Not until an avalanche breaks from the 300-foot névé cliff
above and hurls itself over the precipice with crashing thunder, does
one begin to realize the depth of the colossal recess. The falling
snow mass is several seconds in descending, and though weighing
hundreds of tons, seemingly floats down with the leisureliness of a
feather.
These avalanches were once believed to be the authors of the cirque.
They were thought to have worn back the head wall little by little,
even as a waterfall causes the cliff under it to recede. But the real
manner in which glacial cirques evolve is better understood to-day. It
is now known that cirques are produced primarily by the eroding action
of the ice masses embedded in them. Slowly creeping forward, these ice
masses, shod as they are with débris derived from the encircling
cliffs, scour and scoop out their hollow sites, and enlarge and deepen
them by degrees. Seconding this work is the rock-splitting action of
water freezing in the interstices of the rock walls. This process is
particularly effective in the great cleft at the glacier's head,
between ice and cliff. This abyss is periodically filled with fresh
snows, which freeze to the rock; then, as the glacier moves away, it
tears or plucks out the frost-split fragments from the wall. Thus the
latter is continually being undercut. The overhanging portions fall
down, as decomposition lessens their cohesion, and so the entire cliff
recedes.
A glacier, accordingly, may be said, literally, to gnaw headward into
the mountain. But, as it does so, it also attacks the cliffs that
flank it, and as a consequence, the depression in which it lies tends
to widen and to become semicircular in plan. In its greatest
perfection a glacial cirque is horseshoe-shaped in outline. The Carbon
Glacier's amphitheater, it will be noticed, consists really of two
twin cirques, separated by an angular buttress. But this projection,
which is the remnant of a formerly long spur dividing the original
cavity, is fast being eliminated by the undermining process, so that
in time the head wall will describe a smooth, uninterrupted horseshoe
curve.
In its headward growth the Carbon Glacier, as one may readily observe
on the map, has encroached considerably upon the summit platform of
the mountain, the massive northwest portion of the crater rim of which
Liberty Cap is the highest point. In so doing it has made great
inroads upon the névé fields that send down the avalanches, and has
reduced this source of supply. On the other hand, by deploying
laterally, the glacier has succeeded in capturing part of the névés
formerly tributary to the ice fields to the west, and has made good
some of the losses due to its headward cutting. But, after all, these
are events of relatively slight importance in the glacier's career;
for like the lower ice fields of the Nisqually, and like most glaciers
on the lower slopes of the mountain, the Carbon Glacier is not wholly
dependent upon the summit névés for its supply of ice. The avalanches,
imposing though they are, contribute but a minor portion of its total
bulk. Most of its mass is derived directly from the low hanging snow
clouds, or is blown into the cirque by eddying winds. How abundantly
capable these agents are to create large ice bodies at low altitudes
is convincingly demonstrated by the extensive névé fields immediately
west of the Carbon Glacier, for which the name Russell Glacier has
recently been proposed. It is to be noted, however, that these ice
fields lie spread out on shelves fairly exposed to sun and wind. How
much better adapted for the accumulation of snow is the Carbon
Glacier's amphitheater! Not only does it constitute an admirably
designed catchment basin for wind-blown snow, but an effective
conserver of the névés collecting in it. Opening to the north only,
its encircling cliffs thoroughly shield the contained ice mass from
the sun. By its very form, moreover, it tends to prolong the
glacier's life, for the latter lies compactly in the hollow with a
relatively small surface exposed to melting. The cirque, therefore, is
at once the product of the glacier and its generator and conserver.
Of the lower course of the Carbon Glacier little need here be said, as
it does not differ materially from the lower courses of the glaciers
already described. It may be mentioned, however, that toward its
terminus the glacier makes a steep descent and develops a series of
parallel medial moraines and that it reaches down to an elevation of
3,365 feet, almost 600 feet lower than any other ice stream on Mount
Rainier. A beautiful cave usually forms at the point of exit of the
Carbon River.
West of the profound canyon of the Carbon River, there rises a craggy
range which the Indians have named the Mother Mountains. From its
narrow backbone one looks down on either side into broadly open,
semicircular valley heads. Some drain northward to the Carbon River,
some southward to the Mowich River. Encircling them run attenuated
rock partitions, surmounted by low, angular peaks; while cutting
across their stairwise descending floors are precipitous steps of
rock, a hundred feet in height. On the treads lie scattered shallow
lakelets, strung together by little silvery brooks trickling in
capricious courses.
Most impressive is the basin that lies immediately under the west end
of the range. Smoothly rounded like a bowl, it holds in its center an
almost circular lake of vivid emerald hue--that mysterious body of
water known as Crater Lake. Let it be said at once that this
appellation is an unfortunate misnomer. The basin is not of volcanic
origin. It lies in lava and other volcanic rocks, to be sure, but
these are merely spreading layers of the cone of Mount Rainier. Ice is
the agent responsible for the carving of the hollow. It was once the
cradle of a glacier, and that ice mass, gnawing headward and deploying
even as the Carbon Glacier does to-day, enlarged its site into a
horseshoe basin, a typical glacial cirque. The lake in the center is a
strictly normal feature; many glacial cirques possess such bowls,
scooped out by the eroding ice masses from the weaker portions of the
rock floor; only it is seldom that such features acquire the symmetry
of form exhibited by Crater Lake. The lakelets observed in the
neighboring valley heads--all of which are abandoned cirques--are of
similar origin.
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 altitudes 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 massif, 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 reënforce 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 partition 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 débris-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 massif. It is the shortest of the western glaciers,
measuring only a scant 3 miles. Aside from the snows accumulating in
its ill-shaped cirque it receives strong reënforcements 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 pass 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 névé
fields about the summit, and this cascade, reënforced 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 névés 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
mass--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 débris-laden mass, 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 passages or by a newly
formed crevasse. So abundant is the rock débris released by melting
that the wind at times whips it up into veritable dust storms.
Beautifully regular moraines accompany the ice mass 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
buttress 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 névés 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 altitude 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 assumes the
proportions of a massive 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 passage. 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 buttresses 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 névés 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 altitudes have, of course, been a
controlling factor; nevertheless in many instances their influence has
been outbalanced by topographic features favoring local snow
accumulation and by the heavy snowfalls occurring on the lower
slopes.
[Illustration: 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 assistant 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 groundmass 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 classes 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