A glacier is an extremely powerful agent of erosion, capable of
profoundly altering the landscape over which it passes.
Glaciers erode mainly by two processes, plucking and abrasion. The first
is more active near the head of the glacier, but may take place anywhere
throughout its course; abrasion or scouring is effective underneath most
sections of the glacier, particularly where the ice moves in a
well-defined channel.
[Illustration: MT. OBERLIN CIRQUE AND BIRD WOMAN FALLS (HILEMAN
PHOTO)]
In plucking, the glacier actually quarries out masses of rock,
incorporates them within itself, and carries them along. At the head of
the glacier this is accomplished mainly by water which trickles into
crevices and freezes around blocks of rock, causing them to be pulled
out by the glacier, and also by the weight of the glacier, squeezing ice
into the cracks in the rock. As the glacier moves forward these blocks
of ice are dragged or carried along with it. Usually a large crevasse,
the bergschrund, develops in the ice at the head of a glacier. The
bergschrund of most glaciers in the park consists of an opening, usually
10 to 20 feet wide at the top and as much as 50 feet deep, between the
head of the glacier and the mountain wall. On Sperry Glacier, however,
it is more typical of that found on larger valley glaciers and consists
of several conspicuous crevasses separating the firn area (where the
snow is compacted into ice) on top of Gunsight Mountain from the glacier
proper below (see photo on the cover). It is at this site that plucking
is most dominant because water enters by day and freezes in the rock
crevices at night. This quarrying headward and downward finally results
in the formation of a steep-sided basin called a cirque or glacial
amphitheatre. Because the cirque is the first place that ice forms and
the place from which it disappears last, it is subjected to glacial
erosion longer than any other part of the valley. Thus its floor is
frequently plucked and scraped out to a comparatively great depth so
that a body of water known as a cirque lake forms after the glacier
disappears. Iceberg Lake lies in one of the most magnificent cirques in
the Park. The lowest point on the crest of the wall encircling three
sides of the lake is more than 1500 feet above the water. Prior to 1940
this cirque contained a small glacier. It has been shrinking rapidly for
about two decades, and in the last two or three years of its existence
was hardly recognizable as a glacier. Its disappearance is made more
remarkable by the knowledge that in 1920 the front of the glacier rose
in a sheer wall of ice nearly 100 feet above the surface of the lake.
All that remains of this glacier which once kept the lake filled with
icebergs each summer is a large bank of snow at the base of the cirque
wall at the head of the lake. Other good examples of cirques are those
which hold Hidden, Avalanche and Cracker Lakes. The tremendous cliff on
the south side of the latter rises 4,100 feet from the lake to the
summit of Mount Siyeh. Other notable cirque lakes are Ellen Wilson,
Gunsight, Ptarmigan and Upper Two Medicine.
[Illustration: ST. MARY VALLEY FROM LOGAN PASS SHOWING GLACIAL
PROFILE (HILEMAN PHOTO)]
Rock fragments of various sizes frozen into the bottom and sides of the
ice form a huge file or rasp which abrades or wears away the bottom and
sides of the valley down which the glacier flows. The valley thus
attains a characteristic U-shaped cross section, with steep sides (not
necessarily vertical) and a broad bottom. A mountain valley cut entirely
by a stream does not have such shape because the stream cuts only in the
bottom of the valley, whereas a glacier, filling its valley to a great
depth, abrades along the sides as well as on the floor. Practically all
valleys of the Park, especially the major ones, possess the U-shaped
cross section. This feature can best be seen by looking down from the
head of the valley rather than from the valley floor. Splendid examples
are the Swiftcurrent Valley viewed from Swiftcurrent Pass or Lookout;
St. Mary Valley from east of Logan Pass; the Belly River Valley from
Ptarmigan Tunnel; and Cataract Creek Valley from Grinnell Glacier.
[Illustration: FIGURE 1. IDEALIZED SKETCH OF A GLACIAL STAIRWAY FROM
THE ARETE AT THE CENTER OF THE RANGE TO THE ICE AGE MORAINE AT THE
MOUTH OF THE VALLEY.]
Cirque wall
Glacier
Lake
Moraine
The floors of many of the Park’s major U-shaped valleys instead of
having a more or less uniform slope, steeper near the head than farther
down, as is usually the case in a normal stream valley, are marked by
several steep drops or “steps,” between which the valley floor has a
comparatively gentle slope. Such a valley floor, throughout its entire
course, is sometimes termed the glacial stairway. Most of the steps,
particularly those in the lower courses of the valleys, are due to
differences in resistance of the rocks over which the former ice flowed.
On the east side of the Lewis Range, where the steps are especially
pronounced, the rock strata of which the mountains are composed dip
toward the southwest, directly opposite to the direction of the slope of
the valley floors (Figure 1). Thus, as glaciers flowed from the center
of the range down toward the plains, they cut across the edges of these
tilted rock layers; where the ice flowed over weaker beds it was able to
scour out the valley floor more deeply creating a “tread” of the glacial
stairway. The more resistant rock formations were less easily removed,
and the ice stream, in moving away from the edges of these resistant
strata, employed its powers of plucking and quarrying to give rise to
cliffs or “risers.” Lakes dammed partly by the resistant rock strata now
fill depressions scoured out of the weaker rock on the treads (Figure 1
). These are rock-basin lakes, and where several of them are strung out
along the course of the valley they are referred to as paternoster lakes
because their arrangement resembles that of beads on a string.
Well-known examples of such bodies of water are Swiftcurrent and
Bullhead Lakes, two of the long series which stretches for seven miles
between Many Glacier Hotel and Swiftcurrent Pass. Resistant layers in
the lower portion of the Altyn formation, the upper part of the
Appekunny, and the upper part of the Grinnell[2] normally create risers.
[Illustration: TYPICAL GLACIAL VALLEY WITH CHAIN OF ROCK-BASIN
LAKES. GLENN AND CROSSLEY LAKES IN DISTANCE; UNNAMED LAKE IN
FOREGROUND RESTS IN A HANGING VALLEY AND ITS OUTLET DROPS SEVERAL
HUNDRED FEET TO THE MAIN VALLEY (HILEMAN PHOTO)]
The tributaries of glacial valleys are also peculiar in that they
usually enter the main valley high above its floor and for this reason
are known as hanging valleys. The thicker a stream of ice, the more
erosion it is capable of performing; consequently, the main valley
becomes greatly deepened, whereas the smaller glacier in the tributary
valley does not cut down so rapidly, leaving its valley hanging high
above the floor of the major valley. The valleys of Virginia and
Florence Creeks, tributary to the St. Mary Valley are excellent examples
of hanging valleys. A splendid view of Virginia Creek valley may be had
from Going-to-the-Sun Road near the head of St. Mary Lake. The valley
above Bird Woman Falls as seen from Going-to-the-Sun Road just west of
Logan Pass is a spectacular illustration of a hanging valley. In
addition there are many others, such as Preston Park, on the trail from
St. Mary to Piegan Pass; and the Hanging Gardens near Logan Pass.
[Illustration: REYNOLDS MOUNTAIN AT LOGAN PASS—A TYPICAL HORN]
Even more conspicuous than the large U-shaped valleys and their hanging
tributaries are the long, sharp-crested, jagged ridges which form most
of the backbone of the Lewis Range. These features of which the Garden
Wall is one of the most noticeable, are known as aretes and owe their
origin to glaciers. As the former long valley glaciers enlarged their
cirques by cutting farther in toward the center of the range, the latter
finally was reduced to a very narrow steep-sided ridge, the arete. The
imposing height of the Garden Wall can readily be determined by using
the layer of diorite as a scale. The conspicuous black band formed by
the edge of this layer has an average width of 75 feet. So, from the
porch of the Many Glacier Hotel a Park visitor can readily see that the
Garden Wall, even though five miles distant, is about 4,200 feet high.
The height of other aretes can be just as readily obtained, for the band
of diorite appears on the faces of most of them. In certain places
glaciers on opposite sides of the arete nearly cut through creating a
low place known as a col, usually called a pass. Gunsight, Logan, Red
Eagle, Stoney Indian and Piegan are only a few of the many such passes
in the Park. At places three or more glaciers plucked their way back
toward a common point leaving at their heads a conspicuous,
sharp-pointed peak known as a horn. Innumerable such horn peaks occur
throughout both the Lewis and Livingstone Ranges. Excellent examples
near Logan Pass are Reynolds, Bearhat, and Clements Mountains. Other
imposing horns are Split Mountain at the head of Red Eagle Valley,
Kinnerly Peak in the Kintla Valley, and Mount Wilbur in Swiftcurrent
Valley. The horn peak, because of its precipitous sides, is especially
attractive to mountain climbers. The comparatively recent dates of first
ascents on many Park peaks attest to the difficulties they offer the
mountaineer. Mount Wilbur, despite proximity to Many Glacier Hotel and
camp, was unclimbed until 1923; Mount St. Nicholas succumbed in 1926,
and the first ascent of Kinnerly Peak was made by several members of the
Sierra Club in 1937.
Another feature of the Park which must be attributed partly to
glaciation is the waterfall. There are two principal types, one which
occurs in the bottom of the main valleys and one at the mouth of the
hanging tributary valleys. The former, exemplified by Swiftcurrent, Red
Rock, Dawn Mist, Trick, Morning Eagle and others, is located where
streams drop over the risers of the glacial stairway. In other words,
resistant layers of rock which the former glaciers were unable to
entirely wear away give rise to this type of fall.
Examples of the hanging tributary type of fall which is due directly to
the activity of the glaciers are Florence, Bird Woman, Virginia,
Grinnell, Lincoln, and many others.
[Illustration: TRICK FALLS IN THE TWO MEDICINE RIVER]
No less conspicuous than the mountains themselves are the lakes. In most
instances glaciers have been either directly or indirectly responsible
for the origin of the several hundred in the Park. In general, these
lakes may be divided into five main types, depending upon their origin.
(1) Cirque lakes. This type of lake frequently is circular in outline
and fills the depression plucked out of solid rock by a glacier at its
source. Some of the most typical examples are listed in the foregoing
discussion of cirques.
(2) Other rock-basin lakes. This type, referred to above, fills basins
created where glaciers moved over areas of comparatively weak rock. In
all cases the lake is held in by a bedrock dam. A typical example is
Swiftcurrent, which lies behind a dam of massive Altyn Limestone layers.
The highway, just before it reaches Many Glacier Hotel, crosses this
riser of the glacier stairway.
(3) Lakes held in by outwash. Most of the large lakes on the west side
of the Park fall in this category. The dams holding in these lakes are
composed of stratified gravel which was washed out from former glaciers
when they extended down into the lower parts of the valleys. Lake
McDonald, largest in the Park, is of this type.
[Illustration: ST. MARY LAKE FROM GOOSE ISLAND OVERLOOK]
(4) Lakes held by alluvial fans. St. Mary, Waterton, Lower St. Mary, and
Lower Two Medicine Lakes belong in this group. These bodies of water may
have been rock-basin lakes, but at a recent date on their history
streams entering the lake valley have completely blocked the valley with
deposits of gravel; thus either creating a lake or raising the level of
one already present. St. Mary and Lower St. Mary Lakes probably were
joined originally to make a lake 17 miles long. More recently Divide
Creek, entering this long lake from the south, built an alluvial fan of
gravel where it entered the lake. This fan was large enough to cut the
lake into the two present bodies of water. The St. Mary Entrance Station
at the eastern end of Going-to-the-Sun Road, is located on this alluvial
fan, the form of which can readily be distinguished from a point along
the road at the north side of the upper lake near its outlet.
(5) Moraine lakes. Most lakes with moraines at their outlets are partly
dammed by outwash or rock ridges. One of the prominent examples is
Josephine Lake near Many Glacier Hotel. The moraine which is partly
responsible for the lake is a hill which can be seen from Many Glacier
Hotel. Several of the large lakes on the west side of the Park are also
held partly or entirely by moraines.
Another type of moraine lake, which occurs only at Sperry and Grinnell
Glaciers, has already been mentioned. It differs from all other Park
lakes in having a glacier for part of its shoreline. There are two of
these lakes at Sperry and one at Grinnell. Despite their small size,
they are tremendously interesting, not only because of their relation to
the glacier, but also because they are ordinarily filled with icebergs
throughout the summer. Their surfaces often remain frozen until
mid-summer.
There are several types of minor importance, the principal one of which
is that formed by a landslide damming the valley.
One cannot remain long in Glacier National Park without noticing the
varying colors of its lake waters. In fact this feature is so striking
that ranger-naturalists probably are questioned more about it than about
any other feature or phenomenon. To find the answer we must go again, as
in so many instances, to the glaciers. As the ice moves it continually
breaks rock fragments loose. Some of these are ground into powder as
they move against each other and against the bedrock under the glacier.
Most types of rock, especially the limestones and shales on which the
Park glaciers rest, when ground fine enough yield a gray powder. All
melt-water streams issuing from glaciers are cloudy or milky from their
load of this finely ground “rock flour.”
Water from Grinnell Glacier is so laden with rock flour that the small
lake along the edge of the ice into which the water pours is nearly
white. Much of the silt is deposited in this lake, but enough is carried
downstream to give Grinnell Lake a beautiful turquoise hue. Some of the
very finest sediment which fails to settle in Grinnell Lake is carried a
mile farther to Josephine Lake to give it a blue-green color. Even
Swiftcurrent Lake, still farther downstream, does not contain clear
water.
The rock flour which colors these as well as other Park lakes can also
be seen in the streams. Baring Creek at Sunrift Gorge (see p. 13 in
Motorist’s Guide) is milky with powdered rock from Sexton Glacier.
Cataract Creek along the trail between Josephine and Grinnell Lakes is
noticeably milky, extraordinarily so in mid-afternoon on very warm days.
At such times melting of the glaciers is accelerated and more silt is
then supplied to the streams.
Part of Sperry Glacier, in contrast to Grinnell, rests on a bright red
shaly rock (known to the geologists as argillite) which yields a
red-gray powder when finely ground. Hence the water in several small
lakes adjacent to the glacier has a pinkish tint.
Although a large number of Park streams are fed by glaciers there are
many others, particularly in the south and west sections, which have no
ice as their source. On a trail trip from Sunrift Gorge to Virginia
Falls, one is certain to be impressed by the extreme clarity of the
water in Virginia Creek. For half a mile below the falls the trail
follows this cascading torrent from one crystal pool to another. So
clear is the water that we are apt to mistake for wading pools places
where the water may be five or more feet deep. Snyder Creek near Lake
McDonald Lodge nearly rivals Virginia Creek in clarity. The sources of
these two streams obviously are not melting glaciers.
From the foregoing discussion, it is evident that glaciers constitute
one of the principal controlling factors in the color of the water in
Park streams and lakes. Where there are no ice masses streams are clear,
and where glaciers occur the water possesses many shades varying from
clear blue through turquoise to gray, and in rare cases even pink.
[Illustration: MORAINE NEAR GRINNELL GLACIER IS 120 FEET HIGH. THE
GLACIER EXTENDED NEARLY TO TOP OF MORAINE 50 YEARS AGO. (DYSON
PHOTO)]
Although the former large glaciers of the Ice Age transported huge
amounts of rock debris down the valleys of the Park, the moraines which
they deposited are, as a rule, not conspicuous features of the
landscape. The Going-to-the-Sun Road, however, crosses several
accumulations of moraine in which road cuts have been made. The road
traverses a number of such places along the shore of Lake McDonald.
Because of the large proportions of rock flour (clay) in these
accumulations, the material continually slumps, sometimes sliding onto
the road surface. One of these cuts has been partly stabilized by a
lattice-like framework of logs. The largest excavation in moraine along
the highway is located about three miles east of Logan Pass just below
the big loop where the road crosses Siyeh Creek. The surfaces of many
boulders in this moraine are marked by grooves and scratches, imparted
to them as they were scraped along the side of the valley by the glacier
10,000 or more years ago.
A small moraine is exposed along the exit road from the parking lot at
Many Glacier Hotel. It contains a number of small red boulders, the
sources of which are the red rock ledges in the mountains several miles
up the Swiftcurrent Valley, plainly visible from the hotel.
One of these ancient moraines which has been eroded into a series of
mounds (25 to 100 feet high) extends from Swiftcurrent Cabin Camp down
the valley on the north side of the road to a point near the entrance to
Many Glacier Ranger Station. Some of the cabins are actually situated in
a space between two of the highest mounds.
[Illustration: LOOKING SOUTH ALONG THE GRINNELL GLACIER ICE FRONT.
NOTE CREVASSES ALONG WHICH BERGS ARE BREAKING OFF. (DYSON PHOTO)]
Surrounding all existing Park glaciers are two sets of recent moraines
varying in height from a few feet to more than two hundred. So recently
(probably 800 to 900 years) have the glaciers withdrawn from the older
of these that only sparse willows and other forms of dwarf vegetation
are growing on them.
The younger set of moraines, which has accumulated during the last
several hundred years, consists of unweathered rock on which only small
pioneer plants and lichens have begun to establish themselves. These
moraines are particularly striking at Grinnell, Sperry, Blackfoot,
Agassiz and Sexton Glaciers. On the last few yards of the spectacular
Grinnell Glacier trail all persons who make the trip to the glacier must
climb over the moraine before setting foot on the ice. From this vantage
point on the highest part of this moraine the visitor can look down upon
a huge crevassed mass of ice lying in a stupendous rock-walled
amphitheater, then merely by facing the opposite direction, he will see
unfolded before his view one of the most colorful vistas in the Park.
More than a thousand feet below in the head of a splendid U-shaped
valley lies the turquoise gem of Grinnell Lake. A mile farther away the
blue surface of Lake Josephine stands out in sharp contrast to the dark
green of the spruce which lines its shores. High above he can see the
red summit of Mount Allen carrying its white snowbanks into the deep
blue of a Montana sky. Despite this magnificence the visitor must soon
turn his attention to the tremendous accumulation upon which he stands,
for it is no less interesting than the mountains and lakes. Among the
many boulders which lie along the path are two prominent limestone
blocks each 10 to 15 feet in diameter. The underside of one was grooved
and polished as the ice pushed it across the rock surface underlying the
glacier. The other, partially embedded in the moraine, has a polished
upper surface because the glacier flowed over it for a time. Both these
boulders, although now nearly 300 yards from the ice front, were covered
by the glacier until about 20 years ago.
Because of shrinkage many of the glaciers are no longer in contact with
these newer moraines. In some cases a quarter of a mile of bare rock
surface intervenes between the moraine and the glacier which made it.
A few glaciers have disappeared within recent years, but their moraines
remain as evidence of former glacier activity. One of the most notable
examples is afforded by Clements Glacier, a small body of ice which
existed until about 1938 in the shadow of Clements Mountain at Logan
Pass. Its edge was bordered by a ridge-like moraine nearly a hundred
feet high. Today, the trail from Logan Pass to Hidden Lake skirts the
outside edge of the moraine. Should the hiker leave the trail and climb
the few yards to the top of this moraine he could see it stretched out
before him as a giant necklace encircling the base of Clements Mountain,
but between mountain and moraine, where a few years ago the glacier lay,
he will see only bare rock or drifted snow.
Despite recent rapid shrinkage of glaciers and the disappearance of
some, Glacier National Park still is a land of ice, yet when the visitor
views its present day glaciers and its sublimely beautiful mountain
scenery he should not be unmindful of the powerful forces which, working
during many thousands of years, have brought it all about. Then, and
only then, can he properly appreciate the magnificence which Nature has
so generously bestowed upon us.
[Illustration: CLEMENTS MOUNTAIN AND GLACIER. THE GLACIER HAS SINCE
DISAPPEARED. (HILEMAN PHOTO)]