Within the boundaries of Glacier National Park there are 50 to 60
glaciers, of which only two have surface areas of nearly one-half square
mile, and not more than seven others exceed one-fourth square mile in
area.
All these bodies of ice lie at the heads of valleys with high steep
headwalls on the east and north sides of high ridges at elevations
between 6,000 and 9,000 feet, in all cases well below the snowline.
Consequently, these glaciers owe their origin and existence almost
entirely to wind-drifted snow.
Ice within these glaciers moves slowly. The average rate in the smallest
ones may be as low as 6 to 8 feet a year, and in the largest probably 25
to 30 feet a year. There is no period of the year when a glacier is
motionless, although movement is somewhat slower in winter than in
summer. Despite the slowness of its motion the ice, over a period of
years, transports large quantities of rock material ultimately to the
glacier’s end where it is piled up in the form of a moraine.
[Illustration: FRONT OF SPERRY GLACIER]
The largest glacier in the Park is Grinnell. In 1960 it had a surface
area of 315 acres.
Sperry is the second largest glacier in the Park. Its surface in 1960
was 287 acres. Both Grinnell and Sperry have probable maximum
thicknesses of 400 to 500 feet.
[Illustration: JACKSON GLACIER IS VISIBLE FROM GOING-TO-THE-SUN ROAD
(BEATTY PHOTO)]
Other important Park glaciers, although much smaller than the first two
mentioned, are Harrison, Chaney, Sexton, Jackson, Blackfoot, Siyeh, and
Ahern. Several others approach some of these in size, but because of
isolated locations they are seldom seen. As a matter of fact, there are
persons who visit Glacier National Park without seeing a single glacier,
while others, although they actually see glaciers, leave the park
without realizing they have seen them. This is because the highways
afford only distant views of the glaciers, which from a distance appear
much like mere accumulations of snow. A notable example is Grinnell as
seen from the highway along the shore of Sherburne Lake and from the
vicinity of the Many Glacier Entrance Station. The glacier, despite its
length of almost a mile, appears merely as a conspicuous white patch
high up on the Garden Wall at the head of the valley.
Several of the glaciers, however, are accessible by trail and are
annually visited by many hundreds of people, either on foot or by horse.
Most accessible of all Park glaciers is Grinnell. It can be reached by a
six-mile trip over an excellent trail from Many Glacier Hotel or
Swiftcurrent Camp. Sperry, likewise, can be reached by trail, although
the distance is several miles greater than in the case of Grinnell. The
trip, however, can be broken and possibly made more interesting by an
overnight stop at Sperry Chalet, which is located about three miles from
the glacier. Siyeh is the only other regularly visited Park glacier. It
lies about half a mile beyond the end of the Cracker Lake trail, and can
be reached from that point by an easy walk through grassy meadows and a
short climb over a moraine. Siyeh, however, is less spectacular than
either Grinnell or Sperry, being much smaller and lacking crevasses, so
common on the other two. Few people make the spectacular trail trip over
Siyeh Pass but those who do may visit Sexton Glacier by making a short
detour of less than half a mile where the trail crosses the bench on
which the glacier lies. Sexton is a small glacier, but late in the
summer after its snow cover has melted off it exhibits many of the
features seen on much larger bodies of ice.
Interesting surface features which can be seen at times on any of these
glaciers include crevasses, moulins (glacier wells), debris cones, and
glacier tables. Crevasses are cracks which occur in the ice of all
glaciers. They are especially numerous on Sperry and Grinnell. Moulins,
or glacier wells, are deep vertical holes which have been formed by a
stream of water which originally plunged into a narrow crevasse.
Continual flow of the stream enlarges that part of the crevasse,
creating a well. Several such features on Sperry Glacier have penetrated
to depths of more than 200 feet, and are 20 or more feet wide at the
top.
No one can walk over the surface of Grinnell Glacier without noticing a
number of conical mounds of fine rock debris. Actually these are cones
of ice covered with a veneer, seldom more than two inches thick, of rock
debris, so their name, debris cone is somewhat misleading.
[Illustration: CREVASSE IN SPERRY GLACIER]
This rock material, usually deposited by a stream, protects the ice
underneath from the sun’s rays. As the surface of the glacier, except
that insulated by the debris, is lowered by melting, the mounds form and
grow gradually higher until the debris slides from them, after which
they are speedily reduced to the level of the rest of the surface. They
are seldom higher than 3 or 4 feet.
A glacier table is a mound of ice capped, and therefore protected from
melting, by a large boulder. Its history is similar to that of the
debris cone. After a time the boulder slides off its perch, and then the
mound of ice melts away.
Snow which fills crevasses and wells during the winter often melts out
from below, leaving thin snowbridges in the early part of the summer.
These constitute real hazards to travel on a glacier because the thinner
ones are incapable of supporting a person’s weight. This is one very
good reason why the inexperienced should never venture onto the surface
of a glacier without a guide.
It is probable that the Park glaciers are not remnants of the large
glaciers present during the Ice Age which terminated approximately
10,000 years ago, because it is known that several thousand years after
that time the climate of the Glacier National Park region was somewhat
drier and warmer than now. Under such conditions it is probable that
most, if not all, of the present glaciers could not have existed.