Mount Rainier’s great sprawling cone would seem incomplete without the
glistening sheets of ice that descend its flanks. We have reason to
believe that the volcano has borne glaciers ever since its
origin—sometimes smaller than now, at other times vastly larger. Mount
Rainier probably started to grow during the middle part of the
Pleistocene Epoch, or Ice Age, which began more than 1 million years
ago, but glaciers had covered this part of the mountains even before the
volcano appeared. Masses of rock debris formed by ancient glaciers occur
beneath lava flows from Mount Rainier on the west side of Mazama Ridge
just upslope from Narada Falls, on the north side of Glacier Basin, and
at a few other places in the park.
Mount Rainier may have reached its present size by about 75,000 years
ago. Since that time great icefields and glaciers have formed at least
three times on the slopes of the volcano and in the nearby mountains.
During the first two glaciations, ice completely buried the flanks of
the volcano and the surrounding mountains, except for the very highest
ridges and peaks. These great ice masses slowly flowed down all the
valleys that head at Mount Rainier. The glacier in the Cowlitz River
valley, for example, extended 65 miles from the volcano and reached a
point about 33 miles west of the community of Randle. Deposits of the
younger of these two glacial episodes can be seen in cuts along the
Mowich Lake Road for a distance of about 1½ miles inside the park
boundary. The glacial deposits were originally more widespread, but in
most of the park they have been removed by erosion or covered by the
deposits of yet younger glaciers.
[Illustration: Extent of glaciers in the Cascade Range near Mount
Rainier between about 15,000 and 25,000 years ago. Arrows show the
direction of ice movement; solid black represents modern glaciers on
Mount Rainier. (Fig. 16)]
During the most recent major glaciation of the park, which lasted from
roughly 25,000 to 10,000 years ago, ice again sheathed the slopes of the
volcano, but glaciers in the nearby mountains were smaller than before.
Most of the glaciers originated at the valley heads, where they gouged
out countless bowl-shaped bedrock basins called cirques. Many of the
basins held lakes after the glaciers disappeared. (See frontispiece.)
Hikers on the trail to the Paradise ice caves cross the floor of a
typical cirque at the head of Paradise Valley. From the Sunrise Visitor
Center at Yakima Park you can walk a short distance to a point along the
crest of the Sourdough Mountains and stand at the rim of a deep
north-facing cirque. Ice originating in this cirque and in the cirques
adjacent to it moved northward down the valley of Huckleberry Creek at
least as far as the park’s north boundary (fig. 16).
These glaciers left most valley walls in the park covered with rock
debris. Lateral moraines can be seen along the highway at and just east
of Ricksecker Point (fig. 17). Other glacial deposits are especially
well displayed in roadcuts along the north wall of the White River
valley.
[Illustration: Lateral moraine of rock debris at Ricksecker Point. It
was formed by Nisqually Glacier when the glacier was at least 1,000 feet
thick and about 15 miles longer than it is today. (Fig. 17)]
A little more than 15,000 years ago the long glaciers began to shrink
and recede. By 11,000 years ago there was only about as much ice on
Mount Rainier as there has been within the last century. Then, during a
short period of renewed glacier growth, most glaciers expanded short
distances and new ones appeared in cirques from which ice had
disappeared only a short time before. In some of these cirques so much
rock debris was being dislodged from surrounding cliffs by repeated
freeze and thaw that a rock glacier, consisting mostly of rock fragments
bound together by ice, was formed. A trail to the Huckleberry Creek
valley crosses hummocky rock debris left by such a rock glacier in a
cirque on the southeast side of Mount Fremont. A larger rock-glacier
deposit lies about 2 miles north of Sunrise Point in an east-facing
cirque between The Palisades and Hidden Lake (fig. 18).
In other cirques, where the proportion of ice to rock debris was
greater, the glacier transported the debris a short distance forward and
built a terminal moraine. You can see particularly good examples of
moraines formed about 11,000 years ago near Tipsoo Lake, where the pond
southeast of the lake is dammed by a moraine, and at Mystic Lake. The
ice that formed the terminal moraine at Mystic Lake was a tongue of
Carbon Glacier.
In some places the orientation or altitude of the cirque did not permit
enough snow to accumulate to form a glacier but just enough to create a
permanent snowbank. Rock debris that fell from the surrounding cliffs
rolled down these snowbanks and formed low ridges at their toes. Such a
ridge is called a protalus rampart because it is found just in front of
the apron of rock fragments, called talus, that lies beneath cliffs. A
trail at Sunrise Point leads to protalus ramparts along the north side
of Sunrise Ridge.
During the last 10,000 years, glaciers have been very small by
comparison with the great ice mantles that overwhelmed the park earlier.
However, glaciers have grown larger at least twice just within the last
3,000 years. During both of these periods most glaciers were slightly
larger than they are today, and ice occupied most cirques at altitudes
above 6,500 feet—even some that are now free of ice. The most recent
time of extensive glacier growth began at least 800 years ago, and
various glaciers in the park reached their maximum size between the
mid-14th century and the mid-19th century. Oddly enough, even though all
the glaciers headed on Mount Rainier, they did not all attain their
maximum size simultaneously. The largest terminal moraine of this most
recent glacial period was built by Emmons Glacier in the White River
valley (fig. 19). It is now largely forested, and cores taken from the
trees with a special boring tool that does not harm the tree show ages
indicating that the moraine was stable enough to permit seedlings to
survive on it by the mid-17th century. A similar but smaller terminal
moraine built by Cowlitz Glacier has trees on it that started to grow in
the mid-14th century.
[Illustration: Rock-glacier deposit (light-gray rubble beyond the brown
slopes in the foreground) at The Palisades, which was formed about
11,000 years ago when the climate was colder than it is today. Rocks
fell from the cliffs in such great quantity that a small glacier in
front of the cliffs consisted of more rock debris than ice. The melting
of the ice left a mass of broken rock several hundred feet thick which
covers about 80 acres. (Fig. 18)]
[Illustration: The hummocky end moraine at the left still had blocks of
ice buried in it when this picture was taken in 1954. The front of
Emmons Glacier was near the left edge of the bare moraine in about 1900.
Now the glacier ends 1 mile farther upvalley at the upper right. The
valley floor and moraine were buried by an avalanche of rock debris from
Little Tahoma Peak in 1963. (Fig. 19)]
Nearly all the glaciers gradually decreased in size after the mid-18th
century. Although the shrinkage was sometimes interrupted by short
periods of renewed glacier growth, by 1950 the glaciers at Mount Rainier
covered only about two-thirds of the area that had been buried by ice
only a century before. The overall loss of volume by Rainier’s glaciers,
as well as those elsewhere in the Pacific Northwest, was slowed or
halted by slightly cooler temperatures and higher precipitation starting
in the mid-1940’s. Volume increases in their upper reaches caused the
larger glaciers to grow from year to year, and since the early 1950’s
the terminuses of many glaciers have been advancing. This renewed growth
of glaciers is not unique at Mount Rainier—similar changes have been
observed at other glaciers in the Cascade Range and elsewhere.