A giant boulder of Precambrian gneiss lies among the trees beside the
road leading to Inspiration Point on the north rim of the Grand Canyon
of the Yellowstone (fig. 34). This boulder, measuring approximately
24×20×18 feet and weighing at least 500 tons, is of considerable
interest, not so much for its great size but because it is completely
out-of-place in its present surroundings. The boulder rests on rhyolite
lava flows of Quaternary age, at least 15 miles from the nearest
outcrops of the ancient gneiss to the north and northeast. Obviously,
this seemingly immovable chunk of rock was pushed or carried a long way
by some very powerful transporting agent before it was finally dropped.
A natural force of such magnitude could only have been exerted by moving
ice; in fact, no further proof than this one boulder is needed for us to
conclude beyond question that glaciers once existed in Yellowstone.
There is, to be sure, much additional evidence that the Park region was
extensively glaciated. Deposits of out-of-place boulders (_glacial
erratics_), like the one mentioned above, are found nearly everywhere
(fig. 35) and the mountains and high valleys still bear the vivid scars
of ice sculpturing (figs. 36 and 37).
[Illustration: GIANT BOULDER (glacial erratic) of Precambrian gneiss
near Inspiration Point on the north rim of the Grand Canyon. The
boulder, measuring 24×20×18 feet and weighing more than 500 tons,
was dropped at this locality by glacial ice; it now rests on the
much younger Plateau Rhyolite. The distance that the boulder was
carried or pushed was at least 15 miles. (Fig. 34)]
The principal requirement for the formation of glaciers is simple: more
snow has to accumulate during the winter than is melted during the
summer. If this condition continues for a long enough period of time
(measured in centuries), the snow compacts to ice, and extensive
icefields grow until they finally begin to move under their own weight,
thereby becoming glaciers. Records show that the average year-round
temperature is 32°-33°F along Yellowstone Lake, 35°F at Old Faithful,
and 39°F at Mammoth. Each winter, snow accumulates to depths of 5-10
feet throughout much of the Park. If the average annual temperatures
were to decrease a few degrees or the yearly snowfall were to increase a
foot or so, either change could possibly herald the beginning of another
ice age in the Yellowstone region.
Yellowstone was glaciated at least three times. These glaciations are,
from oldest to youngest, the pre-Bull Lake, Bull Lake, and Pinedale.
Their precise age and duration are imperfectly known, but estimates
based on a few radiometric determinations are: (1) the oldest glaciation
(pre-Bull Lake glaciation) began more than 300,000 years ago and ended
between 180,000 and 200,000 years ago; (2) Bull Lake Glaciation began
about 125,000 years ago and ended more than 45,000 years ago; (3)
Pinedale Glaciation began about 25,000 years ago and ended about 8,500
years ago. The pre-Bull Lake and Bull Lake are known only from scattered
deposits of rock debris (_glacial moraines_) and other features, but the
distribution of these deposits indicates that glaciers were widespread
throughout the region and occurred both between and during eruptions of
the Plateau Rhyolite. The effects of the Pinedale glaciers, on the other
hand, are obvious in many parts of the Park, and the history of this
youngest glacial cycle (described below) is known in much greater detail
than that of the two older ones.
In the early stages of Pinedale Glaciation, an enormous icefield built
up in the high Absaroka Range southeast of the Park area. A glacier, fed
by this icefield, flowed northward down the upper Yellowstone valley and
into the basin now occupied by Yellowstone Lake. At about the same time,
another great icefield formed in the mountains north of the Park and
sent long tongues of ice southward toward the lower Yellowstone and
Lamar River valleys. Smaller valley glaciers flowed westward out of the
Absaroka Range along the east edge of the Park, and still others formed
along the main ridges and valleys of the Gallatin Range, in the
northwestern part of the Park. Thus, many huge masses of ice from the
north, east, and southeast converged and met in the Park. At this stage,
probably about 15,000 years ago, only the west edge of the Park, and
perhaps a few of the highest peaks and ridges within the Park, remained
free of ice. It is interesting to note that although ice moved across
and buried the ancestral Grand Canyon of the Yellowstone, it did not
flow down and scour the canyon (fig. 36). If it had, the canyon would
look much different than it does today (fig. 41).
[Illustration: GLACIATED TERRAIN along the Northeast Entrance road.
The boulders, many of them measuring 10 feet across or more, were
carried into the area by ice flowing down Slough Creek from
mountains north of the Park during the Pinedale Glaciation. As the
glaciers melted, the boulders were left stranded in hummocky,
morainal deposits. Shallow depressions in the irregular topography
are now commonly filled by small ponds. (Fig. 35)]
For the next 10,000 years, the ice thickened and spread out over more
and more of the Park area. The mass centered over the Yellowstone Lake
basin grew to a depth of 3,000 feet or more and dominated the entire
scene; it formed a broad “mountain” of ice which became so high that it
caused more snow to fall upon itself and was cold enough to prevent much
of this snow from melting. Eventually the Pinedale glaciers covered
about 90 percent of Yellowstone (fig. 38).
[Illustration: CANYON PROFILES. Typical profiles of a canyon cut by
a stream (A) and of a canyon gouged by a glacier (B). Glacial
cirques (C) are shown at the head and high on the side of the
glaciated valley. (Fig. 36)]
[Illustration: GLACIAL CIRQUE on east face of Electric Peak,
northern Gallatin Range. During several episodes of glaciation, this
steep-walled amphitheaterlike valley was cut and filled by ice which
fed glaciers moving downslope to the lower right. The cirque floor
is now covered by a thick deposit of rock rubble underlain in part
by ice, and the whole mass is still moving slowly downhill as a rock
glacier. The dark rock at lower right is part of the Electric Peak
stock, composed of diorite (fig. 20) and other kinds of intrusive
igneous rocks. The rocks in the cirque walls are chiefly Cretaceous
shales (light to moderately dark color) with thin sills of igneous
rock (very dark color). (West-looking oblique aerial photograph,
courtesy of William B. Hall, University of Idaho.) (Fig. 37)]
After their maximum advance, the Pinedale glaciers began to melt,
leaving behind the rock debris they had gouged from the landscape and
had pushed or carried along with them. These glacial moraines are now
found in many areas throughout the Park. In places, glacial ice and (or)
rock debris formed natural dams across stream valleys, thereby
impounding lakes. Parts of Hayden Valley, for example, contain layers of
very fine sand, silt, and clay several tens of feet thick (fig. 39) that
accumulated along the bottom of a large lake. This lake formed behind a
glacial dam across the Yellowstone River near Upper Falls. Some of the
glacial dams broke and released water catastrophically, causing giant
floods; the occurrence of one such flood is particularly evident along
the Yellowstone River valley near Gardiner, Montana.
By about 12,000 years ago the thick Pinedale ice sheet had melted
entirely from the Yellowstone Lake basin and most other areas of the
Park, although valley glaciers continued to exist in the mountains until
about 8,500 years ago. Then, following a short period of total
disappearance, small icefields formed again in the heads of some of the
higher mountain valleys. Since the melting of the Pinedale ice, however,
none has descended as a glacier into the lower stretches of the valleys.
Even though a few snowfields persist locally throughout the summers
(except during the warmest years), no glaciers exist in the Park at the
present time.
[Illustration: EXTENT OF ICE in Yellowstone National Park during the
maximum spreading of the Pinedale glaciers, probably about 15,000
years ago. Long arrows indicate direction of strong flowage of ice;
short arrows show direction of less vigorous ice flowage. The
dark-blue area shows the main ice mass centered over the Yellowstone
Lake basin in the southeast corner of the Park. Many of the high
peaks and ridges such as Mount Washburn, which are here shown free
of ice, were glaciated at least once during the past 250,000 years.
Whether they were covered by the Pinedale glaciers, however, is
still an unresolved question. (Based on information supplied by G.
M. Richmond, K. L. Pierce, and H. A. Waldrop.) (Fig. 38)]
[Illustration: FLAT-LYING BEDS of fine sand, silt, and clay near the
mouth of Trout Creek in Hayden Valley. These beds were deposited in
a glacially dammed lake that covered part of Hayden Valley when the
Pinedale glaciers were melting. The height of the streambank is
about 40 feet. (Fig. 39)]
[Illustration: Douglas-fir branch and cones.]
[Illustration: WATERFALLS in Yellowstone National Park. (Fig. 40)
A, Lewis Falls on the Lewis River. The falls cascade over the steep
edge of a rhyolite lava flow.]
[Illustration: B, Upper Falls on the Yellowstone River. The brink of
the falls marks the contact between dense, resistant rhyolite lava
(which forms the massive cliff) and more easily eroded rhyolite lava
containing a high proportion of volcanic glass immediately
downstream, as shown in figure 42.]
[Illustration: C, Gibbon Falls on the Gibbon River. The river
tumbles over a scarp etched in the Yellowstone Tuff. The scarp first
formed along faults at the north edge of the Yellowstone caldera
600,000 years ago, at a point that now lies ¼ to ½ mile downstream.
Continued erosion has caused the falls to recede northward to their
present position.]
[Illustration: D, Tower Falls on Tower Creek. The rocks at the brink
of the falls, and in the vertical cliff beneath, are coarse breccias
and conglomerates of the Absaroka volcanic rocks. The channel of
Tower Creek has not been cut down rapidly enough to keep pace with
the downcutting of the main channel of the Yellowstone River, which
lies a short distance downstream from the base of the falls.]