Except for Old Faithful, the Grand Canyon of the Yellowstone is probably
the best known and most talked about and photographed feature in the
Park (fig. 41). Although not so deep or wide as some of the other great
canyons in America, its sheer ruggedness and beauty are breathtaking.
Here the aptness of the name “Yellowstone” can be fully appreciated and
understood, for the viewer is at once engulfed in a sea of yellow hues
streaked and tinted with various shades of red and brown.
[Illustration: GRAND CANYON AND LOWER FALLS of the Yellowstone
River, as viewed upstream (southwest) from Artists Point on the
south rim. The yellow-hued rocks lining the canyon walls are soft,
hydrothermally altered rhyolite lavas. The rocks at the brink of the
falls consist of less altered and therefore more resistant
rhyolites. The falls, 309 feet high, formed at the contact between
the hard and soft rhyolite units. (Photograph courtesy of Sgt. James
E. Jensen, U.S. Air Force.) (Fig. 41)]
[Illustration: DEVELOPMENT OF GRAND CANYON. Profiles along the floor
of the Grand Canyon of the Yellowstone as it appears today (C) and
as it appeared at two older stages in its development (A and B).
Note particularly the various kinds of rocks through which the
canyon has been cut, and how rock differences have influenced the
location of the two falls. Diagonal lines indicate unaltered
rhyolite; large dots, rhyolite with much volcanic glass; small dots,
hydrothermally altered rhyolite; and circles and dots, Absaroka
volcanic rocks. (Based on information furnished by R. L.
Christiansen and G. M. Richmond; vertical scale is exaggerated about
10 times.) (Fig. 42)]
C: As it appears today
Yellowstone Lake
Hayden Valley
Grand Canyon of the Yellowstone
Upper Falls
Lower Falls
Inspiration Point
North edge of Yellowstone caldera
Confluence of Lamar and Yellowstone Rivers
B: A stage somewhat before 300,000 years ago
Profile of Yellowstone River today
A: A stage somewhat before 600,000 years ago
Profile of Yellowstone River today
At first glance, the canyon may appear to be a giant crack which
suddenly opened up and into which the Yellowstone River then plunged
headlong over high waterfalls at its southwest end. This, of course, is
not the way the canyon formed. Nevertheless, it is apparent that certain
unusual conditions caused the river, after winding slowly through
flat-floored Hayden Valley for about 13 miles, to cut a precipitous
gorge 1,000-1,500 feet deep and 20 miles long (fig. 42C). A full
explanation must be based on all the many events surrounding the
eruption of the Yellowstone Tuff, the collapse of the Yellowstone
caldera, the outpouring of the Plateau Rhyolite, and the various
episodes of glaciation. Geologic studies show that all these events took
place while the canyon was being cut, and that each one played an
important role in its development. Hot-water and steam activity likewise
was a significant factor. However, despite its many complexities, the
history of the Grand Canyon can be divided into a few major stages, as
outlined below:
1. From more than 2,000,000 years ago to about 600,000 years ago, a
shallow canyon was gradually being cut into the Absaroka volcanic
sequence by the ancestral Yellowstone River as it eroded headward from
a point near the present confluence of the Yellowstone and Lamar
Rivers (fig. 33). By the time of the climactic volcanic eruption in
central Yellowstone 600,000 years ago, the head of the “old” canyon
probably had been eroded southward nearly to the place where the north
rim of the Yellowstone caldera was to form later (fig. 42A). This
point now lies about 5 miles below Lower Falls.
2. Ash-flow tuffs that were erupted 600,000 years ago filled the “old”
canyon, and the river recarved its channel, chiefly along its previous
course.
3. A large lake formed behind (south of) the north rim of the caldera,
the damming resulting in part from lava flows of Plateau Rhyolite that
poured out across the caldera floor in this area between 600,000 and
500,000 years ago. Eventually the lake rose and spilled northward into
the head of the “old” canyon, causing additional downcutting in what
is now the lower 15-mile stretch of the canyon.
4. As the lake emptied, the river began to erode upstream into the
thick rhyolite lava flows toward the present site of Lower Falls; the
process was very similar to that of a common stream gully eroding
headward into a hillside. At a stage somewhat more than 300,000 years
ago, the head of the canyon probably lay near the falls, and the river
had cut a channel 400-600 feet deep along this upper 5-mile stretch
(fig. 42B).
5. Approximately 300,000 years ago the canyon area was covered by ice
during pre-Bull Lake glaciation. During and after the retreat of this
ice, sediments accumulated in a lake that occupied the upper reaches
of the canyon between the present site of Upper Falls and Inspiration
Point. Subsequently, very little downcutting was accomplished until
about 150,000-125,000 years ago, when the canyon was eroded nearly to
its present depth.
6. Canyon development was further interrupted by the advance and
retreat of glaciers during Bull Lake and Pinedale Glaciations. During
and since the melting of the Pinedale glaciers about 12,000 years ago,
the canyon has attained its present depth, and its walls have acquired
much of their picturesque erosional form. The Yellowstone River now
maintains a fairly uniform gradient (60-80 feet per mile) throughout
the 20-mile-long gorge, even though different segments of the canyon
were cut at different times and through different kinds of rocks (fig.
42C).
The spectacular erosional development in the upper 5-mile segment of the
Grand Canyon, which is the only part seen by most Park visitors, except
for the very lower end near Tower Falls (fig. 33), has taken place
mostly within the past 150,000-125,000 years. One reason for such a
rapid rate of erosion stems from the fact that this part of the canyon
overlies one of the wide ring fracture zones of the Yellowstone caldera
(fig. 22). The fracture zone extends to great depth, providing a ready
avenue of travel for the upflow of hot water and steam rising in the
Yellowstone thermal system, as described in the following chapter.
Through many thousands of years, the upward percolation of the hot
fluids has caused severe chemical and physical changes (known as
_hydrothermal alteration_) in the rhyolite lava flows. One spectacular
result of the alteration has been the change from the normal brown and
gray color of the rhyolites to the bright yellow and other colorful hues
now seen in the canyon walls (as well as in many other places throughout
the Park). Another significant result of alteration has been the
weakening of the rocks; that is, the altered rocks are softer and less
resistant to erosion than unaltered rocks. Hence, the river has been
able to erode these softer rocks, upstream to Lower Falls, at a very
rapid rate.
The position of Lower Falls, as might be expected, coincides with a
change from highly altered to less altered rhyolite; the difference in
the erosion rates of the two kinds of rocks here is self-evident (figs.
41 and 42C). The position of Upper Falls is likewise closely controlled
by differences in rock hardnesses. The rhyolites on the upstream side
are hard and dense, whereas those on the downstream side contain a high
proportion of volcanic glass which causes them to be more easily eroded
(fig. 42C).
[Illustration: Swan family.]
[Illustration: COMMON KINDS OF THERMAL FEATURES in Yellowstone
National Park. (Fig. 43)
A, Hot springs and terraces colored by algae at Mammoth Hot
Springs.]
[Illustration: B, Castle Geyser erupting in Upper Geyser Basin.]
[Illustration: C, Fountain Paint Pots in Lower Geyser Basin.]
[Illustration: D, pool in Lower Geyser Basin.]