Travel Reference
In-Depth Information
At the same time this was going on, glaciers were coming and going over Yellowstone.
Knowing that during the last glaciation the whole area was buried in ice except for the tops
of some of the highest peaks, we would expect that the canyon of the Yellowstone would have
been filled with ice and would now be U-shaped with almost vertical walls and a flat, rounded
bottom. But it is not.
A possible explanation consists of two parts. First, there has been geyser activity in this
area for at least 480,000 years, and it existed during the glaciation. Either the valley was not
completely filled with ice, or the glacial ice was not moving. Second, to the north a glacier
flowing along the Lamar Valley blocked the Yellowstone River Valley, creating a dam and a
lake that filled part of the Yellowstone Canyon. The canyon was then partly filled by lake sed-
iments—until the Lamar Valley glacier retreated. The ice dam failed suddenly at one point
during the retreat, causing a wall of water about 150 to 200 feet high (45 to 60 m) to flow north
and west, rolling and carrying boulders along until they were dropped northwest of Gardiner,
about 20 miles (32 km) downstream. There is evidence that this type of catastrophic flood oc-
curred more than once.
After each flood event, the Yellowstone River redug its channel in the canyon and contin-
ued eroding to its present depth. Consequently, the Grand Canyon of the Yellowstone was cut
mainly by the river, and all in about 150,000 years, a very short time geologically speaking.
The fast erosion was helped by local deep fractures that let water seep into the earth and be
warmed by the still-hot rocks. The hot water rose toward the surface, and in addition to help-
ing form the lake, reacted with some layers of the overlying rocks, breaking down and chan-
ging their minerals, weakening them. This process resulted in layers of weaker and stronger
rocks and is still going on today. The river exploited this and cut down through the softer
rocks very rapidly. The result is two splendid waterfalls, the Upper and Lower Falls of the Yel-
lowstone
[GEO.18].
There are two falls because two layers of quite resistant, nearly half-million-year-old rhyol-
ite protect weakened, easily eroded rocks below them. The Upper Falls has cut back through a
layer of rhyolite and also a thermally weakened layer below it. The Lower Falls formed where
a different rhyolite flow protects another weakened layer. Both falls will continue to erode the
layers over the coming millennia.
Geysers, hot springs, fumaroles, and mud pots
Most visitors come to Yellowstone to see
geysers.
There are more geysers in Yellowstone than
anywhere else in the world. What visitors also find, however, is a vast collection of places that
spurt, hiss, boil, and bubble, sometimes with a nasty smell, often displaying a rainbow of col-
ors, but always interesting.
