Geological History - Yellowstone Treasures - page 362

6D). Over the past 639,000 years, the cliff has been eroded back a bit, but it still represents
that rim fault.
If you now drive past Gibbon Falls and go north toward Norris, you have moved away
from the caldera and its rim. Road cuts both north and south of the falls allow you access to
very tall, nearly vertical walls of Lava Creek tuff, which is cream colored with hints of pink.
B ASALTIC V OLCANISM . A different type of volcanic rock encountered in the park is basalt. Basalt
contains less silica than rhyolite and is black due to its high iron content. It is much less vis-
cous and flows out as sheets that sometimes spread for miles. Good examples can be seen
near Tower Fall [GEO.12]. (There's a description of columnar jointing in basalt on page 217 . )
Basalt flows occurred before, during, and after all of the caldera-forming events, usually on
the edges of the calderas themselves.
A 1.3-million-year-old basalt lava flow near Calcite Springs extends across the middle of the photo.
Vertical lines in the flow are shrinkage cracks outlining columns formed as the solidified basalt cooled
[GEO.12].
Most Recent History
More geological processes continue to leave their traces on Yellowstone.
Glaciers reshape the landscape
About 2 million years ago, at about the same time that the first caldera event in the Yellow-
stone region occurred, a worldwide climate change brought colder temperatures and, in some
places, glaciers. Vast ice sheets covered much of Canada and spread south to the Ohio River.
Elsewhere, glaciers also formed at high elevations, such as on the broad plateaus of the park,
which average about 8,000 feet (2,400 m) above sea level.