Many features of the present-day landscape of Yellowstone stem from
Pliocene time, about 10 million years ago. At that time the entire
region—in fact, much of the Rocky Mountain chain—was being uplifted by
giant earth movements to heights several thousand feet above its
previous level. This episode of regional uplift accounts in large
measure for the present high average elevation of the Yellowstone
country. Although the precise cause of the uplift is unknown, the uplift
assuredly reflects profound changes that were taking place deep within
or beneath the earth’s crust.
Great tensional forces, operating during Pliocene time, pulled the
Yellowstone region apart and partially broke it into large steep-sided
blocks bounded by _normal faults_ (fig. 13). Some blocks sank while
others rose, commonly on the order of several thousand feet. The
Gallatin Range, in the northwest corner of the Park, for example, was
lifted as a rectangular mountain block along north-trending 20-mile-long
normal faults that border it on each side (fig. 14, section A-A′; pl.
1). In the south-central part of the Park, the differential movements
between several adjacent fault blocks totaled more than 15,000 feet
(fig. 14, section C-C′). Farther south, the Teton Range moved up and the
floor of Jackson Hole moved down along a normal-fault zone that
stretches along the east foot of the range. An enormous offset of about
30,000 feet developed between the two crustal blocks, accounting in
large part for the now incredibly steep and rugged east face of the
Teton Range.
The pronounced rise in elevation of the general ground surface and the
chopping of the region into many mountainous fault blocks caused a
profound increase in the rate of erosion. Once-sluggish streams turned
into vigorous, fast-moving rivers that began to cut deeply into the
Absaroka volcanic plateau. Huge quantities of rock debris were stripped
off and carried out of the area, and at the end of the Pliocene, the
Yellowstone region must have been very highly dissected mountains and
table- and canyon-lands. Much of the landscape may have resembled the
rugged terrain now seen in the Absaroka Range along the east side of the
Park. These mountains (fig. 27), and the Washburn Range in the interior
of the Park (fig. 4), today represent but small remnants of the vast
pile of Absaroka volcanic rocks that once covered all of Yellowstone and
the surrounding regions.
[Illustration: Moose]