Travel Reference
In-Depth Information
We know that worldwide there were about 20 glaciations, or ice ages. Each glaciation lasted
about 50,000 to 100,000 years, with shorter warm periods in between. The glaciers were at
their worldwide maximum about 20,000 years ago, and have been retreating ever since, giving
us the warm interglacial period we now enjoy.
Glaciers had several major effects on Yellowstone, and the last two glaciations left evidence
all around. A small ice cap formed over Yellowstone, and almost the entire park was covered
with ice. Often the ice was about one-half mile (800 m) thick, so that animals and plants were
essentially nonexistent here. Although glaciers are solid ice, they flow slowly toward lower
ground at rates of a few yards (meters) per year. The glaciers were major agents that modified
the landscape drastically by transporting some rocks, digging and scraping away at others,
and interacting with the park's thermal areas. Some examples of this activity follow.
The glaciers' ability to transport rocks is demonstrated in the area of the Yellowstone
Canyon
[GEO.13]
and in the Lamar River Valley
[GEO.14].
In both places, you see boulders
as large as 10 feet (3 m) across that are clearly out of place. We know where they originated,
because they match the 3.5- and 2.7-billion-year-old gneisses that form the Beartooths. These
boulders are called
glacial erratics
(see picture), because they are many miles from the nearest
outcrops of similar rocks.
Glacial erratics: The boulders, some as big as bison, in this Lamar Valley photo were carried by the
glacier from the Beartooth Mountains and left behind when the glacier melted
[GEO.14].
The glaciers carried huge amounts of rock and soil along and dumped it at their downhill
edges where they melted. These deposits (or
moraines)
can be seen in Paradise Valley along
the Yellowstone River north of the park. Often large blocks of ice are left stranded in the mo-
raines. When the ice melts, holes appear that sometimes fill with water. Look for these
kettle
ponds
or kettle holes (pictured) mostly on the north side of the Northeast Entrance Road
