TEXT: INSIDE USGS Number 5
Ken Pierce U.S. Geological Survey
Pleistocene Glaciations of the Greater Yellowstone Area
DR. PIERCE: My name is Ken Pierce, I'm a geologist with the U.S. Geological Survey, actually
presently retired and emeritus with the U.S. Geological Survey, and my field of focus-of
specialization, is in what's called Quaternary geology and geomorphology.
TEXT: Why is the glacial story of Yellowstone so unique?
DR. PIERCE: The glacial story in Yellowstone is really unusual for the Rocky Mountains.
Yellowstone is cold, high and snowy. And all three of these features are very important
to Pleistocene glaciations. The reason it's cold is because it is high, it stands 8,000
feet and higher. The reason it's snowy is that moisture can come way inland from the
Pacific Ocean to Yellowstone by traversing the Snake River Plain. It moves up the Snake
River Plain and Yellowstone has an area we call the Yellowstone Crescent of High Terrain.
It's the Yellowstone Plateau and the surrounding mountains. Moisture-bearing storms come up
the Snake River Plain, rise up onto this plateau and onto the surrounding mountains. And in
rising upward, there is a lot of orographic snowfall in the present and particularly in
the Pleistocene. Yellowstone glaciation started out by ice build-up in this Yellowstone Crescent
of High Terrain and flowing out onto the Yellowstone Plateau. And built-up on the Yellowstone Plateau
to an extent that the Plateau became above the snowline. The build-up reached about 11,000
feet which made Yellowstone one of the highest places in the whole region. And that led to
the extensive glaciation.
TEXT: What clues did you find in the landscape that help tell the glacial story?
DR. PIERCE: Some of the high moments in doing the geology of Yellowstone don't come up very
often but it's really wonderful that they do come up and it changes how you're thinking
about the landscape. One of them was in the Washburn Range. On the top of Mount Washburn
and traversing the whole crest of the Washburn Range, I found there were glacial striations
indicating first in one area called Observation Peak, flow from the Bear Tooth Uplift south
across the Washburn Range onto the Yellowstone Plateau, indicating ice was coming first from
the high Bear Tooths and building up on the Plateau and then other striations at this
location indicating northward flow across the crest of the Washburn Range. So this indicated
two different things. One, the ice built up on the high terrain outside of Yellowstone,
flowed into Yellowstone. It then built up on Yellowstone and flowed at a very high level
northward to form the Yellowstone outlet glacier. And this combination of observations was critical
to understanding how glacial build-up occurred in Yellowstone and how it changed through
time.
TEXT: Where did the ice flow?
DR. PIERCE: When ice built up at the full extent of glaciation, the oldest part of the
glacial sequence we have around the whole periphery of the Greater Yellowstone Glacial
System is at the mouth of the Clark Fork River, at edge of the Bighorn Basin, where the glacial
deposits are about 21,000 years old. At that time, the ice was primarily coming probably
from the Bear Tooths. Then the ice advanced onto the Yellowstone Plateau and was building
up on the Yellowstone Plateau and going over the top of the Washburn Range and way down
the Yellowstone Valley to deposit the 8-Mile and Chico moraines, and that was around 19,000
years ago. And then ice continued to build up on the Yellowstone Plateau, building to
the southwest, toward the source of moisture. The culmination of the glaciation that went
down into Jackson Hole was about 17,000 years ago. One other thing that happened, very much
like this same thing, is ice from the Bear Tooths advanced up the Yellowstone River to
the Canyon Village area and deposited thick glacial moraines in that area that have granitic
erratics from the Bear Tooth mountains in this thick glacial till that is kind of amorphous
but very thick. And at this time, it also dammed the lake in Hayden Valley. And this
is one of the main reasons that Hayden Valley exists as a sagebrush grassland, with its
charismatic megafauna, particularly the bison, that area is on lake sediments in Hayden Valley.
Whereas all the area around there is rhyolite and lodgepole pine, which doesn't have nearly
the interest as the Serengeti of Yellowstone in Hayden Valley.
TEXT: How is the most recent glaciation (the Pinedale) different from the older Bull Lake
glaciation?
DR. PIERCE: The Greater Yellowstone Glacial System shows some major contrasts in the extent
of glaciation between the last, or the Pinedale Glaciation and the next to last, or Bull Lake
Glaciation. And this changes around the periphery of the Greater Yellowstone Glacial System.
For example, in Jackson Hole, the Bull Lake glaciation was very large, it filled all of
Jackson Hole with ice. Whereas the Pinedale only came half the way south into Jackson
Hole. The same thing happens over on the southwest side, in the Ashton area, the Bull Lake is
much bigger than the Pinedale. The same happens in the west Yellowstone area, on the west
side of Yellowstone, the Bull Lake is much bigger than the Pinedale. But if you go north
from Yellowstone, the Pinedale overrode the Bull Lake glaciation. Or if you go northeast
and east, it seems also the same thing happens, the Pinedale was about the same size as the
Bull Lake, and not nearly like it is in southern Jackson Hole. There obviously is a major difference
in the extent of glaciation. And I think that the Yellowstone hot spot may be the reason.
On the leading margin of the Yellowstone hot spot, the terrain is uplifting. From the next
to last glaciation, it's lower and it rises during the last glaciation. So the Pinedale
is big in that setting. Whereas on the trailing edge of the Yellowstone hot spot, the Bull
Lake glaciation occurs and the landscape lowers at the time of the Pinedale glaciation. This
uplift that is occurring on leading and subsiding on training edge, may explain this large difference
between the Bull Lake and Pinedale on the leading edge in which the Pinedale is overridden
the Bull Lake and on the trailing edge in which the Bull Lake is much bigger than the
Pinedale.
TEXT: How did obsidian contribute to your understanding of when the Bull Lake glaciation
occurred?
DR. PIERCE: Another high moment occurred partly in the field in collecting obsidian samples
in what are called the Bull Lake glacial moraines around west Yellowstone, Montana. I collected
these samples and then worked with a colleague Irving Friedman to measure the hydration on
the obsidian samples that were related to the glacial abrasion of these obsidians by
scraping of glaciers as they were depositing the moraines. I measured the hydration on
the moraines and they all came out around 14 micrometers in thickness. This, in combination
with some other observations, indicated that the Bull Lake glaciation in the West Yellowstone
area was about 140,000 years old. And this was a great departure from what we had thought
the Bull Lake was. We, and particularly my boss Gerry Richmond, thought it was around
60,000 years old, and it really was more than twice that old, near 140,000 years old. That
indicated that the Bull Lake glaciation correlated with the Illinoian glaciations of the mid-continent
with the Riss glaciation of Europe and with Marine Isotope Stage VI which is a major cold
interval that shows up by analysis of deep sea cores.
TEXT: And your work on the Bull Lake glaciation led to the Kirk Bryan Award?
DR. PIERCE: In the study of glaciation in Yellowstone, I mapped the northern third and
the area outside of Yellowstone. My supervisor Gerry Richmond mapped the southern part of
Yellowstone, particularly the area with the volcanic rocks. In the process of doing the
mapping and the glacial studies, we ended up with two very different understandings
of the glaciation. One was that the Bull Lake glaciation was 150,000 years old. The other
was that glaciers built up in Yellowstone and went over the top of Mount Washburn. Both
of these differences we couldn't resolve by studies in the field. And I kept wondering,
is there some way I can compare the glacial reconstructions that will help resolve this
difference? I learned there is a branch of science called glaciology, which is very different
from glacial geology. Glaciology is the physics of glaciers, it has to do with like things
like basal shear stress. So I learned about basal shear stress and quite a few features
of glaciology particularly from John Andrews and Mark Myers (with the U.S. Geological Survey
in Tacoma). I was able to use basal shear stress to indicate that my reconstruction
made sense and that there were problems with Gerry's reconstruction. I really have to thank
Gerry Richmond for making it necessary for me to learn a whole new field of science.
And by using this field, to apply it to my study of northern Yellowstone. After I published
this paper in 1979, I received, in 1982, the Kirk Bryan Award, which is the highest award
in my field of geology, particularly for applying this glaciology concept to the reconstruction
in Yellowstone. So Gerry Richmond really did me quite a favor by challenging me and making
me prove my point.
TEXT: How do glacial deposits of the past affect the geothermal system today?
DR. PIERCE: Quite a few of the thermal areas in Yellowstone are also in areas where quite
a thickness of glacial deposits filled the lowland areas. The sands and gravels are probably
pretty important to forming the complex plumbing system of geysers like Old Faithful and Steamboat
Geyser. The porosity and the open parts of the sands and gravels sort of permit the thermal
waters to heat up and get stored, and then triggering causes one of the big eruption
of geysers. The thermal areas they sometimes occur outside the area of glacial deposits
but quite a few of the most important ones are in areas where there are a lot of glacial
sands and gravels and other glacial deposits, including lake sediments and glacial till.
TEXT: Do you have favorite stories from your times in Yellowstone?
DR. PIERCE: The times I've spent in Yellowstone have been wonderful. There are some times
that were a particular strain. And this is kind of an odd situation. I was able to drive
a government vehicle up to the top of Mount Washburn, I hiked several thousand feet down
from where I parked my vehicle, mapping all the time these beautiful glacial striations
that indicate a very high ice cap in Yellowstone flowing north across the Washburn Range. And
then I noticed a thunderstorm was coming up - a big one. And so I started hiking up as
fast as I could. The storm was coming very fast. I was hiking up and my lungs were bursting
and I kept hiking up. I knew I was in a dangerous position, on a ridge where lightning was likely
to strike. I was totally exhausted and completely out of breath by the time I reached my vehicle,
got inside, shut the door, and then I was safe from lightning. But it was popping all
around.
DR. PIERCE: Another time, we were going along the crest of the Washburn Range. And that
area is at timberline or above Timberline. And there are narrow bands of small trees.
I walked through one of these bands of small trees and there on the other side was a grizzly
bear - standing full upright, looking at me. I thought he was 10 feet tall. I knew I was
in a bad position. With me, was a person on horseback (I wasn't on horseback). The highest
thing in the landscape was the back of this horse. And so I got up on the back of the
horse with him and this horse was not afraid at all of the grizzly bear, it kind of challenged
the bear. The bear looked us over and went off. It was a teenage bear, and I don't think
it was acting particularly aggressive but it was a lot bigger than I was.
DR. PIERCE: My favorite part of the area is hiking up onto a bedrock knoll several hundred
feet up, a kind of a sheep's back in the glacial geologic terrane, and I very much enjoy hiking
up there particularly with our dogs and getting on the crest of this thing and looking around
the 360 degree compass at a lot of very interesting and geologically diverse terrain.
TEXT: Special thanks to
Students of Montana State University Linda Pierce
Jake Lowenstern
Images by Dan and Lin Dzurisin Video by Liz Westby
Guest appearance by Jack
TEXT: Interview with Ken Pierce
Produced by Liz Westby 2016
TEXT: For more information on Yellowstone visit
the Yellowstone Volcano Observatory
volcanoes.usgs.gov/observatories/yvo/
USGS 2