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Inside USGS, No. 5, Kenneth Pierce, Pleistocene Glaciations of the Greater Yellowstone Area

TEXT: INSIDE USGSNumber 5 Ken PierceU.

S.

Geological Survey Pleistocene Glaciations of the Greater YellowstoneArea DR.

PIERCE: My name is Ken Pierce, I'm a geologistwith 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 Quaternarygeology and geomorphology.

TEXT: Why is the glacial story of Yellowstoneso unique? DR.

PIERCE: The glacial story in Yellowstoneis really unusual for the Rocky Mountains.

Yellowstone is cold, high and snowy.

And allthree of these features are very important to Pleistocene glaciations.

The reason it'scold is because it is high, it stands 8, 000 feet and higher.

The reason it's snowy isthat moisture can come way inland from the Pacific Ocean to Yellowstone by traversingthe Snake River Plain.

It moves up the Snake River Plain and Yellowstone has an area wecall the Yellowstone Crescent of High Terrain.

It's the Yellowstone Plateau and the surroundingmountains.

Moisture-bearing storms come up the Snake River Plain, rise up onto this plateauand onto the surrounding mountains.

And in rising upward, there is a lot of orographicsnowfall in the present and particularly in the Pleistocene.

Yellowstone glaciation startedout by ice build-up in this Yellowstone Crescent of High Terrain and flowing out onto the YellowstonePlateau.

And built-up on the Yellowstone Plateau to an extent that the Plateau became abovethe snowline.

The build-up reached about 11, 000 feet which made Yellowstone one of the highestplaces in the whole region.

And that led to the extensive glaciation.

TEXT: What clues did you find in the landscapethat help tell the glacial story? DR.

PIERCE: Some of the high moments in doingthe geology of Yellowstone don't come up very often but it's really wonderful that theydo come up and it changes how you're thinking about the landscape.

One of them was in theWashburn Range.

On the top of Mount Washburn and traversing the whole crest of the WashburnRange, I found there were glacial striations indicating first in one area called ObservationPeak, flow from the Bear Tooth Uplift south across the Washburn Range onto the YellowstonePlateau, indicating ice was coming first from the high Bear Tooths and building up on thePlateau and then other striations at this location indicating northward flow acrossthe crest of the Washburn Range.

So this indicated two different things.

One, the ice built upon the high terrain outside of Yellowstone, flowed into Yellowstone.

It then built upon 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 occurredin Yellowstone and how it changed through time.

TEXT: Where did the ice flow? DR.

PIERCE: When ice built up at the fullextent of glaciation, the oldest part of the glacial sequence we have around the wholeperiphery 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 thattime, the ice was primarily coming probably from the Bear Tooths.

Then the ice advancedonto the Yellowstone Plateau and was building up on the Yellowstone Plateau and going overthe top of the Washburn Range and way down the Yellowstone Valley to deposit the 8-Mileand Chico moraines, and that was around 19, 000 years ago.

And then ice continued to buildup 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 yearsago.

One other thing that happened, very much like this same thing, is ice from the BearTooths advanced up the Yellowstone River to the Canyon Village area and deposited thickglacial moraines in that area that have granitic erratics from the Bear Tooth mountains inthis thick glacial till that is kind of amorphous but very thick.

And at this time, it alsodammed the lake in Hayden Valley.

And this is one of the main reasons that Hayden Valleyexists 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 rhyoliteand lodgepole pine, which doesn't have nearly the interest as the Serengeti of Yellowstonein Hayden Valley.

TEXT: How is the most recent glaciation (thePinedale) different from the older Bull Lake glaciation? DR.

PIERCE: The Greater Yellowstone GlacialSystem shows some major contrasts in the extent of glaciation between the last, or the PinedaleGlaciation and the next to last, or Bull Lake Glaciation.

And this changes around the peripheryof the Greater Yellowstone Glacial System.

For example, in Jackson Hole, the Bull Lakeglaciation was very large, it filled all of Jackson Hole with ice.

Whereas the Pinedaleonly came half the way south into Jackson Hole.

The same thing happens over on the southwestside, in the Ashton area, the Bull Lake is much bigger than the Pinedale.

The same happensin the west Yellowstone area, on the west side of Yellowstone, the Bull Lake is muchbigger than the Pinedale.

But if you go north from Yellowstone, the Pinedale overrode theBull 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 southernJackson Hole.

There obviously is a major difference in the extent of glaciation.

And I think thatthe Yellowstone hot spot may be the reason.

On the leading margin of the Yellowstone hotspot, the terrain is uplifting.

From the next to last glaciation, it's lower and it risesduring the last glaciation.

So the Pinedale is big in that setting.

Whereas on the trailingedge of the Yellowstone hot spot, the Bull Lake glaciation occurs and the landscape lowersat the time of the Pinedale glaciation.

This uplift that is occurring on leading and subsidingon training edge, may explain this large difference between the Bull Lake and Pinedale on theleading edge in which the Pinedale is overridden the Bull Lake and on the trailing edge inwhich the Bull Lake is much bigger than the Pinedale.

TEXT: How did obsidian contribute to yourunderstanding of when the Bull Lake glaciation occurred? DR.

PIERCE: Another high moment occurred partlyin the field in collecting obsidian samples in what are called the Bull Lake glacial morainesaround west Yellowstone, Montana.

I collected these samples and then worked with a colleagueIrving Friedman to measure the hydration on the obsidian samples that were related tothe glacial abrasion of these obsidians by scraping of glaciers as they were depositingthe moraines.

I measured the hydration on the moraines and they all came out around14 micrometers in thickness.

This, in combination with some other observations, indicated thatthe Bull Lake glaciation in the West Yellowstone area was about 140, 000 years old.

And thiswas a great departure from what we had thought the Bull Lake was.

We, and particularly myboss Gerry Richmond, thought it was around 60, 000 years old, and it really was more thantwice that old, near 140, 000 years old.

That indicated that the Bull Lake glaciation correlatedwith the Illinoian glaciations of the mid-continent with the Riss glaciation of Europe and withMarine Isotope Stage VI which is a major cold interval that shows up by analysis of deepsea cores.

TEXT: And your work on the Bull Lake glaciationled to the Kirk Bryan Award? DR.

PIERCE: In the study of glaciation inYellowstone, I mapped the northern third and the area outside of Yellowstone.

My supervisorGerry Richmond mapped the southern part of Yellowstone, particularly the area with thevolcanic rocks.

In the process of doing the mapping and the glacial studies, we endedup with two very different understandings of the glaciation.

One was that the Bull Lakeglaciation was 150, 000 years old.

The other was that glaciers built up in Yellowstoneand went over the top of Mount Washburn.

Both of these differences we couldn't resolve bystudies in the field.

And I kept wondering, is there some way I can compare the glacialreconstructions that will help resolve this difference? I learned there is a branch ofscience called glaciology, which is very different from glacial geology.

Glaciology is the physicsof glaciers, it has to do with like things like basal shear stress.

So I learned aboutbasal shear stress and quite a few features of glaciology particularly from John Andrewsand Mark Myers (with the U.

S.

Geological Survey in Tacoma).

I was able to use basal shearstress to indicate that my reconstruction made sense and that there were problems withGerry's reconstruction.

I really have to thank Gerry Richmond for making it necessary forme to learn a whole new field of science.

And by using this field, to apply it to mystudy of northern Yellowstone.

After I published this paper in 1979, I received, in 1982, theKirk Bryan Award, which is the highest award in my field of geology, particularly for applyingthis glaciology concept to the reconstruction in Yellowstone.

So Gerry Richmond really didme quite a favor by challenging me and making me prove my point.

TEXT: How do glacial deposits of the pastaffect the geothermal system today? DR.

PIERCE: Quite a few of the thermal areasin Yellowstone are also in areas where quite a thickness of glacial deposits filled thelowland areas.

The sands and gravels are probably pretty important to forming the complex plumbingsystem of geysers like Old Faithful and Steamboat Geyser.

The porosity and the open parts ofthe sands and gravels sort of permit the thermal waters to heat up and get stored, and thentriggering causes one of the big eruption of geysers.

The thermal areas they sometimesoccur outside the area of glacial deposits but quite a few of the most important onesare 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 yourtimes in Yellowstone? DR.

PIERCE: The times I've spent in Yellowstonehave been wonderful.

There are some times that were a particular strain.

And this iskind of an odd situation.

I was able to drive a government vehicle up to the top of MountWashburn, I hiked several thousand feet down from where I parked my vehicle, mapping allthe time these beautiful glacial striations that indicate a very high ice cap in Yellowstoneflowing 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 veryfast.

I was hiking up and my lungs were bursting and I kept hiking up.

I knew I was in a dangerousposition, on a ridge where lightning was likely to strike.

I was totally exhausted and completelyout of breath by the time I reached my vehicle, got inside, shut the door, and then I wassafe from lightning.

But it was popping all around.

DR.

PIERCE: Another time, we were going alongthe 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 smalltrees 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 onhorseback (I wasn't on horseback).

The highest thing in the landscape was the back of thishorse.

And so I got up on the back of the horse with him and this horse was not afraidat all of the grizzly bear, it kind of challenged the bear.

The bear looked us over and wentoff.

It was a teenage bear, and I don't think it was acting particularly aggressive butit was a lot bigger than I was.

DR.

PIERCE: My favorite part of the area ishiking up onto a bedrock knoll several hundred feet up, a kind of a sheep's back in the glacialgeologic terrane, and I very much enjoy hiking up there particularly with our dogs and gettingon the crest of this thing and looking around the 360 degree compass at a lot of very interestingand geologically diverse terrain.

TEXT:Special thanks to Students of Montana State UniversityLinda Pierce Jake Lowenstern Images by Dan and Lin DzurisinVideo by Liz Westby Guest appearance by Jack TEXT:Interview with Ken Pierce Produced by Liz Westby2016 TEXT:For more information on Yellowstone visit theYellowstone Volcano Observatory volcanoes.

usgs.

gov/observatories/yvo/ USGS2.

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