Inside USGS No. 1, Robert Christiansen, Yellowstone

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Detailed Description

USGS emeritus geologist Robert Christiansen describes his career working on Yellowstone geology from the 1960's through 2014. Bob's work along with his USGS colleagues revealed the details of Yellowstone's explosive volcanic past including mapping and dating of past super eruptions 2.1 million years ago, 1.3 million years ago and 640,000 years ago.


Date Taken:

Length: 00:37:18

Location Taken: Menlo Park, CA, US


Interviewer: Describe your area of expertise
in geology.

Bob Christiansen: As a geologist I have worked
for the survey most of my professional life,

really all of it since grad school.

And my work has always involved volcanoes
in one way or another, sometimes explicitly

working directly on volcanoes, volcanology
or geology of volcanoes, sometimes working

on tectonic problems that involve volcanic
rocks, too.

So I’ve been very interested in how volcanoes
and magnetism are related to tectonics.

So these are two areas that I’ve spent most
of my career working on.

Ok and so where were you educated and what
was your degree in…?

I graduated both as an undergraduate—my
bachelor’s degree and my Ph.

D. were both Stanford.

I graduated in 1956, got my Ph.

D. in 1961 in Geology, not in volcano studies.

However, I was working in metamorphic petrology
and structural geology at that time.

How was it that you began working in Yellowstone?

I was one of a group of people who started
working in Yellowstone in 1965.

And the impetus for this was that Yellowstone
had really not had a comprehensive geologic

study since the late 19th century, surprisingly

There had been involved specific topical studies
but there hadn’t really been an overall

comprehensive study of Yellowstone’s geology
in the modern era.

So a group of us were tapped to undertake
a new geologic map of Yellowstone.

We were divided into five teams.

And rather than just each one mapping his
own area, which is often the way this sort

thing might have been done in a joint project
like that—each person with a separate quadrangle—we

actually worked in topical areas within the
overall framework of Yellowstone geology.

And so I was asked to organize a study of
the young volcanic geology of Yellowstone.

So were was a group of people doing the early
geology, the pre tertiary geology in the northern

part of the park, and a different group of
people working in the southern part of the

park on the same kind of thing, another group
working on the early tertiary rocks, which

is a completely different problem, and I and
some colleagues worked on the quaternary volcanics,

the young volcanics, and then there was yet
another group of people studying the glaciation

and related surficial geology.

In addition to that, there were some other
topical studies that went on simultaneously,

including the geothermal studies that included
drilling, isotopic studies, isotopic geochemistry,

which was usually coordinated with geologic
studies that we were undertaking at that time.

It came about because their funding became
available to do this through a cooperative

program involving the USGS, NASA and the Park

Most of the money actually came from NASA.

And their interest in it was they were at
that time just developing a lot of the remote

sensing instruments, which ultimately went
into satellites and also into interplanetary


But they needed test areas where the geology
was well known, where they can calibrate this

instrumentation, so most of this was done
with some of the instruments flying in aircraft.

And then Yellowstone was one of several of
these so-called ‘ground-truth’ areas and

that was the reason we got started on it at
the time we did.

Describe how you felt the first time you saw
LiDAR at Yellowstone.

Well, I mean, LiDAR is such a marvelous new

When you see the imagery, its just different
from any other kind of technology.

When you see the imagery, it’s just different
from any other kind of imagery we have.

We had to have radar imagery and, of course,
lots of different forms of aerial photography,

but this LiDAR imagery shows details of the
ground surface that you simply cant see, sometimes

you cant even see on the field because the
features themselves are bigger than what you’re

actually seeing right in front of you and
other things like trees are in the way.

So when you actually see it that way, particularly
when you are seeing it, as you suggest, it’s

quite a spectacular thing.

How many years did you work in Yellowstone?

Well I worked in Yellowstone intensively for
a short time in the fall of 1956, but then

every summer for about three months, during
the summer months from 1966 through 1971.

So that was really when the bulk of the geologic
mapping was done.

And it was in Yellowstone and in some of the
adjacent areas outside the park, because the

geology didn’t just magically stop at the
straight-line park boundaries.

So it included work down into Jackson Hole
and over into the eastern part of the Snake

River plain as well.

Then, after that, after doing other things
and then interspersed with other USGS assignments

in one sort or another, I continued to work
in Yellowstone for the next 25 years.

But usually then it was in shorter increments
in the field, a lot more involved in looking

at the chemistry of rocks and thin sections
and looking again at specific topical things,

more collaboration with other geologists on
the geochemistry, and so forth.

What role did collaboration play in your work?

It was very essential, the interaction among
the people working both in the field and then

with our other colleagues who were doing lab-type

The whole project really came to fruition
because of this very intensive collaboration

and the focus of a number of groups at the
same time working on the same overall problem.

Were there epiphanies as the geologic story
of Yellowstone began to unfold?

There were some epiphanies, so to speak, in
terms of what we learned as we were going

through this, particularly during this initial
period, this six years of intensive field


I was very fortunate in that my particular
area of study, the young volcanic rocks, there

had been a fairly recent study, a Ph.

D. study by Joe Boyd, who became a geophysicist
at the Carnegie institution’s geophysical


But his Ph.D. thesis had been to study some
of these same rocks that I ended up studying

with some of my colleagues.

And he had done a wonderful job.

I mean considering that this was a thesis
project and starting from scratch in an area

that had really not been looked at at all,
he studied some of these young volcanic rocks

and he really came to some of the big conclusions.

That is to say, he recognized that the predominant
rhyolitic rocks, particularly around the margins

of the park and the outer parts of the park,
were pyroclastic rocks instead of lava flows.

And this was a very important thing, and he
was the first person who really recognized

that in a big way.

In addition, he recognized that there must
be some kind of—he called it a ‘volcano

tectonic depression’ that is to say, a structural
depression in the central part of the park

somehow related to the eruption of these pyroclastics.

He didn’t have the full picture, what we
now recognize as the Yellowstone caldera.

But there were still many things that he was
not able to recognize in that study, and one

of our first big, in fact, really something
we found on the first day we were in the field,

my colleague Dick Blank and I went out to
look at a particular locality that Boyd had

described quite carefully and he had interpreted
as a vent for some of these pyroclastic rocks

as welded tuffs.

It didn’t sound right to us from his description,
but it sounded like an important locality,

so we went there.

And on the very first day we recognized that
rather than being a vent for the Yellowstone

tuff, as he called it, it was two different
ash flow tuff units, different in age, and

one had been eroded and the other one had
filled in some paleotopography that had been

folded in that older unit.

So from the very first day we got there, we
realized we were dealing with more than one

major pyroclastic eruption.

It turns out that we were three of these major
pyroclastic eruptions, separated in time by

700,000 or 800,000 years or so.

Okay Bob, so tell me what tools did you use
in the field?


The tools that I used in the work that I was
doing, the work I was doing really was the

basic geologic mapping.

The focus of the study was the volcanic geology,
but the tool was really geologic mapping.

So we used topographic maps, aerial photographs,
Brunton compasses to measure various structural

elements, rock hammers to collect samples,
which of course were very important to us,

which would later be either age determinations
that we’ve done on them or chemical analysis

done on thin sections cut.

And so collecting samples, putting lines on
the maps, working primarily from air photographs

originally, and then of course, during the
non field times, a lot of it involved the

use of various ways of getting the information
from the aerial photographs accurately on

the topographic maps to actually create the
geologic maps as such.

Following the fieldwork tell me how the data
was worked up and published

The idea was, of course, each of these five
sort of field-oriented topical studies that

were part of the comprehensive geologic framework
of Yellowstone, each of these would be published

separately but brought together in a series
of US geological survey publications, professional


And so they got published at different times
over a long interval of time.

And mine was actually the last of these to
be published because a great deal of further

work after that initial intensive period of
field study went into that professional paper.

So all of the fieldwork was done.

The bulk of it, probably 85% or 90% of the
work, was done from 1966 to 1971.

The publication didn’t come out until 2001.

So there was really a period of 30 years of
additional fieldwork and additional studies

and interruptions in the work with other things
taking place, all the while working on this

professional paper.

It got to be something of an albatross after
a while.

But that’s the way these things often turn

Was there any pressure to public your work?

I had a great deal of interest in the publication
and I distributed preliminary working copies

of it over a period of years, as revisions
of the manuscript took place, to many colleagues

including not just USGS colleagues but particularly
our colleague Bob Smith at the University

of Utah, who’s a geophysicist.

He and his students were doing a lot of work
during this time of this later phase of my

work, but they were very interested in having
the publication from the overall study of

the volcanic geology.

And so they were among the people I shared
early versions of it with and ultimately had

been able to give the final publications to.

How was it revealed that there were three

We weren’t fully able to delineate the caldera
structures until the mapping was pretty well


As the work evolved, we got parts of the picture.

I would say, within the first two summers
we had a pretty good idea of the Yellowstone

caldera, which is the youngest of the three.

And we realized that it had two resurgent

One of a class of calderas is called ‘resurgent
calderas’, which means that magma coming

back into the near surface, after the initial
partial emptying of the magma chamber and

collapse on the surface to form the caldera,
came back into it and domed up what had been

the caldera floor into a structural dome.

And this class of calderas had been identified
by another Bob Smith who works for the USGS

and his colleague, Roy Bailey, and had been
recognized in many places around the world

in these large calderas.

But this is the first time, I think, that
we recognized one that actually had had these

two separate structural domes as part of a
single caldera.

So it was simply so large that the magma had
generated what amounts to two-ring fracture

zones that had partially coalesced.

We recognized that probably in broad outline
after about the second field summer.

But we really didn’t have the full picture
of all the calderas until the geologic mapping

was completed.

And that’s the important thing to realize
is that the real structure, understanding

these calderas, where they were and the fact
that there were these several calderas, was

learn on the basis of in-the-field mapping,
geologic mapping.

There is a sort of apocryphal story that has
gone around in many different forms that somehow

NASA discovered these from space.

And I think that stems from this idea that
NASA was partially sponsoring this work and

funding a major part of it as part of their
ground trothing of remote sensing instruments.

But they did not discover the calderas.

The calderas were discovered on the ground
by traditional field geologic mapping.

Just to be clear what was the USGS role in
this science?

The USGS' role in all of this was really central.

I mean, we were the people doing the fieldwork.

We collaborated very closely with both the
Park Service and NASA.

NASA basically provided funding and did some
of the remote sensing instrumentation--or

observation, which we then utilized in some
of our work.

We looked at the infrared images, radar things,
some of the aerial photography they took.

We worked on analyzing some of that date for
NASA so that they could utilize, again, to

use the sort of ground-truth concept, in understanding
the nature of the data they were getting with

these instruments.

Sot he USGS was really doing the hands-on
work of what was in the field and relating

the field work to the imagery.

The Park Services roll was primarily in wanted
a comprehensive study done for their own interpretive

programs so that they would have better, newer
information that they could pass on to the


And so they were very active in having us
work with their staff, for example, in giving

them background talks and that sort of thing
and actually working with some of the naturalists

in how to present some of it.

But, really, I would say that it’s the USGS
that was in the active partner in actually

doing most of the work onsite.

So were there seminal publications for USGS
findings at Yellowstone?

Ok, one of the first publication that I think
caught some general attention was one that

was published in 1975 in the journal Science,
and it was an attempt to bring together what

we had learned now in the basis of our initial
period of field work and understanding the

general picture of the volcanic evolution
of Yellowstone in the latest Cenozoic time—say,

in the last 2 million years, roughly—and
bringing that together with the geophysical

data, which had also been acquired during
the same period of time using some of the

same sources of funding.

So we had a new geologic map, we had a new
gravity map; we had a new aeromagnetic map.

A certain number of seismic studies had been
done that is earth passive studies, studying

earthquake distribution seismicity in the

And so bringing this information together
and interpreting it enabled us to interpret

not only the surface geology but how it was
related to what we had felt was the subsurface

magmatic system.

And so this brought together, I think for
the first time, in a paper published in 1975

in which we felt that we had in a number of
ways at least partially imaged the magma chamber

that underlies Yellowstone and is both the
source of the major volcanic eruptions that

have occurred there over this 2 million year
period and the major source of the geothermal

heat which, of course, is what Yellowstone
is best known for is its hydrothermal system.

How was Gordy Eaton involved in this journal
of Science publication?

The person who actually brought this paper
together, that is, fathered the various elements

of the data, particularly the geophysical
data, and worked with me then on the geologic

interpretation of it, was Gordon Eaton.

And at that time, Gordy was working in Reston
for the USGS headquarters and was, I think,

a little frustrated by the fact that he wasn’t
able to do much fieldwork.

But when he started to see some of this geophysical
data, particularly the gravity map that had

just been compiled, he became really excited
about it and recognized that the gravity map

itself was showing a big subsurface system
related to the Yellowstone caldera.

And so he was the one who sort of then gathered
together the other geophysical information

and worked with me on the geologic interpretation.

So Gordon Eaton was the primary author of
that multi authored paper, all the USGS people,

however, who were contributing to it.

When did it become apparent that the caldera
was inflating?

Yeah, probably one of he most interesting
additional pieces of data that came along

after this period of time we’ve been describing
with the early field work with more or less

the completion of the initial geologic studies
and doing the geophysical interpretations

that helped understand the subsurface part
of it.

Bob smith at the university of Utah was interested
in seeing if we could look for signs of contemporary

deformation in the Yellowstone caldera.

He had recognized some of these indications,
particularly changes in the lake levels in

different pats of Yellowstone Lake, which
is a very large lake.

And because it’s so large, and of course
water tending to always seek a level, he felt

that there was indications that the lake basin
itself was being tilted.

And because of this, the lake level was rising
at one end of the lake and falling at the

other end of the lake.

And he was interested in seeing whether we
could actually measure this by some direct


So one of the things I did—at that time
I was in an administrative position in the


I was the coordinator of the USGS geothermal

I managed to get funding together to get the
USGS topographic division involved in re-leveling.

They had leveled the road network in the park,
that is, surveyed it using level instruments

which gave a series of elevations on all the
roads in the park back in the 1920s—I think

’24 or ’25, if I remember correctly.

And there had not been a re-leveling since
that time.

We felt that, with as much deformation as
there appeared to be based on these lake level

changes, that there should be measurable changes
in elevations in the park.

So we finally got the funding together and
got that survey done, and the data was provided

to Bob Smith and his group at the University
of Utah.

And they in turn integrated all this material
into a series of elevation changes, the map

of elevation changes throughout the caldera,
and demonstrated that, in fact, the caldera,

in the roughly 50-year period of time between
these two surveys, had come up nearly two

meters in the center in a large broad, dome
shaped configuration.

So it was a dome shaped uplift that had taken
place during these 50 years, a rather spectacular

amount of uplift, indicating that the magmatic
system in some form was active.

Either the magma was actually intruding the
crust or it was heating the hydrothermal system,

causing it to expand and elevate the crust.

Something was going on.

Subsequent to that, there had been additional
surveys, which showed that, in fact, after

a period of continued uplift over the next
decade or so, there then was a period of stability

for about a year, and then there was subsidence.

So we now know that the Yellowstone caldera
is simply going up but it does up and down

in a sort of breathing motion, apparently,
at times.

Its overall deformation does seem to be an
inflationary one, but it’s not a steady

sort of thing, and there are, in fact, grades
of deflection.

How does the Yellowstone story fit in with
things like Columbia River flood basalts in

the wets?

Ok, yeah.

Yellowstone, we now understand, is the contemporary
expression of a system that has been active

in the earth’s uppermost layers, in the
upper mantel and lower crust, that has been

magmatic ally active over a period of at least
17 million years.

The earliest expression of this appears to
have been related to the vast outpouring of

basaltic magmas in the form of big basalt
lava flows that covered the Columbia River


Most of them erupted from the eastern part
of the plateau, in the northern rocky mountains

of western Idaho and eastern Washington, but
flowed across the Columbia River plateau,

essentially filled the plateau, and spilled
through the Columbia gorge out to the coastal

region along into the sea.

So this vast outpouring of basalts over a
very short period of time—really the biggest

bulk of those came out during less than a
million years, around 17 million years ago


That was the first expression of this.

But then the later expressions have been more
or less along a linear trend, which is now

represented by the eastern snake river plain,
beginning at southwestern Idaho about 14 or

15 million years ago and continuing through
southeastern Idaho, right up to the Idaho

border, and then into Wyoming.

And as I say, the current expression of this
so-called hot spot is the Yellowstone plateau

magmatic system.

But we can find evidence that similar systems
of mainly rhyolitic magmas but associated

basalts of earlier ages form a chain of calderas
now buried by younger basaltic lavas along

the eastern Snake River plain.

So the origin of this so-called hot spot is
a subject of great contemporary interest and

some dispute in geologic literature.

What is the connection between the Grand Tetons
and Yellowstone?

Yellowstone, although it’s actually in the
Rocky Mountains, is really much more related

to the basin and range province than it is
to what we typically think of as Rocky Mountains


Rocky mountain geology for the most part goes
back to what we call ‘laramide’ and earlier,

which is the latest Mesozoic, let’s say
roughly 65 million years ago to something

back even 70 million years and earlier.

So that geology is sort of a framework.

But the basin and range geology, which is
mostly expressed in Nevada and western Utah

and surrounding parts of Oregon and Idaho,
actually encroaches into the central and northern

Rocky Mountains.

And Yellowstone is right where that impingement
of the basin and range tectonics—ongoing,

active tectonics, extensional faulting and
spreading of the earth’s crusts—intersects

with some of these pre-existing structures.

And so the Tetons as we see them now are an
expression of this basin and range topography,

which in turn is an expression of basin and
range faulting with Jackson hole being the

down-dropped black and the Tetons themselves
as being the uplifted block.

If this were in the middle of Nevada, you
would think of it as nothing different than

the Toiyabe range or some other range and
valley system in the central great basin.

How has evolving technology affected your

My own work has been primarily on the geology,
both the field geology and then looking at

some of the analytical work we do on samples
collected in the field in both chemical and

age determination.

I’m working with colleagues on those things.

But most of the technological changes in what
we’ve been able to do and been able to learn

about the Yellowstone magmatic system had
been more in the geophysics.

So we now have much better seismology, for

We have digitals, a broadband seismic instrumentation,
whereas we used to simply have analog, sometimes

just one-component, sometimes three-component

Particularly the deformation studies, which
we used to do primarily by leveling, are now

done by GPS measurements in the field.

And a number of other additional developments
including things like using satellite photography,

comparing different satellite images and getting
‘interferograms’, as they’re called,

which show the difference and changes in surface
features, which again reflects some of this

uplift and subsidence that we talked about
in the Yellowstone caldera.

How did you collaborate with your USGS colleagues
during fieldwork?

Well, certainly one of the joys of that period
of time, particularly in the mid- to late

1960s when we were working as a group of teams
really in the field, we frequently got together.

An even though we were working on different
topical elements, there obviously are very

strong inter-relationships among them.

And so we were working with the people and
getting together, both in a kind of in-the-field

socialization kind of thing as well as scientifically
interacting with these other teams, in particular

with a group of people studying the geothermal
systems, the hydrothermal activity.

Don White, Bob Fournier, Patrick Muffler,
and Al Truesdale were the four people doing

physical and geochemical studies of the hydrothermal
systems during this period of time.

And because that’s so intimately related
to the young volcanism that was my primary

field of study, we spent a lot of time together
and looking at each other’s work.

And seeing what each of us was learning separately
helped the others to understand the framework

of their own work.

So it was a particularly good time, both scientifically
and just in terms of good comradeship.

Did you learn things from the drilling that
clarified the caldera story?


One example of the kind of thing that we learned
from the drilling studies, which of course

were done to study the hydrothermal system,
but they made continuous core drilling of

the entire holes that they went into, which
were sometimes just 100 feet of a few tens

of feet, and sometimes many hundreds of feet—the
deepest one was, I think, 1300 feet—so these

were a great opportunity for us to see that
some of the subsurface geology that was otherwise

inaccessible to us.

So not just the geophysical studies that were
done, too, but some of the shallow subsurface

we were able to observe directly.

The most interesting aspect of this was a
particular drill hole in the lower geyser

basin, or the midway geyser basin as it’s
called, in the western part of the Yellowstone

where we had interpreted that there was a
resurgent dome in the western part of the

caldera but could only see the young rocks
that had covered the original caldera floor

being uplifted.

We were able to interpret the fact that there
were actually two episodes of uplift, one

very close to the time of subsidence.

The caldera subsidence occurred very shortly

There was inflation to form the resurgent
dome on the basis of drill core that was put

down there that would show that there was,
in fact, lava creek tuff, the 600,00-year-old

tuff, present within 10 meters of surface,
within about 30 feet or so of the surface,

which was not to be seen at the surface anywhere.

We had expected it would be there, but actually
have it and to get samples of it and be able

to know which unit it was and so forth was
particularly useful thing to us in interpreting

structure of the caldera.

Did you encounter any wild animals during

We, meaning, Dick Blank and I, who did most
of mapping in the young volcanics, were very

lucky in not having any serious encounters
with wild animals.

We did encounter them and were able to move
in such ways that they were never a direct

threat to us, but there were a few times when
it became a little scary than you would really


One episode I remember in particular with
a field assistant, being out in a back country

area and coming up a trail on our way home
after spending a long day in the field, being

very tired on our way down the trail on our
way home, and here was a grizzly bear we came

around the corner.

There was a grizzly bear sitting right on
the trail as we were headed there.

And as we came around the corner, this grizzly
bear stood up and looked at us, and that was

not a particularly comforting sight.

Fortunately, the bear decided that we either
weren’t interesting or he didn’t want

to mess with us—we certainly didn’t want
to mess with him—so the bear turned around

and went away.

And after some misgivings we finally did decide
to walk on down that trail.

So those near-encounters are entertaining,
if you will, in retrospect, but we never felt

we were in direct danger.

Ah, you’re lucky.

Yeah, right.

There were other people who did have encounters.

And one of the things that—because of what
we were doing, Dick Blank and I did not use


We did everything we did on foot.

However, some of the other people working
in the park did use helicopters, and there

was one instance where the glacial geology
crew actually were working with a helicopter.

The pilot decided to have his lunch in the
helicopter one day, and so then when everybody

was off in the field, a bear got into the
helicopter and there was one instance where

the glacial geology crew actually were working
with a helicopter.

The pilot decided to have his lunch in the
helicopter one day, and so then when everybody

was off in the field, a bear got into the
helicopter and totally tore it up from the

inside, and in fact destroyed the helicopter

So it had to be flow out with assistance later.

Did your family spend summers in Yellowstone
as well?

During that first five or six summers of intensive
field work, my family came out with me in

the field every summer.

So we would usually drive from Denver, where
I was living at that time, up to Yellowstone,

which was for us typically a day-and-a-half's

You could do it in a day, but we usually chose
to extend a little bit.

So in a day and a half we would get up there.

The interesting part of this was we had two
little boys.

We had twin boys who, the first summer we
went up there in June, were about nine months


So they were still crawling.

We were to have a brand-new government trailer
as our permanent quarters, but it had not

yet arrived, so we were more or less stuck
in what they called 'camper cabins' in the

Old Faithful area.

These were really kind of dinghy, old, concrete-floored
log cabins heated only by a wood stove.

And the floors were so dirty my wife wouldn't
let the boys crawl around on it.

So what she did was build a little playpen
with suitcases around the double bed, and

the boys spent about the first week playing
on this double bed.

And it turned out it rained a lot during that
particular week, too.

So indoors was pretty much the order of the
day everyday for several days in a row.

By the end of that week, my wife was somewhat
near wit's end and when our brand-new house

trailer finally arrived.

So we all of a sudden moved into this trailer,
which, probably if you had started directly

in that trailer would have seemed very cramped
and small, but at that point it was clean

and new and big and it just looked wonderful
to her.

So that was actually almost a slight benefit
of having had to spend that rather miserable

first week in the camper cabin.

How old were your boys?

The boys were nine months old when we first
went to Yellowstone.

They had their first birthday in the park
before we came home in September of that year.

What were your favorite locations in Yellowstone?

I think a couple of my favorite locations
in terms of geology--the park as a whole is

such a wonderful place that I don't have any
particular favorites just in terms of what

areas I liked, but in terms of geology, there's
one particular place that really stands out

is Mt. Everts, which is an area just east
of Mammoth Hot Springs.

And this is the locality I referred to earlier
where Joe Boyd had thought there was a vent

for one of the welded tuffs, but where Dick
Blank and I actually went on our first day

there and found that, in fact, what it is
is a place where the base of the Huckleberry

Ridge tuff, the two-million-year-old tuff,
is well-exposed sitting on gravels in a paleo-valley,

but then this whole thing had been cut through
by a younger canyon of Lava Creek, and the

younger Lava Creek tuff, the 600,000-year
tuff, had then come into the eroded surface

of the Huckleberry Ridge.

So what you can see there is the gravel at
the base of Huckleberry Ridge on an erosional

surface, the Huckleberry Ridge itself, well-exposed
with its vitrophyric base, its glassy base,

and then the erosion cut into it and the paleo-valley
with the soil developed on it, and this younger

ash flow tuff that comes into it, actually
incorporating talus off of the older flow

and having come in and chilled against it
so you, again, get a glassy margin for the

younger flow where it contacted the older,
pre-existing ground surface.

It's just a great place to see these kinds
of relationships, and I've taken many geologists

there over the years.

Unfortunately it's a couple hours' walk to
get in, but it's well worth it.

That's one of them.

The other one is Huckleberry Mountain, which

Huckleberry Ridge and Huckleberry Mountain
are parts of the same topographic feature,

which is just outside the south entrance near
when the Snake River comes out of the park

on the south side.

And, again, it's a fairly steep hike up to
it but it's a spectacular exposure of the

Huckleberry Ridge tuff, the two-million-year-old
tuff, which shows all three members which

were in place at slightly different times
but form a single cooling unit in that area.

And you see the contacts between them, perfectly
exposed in and what amounts to the head wall

of the landslide that has made a clean exposure
of relationships that are ordinarily rather

obscure and hard to see in the trees.