- Our first presenter tonight is David Mickelson,
and he'll be speaking on The Geology
of the National Scenic Ice Age Trail.
David retired in 2005 from the Department
of Geosciences at UW-Madison for 34 years.
He taught, among other things,
Glacial Geology, Geomorphology,
and the Geology of the National Parks.
David and his students have worked on
the Genesis of Glacial Deposits in Argentina,
Tibet, Scandinavia, New England, Utah
Alaska and Wisconsin.
His most recent co-authored book,
"The Geology Of The Ice Age National
Scenic Trail", was published in 2011
by the University of Wisconsin Press.
And I believe we have a few copies
here, if anybody would like to get
a hold of one after the program.
And, David, with that I'll turn it over
to you and I'll try to plug in your mic.
- All right, well thanks very much.
I'm gonna spend the next 40 minutes or so
talking about the ice age national scenic
trail, a trail that runs about 1,200 miles
across Wisconsin, about half of that is
finished off-road, the other part is on
road connectors but the plan is it will
eventually, hopefully while I'm still able
to walk around on the trail, to actually fill
in the gaps and have a complete 1,200-mile trail.
If we climb the tower of Potawatomi today and
look up to the northeast, this is the view you get.
But if you had been here say 14,000 years ago,
this is what you would have seen.
So this is in Glacier Bay, Alaska, and you're
looking at a glacier front that's ending
in water, so ice bergs are breaking off
and that's called a carbon margin.
And I'm going to get my cursor here
so you can see, you see the cursor?
Yeah, okay, and, just to give you an
idea of scale, that's a 21-foot long boat
with three people in it right there.
So the glacier did a lot on the land and
obviously because it's the Ice Age Trail,
it does tend to emphasize glacial features.
But I want to step back in time, we gotta
talk a little bit about Wisconsin's geologic
foundations before we talk about these later events.
And the oldest rocks in Wisconsin are about
2.8 billion years old, that's billion not million.
So that's not the oldest, there are older rocks.
There are rocks that are another billion years
older than that, but they're still very old.
And there was a mountain range built
at that time, and the rocks that were,
that we see now at the ground surface
in northern Wisconsin were deeply
buried under that mountain range.
Miles, tens of miles below.
And so they were heated and under
pressure, and they became metamorphic rocks.
So most of the rocks that we see in the
north-central part of the state, get the
cursor again here, up in this area are
metamorphic rocks that have been
subjected to heat and pressure under
a big, high mountain range like that.
The Ice Age Trail crosses those rocks,
but you can only see them in a few places.
Where the Wisconsin river crosses
the trail, where the trail crosses the
Chippewa river, and a few other places.
But for the most part what you see
up there are glacial deposits, not the bedrock.
The next memorable event in the
geologic history of Wisconsin is
volcanic activity in the central part
of the state near Montello, but it was
fairly localized and roughly the same
time, within millions of years,
there was a tropical sea that came
into what is now Wisconsin from the
South, and in this tropical sea was
deposited sand, sand that was being delivered
from what's now North into this sea by rivers.
And that sand produced what's called
the Baraboo quartzite, which I imagine
many of you have been to Devil's Lake
in the Baraboo area, and you can see
features that indicate that this has to
have been deposited by sandstone.
Ripple marks for instance, this is the
bottom of Devil's Lake today, and you
can see the ripples on the sand under
the water, and then these are the ripple
marks in the Baraboo quartzite.
And quartzite is just quartz and grains
welded together by heat and pressure.
So that's another metamorphic rock.
You probably know that it has a purplish
cast to it, it's very recognizable, very
hard, very resistant rock, and so through
millions of years, the Baraboo quartzite
has been more resistant than the rocks
around, and has tended to stay up as
a relatively high hill or ridge.
Okay, then we have a big gap in time again.
We jump up 0.6 billion years and think
of this geologic record as sort of like
a book with a lot of the pages ripped out.
So we get some information and then
we're missing lots of pages, and then
we get some more information, and
then we're missing lots of pages.
So we're missing that time between
1.7 billion and 1.1, and then there's
evidence of a completely different
geologic environment, instead of
squeezing together and a mountain
range running across northern Wisconsin,
what we have now is the earth's crust
being pulled apart, being pulled apart
like is happening along the mid-Atlantic
ridge that you may have heard of.
It's happening in Iceland, it's happening
in East Africa, places where the crust
is being pulled apart because of this
plate tectonic movement, and at that
time there was volcanic activity.
And that was the time that the Lake
Superior basin actually formed.
The other Great Lakes didn't even
exist then, but the Lake Superior
basin formed early by this process,
by this collapse of the earth as it
was being pulled apart.
We had volcanoes that were sort of
localized, not extensive but there
were volcanoes, and you can see
volcanic rock in places like Copper Falls
State Park, and there's actually this
area of these rocks of this age run
down basically from the west end of
Lake Superior, down under Minnesota,
Iowa, Nebraska, and into Kansas.
Now down there, these rocks are
covered by younger ones, but we
can see them at the surface in
Wisconsin and parts of the UP.
So this is at Copper Falls, associated
with those rocks from the volcano are
also some sandstones, but also we can
see this rock, this volcanic rock called
basalt, right out at the other end,
the other, the terminus of the Ice Age Trail.
So the Ice Age Trail starts here in Potawatomi
State Park, and the western end is out on
the Minnesota line on the St. Croix river,
on these basalts, a very late what's called
Precambrian age, so the purple and the
lavender there are these rocks of relatively
young age, and they're not metamorphic.
They're not buried deeply, these are
volcanic rocks that are basically unchanged
since they were deposited.
Then another period of missing record,
from about a billion years, to about 500 million.
So 500 million years of record missing
and then again we have a record of
tropical seas coming in and depositing
sand, much like happened 1,700 million
years ago, rivers coming from the North.
And in places, you can actually see these two
different sand deposits, one on top of the other.
This is on the East Bluff trail at Devil's Lake,
and the lower rocks are these older,
welded-together Baraboo quartzite
sand grains, and just above it, you can
run your hand right along it, over that
missing billion years, we get sandstone
again, but it's not metamorphic, the
grains haven't been welded together.
You can actually pick up pieces of sandstone
and sort of crush it with your fingers.
So it's very different and yet it was
deposited in a very similar ocean basin
just a billion years different.
So if you get down there, try that.
Walk up and see what it feels like to put your
hand on that missing billion years of rock record.
So this is when we start to see fossils
in Wisconsin, the state fossil is a trilobite,
as many of you probably know.
And oftentimes these show up as
just little pieces, fragments, not as
whole fossils, another fossil that we
get in these deposits are tunnels
that were produced by worms, that
have then been cemented together.
And they are actually cemented
enough so they're more resistant
to weathering than the sandstone around.
So the black arrows there are pointing to a ridge
and that represents the worm tube basically.
And then the low areas between are the
sandstone that hadn't passed through
the gullet of the worm and gotten
cemented together better.
So the geography was very different then.
This is a map that shows the little
green Wisconsin as it moves through
time, from the time of these sandstones
up until present time, and of course
it looks like Wisconsin's floating
as an island out there, it isn't.
I mean it had North America around it,
it wasn't just Wisconsin floating alone.
But notice that 500 million years ago
on the left there Wisconsin was below
the equator it was 15 degrees below
the equator, and it was tilted over.
So what's now North was facing East.
So through that time, that last 500
million years, what's happened is
our whole landmass has rotated
90 degrees, so now what's North
is North, and we've also migrated
from well south of the equator to well
North of the equator, basically 45 North.
So substantial changes in where
we sat on the globe through time.
So those rocks that I'm talking about
are called Cambrian rocks, excuse me,
about 500 million years old, the first
fossils we find in Wisconsin, not the first
in the world, but the first we find in Wisconsin.
And they wrap around those older rocks
so that, let me put the cursor on here again,
so what you see here labeled is Cambrian
the gray, that's mostly sandstone, and they
kind of wrap around those older rocks
that are shown in green, and the ice age
trail crosses those up north of Eau Claire.
And you can actually see places where
there are exposures and relatively
close to the trail find pieces of trilobite.
And there are also places down in the
Central part of the state, as you can see,
Dane County, basically northward up to
say southern edge of Marathon county.
Around that you can see some other colors.
You see there's a blue that's called Ordovician
and there's a purplish color called Silurian.
In those the dominant rock types
at least have changed, so by the time
that we get into the Ordovician Age
so we're talking about 450 million years
ago or something like that, something
happened to the rivers that were coming
off the continent to the north into the sea.
And we had the sand diverted somewhere else.
And so we changed from an environment
in the Cambrian probably somewhat like
the lower Mississippi delta area and out
into the gulf today, in other words getting
lots of river sediment, to a situation
where we are here in Wisconsin, was
missed by rivers coming in, and it was
an environment more like we get in Florida.
So Florida we don't get sand coming off the
continent, there are no big rivers there
and so what do we get instead?
We get basically limestone forming
that basically ground up sea shells,
coral, and so on, so when the sand
isn't being dumped in, then we get those
things that are made of calcium carbonate.
That's a mineral that's deposited in the
or, sorry, yeah, deposited in the ocean
basin, either by microorganisms,
by crushed up shells, or as I've said corals.
A very early one was called a stomatolite.
And you can see that these characteristics
are these concentric rings here and they're
produced by algae that secrete calcium carbonate.
And this is what they look like today
on the coast of Australia, these are heads
of algae that get covered in high tide
and the algae are out and photosynthesizing
and so on, and depositing calcium carbonate.
And then in low tide, basically they're
relatively dormant, and then the tide
comes up and they're active again.
By about 450 million years ago,
we're starting to get lots of shells
being preserved as fossils and those
rocks are called Ordovician, and those
are the ones in light blue there.
And actually the fossils don't
change much when we come to the
rocks that are here under Door County,
the ones of Silurian Age, the fossils
change somewhat, here's a wall here
in Sturgeon Bay that shows curled
mollusk here and you can see actually
quite a few fossils, some here.
Almost any building you walk around here,
weathered wall, you can see those fossils.
So over here in this part of the state,
near Sturgeon Bay, Door County,
the rocks aren't actually flat,
they're actually tilted somewhat.
This is very exaggerated, they're
only tilted a few degrees but they
tilt downward toward the East
under Lake Michigan.
And so these Silurian rocks, the
rock name is dolomite, it's just
calcium carbonate and magnesium
carbonate, is pretty resistant to erosion.
And so it's this fairly hard dolomite
that produces a feature called the
Niagara escarpment.
And the Niagara escarpment is really
produces the backbone of Door County
that you can see here, and it goes
across the UP, and then comes back
down, and ultimately it's what holds
up Niagara Falls, and that's where
the name originally came from.
So it's a fairly resistant rock called
dolomite because it's magnesium
and calcium carbonate, and it was
deposited about 400 to 450 million years ago.
Then we got lots of record missing.
We jump from there to 30,000 years ago, okay?
So we're talking 400 million to 30,000.
Huge amount of time missing and that
brings us to ice age Wisconsin.
And the glacier probably came in
here about 30,000 years ago.
It probably came in through a very
barren environment, tundra, permafrost
environment like we get on north coast
shore of Alaska today and is probably
not trees, but probably just tundra vegetation.
And we were at the very southern edge
of this huge ice sheet, you actually could
have stepped onto the glacier here in
Wisconsin and gone on ice all the way
over into central and southern Russia,
all the way to the Pacific Ocean, all the
way to the Atlantic Ocean.
Now over the Arctic Ocean, it would
have been sea ice, it wouldn't have
been glacier, but still you would've
been on ice all the way over to southern Russia.
So a dramatic mass of ice that was here.
And the next slide I'm going to show you
is a little movie that shows the advance
and retreat of that glacier.
So it starts about 30,000 years ago
and that's our best guess, we don't
have a lot of information on where
it was at various times, but this has
been put together by people at the
Geological Survey and myself to try
to at least give an impression of
what it was like.
And what I want you to see is for
the first 10 or 12,000 years, the
glacier's out pretty much where
the earth's maximum, but now about
18,000 years it starts to retreat and advance,
and retreat and advance.
So you get pretty dramatic changes
and that's pretty when a lot of the
action of the glacier was taking place.
A time when the glacier was moving
back and forth, and really producing
lots of the land forms that we see.
So let's take a minute to talk about
how glaciers operate.
You need more snow falling in the
Winter than melts in the Summer in order
to have a glacier, when that happens,
snow falls and accumulates, year
after year, the snow, because of the
weight of the overlying snow, turns
into ice and when ice gets to a thickness
of about 100 feet, it's behavior changes.
Now if you drop an ice cube on the floor,
it's brittle, it breaks, but glacier ice under
a thickness of about 100 feet, starts to
actually flow, flow like cold molasses,
or room temperature tar.
It's no longer brittle so the curvaceous,
the cracks that you see on top of modern
glaciers, those are the brittle result of the
plastic movement that's taking place underneath.
And so as the glacier then moves, it can
pick up debris, it slides along, picks up
debris and carries it out to the outer
edge of the glacier where melting is
taking place, and that material gets dropped.
So that's a fundamental thing is it's
sort of running like a conveyor belt,
picking up debris, carrying it out
to the edge and dropping it.
If we look at the glacier itself,
many times it's very clean ice.
This is in Alaska, and you're looking
through clear ice, and you're looking
at debris down on the bottom of the glacier.
That material is frozen in through the bottom.
So the bottom of the glacier is actually
sort of like coarse sand paper, sliding along
the ground, plowing up material, grinding
away on rocks, and so that's what's doing
the work that the glacier does.
The other thing that happens is we have
lots of water flowing away from the glacier.
And so that material has been carried
by the glacier, gets picked up by steams,
and moved away from the glacier.
And the effect of the water transport
is to separate sizes, the glacier tended
to mix things together, water separates them out.
So you're looking at a stream here where
the glacier would be off to the left, and here
it's sort of one-inch gravel that's being
deposited and the sand, and the finer
grain material is going way downstream
out in the distance, and the coarser
rocks have been dropped somewhere
closer to the glacier, where the stream
was starting.
So those streams tend to separate things
out and that's what produces sand and gravel.
So the sand and gravel that we mine
for aggregate, for asphalt, or for concrete,
that's not deposited directly by the glacier,
that's deposited by streams flowing away from
the glacier or maybe in a tunnel in the glacier.
And because streams have sorted those
materials out by size, and that makes it
a desirable material for aggregate.
The other thing that is particular
about glacial deposits is that
glaciers pick things up and can
carry them long distances.
So for instance this is a wall outside
of Picnic Point, on the UW campus in
Madison, and look at the mixture of
rock types, those aren't rocks that
occur naturally around Madison.
And they haven't been taken here by a truck.
Those were carried down by a glacier
and they're called erratics, when they've
been carried a long distance from one
place to another and dropped.
And so any glaciated area has erratics
of some kind, and what determines an
erratic is simply is it the local bedrock
or is it something that's been carried
from a distance?
And so the erratics that we have
in this part of Wisconsin, basically
down the Green Bay lowland,
probably came from this zone that
you see marked up here in the east
end of Lake Superior.
So it was coming from somewhere
up in this area and then coming
down this way, it was not coming,
for instance, from over in western
Lake Superior this way, at least
during the last glaciation.
So with that little introduction
to geology, let's talk a little more
about the Ice Age Trail and the
kinds of things that you can see on it.
The Ice Age Trail is in red here.
And the Ice Age Trail was established
by legislation, congressional legislation,
back in around 1960 and also established
at that time were nine Ice Age Scientific
Reserve sites, and you can see those
numbered sites on the map as well.
I'll mention a couple of them as we
go along here, in particular I'll mention
number nine, which is the one closest
to where we are now.
But as you can see, the Ice Age
Trail starts up in here in Sturgeon Bay,
runs southward down a feature called
the Kettle Moraine that I'm going to
describe in a minute, and then for the
most part, across the rest of the state,
it follows a feature called the moraine.
And a moraine is a ridge that forms
out at the edge of a glacier, so this
conveyor belt of a glacier is bringing
material out to the edge, the ice melts
and the material gets dropped.
If the edge of the glacier stays
in one place for a while, then that
material can pile up and produce a ridge.
So what you're looking at in this photo
from Greenland is a ridge on the left
side which is labeled N Moraine,
most oftentimes we just call them moraines.
It's a ridge that was deposited when the
edge of the glacier was sitting out there
for a while, now the glacier is retreating
and what you're seeing on the edge
of the glacier is debris-covered ice.
So that's covered with rock and soil
material that's melting out.
If the glacier edge doesn't sit
in one place for a while, if it
keeps retreating, then it will just
deposit a more or less equal thickness
of material and you don't get a moraine.
So you only get a moraine when the
edge of the glacier stays in one place
for a while, that's what the moraine indicates.
Oftentimes moraines end up having
lots of buried glacier ice underneath.
So you're looking at, again, the edge
of a glacier in Alaska and the edge
of the glacier is really out here.
All of this area in here is covered
by sand, and gravel, and rocks, and so on.
It looks like this if you're down on it.
You see a couple of people in there for scale.
And yet there's glacier ice under all of this.
And this material is slowly melting out.
The ice is melting out and the material
on that ice is collapsing.
It's sliding off the slopes into this pond
and then, if there's ice under the pond,
eventually that will become a high spot
and material will slide off that.
So this material is slipping and sliding
around on the surface, and then oftentimes
when that happens, the last pieces of ice
end up melting out and producing a depression.
So here you could see diagrammatically
a couple of pieces of ice that are partly
buried by debris, the debris melts out
and it produces these features called kettles.
And we have thousands of kettles in Wisconsin.
If it weren't for kettles, we wouldn't
have a tourist industry, well Door County
would still have a tourist industry but
up there in Vilas County and so on,
all of those lakes are kettles and of
course when you think about it,
southwest Wisconsin, which was not
glaciated, it's called a driftless area,
doesn't have lakes, right?
I mean it doesn't have lakes like
Vilas County or Oneida County or so on.
So these kettles are very important.
And in moraines, they're abundant and deep.
So this is in Langlade County north of the Antigo.
The Ice Age Trail runs right through this
landscape, you can't actually see the trail.
But lots of ups and downs, ups and downs
through these kettles, some of them
water filled, some are dry, but thousands
of them across that part of the landscape.
So across the northern tier of counties here
that's characterized by having very deep
kettles and so on, and let's see if I can go
back to that, and then down here, as
we get down closer to Baraboo and down
toward Madison, there are kettles
but they're not as deep.
They're much deeper in northern Wisconsin.
The other part of the Ice Age Trail,
other than the one following the moraine,
is one that runs from down in Walworth County,
down here, and follows what's called the Kettle
Moraine, up to Door County, or almost to Door County.
And this is actually the boundary between
two lobes of ice, so ice, as it was flowing
plastically, was controlled by the landscape it
was going over, and it came down the Lake
Superior basin and then down into the
Lake Michigan basin, and then down the
Green Bay a little bit, Green Bay Basin.
And we call those lobes, so the Green Bay
lobe came down to Madison, for instance,
the Lake Michigan lobe went way down into
Illinois, down past Champaign Urbana.
So the lobes were being created by
the underlying shape of the land.
But where the lobes came together,
they produced this feature called
the Kettle Moraine and the Ice Age
Trail is very well established there
because lots of that was public land
to start with, state forest and so on.
This is just a piece of it to get you oriented.
Up in the far left corner of the map
is the corner of Lake Winnebago.
So we're looking at-- well let's see--
Campbellsport is down here at
the southern-- Yeah, so Campbellsport
is right here, Highway 67-- there's an
Ice Age Visitor Center here on Highway 67.
And the feature that you're seeing
running through that map is the Kettle Moraine.
It's characterized by having sort of
two ridges in many places, much like
this moraine forming between two lobes
of ice in Greenland.
So you can see there's a lobe of ice
here and another one over here.
And where it comes together, there's debris
and that's because the two lobes
are bumping into each other and the
debris they're carrying is being
brought up to the glacier surface.
And so we ended up with lots of
debris up on top of the ice surface
and as that debris melted out between
these two lobes, I mean the ice melted
out between the two lobes, we ended
up with a kettle moraine, that in many
places, especially in the North here,
ends up being sort of a double ridge
with a low area in between that has
glacial features that are fairly spectacular
and for which the Kettle Moraine is well known.
One of these features is called
a moulin kame.
A moulin is a circular, cylindrical
hole in the ice that collects water,
and mud, and debris that falls down it.
This is a moulin here in Iceland
and here I'm hoping the students
aren't going to fall down it.
They're seeing who can get closest
without slipping in, that kind of thing.
But at the bottom of the moulin
this is what debris that comes down,
it's water, and mud, and rocks are
tumbling down and forming a
cone-shaped hill at the bottom.
And these are common around modern
glaciers as small features, this one
that you're looking at is only 10-feet
high or so, but in the Kettle Moraine,
these are world-class features.
Look at the size of this one, for instance.
Look at the barn for scale, that whole
hill is a moulin kame.
So, presumably the ice was considerably higher than
that, lots of gravel on top, dropping
down, holes in the ice to produce these.
And this is a map, a topographic map
of that area east of Campbellsport.
And let's see, yeah, so these dark places
here are places where these moulin kames are.
And these are world class, they don't
know of any place in the world where
there are more, bigger moulin kames
than you see here, especially in the Northern
Kettle Moraine, but not just in the Northern
Kettle Moraine, they're certainly in the
southern part of the Kettle Moraine as well.
Lapham Peak for instance just south of I-94
some of you may have been to is a moulin kame.
All that water that goes down through the ice
has to go somewhere, and it goes through tunnels
like this under the ice, and oftentimes the
tunnel just remains empty, but sometimes
the tunnel fills up or partly fills up with sand
and gravel deposited by the stream.
And when that happens, we're left
behind with a ridge that's called an esker.
Eskers are fairly common, especially in the
Kettle Moraine and in parts of northern Wisconsin.
This is one outside of Campbellsport.
There's some pretty nice ones in, oh gosh,
western Ozaukee County, and over into the
Kettle Moraine west of there.
southern Kettle Moraine has some and they're
across northern Wisconsin, as well.
Here's another one, a smaller one.
So these are ridges of sand and gravel deposited
by a stream flowing underneath the glacier.
The Ice Age Trail follows the tops of these
eskers in a number of places, this is in
northern Wisconsin, northwest Wisconsin
actually, up in well north of Chippewa Falls.
A spectacular esker to hike on.
Whitewater Lake has eskers running through it.
This is an air-view of Whitewater Lake,
and all of those winding ridges are eskers.
So those again are tunnel fillings, gravel tunnel fillings.
Yep.
I want to point out another thing that
happens underneath the ice.
There's these tunnel fillings,
but also back under the glacier,
the glacier is sliding along and
streamlining the landscape.
Okay, it's streamlining whatever
is there, and it produces elongate hills
like this, that hill is called a drumlin.
And drumlins are not stream deposited
in the tunnel, that's an esker.
Drumlins are basically hills that are
shaped by the ice, sometimes they
contain some sand and gravel, but
oftentimes they're actually glacial deposits.
A material called till, but they're
elongate hills, they're shaped by
the flowing of the ice itself.
So they show which way the ice was flowing.
Sometimes they're long and narrow
like this, sometimes they're more equidimensional.
This one is in Jefferson County, east of Madison.
All right, y'all still with me here?
(laughter)
All right, so next time you have a
chance to drive for instance from
here toward Madison, when you hit
Beaver Dam, you start to see these
long drumlins, and you'll see drumlins
going parallel to highway 151 all the
way down to Madison, they're dramatic.
Or if you drive from Madison, east to
Milwaukee, you're crossing across them.
So there are many in that part of the state.
There are also drumlins in northern Wisconsin, as well.
Well, the thing I want to finish up with is glacial lakes.
Lakes were very abundant when the glacier
was here, including a large one that sat in
the area to the west of where Lake Winnebago
is now, called Glacial Lake.
I don't know why that always goes that way.
And that was called Glacial Lake Oshkosh.
Was extensive, and if any of you have
driven down around Appleton in that
area, flat ground down there and that
flat ground is lakebed from this Glacial
Lake Oshkosh.
So take 41 down through around Appleton
and down to Oshkosh and Fond du Lac,
you're on that flat lake plain all the way along there.
Lake Michigan itself was at a higher level
at times in the past, so for instance the
lowland where say Two Rivers is was all
under water at that time, and parts of
the area here around Door County
were under water at that time.
Other times Lake Michigan was much
lower than it is today, and we had
rivers flowing from Lake Oshkosh
across the Door Peninsula and down
into Lake Michigan.
And one of those, there were a number
of them to the south here in the Kewaunee River
and so on, but one of them was Sturgeon Bay
which we are sitting along, very close to tonight.
The final event in northeast Wisconsin
was the glacial advance that deposited
a reddish clay deposit like this and it's
well exhibited along the shoreline of
Lake Michigan, right on Manitowoc
County, Kewaunee County line.
And this is a 14,000-year-old buried forest.
It's wood, mostly Spruce, that was growing
and then buried by a late glacial advance
that came into this area, went as far south
as the north end of Lake Winnebago and down
to Two Rivers, and crashed into a Spruce forest.
So by that time climate had warmed,
we had forests growing here, and the
glacier re-advanced into that, and you
can find pieces of wood, the wood is as I
said about 14,000 years old in calendar years.
And it's not petrified, you could put it in the
fireplace and burn it just like any other wood.
Here's a rooted stump, and I've seen
logs, and I think Al Schneider, who was
a glacial geologist up there in the Au Dans
has seen them too, up to like 20, 30-foot long
logs of this Spruce logs splayed out by the
glacier as it came into that forest.
So as the glacier retreated the final time,
lake level here was higher than it is now.
And the land was depressed, and so lots
of Door County was actually under water.
And there are shorelines that run around
the high parts of Door County that indicate
those times of higher lake level.
And if you go up to Potawatomi
State Park, which is just outside of
town here, this is, you see the tower
is labeled in that slide, and there are
raised shorelines, if you follow the
Ice Age Trail from its terminus at
the tower, heading south as if
you were gonna march all the way
to Minnesota, one of the first
geologic features you'll see are
these high shorelines that have
been cut into the dolomite bedrock.
And in this view from air-born what's
called lidar, you can see not only
drumlins here on the west side of
Sturgeon Bay, see those are indicating
the ice flow direction, but also very
nice shorelines here and of course
Sturgeon Bay is this feature right here.
So with that the climate was warming.
We quickly went into a forest vegetation.
We had Paleo-Indians here by that time.
There were almost certainly Paleo-Indians
hunting mammoth and mastodon all along the
ice margin here when it was here that last time.
So I will leave you with that thought and
we've got a couple minutes for your questions.