cc >> Today we are pleased to
introduce and host University of Wisconsin emeritus professor and
senior scientist David Mickelson and his presentation of
"A Tale of Four Lakes." This is a part of our museum,
the Wisconsin Historical Museum's "History Sandwiched In"
lecture series. So, we want to just say
thank you to Mr. Mickelson, and we would like to say that the
opinions expressed today are those of the presenter, and are
not necessarily those of the Wisconsin Historical Society
or the museum's employees. Please join me in welcoming
Mr. Mickelson to the stage. [APPLAUSE]
>> Thank you. Well, I guess you were scheduled
to have a talk on Devil's Lake and the CCC.
I'm a geologist who was called last Friday and asked if I could
give a presentation, but you won't hear about the CCC.
You will hear about more than one lake.
In fact, on Friday it said three lakes, then over the weekend
when I got this together I upped it to four.
So you get four lakes for the price of one.
Glacial Lake Wisconsin is where we'll start, and that's in the
central part of the state and it's a lake that no longer
exists. It existed when the glacier
covered eastern Wisconsin, and all the drainage from the north
was dammed down here near the city of Portage at the east end
of Baraboo Hills. So we'll come back to that, but
let's look at this map. Devil's Lake, we will talk
about, up here, and obviously Glacial Lake Wisconsin is off
the map to the north. Indian Lake, which probably many
of you have been by, is here. It's a county park now.
And, of course, Lake Mendota. So I'll try to talk about each
of those and how they formed because they formed differently.
And they really represent kind of the range of different kinds
of lakes that we have in the state from a geological
perspective. It all goes back to glaciation
because without glaciation we probably wouldn't have any
lakes. In the unglaciated driftless
area of southwest Wisconsin, there are no natural lakes.
That's because over millions of years the stream drainage has
integrated and taken away any lakes that might have been
there. So except for the occasional
Beaver Dam Lake or that kind of pond or that kind of thing,
there were no natural lakes. Any lakes out there now are
dammed by humans. But in the glaciated area, we
have thousands of lakes. We wouldn't have a tourist
industry without those thousands of lakes.
So it's really important, I think, to recognize the
importance of glaciation. And the maximum glaciation was
about 30,000 years ago. And at that time, Wisconsin was
at the very southern edge of this huge ice sheet.
Now, you could have stepped onto the ice sheet here in Wisconsin
and gone all the way to Russia, all the way to the Pacific
Ocean, all the way to the Atlantic Ocean without ever
stepping off of the glacier. So it was a huge mass of ice up
to a couple miles thick up here on the west side of Hudson Bay.
So, what we see down here is just very marginal effects out
at the edge of that ice sheet. Now, you're going to see a
series of changing ice extents here.
And Glacial Lake Wisconsin is right in here, and you'll notice
that it was there, then it kind of disappears, then the ice
comes back and Glacial Lake Wisconsin forms again.
But every time the ice pulls away from the east end of the
Baraboo Hills, Glacial Lake Wisconsin disappears.
So that was, basically, an extinct lake plain, very
different than the other four lakes that I'm going to talk
about. And as you can see, there's a
complicated story of what was happening in the Lake Michigan
basin and Lake Superior basin as well, but I'm not going to take
time to go into that.
There were many lakes at the time the glacier was here that
are called ice-dammed lakes. In other words, they're there
just because the glacier is damming up drainage.
Maybe melt water coming off the glacier or rainfall or melted
snow, whatever it is, wherever the land slopes back down toward
the glacier, water gets ponded there.
And so you can see in this case, in Greenland, here's the edge of
the ice sheet, and the slope of the land is back under the ice
sheet. So we have a lake formed here
and another one here. And these will obviously,
eventually they drain somewhere. As the glacier advances and
covers lower outlets which might be here along the edge of the
glacier, water level rises and eventually it will come out over
the lowest outlet here. And so that's what would have
happened with Glacial Lake Wisconsin.
When the glacier was advancing, it dammed up what we now call
the Wisconsin River, drainage at Portage, right at Cascade
Mountain there, right where the Baraboo Hills end.
Water level rose and finally went out of the basin up here
and down the east fork of what we call the Black River now.
So, that was the outlet for the lake.
It covered a large area. The water down here was probably
about 140-150 feet deep at its maximum.
And there was also smaller lakes in these valleys like the
Baraboo River Valley and so on that were connected to this.
So this was a huge lake going almost up to Stevens Point down
to the Dells and extending westward out to where I-94 and
I-90 separate out here at Tomah. So it covered a big area.
And it was there probably several times as the ice
advanced and retreated. This is probably what it would
have looked like at the time. Lots of icebergs in the lakes in
places where the water was shallow.
Then the glacier came down and probably had a 30-40-foot high
vertical cliff. In other places where the water
was deep, down there where the Dells are now, the ice face
would have been much higher. This is a slide from Glacier Bay
National Park in Alaska, and to give you an idea of scale, that
thing down there is a 21-foot boat with three people in it.
So it would have been spectacular to be out there in a
boat on Glacial Lake Wisconsin when it was there.
Sticking up out of that lake were a variety of islands.
I expect if you've traveled around Wisconsin very much,
you've seen some of these kind of sandstone, sometimes people
call them -- rocks, or stacks, or maces, or that kind of thing
or just bluffs. This is a picture from Mill
Bluff, which is a state park up along the interstate by Mauston,
and this is the view from the top, and you're looking at a
series of these. These are quite spectacular
because they're surrounded by a flat plane and that flat plane
is the former lake bed of Glacial Lake Wisconsin.
So, if you drive up the interstate toward Tomah, just
notice how much of that is just flat surface, some cranberry
bogs on that surface and so on, that's the bed of Glacial Lake
Wisconsin. And likewise, if you go north up
Highway 39 or what was Highway 51, when you get to Coloma, you
come off on to a flat surface and it's flat almost up to
Stevens Point. And, again, you're on that
surface of Glacial Lake Wisconsin.
It's where there's extensive agriculture on those big flat
fields. That's all lake bed from the
Glacial Lake Wisconsin. And there are a number of these
rock outcrops that stuck up as islands up here near Coloma and
some here west of Highway 39. I'm sorry, this is 39.
This is Highway 13. And so there are some of these
that are right out here just west of Coloma, just outside the
ice margin in what was the lake basin.
Now, as the ice kept delivering sand and clay and sediment into
that lake, eventually this part of the lake filled.
So at the end of the story this was actually a plain of sand
that river was flowing away from the glacier out into the lake,
which the edge of the lake had now moved out to the west here.
And so this would have been what we call an outwash plain.
Outwash being simply streams that are flowing out away from
the glacier carrying sand, and of course, the finer grain
sediments, the clay and so on, was getting out to this part,
and some of it went on down into the Mississippi Valley.
So this would be a picture of sort of right at the end when
this part of the lake basin had filled with sand.
And the final disappearance of the lake would have been really
spectacular. As the glacier fluctuated here,
it was still holding in the water here as long as the edge
of the ice was up against the Baraboo Hills here, right near
Portage, Cascade Mountain ski area would be right there, the
interstate goes right by there, that's where the dam was.
But once the water could escape around this hill slope here
between the glacier and the hill, then it would have been
like pulling the plug in the bathtub because all that was
there was ice that would have fractures in it, crevices and so
on, easily broken up. Once that water started to rush
around there, whoop, it would have gone all at once.
And there was a graduate student in geography a few years ago who
did some calculations of what the discharge of water in that
flood would have been. It probably only lasted a few
days for that whole lake to drain.
Remember, it was 140-150 feet deep over the Dells.
So this was a deep lake basin that went very suddenly.
And then it was gone. And it was that flood down the
Wisconsin River Valley that carried boulders.
It carried three-foot diameter boulders down to, well, let's
see. I guess I can't say what size
where, but it carried boulders three feet in diameter a good
way down toward the Mississippi. So, there was a dramatic flood.
And, of course, it would have been carrying ice, chunks of ice
and so on, along with it as well.
He calculated that at the peak when that flood really broke
out, the volume of water coming down was 150,000 cubic meters.
Now, cubic meter, yard by yard by yard, 150,000 per second.
So that's a dramatic amount. He calculated that it would take
about 10 seconds to fill your average NBA arena with that
discharge. So it was a huge flood that came
down there. And it was that flood, that
water that's rushing toward the outlet, that cut the Dells.
I guess most of you know that the Dells are actually referring
to these steep-sided canyons and so on that are cut into the
sandstone, not the amusement park that's all around which
most people think of as the Dells.
[LAUGHTER] But this is the Dells.
This is Witches Gulch actually, which some of you may have been
to. This is up near Witches Gulch.
But all of these channels that were these deep channels that
were cut, Witches Gulch, the one I just showed you, is this one
up here, but there's lots of them all the way up here.
That was probably produced by this rush of water as the lake
drained. So it would have been
spectacular at that time. Let's come a little bit south
and talk about Devil's Lake, which has quite a different
history. If we look at the location of
Devil's Lake, now, just to keep everybody oriented here, so
Glacial Lake Wisconsin was up here and the drainage was down
through the Dells and down around the end of the Baraboo
Hills. So that's where that big flood
was initially dammed, and then where it broke through, it
actually broke through into another big lake that was down
here called Glacial Lake Merrimac.
So there was another lake. And that, when the flood came
into it, then it broke its dam and the whole business drained
down the Wisconsin River. Devil's Lake sits in a valley
that's cut through very hard rock called Baraboo quartzite.
And that's quite different, as you'll see, from the rocks
around it. Baraboo quartzite has sort of a
purplish cast. I expect probably all of you
have been to Devil's Lake and seen these blocks of what we
call talus, blocks that have tumbled off the cliffs above.
Probably many of those blocks came down during the glacial
maximum when we had lots of freezing and thawing, and it's
really water going down in cracks and freezing and
expanding that breaks up blocks like that.
Probably an occasional rock has fallen since, but most of that
came down during the peak of glaciation.
The Baraboo quartzite was deposited about
1.6 billion years ago. All right?
Billion, not million. Billion.
In a shallow sea and it was sand that was being brought into the
sea from a river, washed, very clean sand, and then more
sediment was deposited on top and more sediment was deposited
on top so that eventually those sand grains were heated up
enough so that they became what we call quartzite which is just
quartz sand welded together. We can find some features like
what are called ripple marks in the quartzite.
These are from Devil's Lake State Park.
And this is in Devil's Lake itself.
And you can see these little ripples on the bottom on the
sand on the bottom. Those are being caused by the
waves that are coming in, moving sand grains into these little
ridges. And so that's what was one of
the big indicators that said this was the environment it was
formed in. Shallow water and good
preservation of the ripple marks demonstrates it.
Well, these quartzites are not just contained in the Baraboo
area but are here and there around the Midwest.
So, we get very similar ages to the quartzite in a big area of
what's called Sioux quartzite out here in South Dakota.
Barron quartzite forms the blue hills up north of Chippewa Falls
and Eau Claire. That's of similar age.
There's some up in, gosh, what is it?
I guess it's Langlade County, or maybe eastern Langlade County.
A little bit over here by Waterloo.
And then the Baraboo quartzite here.
They're all about, this slide is a little bit old, I think it's
thought to be more like 1600 million years, or 1.6 billion
years. So there are those same rocks.
It's a different quartzite than Rib Mountain.
Rib Mountain is an older, it's a similar rock type but it's much
older than the Baraboo quartzite or any of the rest of these.
So the quartzite forms sort of a canoe shape.
You see this would be the gunnels of the canoe, and this
would be the middle of it. And it stands up around the
surrounding rocks because not only is it older, but quartzite
is much more resistant to erosion.
So through the millions of years as erosion has taken place, this
resistant quartzite has just stood up higher as the softer
rocks around it have been eroded away.
Now, the peculiar thing is that notice the shape of this canoe
shape isn't really symmetrical. That is, the north side of the
feature here, and I should say the feature is called a
syncline, the rocks underneath are folded down as you'll see in
just a minute, but notice how this edge of the Baraboo
quartzite is quite narrow compared to the south part of
it. And Devil's Lake is right here
in the southern part. And the reason for that is this
syncline is not symmetrical but it's tilted over like this.
So the north side of the syncline, those rocks, the rock
layers have been folded up so that they're standing
vertically. In other words, when you see the
ripple marks on the rock, they're on vertical faces.
So what was horizontal on the sea bottom has now been tipped
up vertically. And I'm going to show you we can
see that in just a minute. On the south side, these rocks,
the tilt on the rocks is much lower so that here is where
they're up at the surface, and here they're tipping at about
30 degrees. So if you take Highway 12, let's
say, and go by the Badger Ammunition plant and start up
into the Baraboo Hills there, you'll notice that the layering
in the rocks is dipping northward at about 30 degrees.
Whereas, if you go up to the north side here, you'll see that
they're just standing, the layers are standing vertically.
This is a photo from the air showing the Baraboo River, here,
and Highway 136. Reedsburg would be up in this
direction, and there's a feature called Van Hise Rock.
This is a quarry in the Baraboo quartzite.
So these are all on the north side of this Baraboo syncline.
And right down here is an old quarry that I believe is state
land now where you can actually park right outside of it and
walk in, and you can see these ripple marks on the vertical
face of the Baraboo quartzite here.
So, that is right here. Highway 136 comes out of
Baraboo, and this is Rock Springs.
In this picture you were just looking at was viewing this
quarry, Van Hise Rock right here, and the quarry where you
can actually walk in and look at those ripple marks is right here
on the west side of Highway 136, just outside of Rock Springs.
Well, to understand the story of why Devil's Lake is there, we
have to go back to look at the shape of the land before
glaciation. This is what's called a
thickness of unconsolidated material map.
Unconsolidated material is just all the sand and glacial till
and all that stuff that sits on top of solid rock.
Okay? And where it's thickest here,
it's dark reddish brown. And so I think you can see that
there is an area here where it's really thick.
That's an old river valley. Long before glaciation that was
a river valley. And it appears if we blow up
this end down here, that it's this valley that comes down here
and comes into Sauk County. And it's kind of split here, but
Devil's Lake sits in one arm of it.
Right there is where Devil's Lake is.
So it's sitting in this old river valley that has nothing to
do with glaciation. This is much older than
glaciation. This is much older than the
sandstone and the limestone that we have around here as well.
So it's a very old river valley, and it's been filled and then
excavated, filled and then excavated several times.
We can find evidence up fairly high on the bluff, this is on
the Potholes Trail on the East Bluff, potholes.
These are the kinds of things you find in the bottom of a
stream. They've been cut by flowing
water. And yet now they're perched way
up high on the East Bluff. How did they get there?
Well, they got there when there was a stream flowing up there,
when this valley was just starting to be cut.
Probably at least a billion years ago.
Way before glaciation. So, up on the East Bluff trail
here in Devil's Lake you can actually see two distinctly
different ages of rock. The Baraboo quartzite, which
I've been talking about, 1600-1700 million years old.
That's that very hard resistant rock.
That's what's forming the bluffs.
But in places on top, you can see a much younger sandstone
called Cambrian sandstone, and that sandstone is about
500 million years old. So it's substantially younger.
And that sandstone actually can be found in the valley bottom
occupied by Devil's Lake. When they drilled a well at the
north end of Devil's Lake, right by the concession stand and that
kind of thing, there's a well there, that actually goes
through the glacial deposits, but then it goes through some of
this 500-million-year-old sandstone before it hits the
quartzite. So what that means is that
valley was there more than 550 million years ago when the
sandstone was deposited. So we had a landscape where
these river valleys were cut, then the sea came in
500-plus million years ago, and that landscape was just
blanketed with sand that became sandstone and mud that became
limestone or dolomite. Those are the rocks that we find
around the Baraboo Hills. A picture I just showed you,
though, is right up here. Just a little patch of this
younger sandstone on top of the quartzite.
You can also see it in Parfrey's Glen.
If you've walked up Parfrey's Glen, there's boulders of
quartzite in the sandstone, and Parfrey's Glen is located right
over here just off the map. So, the Devil's Lake Valley,
then, has been there for a long time.
It was there, it was filled with sandstone 540 million years ago.
That was eroded out, but the quartzite still stood because it
was more resistant. And then in glacial time, so
there was probably a river running through this valley at
the time glaciers first advanced down into southern Wisconsin.
And when the glacier came into the valley, it didn't completely
fill it. It just came in like this one in
Greenland here, and it didn't cover the bluffs.
The bluffs at Devil's Lake are not glaciated, had never been
glaciated. It just poked into the valley,
lake formed out in front that was being dammed by the ice, but
unlike Glacial Lake Wisconsin case, the glacier built moraine
here that dammed the lake. So when the glacier disappeared,
we have a ridge that's called a moraine that's been formed as
the glacier has carried debris out toward the edge, and that
debris piles up to make a ridge. That's a moraine.
And it's a moraine that holds Devil's Lake in now.
So that's why the lake is there. It's there because it's a
moraine-dammed lake. And, of course, it's unusual
because it really has very, very short tributaries just entirely
enclosed within that Baraboo quartzite.
So, here's the map of Wisconsin showing just generalized ages of
things. Northern Wisconsin is all very
old rocks, and then you see here's the Baraboo Hills down
here, also old rocks called Precambrian Rocks.
And surrounded by that are the Cambrian rocks that are mostly
sandstone and then Ordovician rocks that are a mix of
sandstone and limestone. So they're younger.
So we're talking, again, 1.6 billion years for the
dolomite, and then for the rocks around it and the ones down here
under Lake Mendota, 540 to 450 or something like that,
million years. So, these rock types were formed
also in an ocean but, again, substantially later in a time
when we actually had fossils that are being preserved.
So, the dolomite in southern Wisconsin, some of it is
dominated by these fossils called stromatolites.
Those are fossil algae, heads of algae.
They still grow today in Australia This is on the west
side of Australia. And these are calcium carbonate,
the same as what makes up limestone, but these are living
algae heads, and they've got little algae that are living
here and secreting that limestone.
So those are the stromatolites, and they're a very common fossil
in southern Wisconsin. We also have sandstone that was
deposited in southern Wisconsin, and a dominate fossil in there
was trilobite, which is the state fossil.
And there are also some of the dolomite, especially down in far
southwest Wisconsin. This is down by Prairie du Chien
has pretty good fossils of shells that look like mollusks
and so on but they're actually called brachiopods.
But anyway, there's a fossil record.
They're much softer, more easily eroded than the Baraboo
quartzite. Well, if we come down to drop
southward now to Indian Lake, it's here on Highway 19.
This is Highway 12. And this is a completely
different origin. Indian Lake is called a kettle
or a kettle hole. So, these are bedrock hills for
the most part. This was glaciated, but this
represents, this sediment here is outwash.
So that's sand or sand and gravel that's carried by a
stream flowing out away from the glacier.
And this happens to have been a block of ice, glacier ice, that
got kind of left behind and then buried by the outwash.
And then later, probably over a period of several thousand
years, that big block of ice melted out creating a lake.
So, just diagrammatically, just because there are cracks in the
ice and some parts of the ice melt faster than others, maybe
because there's a stream flowing here that cuts its way through
it, for whatever reason, parts of the glacier ice get left
behind covered by sand and gravel, and then when the ice
melts out, it leaves behind a depression.
This is one in Iceland where you can see the piece of glacier ice
is still there surrounded by gravel, but it's just melting
away. And it might take thousands of
years for that to have happened in Wisconsin.
When I was in Alaska quite a few years ago, three of us went
swimming in one of these lakes not knowing that there was a
block of ice about this size just under the bottom.
[LAUGHTER] And once we got in the water,
this thing came and it scared the hell out of me.
[LAUGHTER] I thought we were going to be
swallowed up. So, we have a lot of dry
kettles. We have lots of kettles that are
lake-filled. I would say, I've never counted,
but 99% of the lakes in Wisconsin are kettles, I would
think. Almost every one.
Not all obviously. I've shown you a couple that
aren't, but almost all of them are.
Sometimes they develop peat bogs around them.
Sometimes they just have sandy edges and so on.
Many of the ones up north that people go to in the summertime
are kettles, almost all. This is the view up in Langlade
County north of Antigo looking down at all of these kettles
that are formed. So each of those represent a
melted ice block that was in there.
So, we've talked about ice-dammed lakes, moraine-dammed
lake, Devil's Lake, and now kettles.
If we come down to the Madison area, this is the glacial
geology map of Dane County. The driftless area, the
unglaciated area is out here. The edge of glaciation came
about to Cross Plains here, about to Verona, and right down
just north of Evansville. So all of the area east of there
was glaciated. And green represents glacial
deposits like glacial till. The yellow is sand that was
outwashed, carried by streams, the yellow and the orange.
And the blue is lake sediment here.
And then the modern lakes are in this darker blue.
And you can see, obviously, Lake Mendota and so on.
And you can see from this Lake Mendota was substantially bigger
at one time, and, in fact, this was all one lake we called
Glacial Lake Yahara at the time the glacier was here and the
glacier was retreating. It's then shrunk down to the
present distribution of the lake.
Why is it there? Well, it is an old preglacial
valley. It's not an old valley as old as
the one that Devil's Lake sits in because that's in much older
rock formed much earlier, but this valley did form before
glaciation. And when the ice came down,
there was a river valley under Lake Mendota.
And, again, now we're looking at elevation above sea level of the
top of the bedrock, of the solid rock, and the lowest ones are
this dark red. So you can see under Lake
Mendota that it's deep down to bedrock.
And then there's this narrow valley in here over sort of
under Oscar Mayer that's over this way and down under Lake
Monona and then on down to the other lakes.
Things like Lake Wingra were probably tributaries to that
lake. That would be over in here is
Lake Wingra. And many of the valleys that we
have now going into Lake Mendota were these preglacial river
valleys. So, Lake Mendota is a lake that
is there just because it wasn't completely filled by glacial
deposits. It's not a kettle.
It's not ice dammed lake. The glacial deposits partially
filled it, but there just wasn't enough sediment delivered to the
basin to actually fill it up completely.
And that is why we have the Madison lakes here now.
They're just unfilled parts of this old river valley.
If we take a slice across Lake Mendota, these are the rocks
that are here, sandstone and limestone, that are more or less
horizontal. And then this is the Lake
Mendota basin, and you can see that's where the river valley
was. And then glacial deposits partly
fill it, they just don't fill it all the way up.
And so that's why there's water in there and that's why we had a
lake. If we either had the glacier
delivering more sediment or if we had the glacier here for a
longer time, the lakes might not be here at all because of that.
This is kind of a messy map, but it shows, basically, the same
thing. Here's the Isthmus.
Here Lake Mendota is under here, and this is just showing the
distribution of different rock types, but I think it's too
complicated to look at and understand without looking at a
paper copy. When the glacier started to
retreat though, as I said, there was a much bigger lake here.
The reason is that down by Stoughton the whole system was
being dammed by glacier ice in the moraine.
So just south of Stoughton the glacier ice had debris on it,
and it was holding the water up. So we had this Glacial Lake
Yahara that developed, and it's what makes the flat area under
the airport, for instance, that's the former lake bed, out
here part of Middleton out here, not the main part.
This was a higher lake called Glacial Lake Middleton, but up
under Cherokee Marsh, all the low areas out on the east side
of the Isthmus. If you drive out East Washington
Ave or that kind of thing, there's low areas out there.
Drive out the Beltline, the Coliseum or the Alliant Center
sits on that lake sediment surface.
Lots of the low area out here by where the Yahara river crosses
the Beltline. So it was an extensive lake, all
one lake. You could get in a boat in
Middleton and gone to Stoughton on lake all the way.
You wouldn't have to go through a river.
The hills that stick up in the Isthmus area are features called
drumlins. They're glacial features that
were there as the glacier retreated.
So they would have been islands in that lake, and this was an
old map that was done in the 1890s where they recognized that
these were sand gravel capped by glacial till hills, as are the
ones at the university, Observatory Hill and what we
call Bascom Hill now. So those are drumlins.
Those were there when the glacier retreated.
Another feature I want to mention because people often ask
questions about it is Picnic Point.
What's Picnic Point? I've seen it described as an
esker. This is an aerial view of
Whitewater Lake, and there are ridges.
That kind of looks like Picnic Point.
But these are sand and gravel ridges that are glacial
features. They're eskers that form in a
tunnel underneath the glacier. Okay?
Those are fairly common in Wisconsin, and they often make
islands and lakes like you just saw in Whitewater Lake.
Or they're on dry land, this is up in the northern Kettle
Moraine. But Picnic Point, which sticks
out here, is not an esker. Okay?
It is not an esker. It's not sand and gravel.
It's bedrock. That's a rock ridge with a
little bit of glacial till over it.
And it's there just because it's the ridge between two of the
tributaries of that river valley that were there.
And so if we come back to this map again, here's Picnic Point
and you can see there was a tributary valley that went up
under University Bay and under Shorewood here, and Picnic Point
simply represents kind of the divide, the ridge, between the
main valley that was going up toward Middleton and this
smaller tributary valley that went up under University Bay.
So Picnic Point is not an esker, it's not a glacial feature.
It was glaciated but it's not formed by the glacier.
Okay, and then the lake was at first Glacial Lake Yahara, was
860 feet. And then that forms the surface
around parts of the university. This is the soils building up
here. Dormitories over here.
This is a flat surface that goes out toward the Natatorium and
over toward the hospital. That's all bed of Glacial Lake
Yahara. It's all sand, and this is just
a view of an excavation in it. And you can see the sand that
was deposited in the lake at that time.
It then dropped to an elevation of about 852 which started to
restrict these. And it was dropping because the
ice had melted out of the moraine down here and bedrock
started to control it. So the elevation of this whole
system here is actually fixed by a bedrock, the stream going over
bedrock down by Stoughton. That's what fixes, you can't get
any lower than that very quickly.
So then we started to get all these shallow areas that were
lake bed that then became marsh. And so I guess many of you know
that when Madison first developed, there were all kinds
of marshes that were filled because basically all it was
was some of these drumlins sticking up out of wetlands.
So lots of the wetlands were filled, producing the surface
that we have now. Now, I want to just make a
couple more points here as I wrap up and that is there were
people here at that time. At the time, at least at the
last part of Glacial Lake Yahara being here, there were
Paleo-Indians, and there are several Paleo-Indian sites with
artifacts like this that are found around that 860-foot
shoreline. Not the present lake shoreline,
but the high one. So it's saying that when the
lake was still up at that level, when it was glacial ice still
holding that water up at 860, there were people living and
camping around the edge of that lake.
And, of course, as the lake went down, there were other, younger
occupants. But the oldest ones, these
Paleo-Indians, were up at that highest lake level.
There are bits and pieces of old shoreline, but the trouble is
the landscape here has been so modified by farming and so on,
it's really hard to convince yourself that it really is a
shoreline. You can see things along, going
out to Picnic Point for instance, that might be that
lower 852-foot shoreline, but who knows.
It's been farmed so I wouldn't trust it.
And then the present day Lake Mendota is at about 848.
So it's about a 10-foot difference.
But there are places where you can actually see the shoreline.
One place I would recommend is out here in the Arboretum.
If you take the trail that goes down, I forget the name of the
trail, but it goes down along the lake here, you can actually
identify the shoreline, that higher shoreline that's made up
of a string of boulders that level.
And I want to finally say that we get a record from these lakes
of climate change, and this is a record from about, sorry, 18,000
years ago down to present time. And you can take a core of the
sediment, look at the pollen grains with a microscope, and
look at the abundance and look at back when the glacier was
first leaving, this was lots of spruce pollen, but the amount of
spruce has declined to about 10,000 years ago there was
hardly any spruce left. And it was at that time, about
10,000 years ago, that the amount of oak pollen jumps up
and then has remained fairly common.
So we get a dramatic change in the vegetation after the glacier
left, but we had several thousand years here where we
were dominated by spruce forests and probably like an open spruce
tundra mix that led to the present day vegetation.
So I think I'm basically out of time, so I want to quit there
and see if you have, I guess we have a few minutes for
questions. Any questions?
Yeah. >> There's a, up by Parfrey's
Glen up in that area, there's a big rock, I can't remember the
name of it, but you can go right up to the top of it.
>> Gibraltar Rock in Lodi? >> That's it, Gibraltar.
>> Yeah, Gibraltar Rock is a sandstone hill.
So that's the 550-million-year-old Cambrian
sandstone. It's one of many hills there.
It just happens to be the highest.
There's nothing special about it.
It's not cemented differently or anything different than the rest
of the hills. Yeah?
>> How deep was Glacial Lake Yahara and when did the glaciers
start leaving the Madison area? >> Well, this is the pollen
record from Devil's Lake, probably the same vegetation
here, and so Devil's Lake was ice free by 18,000 years ago,
and Mendota is probably not much different.
Maybe a thousand years younger. >> We usually hear 12,000-13,000
years. >> All right, well that is a
point of confusion. These are radio carbon ages.
It's a method that's used to date the disappearance of the
ice. And there are two ways they are
expressed. And there are so-called
calibrated or calendar ages and there are radiocarbon ages.
And that's the confusion right there because these are calendar
years and if you were looking at radiocarbon years, this would
be more like 14,000. >> Oh, that's confusing because
all the time I see calendar years.
>> I know. It's very confusing.
>> This is calendar years though?
>> This is calendar years. >> The other thing was, the
Yahara River Valley, was it like a big, deep river valley?
Pretty major? >> It's a couple hundred feet
deep. Yes, in fact there's a well
over close to Oscar Mayer that goes through over 300 feet of
sediment before it hits rock. And so that means the river
valley there was 300 feet deep before glaciation.
So we had a very different landscape here than we have now.
>> So what factors decide how fast Yahara River goes down?
>> Well I think the first thing was it was being held by ice
in a moraine down there. When that melted, it went to
852, but from 852 down to 848 or 847 or whatever it is now,
now it's controlled by a dam, but the natural level, that took
thousands of years, because like I say, it's running on
bedrock, and it's slow moving so it doesn't erode very much.
So it's almost stable except for the human activity that controls
it now. >> If there are no more
questions, I'd just like to give a round of applause to our
speaker. [APPLAUSE]