[ Bee buzzing ] [ Birds chirping ]
Mark Jirsa: In order to understand what the rocks are and why they are where they are and why they were important to native peoples and why they are important to us today for that matter, you really have to understand this broader geologic story. But it can be told in two chapters.
Now the bedrock geologic map you see a lot of different colors on there. Each one of those colors represents a different rock type.
And so pink colors on there for example are granite.
You've heard of granite. The green colors are what we call green stone which was a sort of lava. The grays are metasedimentary rock.
These blue and yellow colors down here in southeastern Minnesota are mostly sediments of various kinds deposited in ocean. And the rock that we're interested in today, the Sioux Quartzite, is shown in yellow down here in the southwestern corner of the State. And it is a representation of the earth's crust. Basically you're looking at the earth's crust as though we scraped away all the glacial materials, the sand and gravel and glacial till and lakes and everything else, if that were scraped away. So I'm going to start with the bedrock geology because that's the oldest and geologists tell a story from oldest to youngest. And the geologic map on the left you recognize from the previous slide. The one on the right is a subdivision of the geology by major age groups of rock. And ah here in southeastern Minnesota where we're recording this is rocks that are about 400 million years old.
Here in western Minnesota and southwestern Minnesota are the cretaceous rocks that are about 90 million years old. Bedrock geology in Minnesota as I mentioned, is largely obscured by glacial till. In some places the glacial material can be as much as a thousand feet thick.
What I've shown here in this gray overlay is places where we have no rock outcrop from the crystalline bedrock underneath. And you see Jeffers. Well of course there are some, we know there are some scattered rock outcrop there as there are down here around Pipestone.
There is rock outcropping along the Minnesota River Valley. We've seen some of the ancient gneisses there, down here in southeastern Minnesota and much of northeastern Minnesota. Well so this is southwestern Minnesota.
This shows a lot of different colors here now each of these colors represent different parts of what we think was a micro continent that basically became attached to a larger growing continent a long time ago, back 2.6 billion years ago or so. This continent actually has some of the oldest ages of rock in the State, 3.5 billion years old along the Minnesota River Valley and this rock type called the Morton Gneiss which there's a picture of it right there. On this slide of southwestern Minnesota there's a Sioux Quartzite shown in yellow which is the rock type that the Jeffers Petroglyphs are carved in and on.
[ Birds chirping ] The reason that the Sioux Quartzite is what it is and is where it is, is because this very ancient micro continent that I described that had all of this gneiss, granitic gneiss in it. Granite is a combination of minerals; quartz, which is one of the hardest minerals, feldspar which is fairly soft and easily weathered and a few other minerals that also are fairly easily weathered. This very ancient micro continent that I described that had all of this gneiss, granitic gneiss in it, this granite gneiss basically sat there and weathered for a billion years.
And in that billion years what happened was the chemical weathering weathered out feldspars and so the end result was this huge source of quartz that covered much of the North American Continent at that time. So and it was readily available it's just loose quartz sand, hard, chemically inert, and just waiting and ready for something to move it.
Well, something did and that was a series of rivers 1600 to 1700 million years ago that flowed down and drained sand off of the continent and into this river system, all washed down into what is now the Sioux Quartzite basin. Quartzite's that are all similar to the Sioux in localized either basins or fault bounded river valleys. Based on the sedimentary structures that we can see in the quartzite at Jeffers, see it over at Pipestone and most of the other places, you see bedding types that suggest this sort of model deposition. And that is what's called a braided stream.
Braided streams are common in environments where there are broad terrains bordered by more hilly terrains and there's no vegetation to hold back the sediment. So there is so much sediment washed in to the valley that the valley becomes this flat plane along which the individual stream channels are literally braided looking. The sedimentary structures is what tells you the geologic story here. And if you look at the sedimentary structures you can get a sense that this model deposition may be right.
The things that you see is sort of festoon looking patterns on this cross section those represent troughs, channels. You can kind of get the sense of that as you look at them. You're looking right up a channel sort of anastomose, just sort of inter-finger over time, over this broad sedimentary valley. And this is the sediment logical evidence; the evidence in the rock of this environment, the deposition that we think is related to the stream deposition. As the flow sort of wanes after the rain event is over and the flow begins to slow down.
Then you start to get the kind of sediment movement that produces these things. And of course you recognize those, you've seen them in every lake and ocean you may have been in. And those are ripple marks.
Adjacent to glaciers for example, you see sedimentation that could be you know, a meter of sediment deposited in a day's time, one rain event or one melt event. So that's sort of Chapter 1, that's ancient story of it as explained by way of the bedrock geology.
And of course it is the bedrock on which the petroglyphs are carved and much of that area has relied on. [ Birds chirping ] But there is a secondary story and that is that deals with the glacial time period or the quaternary, it's actually called quaternary time period. The blue area here in northern North America is what was called the Laurentide Ice Sheet which is just one of the very large ice sheets that existed in North America during one of the last glacial episodes.
What happens with the ice as it begins to grow in some snowfall in the central part of the ice sheet is that it sends down lobes of ice right, in several, in different directions at different times. And a lobe will move slowly forward and in some cases the ice could be as thick as a mile thick. But these individual lobes will advance downward and then melt back and a lot of the material that's carried in the ice, the sand and gravel etc. melts out of the ice as the ice melts back. And then there will be a colder period and the ice will advance again. And so there have been these repeated episodes of advance and retreat. So this is just another image showing that, those glacial ice advances in those lobes, roughly 12 to 14 thousand years ago. It's called the Late Wisconsin in time period and in glacial history. The evidence that we have on the quartzite outcrops for glaciation of course includes things like glacial scours, glacial we call them striations, glacial striations and chatter marks.
You can imagine a chatter mark. You actually can see these.
They're where the ice froze down a little bit and then [ pounding sound ] moved a little bit [ pounding sound ] like this and moved periodically and plucked up little pieces of the quartzite. Another good evidence for glaciation and a good depiction of glacial history is present in the wind polished surfaces on various well, either boulders or the rock outcroppings themselves.
In fact on some of the boulders you get what are called ventifacts which are almost pyramid shaped pyramidals. Blocks of stone that have been so windblown on one side that they become flat in that direction. And then either the boulder moved and got flattened on another side or the wind shifted. It's really hard to know which of those is more important. But you know we don't get this kind of activity happening much today. And part of the reason is because during glaciation there was: Number one, again very little vegetation. And Number two, the glaciers because as they melt they drop all the sand and gravel and whatever they contain, there's a lot of sediment available and so that sediment is readily available to be picked up by the wind to polish these surfaces. And Number three, is a phenomena called Katabatic Wind which is created between an ice lobe and the adjacent land mass where there's high pressure over the ice and relatively low pressure over the warmer un-ice covered part of the land. And that pressure gradient causes tremendous wind especially at certain times of the day. And so you could literally get winds from different directions at different times at hundreds of miles an hour.
In order to understand what the rocks are and why they are where they are and why they were important to native peoples and why they are important to us today for that matter, you really have to understand this broader geologic story.
[ Bee buzzing ] [ Birds chirping ]