New North Shore guidebook explains Minnesota's originsby Stephanie Hemphill, Minnesota Public Radio
If you've ever walked along the North Shore of Lake Superior, you might have wondered how the rugged scenery came to be.
A geologist and a naturalist have teamed up to write a guidebook for hikers in the eight state parks along the North Shore.
Duluth, Minn. — The Gooseberry River is typical of the many North Shore streams that cascade into Lake Superior. They all have their origin deep in geologic time.
The book, "Gooseberry Falls to Grand Portage: A Walking Guide to the Hiking Trails in Minnesota's North Shore State Parks," takes you to dozens of places where you can visualize those formative events.
Author Ron Morton says the bedrock here is between one and two billion years old. Back then, Minnesota was at the center of the North American continent.
"And what happened is, a large blob of molton rock rose up from deep within the earth," Morton explains. "And the pressure of that rising lava or magma pushing against the crust caused it to crack open like an over-cooked sausage."
The crack ran from here, along what's now the St. Croix River, all the way down to Kansas. It's called the Mid-Continent Rift.
Volcanoes spewed lava across the land for 23 million years.
"These particular lava flows are called the Gooseberry Lava Flows. And like all lava flows, they have lots of gas," he says. "As the flow moves over the landscape, it starts to cool, and the gas that's in the flow begins to rise up to the top and escape. And as it escapes it leaves behind these little holes -- you can see all the little white spots."
They're white because they're filled with minerals deposited there by groundwater bubbling up through the holes.
"That's where agates come from, of course: the banded, and red, and creamy agates, made out of of silica dioxide, chert, or quartz."
As the lava cooled it shrank, creating vertical cracks that were perfect targets for rainwater to penetrate the rock and slowly break it apart.
"And so the water will work its way down through that, freeze and thaw, and then in the spring just the volume of water coming through there tends to wear it away," says Morton. "And that'll just peel off, either as a whole column or in pieces, forming the step-like shapes of the falls. And so with time as these things break away or fall off, the waterfall actually migrates upstream."
Five waterfalls bring the Gooseberry River down the steep hillside to the lake shore. Geologist Ron Morton says at the Lower Falls, the river seems to slow down.
"The mad rush of river to get to lake is over here," Morton says. And now it's sort of, 'Whew, I made it this far, now I can take my time, and meander down to the lake side.'"
The slow part of the river near the lake is great for wildlife. Migrating birds stop here, and just last year a beaver family made their home in the river. Today there's no sign of them.
The volcanic rock along the North Shore dates from a geologic period called the Precambrian.
"And 'Precambrian' is a really clever geological word meaning 'before the Cambrian,'" Morton explains. "The reason why the time break is put there is because the start of the Cambrian was when fossils appeared in rocks, just immense amounts. Nice time break; the problem is the Precambrian then, when we look at the age of the earth, accounts for about 88 percent of all geological time. And so those are the rocks we're dealing with along all the hiking trails, with the oldest rocks actually up in Grand Portage State Park."
The guide book has a chapter for each state park, with descriptions of the trails, how long they are and how difficult. Co-author Steve Morse describes the flora and fauna of the region.
Maps indicate stopping points where you can pause and read about what's special in that place.
It might be a prominent view point or a geologically important spot with a particular kind of rock or outcrop. Or it could be a series of flowers or types of trees. It could even be historical. Morton and Morse marked these points using a GPS unit.
So people with GPS units can program in the locations and know for sure exactly where they are.
But the guidebook is just as informative and fun for the less technologically-inclined.
- Morning Edition, 09/24/2007, 7:25 a.m.