Pothole Trail Geology Field Trip

"A billion years ago,molten lava cooled, cracked and formed basalt, also known as trap rock."

   

At the start of our school year, during our studies of prehistory, the boys and I talked a little about the last ice age and the glacial effects on Wisconsin terrain.  To follow up on that, Will and I took a geology field trip today and had a gorgeous, gorgeous morning hike on the Pothole Trail at Interstate Park.  For those of you not in this area of the Midwest, I just really have to emphasize that it was a gorgeous morning.  We have had temperatures in the eighties the past few days and it is such a treat-especially since we soon will have our own little ice age to contend with from November until April.

 

Since we spend so much time up at Interstate anyway, Will was familiar with the trail we were on, and some of the information presented about it.  From our geology reading at home, he was well-prepped to answer many of Julie's questions about the dominant type of rock found in the park (basalt), what type of rock it is (igneous) and how it was formed (heat). 

One thing Will and I hadn't really considered about the park's geography before the field trip is that it was not formed by glaciers themselves, but rather from glacial meltwater!  

More than 10,000 years ago, great glaciers covered much of the northern United States, including most of Wisconsin. As the glaciers melted, forming glacial Lake Duluth and overflowing its banks, torrents of water tore through the river valley carving out the gorge we see here today. 

Here is a photo of the Dalles of the St. Croix River, formed by the meltwaters of the Wisconsin glacial period.  Can you see just how gorge-ous of a day it is? We still have a bit of the burgundies and yellows in the trees, too, which you can see in the distance.

St. Croix River Gorge

Julie also enlightened us about the four major lobes of glaciation in our state: the Lake Michigan, Green Bay, Chippewa and Lake Superior lobes. Twenty thousand years ago, the Lake Superior lobe left scratches called striations in the basalt of Interstate Park as it passed through.

To illustrate where each lobe was located, Julie had the kids take out their maps of Wisconsin.  Of course they were all baffled because, duh, no one told them to bring a map!  And everyone had a laugh when she explained it was the backside of their left hands.  Thankfully my son was not among those children who had to be told "Nope, it's your other left hand".

Above the St. Croix Dalles, looking at our maps.

After finishing up with the background info on the glacial melt, we headed further along the trail to see some of the geologic features the park is known for.  How were these attractions formed? On top of the bluffs, covered by waters so many thousands of years ago, swirling whirlpools of water, sand and rocks (grinders) wore potholes into the solid bedrock.  

  Minnesota Interstate Park may claim the deepest pothole (60 feet) as it's main attraction, but Wisconsin Interstate has The Best Example of a pothole in the world.  (Even if it is only 16 feet deep.) In Your face Minnesota Interstate Park!   

Here is Will checking it out.

So what makes it so great?  The fact that there is such a smooth spiraling to its insides.  It really illustrates the grinding, whirl-pooling action that took place to create it.  How long does it take to make a pothole in such a dense, hard rock?  We don't know.  A really, really, really, really, really long time.  

The Best Pothole In The World!

To finish up our time together, Julie talked to the kids about how even today potholes are being formed under the St. Croix River.  She showed them an example of a grinder rock whose job it is to swirl around and around to create these curiosities that are not just the result of a distant event, but are an ongoing natural process.