Saturday, June 30, 2012

Roadside Geology of Minnesota: Twin Cities Adventure

After recently moving to the Twin Cities from Missouri, a good friend of mine from the undergrad geology days came to town and suggested exploring some local outcrops. I grew up near the Cities, and she went on to grad school for psychology (and was therefore suffering from a rock-hammering deficiency), so it sounded like a nice little adventure.

As our guide was my Roadside Geology of Minnesota book by R. W. Ojakangas, which had a section on things to see in the Minneapolis-St. Paul area (starting around P. 277, if you have it).



Because of the fairly recent and extensive glaciation, finding outcrop means going to the river valleys. More specifically, the Mississippi River and its tributaries.

Our first stop of the day was Lilydale Regional Park in St. Paul. Since neither of us had much experience navigating the area, and because the book only had written directions and no map, we eventually found the place, after some sketchy navigation. Here is the map I wish we had:


Lilydale Regional Park (red star).

Exerpt from the Guidebook.

Just up the path from the parking lot is a sign describing the trail and the kind of fossils you can find in the Decorah Shale (the fossils were the reason we decided to go here).  We looked forward to bryozoans, crinoids, and brachiopods from the Ordovician. These were all fairly abundant in Missouri's Mississippian rocks but I had never seen them in Minnesota.


Informational sign at Lilydale Park explaining local fossils.


It also included a handy map pointing out where the fossils beds were located on the trail. I don't think we ever found the areas highlighted as "fossil beds", there was a lot of vegetation, but you couldn't go wrong checking out the valleys carved out by the streams.

Trail map of Lilydale Park. The best fossils we found were in the stream bed by the 2nd bend.

Just off the trail by the 2nd bend we climbed down into the stream valley. Listen for the waterfall.


Down into the stream bed

Making our way to the waterfall

 
She finally got to hammer some rocks

Down in the valley area a bunch of different rocks: limestones, shales, and cobbles of igneous and metamorphic rocks from glacial float washed in. One of the rock horizons in the Decorah Shale was a genuine fossil hash containing mixes of bryozoans (look like twig segments), crinoids (look like life savers), and brachiopods (look like sea shells). The bryozoans really stood out in relief in a lot of the weathered samples, and you could often find bryozoan pieces by themselves in the stream bed.

 
A slab of the Decorah Shale fossil hash containing bryozoans, crinoids, and brachiopods.

Going along the trail brings you to the top of a hill made of crumbled Decorah Shale. Through the trees you can see St. Paul and the Mississippi River.

View from the top of the pile of Decorah Shale

On our way back to the parking lot we ran into another fellow geologist who was fossil hunting. She filled us in on the precise geology of the trail and which formations had the fossils. Along with our Roadside Guide, she told us about a neat spot in the park to check out up another path. This path lead us to a cave which formed at the contact of the St. Peter Sandstone and Platteville-Glenwood Formations. Most people are familiar with the St. Peter Sandstone as the pure sandstone used for fracking, and also as the petrologic standard for a pure sandstone.

 
Diagram from the guide showing the exposed rocks along the Mississippi River

 
Stratigraphic column of Minnesota. This trip was near the St. Peter Sandstone/Glenwood-Platteville Formation contact 

The St. Peter is poorly cemented and crumbles fairly easily when hit with a rock hammer. The overlying Glenwood Shale and Plattevile Limestone are more cohesive and are harder to break apart. Because of this sharp contrast in cohesiveness, when these units are exposed the St. Peter crumbles away while the overlying Glenwood and Platteville form ledges. This was seen very nicely at the cave.

Contact between the easily eroded St. Peter Sandstone (white, incised lower unit) and the overlying resistant Glenwood and Platteville Formations and Decorah Shale


Cave down into the St. Peter Sandstone

After leaving the park we planned out the next stop of the day. Lunch. Anywhere with air conditioning (it was hot out).


To make this a cultural as well as scientific adventure we decided to go to one of St. Paul's well-known eateries, Mickey's Diner, just on the other side of the river. This place has been shown in a lot of movies based out of the Twin Cities, but it is best known by my generation for being in The Mighty Ducks (we actually sat where they filmed the scene). We got some apple pie and french fries while planning our next stop in the comfort of cool air.

Menu at Mickey's Diner

 
Reading up on the next stop while enjoying some pie


The fries were also quite good




The next stop of our trip is a well-visited local attraction: Minnehaha Falls in Minneapolis. Like many cities and parks in Minnesota, the name Minnehaha comes from the Dakota language for "waterfall", mini meaning "water", and haha meaning "falls". This spot was much easier to find:


Minnehaha Falls (red star)

The 50-foot high waterfall is a result of the contrasting resistance to erosion of the softer St. Peter Sandstone and overlying Platteville Formation. Minnehaha Creek, which flows to the Mississippi River, easily erodes away the St. Peter but the Platteville resists and forms a ledge, creating a waterfall which continues to retreat upstream. Although the Platteville resists erosion, blocks of it will fall into the stream valley if too much of the underlying St. Peter is washed away.



View of 50-foot Minnehaha Falls from one of the pathways.


Go down the stairway and you get a good view of the Falls from a bridge across Minnehaha Creek








View from a bridge above the Falls. All of the sparkly objects in the water are pennies and other coins people throw in to make a wish

Always on the lookout, we spotted some interesting geology in the rock walls. I assume much of these are glacial cobbles from the area, but this specific granite boulder had some really well-defined feldspar crystals.

Large feldspar phenocrysts in one of the glacial erratic granite cobbles making up the rock wall

The geology trips I had taken in Minnesota during my undergrad had largely been focused on either glacial features or the pre-Cambrian igneous and metamorphics, while my time in Missouri was heavy on the fossil-bearing rocks, so it was nice to finally see some fossils in Minnesota. I was really hoping to find a trilobite, though...ah well.

Overall it was a good trip, although we didn't get to every site in the Guide. Minnehaha Falls is a nice place to go if you're looking something easy to access. Lilydale Park was a bit more adventurous and required some walking, often off the path. I also recommend visiting Mickey's Diner, they have good pie.

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    1. Thanks Karen! More people would have been fun for sure :)

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