Written by Sarah Dugan, Kroening Nature Center Naturalist
As the waters of the Mississippi recede from their spring highs, now is a great time to visit the river’s edge and see what rocky treasures have been revealed. Although the bedrock of the Twin Cities area is sedimentary, primarily sandstone and limestone, a wide variety of rocks can be found thanks to the glaciers that shaped the Minnesota landscape. Pick up a handful of stones or sand from the beach at North Mississippi Park and you can easily see the differences in color, texture and structure. Whether it’s a flat pebble perfect for skipping or a sparkling stone to collect, each rock carries the story of how it was made.
Granite is easily recognized by its visible crystals in a mix of colors: white or grey quartz, pink feldspar, black mica. These relatively large crystals are the result of melted rock slowly cooling inside the earth. The more slowly it cooled, the larger the crystals. When magma with a similar mineral composition is instead ejected in a volcanic eruption it cools quickly and forms rhyolite (rai-uh-lite). This relatively smooth, opaque rock comes in shades of pink, red or brown. It’s a familiar sight along the North Shore of Lake Superior where it was deposited in an eruption over a billion years ago. In Minneapolis it is easy to find as part of the glacial ‘till’ and along the river.
Basalt is another volcanic rock common in Minnesota. It is similar to rhyolite, but with a dark grey color due to its different mineral content. Both rhyolite and basalt can be found with larger crystals embedded in the base rock, a phenomenon known as porphyry. These crystals grew slowly as the cooling process began underground, then, when the magma containing them erupted, the rock around them cooled quickly, trapping them.
There are several types of sedimentary rock to be found here as well. They vary greatly in texture, from coarse sandstone to smooth, waxy chert. These differences come from differences in the materials that were deposited and compressed to form rock, a process called lithification. As the name suggests, the rough, crumbling texture of sandstone arises from the sand that was laid down over 400 million years ago as the floor of a shallow sea that used to cover most of the state. The pale color of limestone, and the frequency of embedded fossils, reflects its origins as the calcium-rich remains of various aquatic species living in that sea collected atop the seabed. Perhaps less familiar, chert is also formed from the lithification of organic matter, in this case certain silica-rich, microscopic organisms.
The next time you pick up a rock, in the park, at the beach or in your back yard, take a moment to think about what you notice, and wonder about the story it can tell.
Programs this Month:
Nature Tots: Down in the Dirt · Saturday June 1, 10:30-11:30 a.m. | Free| Ages 2-6| Activity #3045. Discover what there is to do down in the dirt. Go for a critter hunt outside to see what’s living under logs and rocks, investigate up close with a magnifying glass, and make a soil-themed craft.
Family Funday: Turtle Time · Saturday June 8, 10:30 a.m.-noon |Free| All ages| Activity #26462. Learn about different Minnesota turtles and what they need to thrive. Join a naturalist to look for turtles in the park and do a turtle-themed craft.
Get Outdoors Day: Bio Blitz · Saturday June 8, noon-3:30 p.m. | Free | All Ages | Activity #7329. Join naturalists and other volunteers to seek out wildlife in the park! Or try orienteering, bird watching or shelter building with our Adventure Hub kits!
Bird Watching: Feathered Fridays · Friday June 14, 9-10:30 a.m. | $5/person | Ages 18+ | Activity #4079.
Learn distinguishing characteristics of birds in the park including plumage, bird songs and calls, and behavior, while strolling through the park with a naturalist. All experience levels welcome, binoculars available.
Adventure Hub Highlight: Shelter Building · Saturday June 15, 1-2 p.m. | Free | All ages | Activity #6475.
Head out with a naturalist to try building some simple trap shelters. Then check out an Adventure Hub kit to try shelter building on your own.
Nature Art: Solstice Crowns · Saturday, June 22, 1:30-2:30 p.m. | Free | All ages | Activity #6487.
Celebrate the longest day of the year in style! Take a walk with a naturalist looking for natural materials to decorate a nature crown to take home.