This is an Earth Cache! No physical hide is here, but rather a lesson on geology instead! There are a couple of physical tradional caches, but have nothing to do with this EarthCache. Using the waypoints at this location and the following lesson please answer the questions below!
The Midwest’s Driftless Region
Most people have never heard of “the Driftless.” Few can describe it. The name comes from something that didn’t happen here. A million years ago (give or take a millennium) a gigantic ice sheet covered most of what we know as the American Midwest and Canada.
As glacier mountains melted and slid north (the last one about 10,000 years ago) they carved up the landscape, leaving massive holes that formed all those famous Minnesota lakes and deposited enormous piles of “drift”—rocks, sand and gravel—across the land. But glaciers did not cover this region of prehistoric land. No glaciers. No lakes. No covering of drift. Hence, “driftless.” And that changed everything.
Without “drift,” the forces of nature—primarily water and wind over time—were free to do their erosive work on the unprotected limestone, sandstone, and layered shale left behind. Huge torrents of water from melting glaciers surged across the landscape towards what became the Mississippi River basin, carving out the valleys and bluffs that give the “driftless” area its distinctive look.
This unique geology has a related scientific name: “karst.” Karst, also rare, is formed when subterranean soft rock, dissolved by running water, becomes a winding underground network of caves and caverns. Think of Swiss cheese made out of rock.
Karst activity produced thousands of deep caves and caverns around here, including the Niagara Cave in Harmony and the Mystery Cave in Forestville State Park near Preston. Both offer public tours.
Let’s go back to those tree-swallowing holes. In karst topography, some caves are very deep; others lie close to the earth’s surface. When a cave ceiling near the surface collapses it creates an opening called a “sinkhole.” There may be as many as 10,000 sinkholes in Fillmore County alone. New ones are still being discovered.
Over time, trees and shrubs often grow up from the bottom of a sinkhole. But there are plenty of sinkholes around here. Watch your step.
Early farmers who found sinkholes on their land just plowed around them or used them as dumping pits for old farm equipment and other refuse, a practice that is now illegal. Today, trained sinkhole searchers sometimes find old relics along with an occasional fossil.
Southeast Minnesota is in the “driftless zone.” What does that mean for the people who live here and for its many visitors? Very cool stuff.
Masonic Park in Fillmore County, Minnesota, is a location that showcases typical karst geology, characterized by a landscape shaped by the dissolution of soluble bedrock, primarily limestone and dolostone.
Key geological features of the park and the surrounding area include:
- Limestone Bluffs/Rock Walls: The park is noted for its scenic limestone cliffs and rock walls along the creek. These bluffs are a surface expression of the underlying Paleozoic carbonate bedrock layers, such as the Oneota Dolomite.
- Karst Features: Fillmore County is known as the heart of Minnesota's karst region, which means water has dissolved fractures in the bedrock over time, creating a network of underground drainage. While specific features within the park itself might be small, the general area has numerous sinkholes, disappearing streams, springs, and caves. There is mention of a small cave nearby in the road right-of-way.
- Creek and Fossils: A creek (likely a tributary of the Root River, which runs nearby) runs through the park, and visitors have reported finding fossils in the area. The presence of fossils is common in the marine sedimentary rocks (limestone, shale, sandstone) of southeastern Minnesota, which were deposited in an inland sea during the Paleozoic era.
- Glacial Influence: The region was only lightly glaciated (part of the "Driftless Area" in this region) and is practically devoid of natural lake basins, meaning the ancient bedrock geology is close to the surface and largely responsible for the topography.
In essence, Masonic Park offers a chance to see the exposed layers of ancient marine sedimentary rock that define the unique, water-carved karst landscape of Fillmore County.
Fossils found in Masonic Park are typical of those found in the Ordovician Period marine sedimentary rocks of southeastern Minnesota. The specific formation exposed around the park is the Cummingsville Formation of the Galena Group, which is known to contain a variety of marine invertebrate fossils.
Common fossils found in this area include:
- Brachiopods: These ancient, shelled marine animals (similar to clams, but different internally) are very common.
- Crinoid parts: Often called "sea lilies" (though they are animals), their stalk segments, which look like small, ornate Cheerios or disks, are frequently found.
- Bryozoans: These colonial, coral-like creatures are also abundant.
- Trilobite fragments: Hard, segmented pieces of these extinct arthropods (relatives of crabs and insects) are common finds, with species like Isotelus found in similar local formations.
- Cephalopods: Fossils of ancient, straight-shelled relatives of modern squid and octopuses can also be found in the general Fillmore County area.
Visitors exploring the creek bed and limestone outcrops within the park are likely to find these types of fossils, which represent the diverse marine life that inhabited a shallow, tropical sea covering Minnesota around 450 million years ago.
Brachiopods, or "lamp shells," are a very common find in the Ordovician rocks. They are filter-feeding marine animals with two shells.
- General appearance: Most amateur collectors will find complete shells that are small (a few millimeters to a couple of centimeters across), subcircular or ovoid, or appearing somewhat like a fingernail in the rock. They can often be distinguished from bivalves by their symmetry: each individual shell has a line of symmetry, but the two shells themselves are often different sizes.
- Common Genera/Species in the region:
- Rhynchonella ainsliei: This species is mentioned as having a distinctive count of costae (ribs) on its shell and is known from these formations.
- Rafinesquina minnesotensis: This is another brachiopod species cited as being present in the general Ordovician shales and limestones of Minnesota.
- Platystrophia and Hebertella (genera): These are common brachiopods often found in the Decorah and Galena formations in the area.
Crinoids
Crinoid fossils, or "sea lilies," are abundant but usually found as fragments. Complete specimens (a calyx with arms and stem attached) are extremely rare in this region.
- General appearance: The most common fossils are individual, button-like plates that made up the stem (columnals). These disc-shaped pieces often have a small hole in the center and can be found loose in the creek bed or embedded in limestone slabs.
- Common Genera/Species in the region:
- Isotomocrinus: A crinoid genus whose stem structure is sometimes found in the Ordovician layers.
- The species are typically identified by the structure of their stem parts and the rare calyx, rather than by a prominent full-body fossil.
- The abundance of these "ossicles" often makes the rock itself a crinoidal limestone.
Ultimately, the fossils you find at Masonic Park are remnants of a thriving shallow marine ecosystem that existed over 450 million years ago.
This bedding plane (surface between two beds) shows a dense accumulation of brachiopod shells.
A selection of various brachiopods, showing different sizes, shapes, and surficial ornamentation.
These two photos show two examples of large D-shaped shells; they are known technically as strophomenids. Both photos show the interior surface of the lower valve.
A selection of individual crinoid columnals and short stacks of columnals, which are the most abundant crinoid fossils.
The white streaks in this photo are crinoid arms that are just barely exposed
A little history
According to history recorded in Fillmore County, George Weisbeck came to the area from Dubuque, Iowa, in 1865 and established a sawmill. It was there that he made walnut furniture,
History records show that Leonard Siebert, a member of Spring Valley Masonic Lodge 58, gave his land to the lodge in June 1915.
The Spring Valley Masons went to work, establishing picnic grounds and building a pavilion. At one time, the pavilion was enclosed, had a fire place and could be used for events year-round.
According to a story written several years ago and on record at the Spring Valley Historical Society, the original stucco structure cost $685 to complete.
Floods throughout the years have forced the pavilion to be simplified. There are still picnic tables available and Fillmore County does maintain the structure.
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Requirements to log this Earth Cache:
Please send me the answers to the following questions either through the "Send Message" link, or email me through my profile. Please indicate that you are sending answers to this Earth Cache either by name or GC Code. Thank you!
Question #1 At the posted coordinates.
#1. How do you think that this rock/wall got its shape?
What type of stone would you think the rock/wall is?
What is your estimate on how tall the rock wall is?
Question #2 Fossils
#2. At either of the waypoints: Find a fossil, identify it and post a picture of your fossil in your log and describe to me if it's Brachiopods, Crinoid or Bryozoans.
At the posted coordinates take a picture of yourself or personal item or your group with the rock wall showing in the background.
Have Fun and Be Safe👽🛸