Written in Stone: The Fossil Record of the Driftless Area

Located among the steep hills and deep valleys of the Driftless Area, fossils abound, many of them remnants of ancient sea creatures. Join Jon McGinty as he takes a closer look at why this area around the Mississippi is so ripe for fossil hunters.

The Driftless Area of the Upper Midwest covers about 24,000 square miles of southeastern Minnesota, northeastern Iowa, southwestern Wisconsin and northwestern Illinois. It consists of an eroded plateau with many deep river valleys, steep hills, forested ridges, caves, sinkholes, springs and waterfalls. This landscape is in sharp contrast to the relatively flat and featureless terrain that surrounds it in all four states.

“It’s incredibly beautiful, and it almost seems out of place,” says Josh Mathews, director of paleontology at Burpee Museum of Natural History in Rockford. “My family is from the area, and I spent many weekends hunting turkeys there when I was younger.”

Josh Mathews, of Burpee Museum (left) and Steve Simpson (right), retired HCC professor.

Beginning about 2.6 million years ago, the North American continent was periodically covered with huge sheets of ice that moved away from the polar regions and then retreated during warmer intervals, in cycles that lasted tens of thousands of years. Such continental glaciers occurred when the climate experienced long, snowy winters during which more snow accumulated than could melt in the short, cool summers. Eventually the layers off snow compacted to solid ice and became so massive they flowed outward.

“They spread like pancake batter,” says Steve Simpson, retired geology/geography professor from Highland Community College (HCC) in Freeport. “Driven by gravity, they moved downhill and outward from the center.”

At times, this ice covered all of the upper Midwest except southern portions of Indiana, Illinois and Ohio – and the Driftless Area. As the glaciers advanced, they scraped away hilltops and ground up the surface rock layers, and when they retreated (melted), they deposited whatever rocks and debris they contained, covering the bedrock beneath with many feet of sediment called “glacial drift.”

For various reasons, the last glacial advances and retreats in the Pleistocene era (2.6 million to 11,700 years ago) surrounded but never covered the Driftless Area. As its name indicates, the Driftless Area didn’t receive the deposits of glacial drift that flattened out most of the upper Midwest.

One hypothesis to explain why this happened is based on the porosity of the surrounding bedrock.
“When glaciers advance, they actually move on a layer of melt water,” says Mathews. “The permeable bedrock could have ‘de-watered’ the glacier bottom, eliminating the lubricating layer it needed to advance.”

Another factor, as explained by Simpson, deals with the pre-existing rock types and structures of the upper Midwest prior to glaciation.

“The structure north of Wisconsin, in what is now Lake Superior, is a massive trough of hard rock called basalt that diverted the flow of ice to the west into Minnesota and Iowa,” he explains. “Farther east, a region of soft shale allowed the glacier to carve a deep trough straight south toward the present-day Ohio River. This trough now forms the basin of Lake Michigan.”

If you wish to examine the fossil record in undisturbed rock layers in central Illinois or northern or eastern Wisconsin, for example, you have to excavate through several hundred feet of glacial drift to reach them. But in the Driftless Area, these layers are uncovered, exposed by ancient rivers that have been eroding them for millions of years.

“Most of the rock exposed in the Driftless Area belongs to the family called sedimentary,” says Simpson, “specifically sandstone, shale, limestone or dolomite. [see graph on page 90] Sedimentary rocks are of particular interest to paleontologists since they often contain evidence of past life and major events that shaped the land.”

Mineralized evidence of ancient life, called fossils, form under specific conditions, so it’s likely that only a small percentage of organisms are ever preserved.

Steve Curran

According to Steve Curran, geology/geography instructor at HCC (Simpson’s replacement), there are two things usually necessary for a fossil to form. The first is a rapid burial, and the second is a creature with “hard” parts, like a skeleton or shell.

“The soft tissue or flesh decomposes rapidly and seldom leaves a trace,” he says.

Such burial can take place in an ocean, near a river or in a desert. Most fossils found in the Driftless Area are remnants of ancient sea creatures that inhabited a warm, shallow sea which covered much of the interior of North America from about 600 million years ago to nearly 250 million years ago (Cambrian through Permian periods).

Relative vs. Absolute Time
Scientists who study the history of the earth in rocks use two main methods to measure age. Rock layers and the fossils they contain can be dated relatively on the basis of where they are found in a stack of layers, older ones toward the bottom, younger ones toward the top. They can be dated absolutely by using the radioactive decay rate of an element in the sample. Normally, scientists use a combination of both to determine the age of rocks and fossils.

Geologic time is measured backward from the present, using larger to smaller time spans: Eons, eras, periods and epochs. These measurements are sometimes delineated by major extinction events.

Rock layers, or formations, are distinguished by type (i.e. sandstone, shale) and are usually named for a location where they were first discovered or studied (Niagara dolomite, St. Peter sandstone.)

When these creatures died and settled to the bottom of the sea, layer upon layer of sediment covered them up. Eventually the weight of many layers consolidated the material into stone. After burial, water containing dissolved minerals often seeped through the sediments and the remains of the living material was replaced by minerals, or the original material was dissolved, leaving a mold of the organism. Hence, they became fossilized.

“In some settings, such as swamps and highly productive marine environments, organic material accumulated so rapidly that it didn’t get a chance to decompose, or oxidize,” says Simpson. “Huge amounts of this organic material was buried over time. This led to the formation of fossil fuels – coal, petroleum and natural gas.”

Norlene Emerson

“During this period of a shallow inland sea, the North American continent was rotated clockwise 90 degrees from its present orientation,” says Norlene Emerson, retired geology professor from UWP-Richland in Wisconsin. “We were located about 15-20 degrees south of the Equator. The landscape was very subdued, so even slight changes in the sea level would move the shoreline quite a distance.”

As the shoreline moved, the type of rock layers created beneath the water depended on its depth. Large particles like gravel and sand were deposited near the shoreline where water activity was strongest. This made sandstone. Smaller silt and clay particles settled in deeper, quieter water, and made shale. In warm, shallow water far from shore, calcium carbonate (lime) from the shells of millions of sea creatures accumulated to make the abundant limestone of the Midwest.

These bedrock layers exposed in the Driftless Area were deposited during a time span from about 500 million years ago (the early Cambrian period) to about 420 million years ago (the Silurian period). But the rock layers younger than the Silurian period are missing from this area.

“There is a huge unconformity in southwest Wisconsin and northwest Illinois,” says Mathews. “We are missing about 450 million years of the rock record here, except for the Pleistocene epoch (1.8 million years ago).”

These missing layers include the periods when dinosaurs roamed all over the planet, including here in the Midwest. Apparently, most of the time after the Permian period (248-290 million years ago), the Upper Midwest was above sea level, and sedimentary rocks were not deposited. The layers that did exist here were eroded away by rivers and streams.

“My least favorite, but most-asked question by locals when they find a fossil in their backyard is, ‘Is this a dinosaur bone?’ and the answer always bums them out,” says Mathews. “But the cool thing is that it could be a tooth of a woolly mammoth or a saber-toothed tiger, since they were all here in the Pleistocene epoch, but it’s not a dinosaur bone. They were probably here at one time, but that fossil record has been washed away.”

What remains is a snapshot of the forms of life that were here in the ocean. All were invertebrates and preceded fish in the evolutionary timeline. Most became extinct, like trilobites (Wisconsin’s official fossil), and receptaculites [re-sep-tak-u-lite-ez], a mat of algae that resembled the center of a sunflower after the seeds are removed. But others have modern-day relatives in today’s oceans: brachiopods, bivalves (clams), gastropods (snails), and cephalopods (octopus and squid), to name a few.

“Many of these creatures were attached to the sea floor or had limited mobility,” says Emerson. “They also created and lived near small coral reefs – just like people congregate in cities, with fewer people in rural areas. They were also sensitive to the amount of sunlight that reached them (so photosynthesis could take place in their food sources), the salinity and temperature of the water.”

Curran, like Simpson before him, leads two geological field trips a year for HCC students to view the rock and fossil record in the Driftless Area, one in October in northern Illinois and one in November in southwestern Wisconsin, with about 15 students per trip. For 40-plus years, Emerson also led numerous field trips for college students, professional scientific organizations and the general public, to view the local geology. All agree it’s harder than it used to be for amateur collectors to find fossils.

“The 1850s to the 1950s were ideal for collectors,” says Simpson. “Quarry operations were labor-intensive, using picks and shovels. A collector could ask quarry workers to set aside specimens and later pay them for their trouble. Now liability issues prevent collectors from even being on the property.”

Public access along road and railroad cuts are still potential locations for adding to one’s fossil collection, but vehicle and train traffic can make the experience problematic. State parks work to preserve geologic formations for the public and prohibit collectors from taking samples. Curran, Simpson and Emerson recommend visiting state parks in the Driftless Area to view fossils: Blue Mound, Devil’s Lake, Governor Dodge, and Wyalusing in Wisconsin; Apple River Canyon, Castle Rock and Mississippi Palisades in Illinois.

They also agree that the best places to view fossils is in established collections on display in local museums.

When she retired, Emerson donated most of her personal collection to the Field Museum in Chicago, the Milwaukee Public Museum and the UW-Madison Geology Museum. Nearly every local college has a collection in its geology department; HCC’s in Freeport is curated by Curran.

But nothing can compare to the thrill of personal discovery. Snails and seashells might not be as exciting as dinosaurs, but finding something that existed 400 million years ago is a thrilling experience nonetheless.

“You become the first human being to see something that was living here, got buried and now is exposed to sunlight after such an incomprehensibly long time,” says Mathews.

One of the best exhibits of ancient life in this area is at the Burpee Museum of Natural History in Rockford.

“As you walk in the front door, you see a diorama of an Ordovician sea, with the actual fossils on display below,” says Mathews. “It includes cephalopods, brachiopods, trilobites, and sponges – a very diverse assemblage. It resembles what you would see if you visited a reef in the Bahamas, but with prehistoric animals. The exhibit is a walk through time, starting with the Ordovician period, and small offshoots of Silurian and Devonian exhibits, followed by a life-size Carboniferous coal forest.”

The Dinosaurs of Burpee

Since 2001, Burpee Museum has sponsored summer expeditions to search for and excavate dinosaur fossils in the West. The first expeditions went to Ekalaka, Montana, to work at the Hell Creek Formation, an area of the Upper Great Plains filled with the remains of dinosaurs that lived at the very end of the dinosaur age.

Within the first two years, Burpee groups found and excavated the near-complete skeleton of a juvenile Tyrannosaurus Rex, later named Jane. Later expeditions found a sub-adult Triceratops, later named Homer. Both are currently on display at the museum.

Since 2007, Burpee has sent similar annual expeditions to south-central Utah to the Hanksville dinosaur bone bed. By the end of that first summer, more than 200 new bones had been discovered. Some of the discoveries were made by students and amateur fossil volunteers. Since then, more than 1,000 bones have been removed from the quarry, and just as many remain to be uncovered.

“Hell Creek is one of the most heavily explored dinosaur formations in the world,” says Mathews. “Jane is considered to be the best preserved and most complete example of a sub-adult T-Rex in the world.”

But discovering the bones and fossils is only the first step in a long and careful process to prepare them for display at Burpee. Excavation includes cutting the rock layers around the fossil and transporting it and the surrounding rock back to the museum. In the basement Paleo Lab, teams of trained preparators carefully and patiently remove the fragile specimen from the encasing rock and sometimes create missing parts to simulate the entire piece. Museum visitors can see all this activity through large windows, which can be opened so that preparators can speak to viewers as they work.

Members of the public are invited to participate on these summer dig sites, three weeks in May and June to Utah, and two weeks in early August to Montana. Find fees and schedules at burpee.org.
New technologies have been added to the modern paleontologist’s toolbox, such as helicopters to reach remote sites and CAT scans to analyze the interior of excavated samples.

“Jane has a big bump on one of her toes,” recalls Mathews. “We have a radiologist on our Board who took it to a local hospital for a CAT scan. The image revealed it was osteomylitis, an incredibly painful growth. We don’t know how Jane died, but that growth might have prevented her from hunting successfully and could have led to her death.”

At the top of the Hell Creek formation is a thin gray layer within the rocks. Called the K-T or K/Pg boundary, it is a layer of volcanic ash laid down by the huge meteorite that slammed into the ocean near the Yucatan peninsula 66.6 million years ago. It was the event that precipitated the mass extinction of dinosaurs.

Paleofest 2022

Burpee’s annual celebration of the science of paleontology returns for the 24th time this March 4-6. This internationally attended festival draws nearly 2,000 scientists, students, kids and families – people at all levels of interest in fossils and the stories fossils tell.

“This year’s keynote speaker is Dr. Joe Sertich, Curator of Dinosaurs at the Denver Museum of Nature and Science,” says Mathews. “He’ll speak on Saturday evening about dinosaurs from the Grand Staircase-Escalante region of southern Utah.”

In addition to lectures by scientists on a diversity of topics, there are hands-on workshops and family-friendly activities. Kids will use a “paleo-passport” to get stamped at each activity they attend, then receive a small prize at the end. A variety of packages are available. Learn more and purchase tickets at burpee.org.