Geological Wonders

Walk through geologic time at Minnesota's state parks.

By Heather Anderson, Jon Ellingson, and Paul Purman

Minnesota’s geology is amazingly varied. Some of our landscapes have formed recently, in the past 20,000 years. Some rocks in Minnesota are among the oldest known on Earth; gneiss at Lac qui Parle State Park might be as old as 3.6 billion years old.

To see some of this geologic diversity, all you have to do is visit Minnesota state parks. Below are just a few examples of the marvels you may encounter.

2 billion years ago

Hill Annex Mine State Park gives a close-up look into the geology of the Mesabi Iron Range, as well as Minnesota’s mining tradition.

Iron precipitated out as sediment in shallow seas about 2 billion years ago. During the much more recent Cretaceous Period, about 80 million years ago, another shallow sea covered the iron formations. When the sea receded, it left a fossil-rich layer. During the "recent" ice ages, beginning about 2 million years ago, glaciers deposited additional debris. Looking down into the mine, you can see the thick cover of various glacial materials as a light-colored layer over very thin Cretaceous beds and thick, red iron ores.

The park offers fossil-hunting trips and tours of the mine. The visitor center has extensive geology and mining exhibits.

2 billion years ago

Jay Cooke State Park offers astonishing views of what can happen to mud after 2 billion years. The mud is found today in the form of slate and graywacke (a kind of sandstone).

Approach the park from the west on Highway 210 and pull over after crossing the bridge over the St. Louis River. Here, just outside the park, is a spectacular view of mud compressed and hardened into shale, then slate. Look south and west of the bridge, back across the river, and you will see a large slab of slate, folded as though a giant had bent it. North and west of the bridge, you can see a band of white quartz, squeezed in molten form into a large crack in the slate. There are many such bands, or "dikes," in this area, most of dark basalt.

After entering the park, stop at the swinging bridge. The St. Louis River here follows fractures in the slate, forming ruler-straight channels. If you’ve never been on a swinging bridge, bounce or rock on it for fun, then stop into the park office to check out the displays that explain local geology.

Don’t leave Jay Cooke without a visit to the overlook at Oldenburg Point (marked on the park map). Here you experience the river valley below as a soundscape as much as a landscape, as the wide waters rush over angular blocks of slate.

1.1 billion years ago

Tettegouche State Park has a no-fee rest area, a natural stopping point for North Shore travelers to stretch their weary legs and take in a view of the park’s history.

Take a short walk to the hiking trail overlooking Lake Superior and you will see Shovel Point to the north and Palisade Head to the south. These massive cliffs of reddish rhyolite and dark basalt are remnants of the ancient volcanic eruptions that occurred as the continent split apart more than a billion years ago. A series of stairs takes you down to the mouth of the Baptism River for a closer look at exposed volcanic rocks.

Hike the trail to Shovel Point or take a short drive up to Palisade Head. Looking inland from either place, you will see a series of wooded knobs of erosion-resistant diabase and anorthosite, formed when molten material welled up under and through cracks in the volcanic rocks. Today the Superior Hiking Trail winds through these hills.

450 million years ago

Frontenac State Park sits atop limestone bluffs 400 feet above Lake Pepin.

Sedimentary bedrock, primarily sandstone and limestone, characterizes southeastern Minnesota. These rocks are remnants of ancient seas that covered the region 450 million years ago. Sandstone indicates a near-shore sea environment (sand, relatively heavy, won’t travel far before sinking). Limestone, made of the remains of marine creatures, tells of deeper, offshore environments.

From the parking area above Point-No-Point, a quarter-mile interpretive trail leads you past several overlooks, under dolomite limestone cliffs, and through an old quarry. Watch your step, and keep an eye on small children: The slopes are dizzyingly steep, and timber rattlesnakes have been seen here. Hikers can continue down to Lake Pepin, a 22-mile-long widening in the Mississippi. The lake formed when Wisconsin’s Chippewa River washed glacial debris into the Mississippi riverbed about 8,000 years ago, creating a partial dam that blocked the river’s flow.

18,000 years ago

Mille Lacs Kathio State Park is located on a large glacial landform called a terminal moraine. This hilly moraine formed when a glacier from northeastern Minnesota stopped advancing about 18,000 years ago. Like a conveyor belt, the glacier carried rocks, sand, and clay, which dropped off the melting edge and piled up to create rolling hills along the western shore of Lake Mille Lacs. Interpretive exhibits and many miles of hiking trails allow you to explore this geologic history.

15,000 years ago

Scenic State Park has an interpretive trail along a sinuous ridge between lakes Coon and Sandwick. Rivers of meltwater carved tunnels in glacial ice 15,000 years ago, forming this spectacular glacial landform, called an esker. Meltwater rivers ran atop, inside, or below the glacier. The flowing water transported silt, sand, and gravel. Over time the tunnel of water filled with sediment, and the glacier melted, leaving behind the meandering ridge.

Other state parks with easy-to-spot eskers include Myre-Big Island, Savanna Portage, and Glacial Lakes.

10,000 years ago

Driving through the northern section of Minneopa State Park, you cross a wide sandstone terrace, exposed about 10,000 years ago by the massive flow of Glacial River Warren as it eroded the thick glacial till and scoured into the much older Jordan Sandstone. Glacial River Warren was the drainage channel for the vast Glacial Lake Agassiz, formed from melting glaciers.

From the Seppman Windmill overlook, you can see that Glacial River Warren cut a valley a mile wide and hundreds of feet deep. It carried away clay, silt, sand, gravel, and most boulders, but left some large rocks. Now scattered across the terrace, these glacial erratics came from Canada and North Dakota, as well as other parts of Minnesota. With a field guide, you can identify rocks and guess where the glacier picked them up.

10,000 years ago

Interstate State Park has the greatest concentration of glacial potholes in the world, formed about 10,000 years ago, in the waning days of the last ice age.

The area first took shape with the formation of massive quantities of basalt lava. Later, seas covered the basalt, laying down sandstone. Ten millennia ago, glacial St. Croix River provided an outlet for Glacial Lake Duluth (where Lake Superior is today, but with lake levels much higher).

As the torrent poured south, carrying heavy loads of mud and sand, the river eroded a deep channel into the basalt, leaving stone ramparts on either side of the river. Atop these steep walls, known as dalles, the raging waters swirled rocks that drilled into the basalt, forming potholes--some 60 feet deep.

From the north parking lot, handicapped-accessible trails lead to more than 200 potholes.

10,000 years ago

Buffalo River State Park has 10,000-year-old beaches--low ridges of sand and gravel. These are the beaches of Glacial Lake Agassiz, a vast expanse of meltwater caught between glaciers to the north and a large moraine to the south. Lake levels varied over the years, creating many beach ridges.

To see the slight but definite rise from the flat lake bed (today’s Red River Valley) to the ancient shoreline, start at Glyndon and take U.S. 10 east. Turn south into the park and look to your left for a large boulder. That marks Campbell Beach. The park’s dam marks a spot where the river cuts through another beach ridge. See if you can follow its slight elevation. As you hike, imagine Agassiz’s icy waters lapping at your feet.

Today

Forestville/Mystery Cave State Park is a good example of the fact that all landscapes are geologically active in some way.

As you drive into the park, notice the steep bluffs, created as hundreds of feet of limestone accumulated at the bottom of large shallow seas nearly 500 million years ago. As recently as 10,000 years ago, large channels of glacial meltwater cut valleys through the limestone. As you hike, keep your eyes open for fossils of sea creatures. But remember, state parks preserve natural resources for everyone, so collecting of rocks or fossils is prohibited.

A guided tour of Mystery Cave is a must, and is handicapped accessible. Your tour will bring you through a maze of caves formed by slightly acidic groundwater running through and enlarging cracks in the limestone. The caves started to grow about 1.5 million years ago and continue to grow today. Water dissolves the rocks, eventually carving large tunnels. As the water runs through the limestone, it picks up calcium carbonate. When the water enters the cave, it redeposits the calcium carbonate, thus forming stalactites (down from the ceiling), stalagmites (up from the floor), and other beautiful geological features.

Two fine references are Geology on Display: Geology and Scenery of Minnesota’s North Shore State Parks and Minnesota’s Geology, available at libraries and bookstores. To learn more about geology, visit the DNR web site: www.dnr.state.mn.us/minerals/digging_minerals.

Heather Anderson and Jon Ellingson are geologists and Paul Purman is an information officer, all with the DNR Division of Lands and Minerals.