Jul
21
2009

Geology and nature along the North Shore

Middle Falls at Gooseberry Falls State Park: Three separate lava flows form the waterfalls.
Middle Falls at Gooseberry Falls State Park: Three separate lava flows form the waterfalls.Courtesy Mark Ryan
Recently my wife and I took a day trip to the North Shore of Lake Superior to hike around, take pictures, and see some great geology. Scenic Highway 61 follows the lakeshore all the way into Canada, and along the way several state parks present some of the most stunning scenery and spectacular geology you’ll find in Minnesota.

In Duluth, a brisk wind made the lake a bit choppy down around the harbor entrance and small white caps rolled into the beaches of Park Point, but along the North Shore the wind subsided and the lake was placid and sparkling. It was a beautiful day, with bright sun and cool to moderate temperature, and air that was an especially clear. Across the lake, Wisconsin stood out in bold silhouette on the south shore.

Our first stop was Gooseberry Falls State Park just north of Two Harbors. One of nine state parks along the North Shore, Gooseberry is located just beyond the tunnels that cut through massive diabase ridges known as Silver Cliff and Lafayette Bluff. Diabase is an intermediate volcanic rock that due to its cooling rate, contains coarser-grained crystals than faster cooling basalt and smaller crystals than slower cooling gabbro.

Columnar jointing at Gooseberry: Polygonal jointing is evident across the top flow of the Middle Falls. The joints formed during cooling of the basalt a billion years ago.
Columnar jointing at Gooseberry: Polygonal jointing is evident across the top flow of the Middle Falls. The joints formed during cooling of the basalt a billion years ago.Courtesy Mark Ryan
Gooseberry offers easy parking and paved walking trails to view the three main waterfalls (Upper, Middle, and Lower). Despite the brisk afternoon air temperature, the waters of the Gooseberry River were warm and kids were having no problem swimming or standing under the falls. Three tiers of lava flows give the falls their structure partially due to the columnar jointing that took place when the basalt was first cooling from the outside in and shrinking into hexagonal joint structures in the process. The columnar jointing is easily viewed atop the Middle Falls. Down river the upper most flow makes up the ledges along the lakeshore.

You might wonder why so much volcanic rock is found along the North Shore. The reason’s because 1.1 billion years ago, the North America continent began to split apart in what’s called the Midcontinent Rift System. From Michigan up through the middle of Lake Superior, down Minnesota and into Kansas, the continent began to separate, releasing massive amounts of flood basalts and explosive rhyolites across the region during several eruptions. The level of volcanism was incredible, lasting for several million years. In some places along the North Shore it’s estimated that the lava flows are piled up 18 miles thick! But for reasons unknown (and lucky for us) the continental division was aborted, leaving us with interesting geology, several rugged parks, and gorgeous scenery.

Palisade Head and Shovel Point: The extrusive igneous rock, rhyolite, formed these high cliffs. Both were created from the same lava flow but the area between them has been eroded away.
Palisade Head and Shovel Point: The extrusive igneous rock, rhyolite, formed these high cliffs. Both were created from the same lava flow but the area between them has been eroded away.Courtesy Mark Ryan
Our next stop was Palisade Head located within the boundaries of Tettegouche State Park. A steep road takes you to the parking area at the top of the promenade. There are a couple low walled overlooks built from local rock for viewing but you can also hike around if you want. The view is fantastic. Shovel Point, located in the park proper, can be seen sticking out over the lake to the northeast with the Sawtooth Mountains in the background. Both Palisade Head and Shovel Point were created from the same lava eruption, but one quite different from those that created Gooseberry. Whereas flood basalts created Gooseberry, an explosion of rhyolite formed Palisade Head and Shovel Point. Rhyolite is chemically similar to granite but rather than being an intrusive rock (one that cooled slowly underground) as granite is, rhyolite is extrusive, meaning it cooled relatively quickly on the surface. Evidence shows the lava flow that created both Shovel Point and Palisade Head was one of great explosive energy. As the mass of magma neared the surface pressure dropped and volatiles in its mixture (such as water) expanded its volume and punched it through a vent in the Earth’s crust in a hail of molten ash and rock. The molten material settled into a pancake-shaped mass that eventually cooled into a highly erosion-resistant rock. But the relatively rapid cooling from the outside in once again caused columnar jointing, like that seen in Gooseberry.

Face to face with North Shore geology: Palisade Head's sheer cliffs challenge local rock climbers.
Face to face with North Shore geology: Palisade Head's sheer cliffs challenge local rock climbers.Courtesy Mark Ryan
The jointing and high cliffs that reach up 300 feet above the lake make Palisade Head a favorite site for rock-climbing. A group was doing just that when we were there.

Cobble beach at Sugarloaf Cove: Glaciers deposited most of the loose beach stones. Wave action shaped and sorted them.
Cobble beach at Sugarloaf Cove: Glaciers deposited most of the loose beach stones. Wave action shaped and sorted them.Courtesy Mark Ryan
Our final stop was Sugarloaf Cove just outside the town of Schroeder. This preserve is a recent addition to the scenic North Shore. From 1943 to 1971, the cove was used by a paper company to ship rafts of pulpwood across the lake to paper mills in Ashland, Wisconsin. But today practically all traces of the business have disappeared and the 34-acre site is being preserved by the State of Minnesota and the Sugarloaf Interpretive Center Association for educational purposes. Seven acres have been designated as a State Scientific and Natural Area. A fenced area protects a native plant restoration project where volunteers planted over 12,000 native trees, plants, flowers, and grasses.

Rock sorting at Sugarloaf Cove: Lake Superior's wave action sorted beach deposits along Cobble Beach. They're all along a 50-yard stretch of the beach.
Rock sorting at Sugarloaf Cove: Lake Superior's wave action sorted beach deposits along Cobble Beach. They're all along a 50-yard stretch of the beach.Courtesy Mark Ryan
There’s a one-mile trail that takes you through a pine plantation and alder thicket and down along the lake’s rocky shoreline and to the cove. Across the cove is Sugarloaf Point, which used to be an island but is now connected to the mainland by a tombolo, a shoreline feature formed from wave action depositing sediment over a shallow area of the lakeshore. As you walk the beach toward Sugarloaf Point you’ll notice the beach deposits become increasingly larger in size, going from fine sand to large cobbles, and even an occasional boulder. Glaciers carried most of these stones from areas north and northeast, smoothing their edges in transport and depositing them here as glacial till. Lake Superior’s wave action continued rounding the stones, at the same time sorting out the sand, gravel, and cobbles by size.

Multiple lava flows at Sugarloaf Cove: Zeolites (close up in right photo) show the top of one basalt flow in contact with the bottom of another.
Multiple lava flows at Sugarloaf Cove: Zeolites (close up in right photo) show the top of one basalt flow in contact with the bottom of another.Courtesy Mark Ryan
On the south side of Sugarloaf Point several lava flows can be seen piled one atop the other. When lava poured out on the surface a billion years ago, gases within the molten mixture formed bubbles that rose to the top of the flow much like bubbles forming on top of a pancake. As the molten material cooled into rock, these gas pockets – known as vesicles – became trapped. Additional pulses of lava soon covered the old flow and eventually everything became buried under younger sediment. At some point, hot ground water percolated through the buried masses of basalt and deposited different minerals in the vesicles in the process. These mineral deposits are known as amygdules and often appear as lighter crystals (zeolites) or agates inside the basalt layers. So when you see these lighter crystals in a rock sequence at Sugarloaf you know you’re looking at the top of a lava flow.

Lichen and cinqfoil on Sugarloaf Point: Along with wave action, several species of lichen and other plants are constantly at work eroding the North Shore basalts. The Sawtooth Mountains can be seen in the distance.
Lichen and cinqfoil on Sugarloaf Point: Along with wave action, several species of lichen and other plants are constantly at work eroding the North Shore basalts. The Sawtooth Mountains can be seen in the distance.Courtesy Mark Ryan
Vegetation on the point is very fragile, and visitors are requested not to walk out on it. At the far end of the point several lichens are working hard breaking down the basalt, along with the constant wave action of Lake Superior.

After this trip I’m anxious to go back and visit the other state parks along the North Shore. The area has so much to offer in terms of nature and geology. You can learn more by checking out some of the links below. There’s also a great book by geologist John Green titled Geology on Display that’s all about the geology of the state parks along the North Shore. If you get a chance this summer to visit these or any of the other beauty spots along Superior’s North Shore, take it - you won’t be disappointed.

LINKS

Jay Cooke State Park
Split Rock Lighthouse State Park
Crosby Manitou State Park
Temperance River State Park
Cascade State Park
Judge C. R. Magney State Park
Grand Portage State Park
More about the geological history of northeastern Minnesota
Restoration of Native Plant Communities at Sugarloaf Cove

Your Comments, Thoughts, Questions, Ideas

Thor's picture
Thor says:

Is that a current photo of Gooseberry Falls? It must be exceptionally dry up on the North Shore this summer.

posted on Tue, 07/21/2009 - 4:46pm
mdr's picture
mdr says:

Yeah, July 8th. It's dry just like many places in Minnesota this summer.

posted on Tue, 07/21/2009 - 6:06pm
samanthapha's picture

Oh i remember going to gooseberry falls last year in august. And it was dry up, there wasn't much water.
But it is still very pretty over there. And Lake Superior is very pretty, yet very cold.

posted on Thu, 07/23/2009 - 4:04pm

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