Ice Age: Where There's Water, There's a Way

The Ice Age

Wisconsin shore St. Croix River Dalles

The Wisconsin glacial period lasted from about 85,000 - 11,000 years ago.  During this time ice covered most of North America north of 38 degrees north latitude to a depth of 2 miles.  As the ice sheet melted about 10,000 years ago, the meltwater filled parts of the basins of modern Lakes Superior, Huron and Michigan.  The natural drainage of this lake was to the east, but a lobe of the glacier blocked that exit.  Water from an area of ice sheet the size of Minnesota and a mile deep filled glacial Lake Duluth until water level was 400-500 feet above the level of modern Lake Superior.

Rounded and polished boulders

When the water overtopped the hills forming a natural dam on the southwest side, the water ran in torrents down the modern Bois Brule and St. Croix River Valleys for years.  The water cut easily through Cambrian sandstones, but ran over the top of harder rock.  South of here, the rush of water cut deeply into sandstone forming a huge waterfall.  The erosive action of the water cut away the foot of the fall and marched upstream and the lip broke loose, eventually cutting into the hard basalt that would form the narrow chute we call the Dalles of the St. Croix River.  The force of the water running by here is likened by park interpreters as that of a fire hose.  Only the water was full of sand, rocks and boulders.  These can be seen in the smoothness of the rocks polished by the action of that sediment-laden water.

But Doctor Hoffman, where did the sand, rocks and boulders come from?  Wow, great question, glad you asked.  As the Midcontinent Rift formed, water and cycles of freezing and thawing wore down the basalt and the dirt filled the rift valley.  In lakes and rivers of the rift valley, sandstones and shales were deposited.  Add to that the material that was scooped out by glaciers during the ice ages and deposited at the margins of the glaciers as they melted.  Turn on the jets from an overtopped glacial lake dam, and it scours out the loose stuff just like a power washer pushing dirt across your porch or driveway.

Fun Potholes

Small pothole cut through rock

As a torrent of water runs over a rocky bed, there are areas where eddies form.  These swirling circular currents will start to scour out a depression in the rock with the sand and rocks that it moves.  If a rock rolls into the depression and cannot move out, the water may be able to rock it and roll it around, smoothing the rock into a round grindstone.  These grindstones can then swirl around the perimeter of the hole, cutting it deeper.  Interstate Park has the highest known concentration of these features and some of the deepest known potholes.

Pothole

The flow of water out of Lake Duluth may have continued for hundreds of years.  Only when the Superior lobe of the Laurentide Ice Sheet melted did the water flow downhill toward the east.  One can only imagine how much water and force was required to carve out the St. Croix Dalles and these potholes.  It must have sounded much like it would standing at the bottom of Niagara Falls.  The small pothole figured above is only about six inches across and about two feet deep, tunneling through the entire rock layer.  There are over a hundred potholes in the boundaries of the park that are much larger than this.  We were actually standing in a twenty feet deep pothole when we took the picture of the left, which shows another pothole that was cut even deeper.

Grindstone

Conglomerates

Conglomerate on rocks

Scoured potholes, grindstones, a gorge cut through tough rock, and rounded boulders all point to a huge flow of water through here.  Not just water, but water full of boulders, rocks, pebbles, and sand.  How can we be sure? As we were hiking through the woods, we came upon some rocks at least 30 feet above the current level of the water covered with conglomerate.  This mixture of sand and pebbles solidified after one of these tremendous floods and cover the rocks in places.  The water flowing out of glacial Lake Duluth would have been chock full of this stuff, ideal for cutting through rock.  The pebbles in the photograph range up to about 1 inch in width.

Creation of the Park

The basalt in this area was perfect for making gravel for building material, a business venture proposed by a group of St. Paul businessmen in the 1860s.  Public interest to preserve the scenery was piqued and influential people like George Hazzard and William H.C. Folsom led the push for preserving the Dalles.  George Hazzard was a travel agent, Chamber of Commerce chair and secretary of the Red Rock Camp Meeting Association, which pushed for the combined Minnesota and Wisconsin park idea.   Folsom led the legislative push to establish the Minnesota side of the Park.  The Minnesota side was established in 1895 and was the second Minnesota State Park, while the Wisconsin side was established in 1900 as the first Wisconsin State Park.

Deep pothole

Rock with several small potholes

Getting There

Waypoint: Latitude 43.3936727 N; Longitude 92.6709847 W
Street Address: US-8/St. Croix Trail and Milltown Rd., Taylors Falls, MN 55084


On I-35 north of the Twin Cities, take Exit 132 to Taylors Falls (US-8 East).  The entrance to the Park (Milltown Rd) is in Taylors Falls just west of the bridge to Wisconsin.  There is another part of the park in Wisconsin (hence the name Interstate State Park)

Pothole filled with debris

Breaking Up is Hard to Do: Continental Rift

Wisconsin side of St. Croix Dalles as seen from Minnesota

One of my favorite questions to ask students in Biogeography or Evolution is "What evidence do you see around you of a changing Earth?"  It is an important question; one of the many that were just starting to be addressed as Charles Darwin set foot on the HMS Beagle.  Our time on Earth is so fleeting and the time required for massive change is so long, that it is no wonder that many people assume that the Earth is essentially as it always has been.  If only time machines were a thing.  Where are Doc Brown and Marty McFly when you need them?

Nadienne blending in

The evidence is there if you open your eyes and ask the right questions.  Many places exist that act as our time machine, allowing us to see structures on Earth that have been hidden for hundreds, millions, or even billions of years.  One of my favorite evidences for a changing Earth has to be Interstate State Park, which flanks the St. Croix River in Minnesota and Wisconsin, just north of the Twin Cities.  Here, Ice Age floods have scoured out a record of a time when the North American Craton (core continental crust) began to tear apart into new continents about 1.1 billion years ago.

Volcanic Rocks...In Minnesota and Wisconsin?

Pillow lava with flood basalt deposited on top

A lot of people are surprised to hear that there are deposits of basalt or other volcanic rock in places where there are no obvious volcanoes, but they are here.  When I walked down into one of the park valleys, even knowing that I was looking at basalt I was still a bit surprised to see signs of classic pillow lavas.  This lava breaks out to the surface underwater, and the water cools the surface of the lava, but the inside keeps flowing which causes big blobs of basalt to form as in the bottom half of the picture at right.

Bubble pattern shows successive volcanic episodes 

Reading the record in the rocks (yes, I do see the world in equation form and process diagrams), it is possible to see evidence for at least seven basalt flows at Interstate State Park.  When a basalt erupts to the surface, gases escape from the molten rock, and bubbles of gas are trapped in the thicker cooling rock at the surface of the deposit.  Looking closely at the rocks, you can see a layer of bubbles from one eruption, then see a smooth deposit with few bubbles from the next eruption, then the incidence of bubbles increases as you move upwards, until the cycle repeats itself.

Multiple volcanic episodes seen...bubbles...no bubbles...bubbles...no bubbles...bubbles

Some of those bubbles can be quite

Amygdules in basalt

large.  Over the course of a billion years, and under a pool of water like Lake Superior and its ancestral precursors, water percolates through the rock and drags minerals into these cavities.  When the concentration of mineral gets high enough, they crystallize.  Several of the rocks here contain amygdules of quartz and feldspar.  In the Lake Superior basin, many of these deposits are banded in beautiful reds and browns from iron, forming the famous Lake Superior agates.  These rocks were released from their lava prisons by cycles of freezing and thawing, then tumbled and transported south by glaciation.  We find them (rarely) in northern Missouri, and they are relatively common in the Mississippi River valley north of Missouri.

A large Lake Superior agate

The Midcontinent/Keweenawan Rift

Arms of rift in red with direction of rifting indicated

Most of the rocks in this volcanic system lie buried, but the metals that they contain create a gravity high that can be measured.  The system runs from Lake Superior east into Michigan, southwest into Kansas/Oklahoma and perhaps north to Lake Nipigon in Ontario, Canada.  The basalt ranges from two to twelve miles in thickness, and is buried by as much as six miles of sediment in some places.  Surface exposures are found only around Lake Superior and south through the St. Croix River valley.

The crust of the Earth is relatively thin and broken into a variety of continental and oceanic rocks.  These plates move with respect to each other, driven by the tremendous heat engine that is the mantle.  Seafloor spreads from an upwelling of magma that seeps through to the surface depositing igneous (volcanic) rock that is relatively dense.  Deposition of new material here will push two plates apart.  Heated magma will also rise and strike the bottom of the plate, dragging it along by conveyor action.  Eventually, two plates are going to collide with each other.  Oceanic crust is denser than continental crust, and tends to slip under the continental crust, pushing on the edge of the continent, elevating the continental edge.  A deep trench is evident where this subduction is taking place.  Gravity and density work together and pull the subducting plate downwards.

Stages of rifting - Midcontinent Rift stopped between steps  5 and 6

As the oceanic crust melts, the water in it superheats and may start melting the continental crust, far away from the subduction zone.  The rising plume of magma will cause a dome to form in the crust, eventually the heating by the magma will cause the lithosphere (crust and upper layers of mantle) to become more plastic and stretch.   The lithosphere will sag under the weight of the overlying crust, causing faults (breaks) to form in the crust, forming a series of valleys and ridges.  Magma may find its way to the surface in the region of the mantle plume or intrude into pre-existing rocks further away.  The valleys may fill with water, as with the Rift Valley Lakes in East Africa.

As igneous rocks are deposited in the rift, the rift widens.  If rifting continues long enough, a new ocean will open up, separating the continental land mass.  This process is easiest to see in East Africa, where a triple-junction has opened from a mantle plume under Ethiopia.  Extension of the rift to the northwest is tearing the Arabian peninsula free from Africa, and the Mediterranean Sea will be connected to the Indian Ocean, as well as the Atlantic. Rift extension to the southwest will separate the African Plate into the Nubian Plate and Somalian Plate, with an oceanic corridor between them.

East African Rift triple junction

Round and round

Pillow lava overlain by flood basalt

This predicts a cycle of events in which oceanic widening pushes continental landmasses together into a supercontinent.  Rifts that appear in the supercontinent results in oceanic intrusion and the landmasses float apart into separate continents.  Continued seafloor spreading pushes the landmasses together again and another supercontinent  forms.  Currently, spreading of the Atlantic Ocean basin is pushing the Americas away from Europe and Africa on the east, while the Pacific Ocean is closing on the west and the Americas and Asia are getting closer.  The 2011 earthquake in Japan caused Japan to shift closer to North America by 13 feet.  The usual rate of plate movements is about the same as the growth of your fingernails.

Rodinia reconstructed showing mountain building areas (green)

The Midcontinent rifting took place when the continents were assembling into the supercontinent of Rodinia.  Minnesota was located very close to the equator and directions were rotated about 90 degrees from present (west now was north then).  The mantle plume for the Midcontinent Rift was under present-day Lake Superior.  The rifting resulted in quite a bit of volcanic action which is evident on the surface in the north.  Pillow lavas which were deposited at the bottom of rift lakes were overlain by later flood basalts that erupted from fissures in the Earth.  From Iowa south, the volcanics rose up into pre-existing rocks, but probably did not break through to the surface.

Continental movement prediction for the next 250 million years

Rifting lasted for 15-22 (ish) million years, then stopped.  This break nearly opened up a way to the ocean; it is the deepest rift known to have healed.  Geology of the region suggests that the crust in this region thinned to 25% of its pre-rifting thickness.  The region of Lake Erie was about continents edge at the time of this rift.  The East Coast of North America was added through later collisional events.  During the formation of Rodinia, it is thought that the North American craton collided with the Rio Plato, Amazonia and Baltica cratons, resulting in the formation of mountains.  This mountain building episode is called the Grenville orogeny, and may have put enough pressure on North America to halt the spreading of the rift and reverse it.  Many blocks of reverse faulted crust can be found, being pushed upwards by compression.

Rio Grande Rift

Continents will continue to move, stretch and break as long as there is molten rock on the interior of Earth.  There is some thought that the New Madrid fault zone between Missouri and Tennessee is the product of a failed rift.  Active rifts abound, famously including the East African Rift.  The magnificent lava fountains of Iceland are formed by the Mid-Atlantic Rift reaching the surface of the water.  In North America, there are several active rift zones, including Death Valley and the Rio Grande River Valley.  Hotspot plumes of mantle trace a path in the Hawaiian Island chain and across Idaho into present day Yellowstone National Park.  Present models suggest that 250 million years from now, the landmasses of Earth will again coalesce into a supercontinent, this time called Pangaea Proxima.

Getting There

Waypoint: Latitude 43.3936727 N; Longitude 92.6709847 W
Street Address: US-8/St. Croix Trail and Milltown Rd., Taylors Falls, MN 55084


On I-35 north of the Twin Cities, take Exit 132 to Taylors Falls (US-8 East).  The entrance to the Park (Milltown Rd) is in Taylor Falls just west of the bridge to Wisconsin.  There is another part of the park in Wisconsin (hence the name Interstate State Park) just across the bridge at Taylors Falls.

Further Reading

Interstate Park - MN DNR

Midcontinent Rift System in Iowa - Aerial Surveys