Minnesota’s Tropical Beaches
Imagine a sandy, tropical seashore extending across southern Minnesota—part of a vast,
shallow sea that covers much of North America. The sandstone, shale, and limestone rock
layers that you can see exposed across much of the Twin Cities area tell the story of the
conditions that existed hundreds of millions of years ago, during the early Paleozoic era.
Although many people are not aware of the geologic history that made the Paleozoic
bedrock in Minnesota, the rocks are familiar to anyone who has seen the bluffs along the
Mississippi, St. Croix, and Minnesota rivers in southeastern Minnesota.
To understand how the bedrock of southeastern Minnesota was formed, you need to
know processes: deposition and erosion. The earth is broken down by wind, water, sun
and ice into smaller particles. Erosion is the loosening and movement of those particles.
The dust we see in the air, the muddy water in streams, and even the potholes in roads are
evidence that the land is being leveled and eroded. Deposition is the dropping of those
particles by the wind or water into layers. These layers are also called beds, which is why
they are called bedrock after they have been cemented and squeezed into rock over time.
Small grains of sand settle to form sandstone, very small particles settle to form shale,
and chemicals like carbonate that form the shells of clams and other ocean organisms can
form rock called limestone.
In earliest Paleozoic time, North America was on the equator, with Minnesota a little
south of the equator! Minnesota was a low-lying, mostly flat area. Although the climate
was probably tropical, you wouldn’t have seen any palm trees or tourists, because land
plants and animals had not yet evolved! The land had nothing but some primitive algae
and bacteria. Sea level began to rise much higher than it is today, and most of North
America was covered by ocean. As a result, about 520 million years ago, southern
Minnesota, Wisconsin, and Iowa became a shallow sea with islands at Baraboo, WI and
Near Taylors Falls (Figure 1). Over the next 200 million years, sediments (sand, smaller
particles and carbonate chemicals) eroded and settled into the sea in more or less flat
layers. These layers were buried by other layers and were eventually cemented into layers
of bedrock (Figure 2). Shells of snails and evidence of other organisms were preserved in
this bedrock and became fossils that you can still see today in some of the layers of
Paleozoic rock.
Figure 1. This is a map of Paleozoic southern Minnesota and parts of Wisconsin and Iowa
when a shallow sea covered much of North America. At times, when sea level was much
higher than shown here, most of Minnesota was covered with water. At other times, sea
level was much lower and all of Minnesota was dry land. The line from B-B’ shows the
area from Figure 4 as it would look viewed from the side in a cross section.
Figure 2. This is a model of how deposits were made in the shallow sea shown in Figure
1 during the early Paleozoic Era. Sand and smaller particles were carried by rivers to the
shoreline. Storms and waves at the shoreline sorted the particles so that the bigger
particles like sand were deposited at the beach and shoreface, and the smallest particles
like clay were carried out further off shore into the deeper water where the water was
calm.
The Paleozoic rock layers are more than 1,500 feet thick in some places, and were
deposited over a time span of 200 million years during three geologic time periods called
the Cambrian, Ordovician, and Devonian (see Figure 3).
Figure 3. Cross section of bedrock at the Minnesota-Iowa border
The rocks can tell us about the conditions during the time that they were formed. The
earliest rocks from the Cambrian period are sandstone. Picture a sandy coast like the Gulf
of Mexico—but without the palm trees and tourists. The waves continually roll back and
forth, sorting the sand grains and rounding them until they are almost all the same size.
At the same time, smaller grains and clay-sized particles could float much further out into
deeper water before they fell and settled to the bottom (see Figure 2). The sandy shoreline
layers became sandstone, while the deep water layers became shale.
Sea level changed during the Paleozoic, like it is changing now with global climate
change and glaciers melting. As the sea over Minnesota got deeper, nearly all of
Minnesota and surrounding areas were flooded and limestone was deposited at the
bottom of these deep seas.
By looking at a cross-section diagram of the deposits in Lillydale Park in St. Paul, you
can see how the water level over the Twin Cities changed over time. Sandstone was
deposited when the shoreline of the ocean moved over the area and can be seen as the St.
Peter sandstone (Figure 4). The water became deeper as sea level went up, and shale was
deposited. When the water was deeper and the area was more like a coastal shelf, the
Glenwood shale was deposited over the St. Peter sandstone (Figure 4). When sea levels
were highest, and the sea water was deepest over Minnesota, sand was not able to go all
the way from the shore out to the deepest sea floor. The rocks that were deposited in the
Twin Cities area when sea levels were highest were made of carbonate chemicals that fell
out of the sea water and the remains of the animals that fell to the bottom and became
fossils. These rocks formed in the deepest water became the Platteville limestone and
were deposited over the Glenwood shale (Figure 4).
Figure 4. Layers of rock at Lillydale Park in St. Paul. The oldest layers are at the bottom,
and the newest layers are at the top. The numbers on the left show elevation above sea
level.
The sea became shallower again and the Decorah shale was deposited, and when sea
levels rose again and the sea got deeper again, the galena limestone was deposited.
There is no bedrock in the Twin Cities area that represent the time period after the Galena
limestone (newer than 350 million years old). This is because the oldest rocks (Figure 4,
Shakopee dolomite) are buried the deepest, and the youngest rocks are closest to the
surface (Figure 4, Galena limestone). Rock that was even younger (closer to the surface)
than Galena limestone got worn down and scraped off or left as a mixed up layer called
glacial till (Figure 4).
Fossils are evidence of organisms buried and preserved in sediments. They are hard parts
like shells and bone usually, but may also be tracks, imprints, and burrows. The shallow
sea that covered Minnesota during the Ordovician period contained a variety of salt water
organisms including corals, clams, snails, bryozoans, brachiopods, cephalopods, and
trilobites (see Figure 4). Many of these groups of organisms can be found in today’s
oceans, but many of the individual species from the Ordovician period are extinct or very
rare in today’s oceans. Some groups, like trilobites, are extinct altogether.
The Decorah shale is the best bedrock in the Twin Cities area in which to collect fossils.
Life was abundant in the calm water environment in which these sediments were
deposited. This type of environment was also good for preserving fossils, and many can
be found whole.
The St. Peter sandstone has very few fossils because it was a shoreline deposit. Waves
and oxygen broke down organisms very quickly and didn’t preserve them well.
The Galena and Platteville limestone layers were deposited in clear water under calm
conditions, and both contain many fossils. They are difficult to get out of the hard
limestone, but can be found on the bedding planes where the rock can be pried apart.
Descriptions of the Ordovician fossils that can be found in the Twin Cities area are in
Appendix 1. Drawings of what the fossils look like can be found in Appendix 2.
Appendix 1. Ordovician Fossils of the Twin Cities Area
Appendix 2: Illustrations of Ordovician Fossils in the Twin Cities Area
This article was adapted by John Roper-Batker for the Minneapolis Public Schools from
the following sources:
“Minnesota at a Glance: Ancient tropical seas—Paleozoic history of Southeastern
Minnesota” by A.C. Runkel, 2002, Minnesota Geological Survey--University of
Minnesota
“Minnesota at a Glance: Fossil Collecting in the Twin Cities Area” by J. Mossler and S.
Benson, 2006, Minnesota Geological Survey--University of Minnesota
“Minnesota Geologic Timetable” adapted from “Digging Into Minnesota Minerals” MN
DNR, Minerals Division 1995
“Geology of Minnesota: A Guide for Teachers” by G.B Morey and H. Dahlberg, 1995,
MN DNR
Minnesota’s Natural Heritage, by John Tester, 1995, University of Minnesota Press