Download REGION II: Southeastern Minnesota

Southeastern Minnesota
GEOLOGIC OVERVIEW
Buried very deep under the surface of Southeastern Minnesota are ancient igneous rocks similar to
the North Shore. On top of all these older rocks are several hundred feet of rock laid down in
shallow seas about 600 million years ago. There is also some glacial debris brought in from other
areas. The entire area has been eroded for the last 300 million years by wind, water and ice. Deep
river valleys carved by glacial melt-water are the major surface features. These valleys easily show layers
of rock that would normally be buried.
Most of Minnesota has been covered by glacial ice several times in the last 15,000 years but one thing
separates Southeastern Minnesota. The last ice age – the Wisconsin - did not cover Southeastern
Minnesota. For this reason there is much less glacial sediment and more deeply carved river valleys
compared to the rest of the state. When the climate began to warm up and the glaciers elsewhere
began to melt, the glacial melt water flowed over this area and carved deep valleys. These deep river
valleys show the history and evolution of life in Minnesota through fossils and layers of rock that are
shown instead of being deeply buried.
Surface water, wind and glaciers are not the only force that has shaped Southeastern Minnesota.
Naturally acidic groundwater has slowly eroded bedrock and created incredible land features such as
caves and sinkholes. These are the forces that have shaped this region.
ROCKS
Rock cycle processes in the past include deposition of sediment at the bottom of shallow seas and
erosion and weathering by flowing glacial melt water. Rock cycle processes that are occurring today
are normal weathering and erosion by precipitation, runoff, and rivers. In Southeast Minnesota there
is also weathering and erosion of underground rocks from naturally acidic groundwater.
Shale: During the Cambrian and Ordovician time periods, about 600 million years ago, most of
Minnesota was covered by shallow seas. The oldest (and deepest) rocks are shale. The greenish-grayblack sedimentary rock was formed by mud-like clay particles falling out of ocean water making thin,
flat layers on the bottom that hardened into rock. The greenish color is thought to be from the
digestive processes of marine worms.
Sandstone: Sand with iron and calcite minerals layered on the bottom of the ocean. They were
pressed and cemented together to form this sedimentary rock. These sandstone layers range from
white to yellow to red. The Jordan layer of sandstone holds a significant amount of groundwater and
much of this area gets drinking water out of this sandstone.
Dolomite: This gray sedimentary rock is dominated by the minerals calcium and magnesium.
Dolomite is also formed as these minerals dropped out of ocean water in layers and cemented into a
hard rock.
Limestone: As the ocean water continued to deepen and quiet, calcium carbonate was deposited
from the warm seawater and the shells of many marine animals. These materials were pressed and
cemented together to form rock. Limestone in Southeastern Minnesota is a yellow-tan-gray
sedimentary rock.
FOSSILS:
Marine animals were common in the shallow seas of Minnesota 500-600 million years ago. Marine
fossils may be found in practically any sedimentary rock cut or quarry in Southeastern Minnesota.
Park along a road cut (where you can see rock layers) and look at the ground in the ditch. Rainwaters
continually wash fossils out of the soft rock into the ditches. The sedimentary rocks that you find
fossils in formed on the bottom of an ocean, to touch a fossil is to go back in time.
MAJOR FEATURES
Driftless Area: Extreme Southeastern Minnesota has a rugged landscape of broad valleys and ridges.
It was apparently never covered by glaciers (glaciated) or was glaciated so early that no evidence
remains. Soft sandstone hills and spires (spikes) are common here but they are not found in the rest
of Minnesota where glaciers eroded them. Large, old river valleys were deepened and widened from
glacial melt waters but were never scoured or buried in glacial sediment. Why wasn’t this area glaciated?
Highlands to the northeast and northwest deflected the southward flowing ice.
The Root River Valley near Eagle Bluff Residential Environmental Learning Center will show you
one of these wide river valleys that was carved by glacial melt water thousands of years ago, not the
current Root River. The Root River is a post-glacial river.
Karst Topography: Nine counties in Southeastern Minnesota have a karst landscape, including
eastern Rice County. Karst is a landscape that develops in areas with limestone and dolomite at the
surface that is dissolved by naturally acidic groundwater. Six features are dominant in karst regions.
- Streams that disappear underground
- Valleys with no streams
- Caves
- Springs
- Sinkholes
- Little flowing surface water
Karst forms naturally when limestone is dissolved by slightly acidic water. Groundwater mixes with
carbon dioxide to form a weak acid (carbonic acid). Carbon dioxide gas comes from the air and from
bacteria decomposing in the soil.
Below the water table, limestone is always in contact with the groundwater. This contact speeds up
the dissolving of the rock. Cracks and holes are formed in the rock. Larger spaces are called
passageways, caverns, or caves. If a cave roof falls in, the hole at the surface is called a sinkhole.
Water moves fast in karst regions. In non-karst areas, water must pass slowly through pore spaces of
rocks and soil, slowing the water down. Cracks and holes in karst rocks let water through easily. Pollutants
can also get into the groundwater easily. Runoff from storms can become groundwater in hours or
minutes. Sewage, fertilizers, oils, and other harmful materials can damage groundwater very quickly
in karst regions. This pollution puts our greatest natural resource, groundwater used for drinking
water, at severe risk.