GEOLOGIC HISTORY

The geologic units exposed within Dodge County consist of bedrock formations, glacial drift, and modern sediments.  There are 
seven sedimentary rock formations exposed in Dodge County:  St. Peter Sandstone, Glenwood Shale, Platteville Limestone, Decorah 
Shale, and the Cummingsville, Prosser, and Stewartville Limestones which form the Galena Group.  These rocks were deposited in the 
Ordovician Period (458 to 447 million years ago).  During this time, southeastern Minnesota, including Dodge County, was submerged 
under a large continental sea.  As the sea transgressed over the land, one unit to be deposited was the St. Peter Sandstone.  This unit 
represents a marginal environment where large beaches and sand bars formed near the shore of an encroaching sea.  As the sea level
changed, becoming deeper (the sea transgressed) or shallower (the sea regressed) over millions of years, different sedimentary rocks 
were deposited.  Shales represent a near shore depositional environment and limestones represent a calmer, deeper, off shore 
environment.  From the time of deposition, these sediments have undergone an extensive history of deposition and erosion.

Much later in time, during the Quaternary Period (2.5 million years to Present), continental glaciers advanced over most of Minnesota.  
These continental ice sheets originated in Canada and slowly moved southward through Minnesota.  Several different glacial advances 
originated from the northeast, flowing through the Lake Superior area.  Many other glacial advances originated from the northwest, 
flowing through the Winnipeg area.  As these glaciers advanced, they picked up (eroded) bedrock and other surface material along their 
path.  These glaciers continued to advance, transporting some of this material into Dodge County and throughout Minnesota.  The 
sediments contain indicator pebbles that were transported from a specific bedrock source area.  As the glaciers melted (receded), they 
deposited sediments that had been eroded and transported from the northern areas.

Most of the county is covered in glacial material that was deposited from about 300,000+ to 130,000 years ago.   The thickness of 
these deposits varies from several feet to several hundreds of feet.  Generally, the glacial material is thicker in the west and thins to 
the east.  Because the glacial material is thin to the east, the eastern landscape is most influenced by the pre-glacial landscape:  
stream dissected bedrock valleys.  The dissection is dendritic or leaf-like in shape and primarily follows a west-east trend.  For 
this reason, the eastern landscape is very rolling and the farm practice of terracing is observed along the sides of some hills.  The 
western landscape has little topographic relief.  Here the glacial material is very thick and underlying bedrock terrain has little to no 
expression on the surface.  

The older glacial deposits have been subjected to hundreds of thousand years of terrestrial erosion associated with glacial and 
non-glacial climate changes.  Both wind and water have eroded the landscape to produce a wind-scoured surface that may be 
overlain by loess in some areas.  Loess is wind blown silts and clays.  Most of the loess can be observed in the northeastern portion 
of the county.  Some of the loess was derived from wind blowing off of the glacial lobe called the Des Moines Lobe.  This lobe flowed 
from Manitoba, down the Minnesota River valley, and south to Iowa.  The very eastern margin of the lobe flowed onto the very western 
margin of Dodge County (~14,000 years ago).  When the Des Moines Lobe retreated, the meltwater deposited outwash in the valleys of 
existing streams and rivers.  Today, sediments are continually being eroded and deposited. These sediments form as the result of 
recent geological processes.  Modern streams are creating fans, terraces, and bars that are constantly changing in size and shape.  
