GEOLOGICAL HISTORY

The geological units exposed within Benton County consist of bedrock formations, glacial drift, and modern sediments.  The oldest geological material in 
Benton County consists of granitic rocks that were formed during the Precambrian Age (4.65 billion to 570 million years ago).  The largest structure 
within the county is the Foley Batholith, which primarily consists of pink granite.  This batholith was formed as a very large body of igneous rock cooled 
deep below the earth's surface.  These rocks are thought to have formed during the Early Proterozoic (2.45 billion to 1.75 billion year ago).  The only 
other bedrock unit exposed in the county is the gray granodiorites in the extreme southwestern corner of the county.  Several other bedrock units, such 
as diorite, gneiss, and schist, are found within Benton County; however, they are buried under thick accumulations of glacial material (Figure 1).  There 
are also several faults and dikes throughout the county.  The faults caused displacements in the rock bodies, ranging from a few feet to thousands of 
feet.  The dikes within Benton County are primarily made of diabase that formed as hot, molten magma cut through and filled cracks in the bedrock units.  
For more details on the bedrock geology, see Jirsa, et al., 1995.

Much later in time, during the Quaternary Period (2.5 million years ago to Present), continental glaciers advanced across almost all of Minnesota.  These 
continental ice sheets originated in Canada and slowly moved southward through Minnesota.  Several different glacial advances occurred from the 
northeast, through the Lake Superior area, while another glacial advance occurred from the northwest, through the Winnipeg area.  As these glaciers 
advanced, they picked up (eroded) bedrock and other surface materials along their path.  These glaciers continued to advance, transporting some of this 
material into Benton County and throughout Minnesota.  As the glaciers melted (receded), they deposited sediments that had been eroded and 
transported from the northern areas.

Benton County was completely covered by a large ice lobe that came from the northeast called the Superior Lobe.  As this ice flowed over the landscape, 
it created features called drumlins.  Drumlins are basically elongated oval ridges of compacted glacial till that were built under the margin of ice and were 
shaped by its flow.  The orientation of these drumlins shows that the ice flow was towards the west in the northern part of the county and towards the
southwest in the southern part of the county.   As the glaciers melted (retreated), they deposited the sediments that had been eroded and transported 
from the northeast, including volcanic rocks, such as gabbros and basalts, and other material from the Lake Superior region.  

As the glaciers melted, large streams formed that sorted the material as they flowed, carrying away the lighter silts and clays while depositing the 
heavier and coarser sand and gravel material (often termed outwash).  The remaining unsorted material that was incorporated in the ice was deposited 
as a red sandy till.  Later, a smaller ice lobe from the south just barely made its way into the southwest part of the county.  This was a northward 
extension of the Des Moines Lobe ice called the Duelm bulge.  As the Duelm bulge retreated, it left behind a very thin layer of till on top of the Superior 
Lobe sediments.  This Des Moines Lobe material is very different than the Superior Lobe material.  It came from the northwest and consists of 
carbonates, granites, mafics, and shale that were picked up from the Winnipeg, Manitoba, area.  As this lobe melted, a large outwash channel called the 
Duelm Channel formed.  Due to its size and amount of flowing water, this channel was able to carry large amounts of sand and gravel (primarily sand) 
to the east and finally south into Sherburne County.

After all the glacial activity ceased and the ice melted (~12,000 years ago), modern sediments, such as alluvial sand bars, beaches, flood plains, fans, and 
terraces, began to form.  These Holocene (10,000 years ago to present) sediments continue to be deposited today as a result of recent geological 
processes.  The alluvial sand bars within the Mississippi River are constantly changing in size, shape, and even existence.  The beaches, alluvial flood 

plains, fans, and terraces are modified continuously; however, they most actively change in the spring and summer.  The eolian (wind blown) material 
continually changes the shape of the landscape on a large (e.g., dune features) and small (e.g., ripple marks) scale as the wind continues to blow all 
year long.  Geological processes will continue to modify the landscape of Benton County now and into the future.


REFERENCES

Jirsa, M.A., Chandler, V.W., Cleland, J.M., and J.P. Meints, 1995, Bedrock Geologic Map of East-Central Minnesota, Minnesota Geological Survey, 
Open-File Report 95-1, Plates 1 and 2, Scale 1:100,000.

Meyer, G.N., Knaeble, A.R., and J.B. Ellingson, 2001, Surficial Geology of the St. Cloud 30 x 60 minute quadrangle, Central Minnesota, Minnesota 
Geological Survey, Miscellaneous Map Series Map M-115, Scale 1:100,000.

Meyer, G.N. and A.R. Knaeble, 1995, Surficial Geology, Geological Atlas of Stearns County, Minnesota, Minnesota Geological Survey, County Atlas 
Series, Atlas C-10, Part A, Plate 3.

Meyer, G.N. and H.C. Hobbs, 1993, Quaternary Geologic Map of Sherburne County, Minnesota, Minnesota Geological Survey, Miscellaneous Map Series 
Map M-77, Scale 1:100,000.
