Gustavus Adolphus College

Geologic History of Southern Minnesota

Southern Minnesota was first formed about 4.5 billion years ago with the beginning of the Precambrian Era. As with the rest of the earth's crust, the North American continent was just beginning to form out of tectonic plate activity, shaping outcroppings of gneiss and volcanic rock that are now the deep foundations of the continent (Minnesota DNR, 2005).

In the middle to late Precambrian period (1,110–1,090 mya), Minnesota experienced a great deal of volcanic activity. Lava that reached the surface during this period formed overlapping layers of basalt and dolomite. A surface layer of this rock can be found today at the Kasota Prairie (DNR, 2005).

During the Paleozoic Era (600–225 mya), Minnesota was covered with advancing and retreating seas, depositing sea life that can be found in the fossil beds of the southern part of the state today.

The current face of the Minnesotan landscape was formed in the quaternary period (2 mya to present). Immense glaciers formed when the changing climate brought more snow than could melt over a long period of time. As the Laurentide Ice Sheet moved across what is now the Midwestern United States, it sculpted the earth's surface by picking up large pieces of rock and depositing them in other areas (Changnon etc. 2002). Huge chunks of ice would fall off the glacier in its passing, leaving the ice buried in a pile of rock debris called glacial moraines. As the ice melted and left small depressions filled with fresh water, it formed what are known today as prairie potholes. Minnesota once contained over 10 million acres of wetland after the latest glaciation, and there may have been as much as 20 million acres of prairie wetlands found across the Great Plains (Spieles, 2001).

The southwestern corner of Minnesota, dubbed the "Prairie Pothole Region" (PPR) includes Oak Leaf Lake, an example of a local prairie pothole. We can get a glimpse of what the land cover and vegetation might have looked like before Western settlers arrived in the mid-1800's by looking at a vegetation map by Francis J. Marschner in 1930. The map was based on 1847–1907 public land survey notes, and serves as an important record of what the state looked like before wide-spread settlement changed its face:

1. Presettlement Land Cover of the 7 Mile Creek Watershed

Notice that the majority of the watershed consisted of prairie, with a few lakes and scattered wetlands.The middle swath of green and brown is the floodplain forest, surrounding what we know today as Seven Mile Creek.

Settlement of Southern Minnesota

The first settlers of southwestern Minnesota arrived in 1853, and the population has grown to over 50,766 people in Nicollet and Nobles counties combined (US Census Bureau, 1997). Settlers in the 19th century brought with them a need to live and work on the land around them. The fertile prairie soils provided perfect substrate to plant crops such as wheat, corn and beans. Prairie pothole regions were seen as undesirable, but were considered usable once drained (Spieles 2001). The Federal Swampland Acts of 1849 placed wetlands management under the jurisdiction of the state, and the state of Minnesota was quick to encourage the draining of wetlands in the name of economic growth. Most of the drainage in the state occurred from 1858 to 1920 (2001). Not until the 1950s did a large majority of people become actively concerned about the ecological integrity of Minnesota's natural habitats, but by then most of the state's original wetlands habitat had been drained and prairies converted to farmland (Spieles 2001). An example of a drained lake bed can be seen at the Mudd Lake Bed in the northeastern corner of the watershed. Compare the aerial view of the dry lake bed in 1938 and 1990:

2. Compiled from 1938 photos by students at Gustavus Adolphus College

3. 2003 aerial photograph

The Minnesota River was originally carved by the Glacial River Warren, and the floodplain habitat has morphed along with river. In the 1900s, settlers cut down much of the large stand growth for steamboat fuel (Johnson-Groh, pers. comm. 2005). Agricultural run-off contributed to the Minnesota River being the most polluted in the state in the early 1990's (Johnson-Groh, pers. comm. 2005).The floodplain is highly affected by changes in other ecosystems, such as increased flooding due to the loss of wetland reservoirs. Deciduous forest is found on the edge of its range in southwestern Minnesota; it is limited in the north by decreasing temperatures and in the west by fire and limited moisture (Johnson-Groh, pers. comm. 2005). It is almost always found in conjunction with a ravine or riverbed where it is protected from fire (Johnson-Groh, pers. comm. 2005). Deciduous forest has not been converted to agriculture in the extensive manner that prairie has, and it is more highly threatened by fragmentation. In fact, forest habitat is increasing in many areas due to the suppression of natural fire (Johnson-Groh, pers. comm. 2005).

Southwestern Minnesota is part of the Northern Tall Grass Prairie Ecoregion (Gagnon 2004). Tall-grass prairies are considered one of the most endangered ecosystems in North America.  Of the original tall and mixed grass prairies in this region, only 0.3% are left intact. In Nobles County alone, 1,000 of the original 137,000 acres of wetlands remain, resulting in fragmented and disturbed ecosystems (Johnson-Groh, pers. comm. 2005). Over 95% of the original prairie, savanna, forest and wetland systems of the ecoregion have been converted to farmland (Gagnon 2004). Most areas that do remain are small and isolated, both of which are characteristics that increase ecosystem vulnerability.

This is what the watershed looks like about 150 years later:

Today, we see that the majority of the watershed has been converted to farmland, growing mostly corn and soybeans (see virtual tour). The black spots are farmsteads, and the beige color shows their surrounding property. The light green remains as floodplain forest surrounding Seven-mile Creek, which is where the current county park is located. Only Oak Leaf Lake remains as a significant lake in the north-eastern corner of the watershed. The map shows that some wetlands remain on the western half of the watershed—only a fraction of what once was.