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Home LEARN Lake Superior Clay Plain

The Lake Superior Clay Plain

Life on Clay in the Lake Superior Basin
By Tom Fitz

Much of the natural history of the western Lake Superior basin is connected to the thick clay deposits that accumulated here at the end of  glaciation 9,500 years ago. This clay controls the way water flows through  and off the land and gives local soils their particular characteristics. And since soils are one of the dominant factors determining what ecosystems exist in a region, the clay particles under our feet contribute significantly to making this area what it is today.


Clay is created at Earth’s surface when rocks are weathered by the chemical action of water and atmospheric oxygen. Rocks that originally formed deep in the Earth are gradually broken down to minerals that are more stable at Earth’s surface, such clay minerals.

The clay created by millions of years of weathering formed a surface layer until massive glaciers advanced from the northeast starting about 2 million years ago and scraped up clay and other pieces of weathered rock. Then, as the glaciers melted back and the western Lake Superior basin gradually emerged from under the ice, the eastern part of the basin remained buried in ice. This caused the impounding of a large lake in the western part of the basin. Geologists call this lake Glacial Lake Duluth. Its surface was at an elevation of 1100 feet, 400 feet above the surface of Lake Superior today.

Sand and coarse sediment washed into the lake from the melting glacier
and were deposited as beaches along the coast of this glacial lake. Finer
sediment, particularly clay, settled slowly into the depths of the cold deep water. Once the glacial ice margin had melted further to the north, the eastern outlet of Lake Superior at Sault St. Marie was exposed and the water level in the lake fell to its present level. That left the old lake bed and all its clay exposed and ready for soils and life to take hold.


The term “clay” refers to both a size of particle and a specific microscopic structure. In size, clay particles are tiny, the smallest of all sediments, too small to be seen without a microscope. At that level,
clay particles resemble tiny sheets, which means that even a small amount
of clay has a vast total surface area. For instance, just one ounce of clay has a total surface area of 300 square feet.

And because of their particular crystal structure and composition, their surfaces have a slight negative electronic charge. Therefore, anything with a positive charge will stick to clay, giving it the capacity to hold vast amounts of plant nutrients like calcium, or for that matter, water. Attraction between water molecules and the surface of clay particles is why clay is so impermeable and holds onto water for a long time in the spring or after a rain. Most clay is white or grey when it is pure, but the clay in this region is brick red because of the iron oxides that are mixed in with the microscopic clay particles.


Soils are created by the slow, steady accumulation of organic material that forms a layer of topsoil and transforms old sediments to soil. But underneath the thin cover of organic soil sits the clay that has been here for 9,500 years. That means the soils themselves are clay-rich and the material immediately below has the impermeability and water-holding capacity of solid clay.

The abundance of clay makes it so that water does not drain downward to the water table and flow away with groundwater as it does in more-permeable sediments like sand. The water either sits on the surface, runs off
as surface water, or is held within the clay. It also makes life a challenge for plants because the poor drainage means plant roots have to be able to withstand times of water-saturated soils.

On the other hand, in the heat of summer, clay soils have better water retention than do sandy soils. Another benefit is that the charges on clay articles allows them to hold abundant plant nutrients.These nutrients are slowly released to soil water and made available to plants. Thus, clay oils, especially those with adequate organic humus, tend to have enough of most plant nutrients. Ecosystems are based on the plants living in an area, and since the plants are dependent on the soils, the clay has a huge influence on the entire ecosystem.


The poor drainage of clay soils creates problems for people living here too. For example, septic system drain fields, which require water to be able to flow away through the sediments, don't work well and often have to be engineered with more-permeable material like sand brought in from elsewhere. Another problem is that clay expands when wet and contracts when dry, making it unstable material on which to build a foundation.  Over time, buildings tend to sink slowly into clay and roads become cracked and broken in just a few years.

There are some environmental problems associated with clay soils too. The  slipperiness of clay when it is wet makes steep hill slopes unstable and susceptible to slumping. Clay is easily roded from steep slopes and washed into streams and rivers where it remains suspended by even the most gentle of currents. Excessive amounts of clay affects the habit of aquatic organisms by decreasing the clarity of the water, and affecting many other water quality factors. Although erosion is a natural process that has been taking place throughout geologic time, it is a process that can be exacerbated by the actions of people, especially where slopes have been artificially oversteepened.

As is so common in the natural world, what happens at the hidden microscopic scale affects what goes on at much larger scales. In the Lake Superior basin it’s the special charged surfaces on microscopic clay particles that ultimately control how water flows off the land and what trees and animals live here. 

Tom Fitz is an associate professor of geoscience at Northland College in Ashland, WI.  He can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

WDNR Information on the Lake Superior Basin 


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