Sediment Cores

Every year, little particles drift their way down to the depths of Lake Mendota. Sometimes they are the shadows of once-living beings––algae and milfoil and zooplankton––falling to the bottom where parts will decompose, be eaten, and finally rest. Waves and wind also pull a different form of matter to the bottom: sandy beaches, eroded rocks, unstable banks, loose dirt. These inorganic materials combine with the once-living to create a history of the waterways settled into the lakebed––a process called sedimentation.

We can read this history. Using tools like the piston-operated corer or the freeze corer, scientists plunge these instruments into the lakebed to remove cylinders of sediment, ranging anywhere from a meter to ten meters tall. Each sediment core is a timeline of the lake’s life––it contains the changing levels of chemicals and nutrients of Lake Mendota, gives us clues to the lake’s food web over the years, and captures pollen grains that help us envision what plants were growing along the shores. Cores show us what was occurring before our lives, before the institutional study of limnology (freshwater) was developed along the shore, before the Ho-Chunk began farming corn and squash, before humans were even here at all.

In November of 1968, a 990cm sediment core was taken in Lake Mendota. This nearly 10 meter-long core captured the sediment of almost 15,000 years. That means that at the deepest point of this core, the last glacier was still retreating across Wisconsin and mammoths and mastodons were still roaming the terrain.

Over the past 15,000 years, the climate has been slowly warming, which has shifted Wisconsin through the different biomes pictured above. This shift is not to be confused with the very rapid climatic warming in recent centuries due to human activity. Illustration by Julia Buskirk.

For the majority of this core––928 cm of it to be exact––the sediment is a pale color. This rock is called “buff marl” by geologists, a rock that’s a combination of clays and limestone. These 928cm of buff marl represent roughly 14,800 years of Lake Mendota's life. Only a centimeter of sediment accumulated on the lakebed every 16 years, as humans moved into the area after the last glacier, as the climate slowly warmed, as boreal forest biomes shifted to hardwoods shifted to prairie.

About three fourths of the way down the core, somewhere around 9,500 B.C.E., hundreds of fluted spear points were chiseled. This gives us the timeframe of when the first humans had officially made Four Lakes their home. In the next meter of core, trade networks were developed throughout the continent, bringing North Michigan copper, Wyoming obsidian, and Atlantic seashells to the shores of Glacial Lake Yahara, the large continuous lake the last glaciers had left behind. By 5,000 B.C.E. though, less than half way down the core, this large waterway had settled into the familiar shapes of Mendota, Monona, Waubesa, and Kegonsa.

It was around these Four Lakes that the ancestors of the Ho-Chunk would build the first conical mounds, about a quarter from the top of the core in 500 B.C.E. In the coming thousand years, Ho-Chunk created Effigy mounds in the shapes of animals and spirits to celebrate their gods and honor their dead. They grew squash, corn, potatoes, pumpkins, and other goods. Their camps developed into large communities around the shores of Mendota and the other linked lakes.