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Geology of Horicon Marsh

Over the past 1 million years or so, the great glaciers of the Ice Age have come and gone at least four times. With each glacial advance, the land lay buried under a massive continental ice sheet. Each retreat of the ice mass left behind an entirely altered landscape. Every major ice advance is named for the state where the evidence is best preserved. The oldest is known as the Nebraskan glaciation. Next came the Kansan and Illinoisan glaciations. Between each ice advance, the earth experienced a warmer climate. It is quite likely that our present climate is merely a warm period between periodic glacial advances, although human impacts to the world's climate may be upsetting these natural cycles.

The last such ice advance is known as the Wisconsin glaciation. It began about 75,000 years ago and ended within the last 12,000 years. In its wake, it left behind the landscape we know today. Written in the hills, valleys and gently rolling countryside, the tracks of the glaciers can still be clearly seen. In our state remain some of the finest glacial landforms and features to be found anywhere in the world.

The great Horicon Marsh is one of many outstanding natural features formed by the Wisconsin glacier. The ice front invaded Wisconsin along several major pathways. Five separate lobes of ice pushed their way across this land. One of these, known as the Green Bay lobe, encroached upon eastern Wisconsin, where it carved out Green Bay. As it moved inland, it etched out the Lake Winnebago Basin and to the south, the Horicon Marsh and Rock River Basin. It reached as far south as the Madison area before it began to retreat due to warming global climates.

As the Horicon basin was being carved out of the land the advancing ice also created a series of elongated hills, called drumlins. The two large hills comprising Quick's Point, the location of the Horicon Marsh overlook, are drumlins. Note the sharp rise of these hills on the north side and the long sloping sides, which extend down Palmatory Street from the overlook to the hiking trail parking lot. The islands located within the marsh are drumlins as well. They have the same orientation as the upland hills but have been mostly buried by sediment as the marsh developed. Dodge County and the surrounding area has the highest concentration of drumlins in the world!

Upon their retreat, the glaciers receded in successive stages, creating a moraine to mark each temporary halt of the ice front. One such recessional moraine is located at the south end of the marsh. This natural earthen dam temporarily held back the meltwaters of the receding glacier, creating a post-glacial lake where the marsh lies today. The Rock River formed its headwaters around this lake. Glacial Lake Horicon, as it is known, found its outlet across the moraine. With time, the river eroded away this natural dam, draining the lake. As the water levels dropped, fine silts and clay settled into the basin while peat accumulated from abundant plant growth. This resulted in this once great lake giving way to becoming a tremendous marsh. As this lake evolved into a marsh, it became increasingly richer as wildlife habitat. As a result, it has supported wildlife and people for thousands of years. Due to this tremendous geological history, Horicon Marsh has been included as one of the nine units of the Ice Age National Scientific Reserve as an excellent example of an extinct glacial lake.

The Ice Age National Scientific Reserve

Horicon Marsh Wildlife Area is a unit of the Ice Age Reserve system, which is an affiliated area of the National Park System. Unlike our national parks, the reserve consists of nine separate units located throughout the state and is operated by the Wisconsin Department of Natural Resources. Each of these units contains some uniquely representative evidence of the great Ice Age, also known as the Pleistocene Era.

On May 29, 1971, an order was published in the Federal Register formally establishing an Ice Age National Scientific Reserve [exit DNR] in Wisconsin. The purpose of the reserve is to protect, preserve and interpret our glacial heritage, which is most evident and impressive in Wisconsin. The reserve is a cooperative venture of federal, state and local governments. Some 50,000 acres of land are involved, of which more than half are already in public ownership. The Wisconsin DNR manages the reserve. The individual units are state parks, forests and wildlife areas.

Horicon was selected for inclusion in the Ice Age Reserve because it is an outstanding example of an extinct post-glacial lake. Public naturalist programs are conducted at the marsh during the spring and fall seasons and focus on the area's geology, history and wildlife.