A dawn wind stirs on the great marsh. With almost imperceptible slowness, it rolls a bank of fog across the wide morass. Like the white ghost of a glacier, the mists advance riding over phalanxes of tamarack, sliding over bogmeadows heavy with dew. A single silence hangs from horizon to horizon.

—Aldo Leopold

The great marsh that Aldo Leopold venerates in “Marshland Elegy,” an essay that appears in his classic A Sand County Almanac, is the Horicon Marsh.¹ It is indeed a wetland to be venerated, one of the largest freshwater cattail marshes in the world, and host to a spectacular biodiversity of wetland plants and animals. It occupies 31,000 acres, most of it in Dodge County with its northern tip lying in Fond du Lac County. Roughly oval-shaped, it is 13 miles long and up to 4 miles wide. The marsh is fed by the headwaters of the Rock River—three branches that drain 300 square miles of land, funneling surface water into the marsh. The great marsh owes its existence to the glacier that Leopold imagined seeing one morning in that ghostly bank of fog.

While most of the glacial deposits that lie under the marsh are between 12,000 and 16,000 years old, the story of how the marsh was formed is much older. It rests in the same lowland that contains Green Bay and the Fox River Valley, so the bedrock beneath it is composed of Maquoketa shale overlying layers of Ordovician dolomite. During the Ice Age, the lowland was carved deeper with each passage of a glacier over the area. Most of the work was accomplished during the Wisconsin glaciation, but geologists have found evidence that earlier glaciers helped to shape the land. A buried channel beneath today’s East Branch of the Rock River, which flows from the east into the marsh, likely carried meltwater from one of those earlier ice sheets. And deep layers of silt and clay in the Horicon Marsh make it clear that a large lake occupied the marsh basin before the Wisconsin glaciation.