Fire & Ice: Duluth’s Geologic Past

The footprint of Glacial Lake Duluth over an outline of today’s Lake Superior and its largest tributary, the St. Louis River. [Map by Matt Kania, Map Hero Inc.; image courtesy Minnesota Historical Society Press]

The forces that shaped Duluth’s geography occurred during the planet’s formation several billion years ago, in the Precambrian era. Basalt and granite found along Lake Superior’s western shore indicate volcanic and seismic activity, and according to geologist John C. Green, the landscape once featured “great mountain ranges, perhaps rivaling the Alps, which have since been eroded to the nub.” Sandstone and slate deposits along the lake’s south shore and up the St. Louis River to Cloquet speak of a vast sea that once covered the region. More volcanic eruptions and earthquakes were followed by more erosion.

About 1.8 million years ago a giant plume of molten rock melted and spread as it neared the surface, creating cracks in the earth’s crust that allowed magma to burst through in what Green describes as “huge fountains of intensely glowing lava spurting up from fissures that extended for miles across a barren plain” forming a “huge, pancake-like lava flow” that spread for 24 million years.

The result was the Midcontinent Rift System, a geologic feature portions of which have been mapped in a dozen US states, with southern Ontario at its apex and stretching as far south as Oklahoma and Mississippi. Lake Superior sits near the top. As volcanic activity subsided, the heavy basalt it produced sank at the basin’s center, forming a shallow sea that over the eons filled with sediment carried by streams, eventually creating a hard basalt bowl filled with soft sandstone. The region then stabilized until about two million years ago, when it started getting cold and fire finally gave way to ice.

It took a while, but during the Pleistocene epoch—roughly 2.5 million to 11,500 years ago—glaciers eventually reached as far south as modern Kansas. Throughout several periods of glacial advances and retreats, the ice and harder rock trapped within the glaciers scoured out the sedimentary rock, leaving behind basins that would be later filled by rivers and streams created by melting glaciers. The last of these formations—the Laurentide Ice Sheet—left behind large, proglacial lakes that developed into today’s Great Lakes. One, glacial Lake Duluth, essentially covered the western end of modern Lake Superior west of Michigan’s Upper Peninsula and Isle Royale. Lake Duluth sat about five hundred feet higher than Lake Superior does today and covered a much larger area, but during the past 10,000 years the lake drained via the St. Croix River watershed, at one point dipping so low that its western shore sat near today’s Silver Bay, fifty-five miles northeast of Duluth, before it rose to its current level. Most of Duluth’s Skyline Parkway follows the shoreline of glacial Lake Duluth.

The eventual draining of another glacial lake, St. Louis, created the river for which it is named. The St. Louis begins roughly seventy-five miles north and east of Duluth at Seven Beaver Lake and flows roughly southwesterly to Floodwood, forty-six miles west of Duluth, before turning southeasterly, then eastward, then south to Cloquet and on to Carlton. There it turns east and begins a steep descent through a gorge, creating a series of rapids or dalles (French for “gutters”) which drop dramatically as the river makes its way down to Duluth’s Fond du Lac neighborhood. There its waters become navigable, turning north as they pass Gary and New Duluth and widen into an estuary that includes Spirit Lake and spreads east past Grassy Point to become St. Louis Bay before emptying into Lake Superior. Along the way the river covers 192 miles, dropping from 1669 to 602 feet above sea level while creating a 3648-square-mile watershed.

Together Lake Superior and the St. Louis, with help from the Nemadji River, have created a natural haven. Over the eons, silt carried by the St. Louis and Nemadji collided with sand stirred up by Superior’s natural, clockwise rotation, eventually creating four distinct sandbars. Duluth’s Rice’s Point and Superior’s Conner’s Point once formed the very western shore of the lake, but have since been supplanted by Minnesota and Wisconsin Points, together the largest naturally formed baymouth bar on the planet. These four bars formed a large, natural, and protected harbor that would eventually make them an ideal place to build a city—or two.