Geology

Beginning some 2.7 billion years ago and lasting for over a billion and a half years, the shifting of tectonic plates and volcanic activity created immense mountain ranges in Minnesota [1]. During this time, the Canadian Shield, a layer of bedrock underlying most of Greenland, half of Canada, and extending into the northeast United States, was formed.

In Minnesota this massive slab extends down to the upper reaches of the Minnesota River Valley, where some of the planet’s oldest rock, Morton Gneiss (estimated at 3.6 billion years), is exposed in several places. The Canadian Shield is also often revealed along Lake Superior and the Boundary Waters [2], lending a special beauty to the northeast.

Also during this time, iron particles settled at the bottom of the great sea that covered the state forming the Vermilion, Mesabi, and Cuyuna Iron Ranges. Over the rest of the Precambrian era, seas continued to sweep in and out and, along with the wind and ice of early glaciers, wore away the once-mighty mountains.

During the Paleozoic era (540–245 million years ago), when animals evolved, and the Mesozoic era (245–66 million years ago), the age of dinosaurs, Minnesota was floating down around the equator and had a tropical climate. As the North American continent broke away from the supercontinent Pangaea, it drifted north to cooler climes.

Two million years ago the Quaternary period, commonly referred to as the Ice Age, began. During this time the Laurentide Ice Sheet, centered near Hudson Bay, advanced and retreated four times. The first three glaciations, separated by long ice-free periods, reached well into the middle of the United States, covering all of Minnesota. Despite such a long, active, and violent past, Minnesota as we now know it didn’t begin to really take shape until about 75,000 years ago when the last advance of ice, the Wisconsin Glaciation, swept south. The ice sheet wiped away just about everything in its path, though it spared the southeast and southwest corners.

The glaciers began their latest retreat about 12,000 years ago. As they melted away, the various glacial lobes deposited the countless tons of rock and other earthly debris they had picked up on the trip south. This moraine formed most of Minnesota’s hills and, when massive chunks of ice got buried in it, the lakes and wetlands. The meltwater cut most of Minnesota’s riverbeds, including those of its four largest rivers: the Mississippi; its first major tributary, the Minnesota; the St. Croix; and the Red River of the North.

While gently rolling hills cover most of the state, its four corners are each a unique exception. The northeast has many steep, rugged hills, the final remnants of Minnesota [1]’s former mountains, while the Red River Valley in the northwest is improbably flat. The southeast corner has many deep, highly eroded valleys with much exposed bedrock and many caves. The southwest corner is more or less flat except for some deep river valleys and the long, tall Buffalo Ridge, a plateau that stretches well into South Dakota.

Perhaps Minnesota’s most unusual geological quirk is that its waters empty in three different directions: south to the Gulf of Mexico via the Mississippi River, east to the Atlantic Ocean via Lake Superior, and north to Hudson Bay via the Red River of the North.