Outline of "Minnesota's Geology"

Pages 96-121

Quaternary Geologic History of Minnesota

and Lecture Notes

Introduction

A large boulder of dark-colored greenstone sitting on the floodplain of the Chippewa River near Montevideo was long recognized as out of place in an area of pink granite-gneiss bedrock. Residents referred to it as the Montevideo Meteorite. Other boulders of out-of-place rocks in Minnesota were given special attention and honor by native Americans, such as the Three Maidens in Pipestone National Monument (a granite in an area of Sioux Quartzite bedrock) and the Red Rock in the village of Red Rock (a granite from the St. Cloud in an area of sedimentary bedrock). We now know, of course, that these boulders were carried south from their sources in northern Minnesota and Canada by glaciers and were stranded at the surface when the glaciers retreated. Such out-of-place boulders are called erratics and are an important piece of evidence showing that Minnesota was at one time glaciated.

The Quaternary Period of the Cenozoic Era includes the time from 2 Ma to the present. The interval of time from 2 Ma to 10,000 years b.p. is called the Pleistocene Epoch, and includes the history of the Great Ice Age. Nearly all the landscape of Minnesota was developed during this time, either by glacial erosion and deposition, or by stream erosion and deposition.

During the Quaternary, climate change marked by abnormal cooling and warming cycles brought about profound changes of flora and fauna, both on land and in the sea. The record of sedimentation was profoundly affected and the levels of the world ocean fell and rose depending on the the cyclic advance and retreat of glaciers which formed in response to climatic cooling. Relatively complete records of the Pleistocene Epoch are preserved in deep ocean sediments, in contrast to the incomplete record of glacial sediments preserved on the continents where erosion has fragmented the record.

Glaciation

The Glacial Theory

In 1837, Louis Agassiz proposed that the exotic boulders strewn across Europe, together with striated bedrock, were the result of glaciers which have since melted. Agassiz based his idea on observations of the effects of modern glaciers on the landscape of the Swiss Alps.

In 1846 Agassiz move to the US as a professor of geology at Harvard, and publicized his theory among North American Geologists. Within a short time, the glacial deposits of the US and Canada were being mapped and the history of glaciation in North America was being worked out.

As more and more deposits were mapped, geologists began to realize that rather than one prolonged episode of glaciation, ice sheets advanced and retreated in a cyclic manner. Primary evidence for cyclic advance and retreat was found in the vertical sections of glacial deposits, which showed multiple sheets of till separated by non-glacial deposits and soil zones indicating prolonged intervals of weathering under more warm and moist climates.

In 1872, Winchell, the first head of the Minnesota Geological Survey, began a program of mapping glacial deposits in Minnesota, together with Warren Upham, a New England glacial geologist. By 1883, they outlined the extent of the major moraines and lobes of ice that advanced into Minnesota from the Laurentide Ice Sheet which was centered over Hudson Bay in Canada. They were also able to place Minnesota's glacial deposits into a framework of glacial history for the mid-continent of North America.

Vertical sequences of glacial strata in North America indicate four major advances and retreats of the Laurentide Ice Sheet. These advances and retreats are given names in the calendar of the Pleistocene Epoch

Modern studies of glacial strata suggest that even this calendar must be revised, as more complicated vertical sequences of interbedded glacial and non-glacial deposits are being discovered and integrated into the framework of glacial history and climate change.

Minnesota's Glacial History

The Laurentide Ice Sheet, centered over Hudson Bay in Canada, grew in size and shrank with alternating cooling and warming of the climate during the Pleistocene. Cooling a warming cycles appear to have been caused by cyclic changes in the ellipticity of the earth's orbit about the sun, together with changes in the tilt of the earth's rotational axis and the degree of wobble of the rotational axis. The ice sheet formed in a climatic belt where not only was cold weather the rule, but also where sufficient precipitation was present to form the glaciers which coalesced to form the ice sheet. The ice advanced outward from the maximum center of thickness of the ice sheet, and in the US, that direction was generally from north to south. Lobes or tongues of ice advance outward at the edge of the ice sheet and moved through lowlands which channeled the flow.