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2012 Fall Issue 7 (November 2, 2012)

ArbNotes

November 4, 2012
By Callum McCulloch

Fall weather has settled in and temperatures are cooling off--but what would it be like if average temperatures were freezing all year?

For much of its geologically recent history, Minnesota experienced long periods of glaciation. The Wisconsin glaciation, an advance of the Laurentide Ice Sheet, is responsible for shaping the geomorphology of Minnesota in the last 110,000 years.

The most prominent features of glaciation still visible in the Arb today are attributed to two major advances of the Wisconsin glaciation--the Cary Substage and the Mankato Substage.

The Cary advance covered most parts of Minnesota with thick continental ice and transported glacial erratics from the Duluth Complex in northern Minnesota to the Arb.

A number of transported gabbro, granite and gneiss boulders are located throughout the Arboretum. Some boulders were placed in recent years as teaching aids, but some boulders are original glacial deposits. The best example of a glacial erratic deposit is the Duluth Complex gabbro located between the softball fields and Highway 19.

The other piece of relevant glacial history is the Mankato Substage of the Wisconsin glaciation. After the Cary Substage retreated, the Mankato glacial advance extended to Des Moines, Iowa, which is why this lobe is called the Des Moines lobe.

The Des Moines lobe was not as thick as the Cary advance and did not significantly erode the landscape. When the Des Moines lobe melted 14,000 years ago, ice did not melt at a uniform rate—large blocks of ice dotted the landscape.

Sediments filled around these large ice blocks and created glacial kettle lakes, like Kettle Hole Marsh in the Arb.

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