Thomson Dam 2006 

Information of the geolgy on this page is from the 2005 MN DNR Field Trip Guide

Authors Heather Arends, Chuck Corwin, John Heine, Richard Patelke, Rick Ruhanen and Mark Severson

In the summer of 2006 low water levels allowd a much closer inspection of the Thomson Dam area. The rocks of this area are classified as metasedimentary because they have been changed (metamorphosed) and deformed by Penokean Orogeny (mountain building) about 1.85 billion years ago. It is this same event formed the back arc basin and the iron deposits at the edge of this basin.

The rocks are mostly graywacke (sandy mudstone) and slate made from sea sediments (mud) deposited about 2 billion years ago at what was the edge of the North American continent. The rocks have been deformed and metamorphosed by the Penokean Orogeny and by the midcontinent rift that forms the basalt rocks at Lake Superior and other areas.

In the photo at the right the layers of mud that became rock have been tilted to be almost verticle by the geologic processes that came later.

The Penokean was a Rocky Mountain-type mountain belt fromed across central Minnesota and northern Wisconsin. The remnants of this mountain include granite bedrock near St. Cloud.

The slate and greywacke still show the signs of their origin with layers visible as well as many ripple marks that are made in areas of moving water. Careful study of the ripple marks indicate that the water was traveling in two directions (as in waves) not in one direction (as would be in a river situation) that provide evidence of shallow sea environments.

There are places where diabase rocks (basaltic composition with mineral size between that of gabbro and basalt) from the younger midcontinent rift. These formations are called dikes because they cut accross the layers of the original rock. The grain direction of this newer (younger) rock is much different from the older slate surrounding it. The joints perpendicular to the edges of the rock are evidence of the cooling of the liquid magma as it turned into diabase.
The photo show a small tape recorder next to a drill hole. This area is often studied by UMD geology students as well as others who make field trips here. The drill hole is cause when diamond tipped drills cut out samples of the rock for testing. While some geology information needs to be gathered in the natural environment many tests can only be done by collecting samples and bringing them into a lab.


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