Driftless Area

October 23, 2014

The drive through the rest of Wisconsin to Minnesota was through areas where, as Frederick said, the foliage was past its peak.

Only evergreens provide color.

There are lots of these birch trees.

In Tuesday’s blog we showed a couple of pictures of some odd shaped rocks and commented that they could be a geo-site. Then yesterday, researching the geo-sites we will be able to visit as we travel down the Great River Road, we saw that one of the geo-sites in Wisconsin was the very type and location of the rocks that we showed in the picture. In other words, we took a picture of the Wisconsin geo-site without even knowing, at the time, that it was a geo-site. It is true that Mr. Spock did not have his picture taken at the site, but rest assured, he was in the car and was part of our conversation about this interesting area that was spared the destructive effects of continental glaciation.

During the Pleistocene epoch, mile-thick tongues of ice possessing the capacity to redue anything standing in the way to sand and dust repeatedly overwhelmed the northern portion of the contiguous United States. The present-day courses of the Missouri and Ohio rivers mark the southern-most reach of these invasions. For reasons still subject to debate, a 14,500-square-mile area centered on southwestern Wisconsin escaped these repeated onslaughts of frozen water. Various glacial tongues bypassed this Denmark-sized region in a gigantic pincerlike military style of advance, then rejoined forces to the south and enveloped it within a wall of ice. Today this is called the Driftless Area because it lacks the ground-cover debris recognized as the footprint of glacial movement. Numerous topographic sentinels, such as chimneys, crags, towers, pinnacles, mesas, and buttes, provide further proof that this area has never been subjected to the run-amok process of glaciation. 

Wisconsin geo-site from the highway.

A single remaining tower.

A mesa and butte terrain spared tom the all-consuming destuctive effects of continental glaciation.

In the early afternoon we checked into the campground at Jay Cooke State Park. The reason for our being in the Jay Cooke State Park, on the outskirts of Duluth, is to visit one of the three Minnesota geo-site called Thomson Dikes, located within and adjacent to the Park. 

In comparison to other planets of the solar system, Earth appears to be unique. Thick thermal curets constantly churn its interior, plate tectonics movement continually rearranges the geographic design of its crust, and oceans form and then are destroyed, while mountains and canyons are but ephemeral elements of the scenery. Translated into geologic headlines of the 21st-century, this omnipresent interplay of land and water would read: Atlantic Ocean expanding, the Pacific shrinking; Alps and Himalayas growing, Appalachians declining; Africa splitting up, California slowly sliding offshore, and the Mediterranean being quietly squeezed shut. 

This park graphically shows these types of changes. 1.8 to 2 billion years ago, during the Precambrian time, a sea covered this area and deposited sediments of both mud and mixtures of sand and gravel. This deposition went on for a long enough period of time to leave mud and sand/gravel deposits hundreds of feet thick. Over time these deposits solidified, the mud becoming shale and the sand/gravel deposits becoming sandstone. Shale became the most common rock in the formation. 1.7 to 1.8 billion years ago, earth movements, along with the resulting heat and pressure, during the Penokian mountain-building period brought about the metamorphism of the shale into slate, and the sandstone into graywacke. At the same time, heat and pressure folded the layers of rock into huge waves extending about 1/2 mile from peak to peak. This formation of rock is called the Thomson Formation and is the rock in the bed of the St. Louis River in Jay Cooke State Park. During the Late Precambrian, 1.1 billion years ago, a period of volcanic activity formed much of the igneous rock of the north and south shores of Lake Superior. At this same time molten rock was forced up through fractures of the Thomson Formation. These igneous intrusions are mostly a fine-grained, black rock called diabase.

 Behind Mr. Spock's right shoulder is a small anticline in the Thomson Formation. This is one of the contorted roots of the Penokean orogeny, a mountain building event that stretched from Minnesota into Ontario.

The St. Louis River just before it enters the Jay Cooke Park. Notice the Thomson dam behind the bridge.

Above the dam is this placid reservoir.

The University of Minnesota uses this area for their whitewater center.

During the Pleistocene Age, 2 million to 10,000 years age, ice from glaciations covered the entire area several times. This ice had three main effects. First, it depressed the surface of the earth through its great weight. Second, it scraped the surface of the area, removing previously deposited soils and weathered rock. Third, it created new deposits as the ice retreated. A huge glacial lake, Glacial Lake Duluth, was formed as the ice receded and the drainage of melt waters was blocked by ice to the east. A thick deposit of fingered clay formed on the bottom of the lake, and remained after the lake drained away to cover much of this area. Runoff from glacial lakes and melting ice to the north and west began to cut down into this clay and into the rocks on the Thomson Formation beneath it. The St. Louis River today, flows within the valley that began to form at that time.

The St. Louis River downstream from the dam.

Next to the dam where there was once a water wheel for the brick making factory.

In the last 10,000 years, the river has continued to cut slowly down into the rock of its bed as inflows to Lake Superior. Most of its route throughout he park is parallel to the folds of the Thomson Formation, following its eroded path through the less resistant layers of the folded beds. The diabase dikes which cut across the layers of the formation are soft, easily eroded rock, and at times become channels of the river, but generally do not effect the course of the river. 

An exposed dike at the base of the bridge.