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Geologic Provinces of the United States: Laurentian Upland Province - Superior Upland
The core of the continent
Every continent has a core of very ancient metamorphic rocks. The Superior Upland Province is the southern extension of the Laurentian Upland Province, part of the nucleus of North America called Canadian Shield.
The basement rocks of the Laurentian Upland Province were metamorphosed about 2500 million years ago in a mountain-building collision of tectonic plates called the Kenoran orogeny.
The oldest rocks
The rocks of the Superior Upland are mostly Precambrian metamorphic rocks and overlying Paleozoic rocks (Cambrian) covered by a thin veneer of glacial deposits left behind when glaciers melted at the end of the Pleistocene Ice Age.
If we could strip away all of the younger rocks deposited on top of buried Precambrian basement, you would see a landscape of low relief. The topography of the Precambrian rocks is very subdued, with barely 500 feet difference between the highest point and the lowest. Clearly, this region was exposed to a very long period of erosion in the very distant past which beveled the original mountainous surface to a gently undulating surface. The present surface is not much different. Hills rise just a few hundred feet above the surrounding countryside. The highest of these, such as Rib Hill, Wisconsin, are made up mostly of resistant quartzite or granite.
Records in the rocks
The structure of Superior Upland rock is quite complex. Folds and faults, most dating back to Precambrian time, record several episodes of mountain-building. The plate collisions that formed the core of our continent left behind a striking structural trend. Ridges and valleys are strongly aligned along this northeast-southwest trend. Lake Superior is a splendid example of this northeast-southwest structural trend. Ridges of erosion-resistant rock rise above valleys and carved into weaker rock units.
Glaciers add the final touch
The effects of repeated glaciation have left their mark on the Superior Upland. The present glacial topography is the product of the most recent glaciation that ended just 10,000 years ago. During the Late Wisconsin, the last glaciation of the Pleistocene Epoch, a massive continental ice sheet grew first in the north, then gradually expanded southward. Several thick finger-like lobes of glacial ice engulfed the region as they moved through the Superior basin. Rocks of all sizes were plucked and scoured from farther north and carried along by the icy mass. When the continental ice sheets melt, they left behind an assortment of rock called glacial drift that covers much of the Superior Upland landscape.