Duluth Complex - Contemporary Landforms

Contemporary Landforms

The Precambrian bedrock of the Duluth Complex and the North Shore Volcanics are not buried beneath layers of later sedimentary rock, as is common further south; much of this bedrock is close to or at the surface. Glaciers scoured away earlier soils, and as is typical of the Canadian Shield, the new topsoils are thin and poor, being derived from the rock beneath or nearby rather than from deep layers of glacial till, which is intermittent and relatively shallow over most of the region. Consequently much of the bedrock is exposed, except for the sediments and glacial till in the watershed of the Saint Louis and Cloquet Rivers inland on the west.

Gabbro outcroppings anchor both ends of the complex. They dominate the city which gave the Duluth Complex its name, and also form part of Pigeon Point, the easternmost point of Minnesota. In between, Superior's shoreline from Duluth to the international border has been likened to one long volcanic outcrop, albeit interrupted by parts of the Beaver Bay Complex, such as the anorthosite cliffs at Split Rock Lighthouse adjacent to basalt flows. Prominent relics of volcanism include rhyolitic cliffs at Palisade Head, basaltic lava flows at Gooseberry Falls, and the Sawtooth Mountains further east. Along the lakeshore can be found quartz-banded thomsonite and agate gemstones created by mineral infilling of gas cavities formed when the lava flows cooled.

Interior highlands include Eagle Mountain and the other Misquah Hills. Most of the eastern part of the Superior National Forest and its Boundary Waters Canoe Area Wilderness (BWCA) is located on the Duluth Complex, and its exposed Late Precambrian bedrock created many of the characteristic features of the region. The inland lakes lie in hollows formed by differential erosion of the gabbro intrusions. These depressions were given their final form by glacial scouring during recent ice ages, creating the irregularly shaped and rocky-shored lakes which are hallmarks of the wilderness.