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April 2003Fire and ice: A geological legacyNancy Larson & Karen Plass |
The major forces of Lake Superior’s geological past – fire and ice –reflect physical diversity. Broadly speaking, the North Shore in Minnesota and Ontario features exposed lava, while the South Shore in Wisconsin is sandstone, sand and clay. East into Michigan, the South Shore displays clay, shales, exposed lava, conglomerates and sandstone.
About 1.1 billion years ago, North America split along the Mid-continent Rift. Lava oozed into the depression that was to become Lake Superior.
Ice came next. During the Wisconsin (most recent) glaciation, the Lake Superior Lobe, an ice mass up to a mile thick, scraped its way down the length of the valley that was to become the Lake Superior basin. Twelve thousand years ago, this glacier melt, left behind Glacial Lake Duluth, roughly 600 feet higher than modern Lake Superior.
While Lake Superior’s North Shore is rich in dramatic beauty, its South Shore abounds with aquatic and wetland habitat. The South Shore, especially in Wisconsin, features most of Lake Superior’s coastal wetlands. The Lake Superior basin still is rebounding from the weight of the glacier, rising faster at Wawa, Ontario, than at Duluth-Superior (by almost 15 inches per century). This rising water level along the southwest shore is creating estuaries and coastal wetlands.
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A sandy slope greets visitors at the mouth of the Brule River as it meets Lake Superior. |
Visitors to Minnesota’s North Shore see the results of the ancient Precambrian lava flows in spectacular landforms such as Shovel Point in Tettegouche State Park, and nearby Palisade Head. In Wisconsin, Apostle Islands National Lakeshore visitors delight in the colors and sculpted shapes of weathered sandstone. The islands are actually the tops of submerged sandstone hills, which run up the spine of the Bayfield Peninsula. Pictured Rocks National Lakeshore in Michigan’s Upper Peninsula also features a spectacular sandstone shoreline.
The Wisconsin waters of Lake Superior include a healthy slice of the shallower “littoral” zone, which is more productive than the cold, deep water that characterizes most of the lake. The Apostle Islands area, in particular, provides high quality spawning grounds and nursery habitat for fish.
Young and old
The eastern Lake Superior basin in Wisconsin contains the rugged remains of an ancient mountain range – the Penokees, home of Copper Falls State Park. The Bad River begins here. Closer to the lake, (along the spine of the Bayfield Peninsula) and extending westward, glacial action left behind a geologically young landscape: sand and red clay.
Stream valleys cut deep into sand and clay, creating special problems in many of Wisconsin’s Lake Superior tributaries. Water runs off clay so quickly, that these streams tend to "flash." The water levels rise dramatically and erode streambanks after rain.
Human activity, from extensive early logging to today’s road building have dramatically increased runoff and flashiness. Turbid water colored by suspended clay may be seen along wave-pounded clay shores and in post-rainstorm plumes flowing from rivers such as the Nemadji in Superior, Fish Creek entering Chequamegon Bay, the Bad east of Ashland, and the Ontonagon in Michigan. Although deposits of eroded sand are less obvious, fish biologists consider sand a greater problem, because it can bury fish habitat.
