Ice Age Michigan: Geology, Glaciers, and Mammal Adaptation

History of Ice Age Michigan

Twenty thousand years ago, mile-thick glacial ice sheets that extended from Canada to the Ohio River covered Michigan and most of northern North America. It took more than 12,000 years for the ice to melt, leaving Michigan a glacially scarred landscape with the Great Lakes. Four huge continental glaciers that formed over the Midwest eventually shaped the state's features. The last one, known as the Wisconsin Glacier, occurred about 14,000 years ago. This mass of ice swept across almost four million square miles, dragging millions of tons of earth and rocks over the landscape.

Michigan's geology is also characterized by a major horizontal break between bedrock geology and surficial geology. Much of the state is veneered by deposits of Pleistocene age — the results of glacial and glaciofluvial depositional processes. These sediments are unconsolidated tills, gravels, sands, silts, and clays. They effectively mask much of the bedrock geology, particularly in the Southern Peninsula. With a few exceptions, the spectacular metallic mineral deposits for which Michigan is justly famous occur in rocks of Precambrian age. The younger Paleozoic rocks are by no means devoid of mineral deposits, but their variety is smaller and their mineral assemblages are more severely restricted.

Michigan's geology and fossil record demonstrate that the state was not always underwater. The oldest known vertebrate fossils are Devonian fish, such as placoderms and primitive sharks. There are also fossil land plants from the Devonian and Mississippian periods, more than 300 million years old. Lungfish burrows from the Pennsylvanian Period also indicate that dry land existed approximately 280 million years ago.

As the climate began to warm, soil and rocks were carried away from the melting glacier, creating low-lying hills across the terrain. As this mix of pebbles, stones, soil, and boulders dried, it formed a mixture of sand, silt, and clay. As the glacier continued to disappear, plants began to appear on the drying landscape. The glaciers eventually vanished entirely, having carved and shaped the landscape as the climate dramatically changed. As the glaciers created hilly belts, rolling plains, and uplands, the retreating ice blocked drainage channels, creating the many lakes, swamps, and marshes that characterize modern-day Michigan.

It is worth noting that Michigan is divided into two areas — the Northern and Southern Peninsulas — which are geographically separate and exhibit different bedrock. The western part of the Northern Peninsula is Precambrian and Cambrian in age, while the eastern part and the Southern Peninsula are younger, dating to the Ordovician through Pennsylvanian periods.

Looking further back into prehistoric Michigan's history, during the Devonian Period approximately 350 million years ago, Michigan is believed to have been near the equator. The area was covered by warm, shallow seas containing coral reefs. Eventually, the coral became fossilized, producing what is now known as Michigan's state stone — the Petoskey Stone. The stone is found only in rocks from the Gravel Point Formation, a Middle Devonian (345–395 million years ago) limestone formation that outcrops near Petoskey, close to the northern tip of Michigan's Southern Peninsula. Ice Age glaciers later gouged Petoskey Stones from the bedrock and spread them across the landscape.

Most Michigan fossils represent either the ancient Paleozoic Era or the more recent Pleistocene Epoch — the Ice Age. Michigan's oldest fossils, however, actually predate the Paleozoic Era. Algal stromatolites — fossils of plantlike creatures called algae — found in the Canadian or Laurentian Shield in northern Michigan date back to the middle of the Precambrian Era.

Geological Structure of the Michigan Basin

In early 2000, a news report highlighted a forest discovered in Michigan's Upper Peninsula that was submerged underwater. The belief is that rapidly melting ice and sand drowned the spruce trees and unintentionally preserved them. The wood was prevented from rotting by a rush of fine-grained sand, which precipitated fossilization. Based on preliminary evidence, scientists believe that the sudden and dramatic climatic changes that took place were not always detectable in advance. This may help explain why some mammals became extinct.

The area's bedrock is Paleozoic bedrock deposited in marine and near-shore environments. This Paleozoic bedrock was deposited in a crater basin — known as the Michigan Basin — which was occupied by marine waters from the Silurian through Pennsylvanian periods. Mississippian and Devonian bedrocks are nearest the surface in the south and along the Great Lakes shorelines; Pennsylvanian bedrock is near the surface in the north, at the center of the Michigan Basin.

According to the Michigan Department of Environmental Quality, Geological Survey Division, "within Michigan, the oldest Precambrian rocks have been subjected to at least three major periods of crustal deformation and mountain building and to at least three to four additional major or minor deformational episodes. Metamorphism of varying degrees of intensity accompanied many of the disturbances and transformed sedimentary, intrusive igneous, and volcanic rocks into their metamorphic equivalents. Thus basalt became greenstone; granite became granitic gneiss; sandstone was converted to quartzite; limestone to marble; and shale became slate or mica schist."

The Paleozoic rocks of Michigan do not represent a completely continuous record of Paleozoic sedimentation. At several points, uplift interrupted the general sinking of the basin, and erosion — or at least nondeposition — characterized those particular intervals. Thus, the stratigraphic sequence contains time gaps, some local and some regional, including gaps in the Early Ordovician, Early Devonian, and Late Mississippian periods. The post-Pennsylvanian geological record for Michigan is divided into two main parts: "the lost interval," a nearly unrepresented period of time between the end of the Pennsylvanian and the youngest Pleistocene glaciation, and the Pleistocene glacial epoch itself. Missing are rocks of the youngest Paleozoic Period (Permian), nearly all of the Mesozoic Era (Triassic, Jurassic, and Cretaceous), and all of the Cenozoic except for the Pleistocene. The complete Pleistocene Epoch involved four major glacial periods, but only deposits from the youngest — the Wisconsinian — are represented as surficial deposits in Michigan.

Mammal and Ice Age Environmental Factors

The melting glaciers of the Ice Age climate had provided tall, lush vegetation for giant mammals to feed upon. As the climate became warmer, lakes and rivers began to dry up, and so did the vegetation that sustained the large woolly mammals. As food sources diminished, so did the survival rate of most mammals weighing 100 pounds or more. Some of these extinct mammals included mammoths, horses, camels, and saber-toothed tigers. The stag-moose (Cervalces scotti), an extinct deer slightly larger than the modern moose, was found in deposits in midwestern America indicating that it probably preferred swamps, bogs, and other wetlands in tundra and spruce parkland environments — a habitat similar to that preferred by the modern moose. Unfortunately, the stag-moose became extinct sometime between 11,000 and 10,000 years ago.

During the last great Ice Age, the advancing ice forced tundra animals farther south, and increasingly cold conditions altered the vegetation, which also adapted to climatic changes. Most of the adaptations observed in Michigan's mammals were driven by warming temperatures, changes in the landscape, and severe deforestation in some areas.

The sperm whale represents a prehistoric carnivore that still exists today, and Michigan holds one of the largest fossils from a prehistoric sperm whale. Even during prehistoric times, the Great Lakes were larger and connected to the Atlantic Ocean — which may help explain why the sperm whale survived. Today, sperm whales are found far from land in very deep waters, an environment they probably also inhabited during the Pleistocene Epoch. In 1930, Michigan geologist Russel C. Hussey reported on several discoveries of whale bones in Michigan. There is speculation that sperm and finback whales entered the glacial Great Lakes via the Mississippi River. The fossil whales and walrus of Michigan support the theory that the land of the Great Lakes region emerged from deep-sea conditions quite recently.