The Great Lakes are Born Glacial Retreat The Great Lakes are Born

Retreat of the Ice Margin Main lines of evidence for process come from the end moraines left behind The Saginaw lobe was thinner than the Lake Michigan and Erie lobes and therefore melted faster and was the first to begin its retreat.

Drainage ran southward to the Gulf of Mexico

Drainage continued southward to the Gulf of Mexico

Retreat of the Ice Margin As the margins of these lobes retreated, glacial meltwater and precipitation drained southward to the Gulf of Mexico because higher land or glacial ice blocked flow in other directions Sand and gravel, which these meltwater streams could not carry, was laid down as outwash, in broad, flat outwash plains Ice-marginal lakes (or proglacial lakes) formed where the land in front of the ice margin sloped toward the ice, allowing meltwater to pond directly in contact with the ice.

Proglacial Lake Formation

Retreat of the Ice Margin The retreat of the margins of the Michigan and Huron-Erie lobes resulted in the impoundment of water between the ice margin and moraines formed previously by the glacier Tops of moraines were at higher elevations than was the ice margin A series of lakes came into existence, one side of which lay against the ice margin These lakes are called proglacial lakes First phase of the complex development of the present Great Lakes.

Glacial retreat and the formation of proglacial lakes along the ice margins. Crustal Rebound - As the glaciers melted back the depressed crust began to slowly rebound, tilting the proglacial lakes to the north.

Ice Flow - weight of ice causes it to flatten down and push outward Ice Flow - weight of ice causes it to flatten down and push outward. Compresses continental surface. Land around Great Lakes is still rising a few feet per century as a result of the ice melting away. Also called crustal rebound

Proglacial Lake

Proglacial Lakes Lake Chicago Lake Saginaw Lake Whittelsey Lake Keweenaw Lake Algonquin Early Lake Erie Lake Iroquois Early Lake Ontario Lake Minong Lake Chippewa Lake Stanley Lake Hough Early Lake Nipissing Lake Barlow

Other Important Proglacial Lakes Glacial Lake Agassiz Glacial Lake Ojibway

Steps to the modern Great Lakes 13.8 kya first proglacial lakes to form Glacial Lake Maumee (M) from the Huron-Erie Lobe Drained to the southwest, out the Maumee-Wabash-Ohio to the Mississippi River Glacial Lake Saginaw (S) Drained westward into Glacial Lake Chicago. Glacial Lake Chicago (C) from the Michigan lobe Drained west and south through Chicago-Des Plaines-Illinois rivers to the Mississippi River (P)

Steps to the modern Great Lakes 12,500 years ago Another lower drainage way referred to as the Ubly Outlet (U) was uncovered in southeast Michigan by the retreating ice margin Carried water northward from the Erie basin. Formed Glacial Lake Whittlesey (W) Drained into Glacial Lake Saginaw

Steps to the modern Great Lakes 11,000 years ago Most, if not all, of the basins of Lakes Michigan and Huron were deglaciated One large body of water, Lake Algonquin (A), resulted from the coalescence of Glacial Lakes Saginaw and Chicago. This lake was more extensive than the present Lakes Michigan and Huron (582 ft or 175 m above sea level) because its altitude was more than 20 feet higher (605 ft or 182 m).

Steps to the modern Great Lakes 11,000 years ago Lake Algonquin Drainage out of Lake Algonquin changed Went through Glacial Lake Iroquois into Atlantic via the Hudson River of New York Glacial Lake Duluth (D) formed Predecessor to Lake Superior

Steps to the modern Great Lakes 9500 years ago Complete deglaciation of Great Lakes area Final stages of modern lake formation Glacial Lake Duluth grew in area The present-day Great Lakes formed as the earth’s crust, depressed from the weight of the ice sheets, rebounded after deglaciation. This formed the St. Lawrence River

Steps to the modern Great Lakes About 9,000 years ago, the early stage of Lake Superior, called Lake Duluth, drained southwest out the St. Croix and Mississippi rivers, along what is today the Minnesota-Wisconsin border.

Steps to the modern Great Lakes About 7,000 years ago, as the last ice left Lake Michigan, the land to south of the lakes had risen high enough that the lakes no longer drained south to the Gulf of Mexico. Lake Ontario came into being, and the Niagara River became Lake Erie's outlet.

Steps to the modern Great Lakes Lake Huron continued to drain eastward out the Ottawa-St. Lawrence rivers until about 5,000-6,000 years ago. Lake Michigan continued to drain out the Illinois River where Chicago now stands until about only 3,000 years ago, when the Great Lakes finally assumed their present shapes.

Steps to the modern Great Lakes 4.5 – 4.0 kya As the glacier retreated into Canada, it temporarily made Lakes Superior, Michigan and Huron into one huge body of water called Lake Nipissing Had three outlets – Ottawa-St. Lawrence Rivers Detroit-St. Clair Rivers Illinois-Mississippi Rivers.

Nipissing three-outlet phase of the Great Lakes, showing the Illinois River, North Bay-Ottawa River, and St. Lawrence River outlets. (From Kelly and Farrand, 1987.)

Steps to the modern Great Lakes Crustal rebound continued Only the St Lawrence outlet remained after crustal rebound closed the North Bay outlet, and Port Huron outlet to Lake Huron eroded below the level of Lake Michigan’s sill at Chicago (at about 3500 BP). Chicago outlet also closed

Steps to the modern Great Lakes Modern Great Lakes are the result of Erosion and deposition by the glaciers Blockage of drainage by glacial ice Subsidence of the land beneath the glacier's immense weight, followed by rebounding of land during and after glaciation Postglacial lowering of the lake outlets by erosion