Presentation on theme: "The Great Lakes are Born"— Presentation transcript:
1The Great Lakes are Born Glacial RetreatThe Great Lakes are Born
2Retreat of the Ice Margin Main lines of evidence for process come from the end moraines left behindThe Saginaw lobe was thinner than the Lake Michigan and Erie lobes and therefore melted faster and was the first to begin its retreat.
4Drainage continued southward to the Gulf of Mexico
5Retreat 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 directionsSand and gravel, which these meltwater streams could not carry, was laid down as outwash, in broad, flat outwash plainsIce-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.
7Retreat 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 glacierTops of moraines were at higher elevations than was the ice marginA series of lakes came into existence, one side of which lay against the ice marginThese lakes are called proglacial lakesFirst phase of the complex development of the present Great Lakes.
8Glacial 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.
9Ice 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
12Proglacial Lakes Lake Chicago Lake Saginaw Lake Whittelsey Lake KeweenawLake AlgonquinEarly Lake ErieLake IroquoisEarly Lake OntarioLake MinongLake ChippewaLake StanleyLake HoughEarly Lake NipissingLake Barlow
16Steps to the modern Great Lakes 13.8 kya first proglacial lakes to formGlacial Lake Maumee (M) from the Huron-Erie LobeDrained to the southwest, out the Maumee-Wabash-Ohio to the Mississippi RiverGlacial Lake Saginaw (S)Drained westward into Glacial Lake Chicago.Glacial Lake Chicago (C) from the Michigan lobeDrained west and south through Chicago-Des Plaines-Illinois rivers to the Mississippi River (P)
18Steps to the modern Great Lakes 12,500 years agoAnother lower drainage way referred to as the Ubly Outlet (U) was uncovered in southeast Michigan by the retreating ice marginCarried water northward from the Erie basin.Formed Glacial Lake Whittlesey (W)Drained into Glacial Lake Saginaw
20Steps to the modern Great Lakes 11,000 years agoMost, if not all, of the basins of Lakes Michigan and Huron were deglaciatedOne 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).
23Steps to the modern Great Lakes 11,000 years agoLake AlgonquinDrainage out of Lake Algonquin changedWent through Glacial Lake Iroquois into Atlantic via the Hudson River of New YorkGlacial Lake Duluth (D) formedPredecessor to Lake Superior
24Steps to the modern Great Lakes 9500 years agoComplete deglaciation of Great Lakes areaFinal stages of modern lake formationGlacial Lake Duluth grew in areaThe 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
26Steps 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.
28Steps 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.
30Steps 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.
31Steps to the modern Great Lakes 4.5 – 4.0 kyaAs the glacier retreated into Canada, it temporarily made Lakes Superior, Michigan and Huron into one huge body of water called Lake NipissingHad three outlets –Ottawa-St. Lawrence RiversDetroit-St. Clair RiversIllinois-Mississippi Rivers.
32Nipissing 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.)
34Steps to the modern Great Lakes Crustal rebound continuedOnly 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
40Steps to the modern Great Lakes Modern Great Lakes are the result ofErosion and deposition by the glaciersBlockage of drainage by glacial iceSubsidence of the land beneath the glacier's immense weight, followed by rebounding of land during and after glaciationPostglacial lowering of the lake outlets by erosion