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Melting of the Laurentide Ice sheet from its
glacial maximum to about 8,000 years ago.

Although there appears to be a recent pollution-driven trend to global warming, the last 2 million years of earth history are known as the Ice Age. This long period of colder climates is referred to as the Quaternary epoch by geologists. During this time glaciers throughout the world (especially those in the Northern Hemisphere) have alternatively expanded and contracted, due to periodic wobbling of the earth's axis, changing aspects of the earth's orbit around the sun, and shifting locations of warm and cold water ocean currents (HDYK-ENV). In North America several cycles of Laurentide Ice sheet advance and retreat played a major and dramatic role in shaping landscapes from present-day New York City through Illinois to Alaska. During the coldest part of the last glaciation about 18,000 to 20,000 years ago, the Laurentide ice sheet reached its maximun southern limits and stretched from present-day Long Island, New York (itself a terminal moraine), through flat fields of central Ohio, Indiana, Illinois and Iowa (HDYK-ENV).

As the ice sheet advanced and retreated - sometimes gradually and sometimes catastrophically - it delivered monstrous volumes of water and sediment to midwestern rivers which created a variety of landforms in river and tributary valleys. Near the glacial terminus glacio-fluvial processes deposited sediments that formed broad outwash plains. Abandoned channels, levees, splays, and other floodplain features were created on the river valley's exposed terraces. In the Pleistocene and early Holocene, many river valleys were so choked with sediment, that rivers often flowed in a complex of braided channels rather than in the incised, single meandering channel we see in the Mississippi and Illinois River valleys today.

Braided channel on outwash plain of a modern glacier.

As glacial retreat continued, river levels lowered, channels erroded and became incised into older deposits, and new floodplains were formed. This left old terraces (floodplains of older, more elevated rivers) perched at higher elevations along valley walls.

At times, water levels were so high that icebergs broken from the glacier terminus to the north (in central Illinois, Iowa, and Wisconsin) floated up Mississippi tributaries depositing exotic sediments scraped by the glacier from land far to the north. At times so great was the volume of water being carried, that the Mississippi River blocked the flow of tributaries causing even major rivers like the Illinois to flow upstream!

Small, sediment-rich iceberg floating near glacial terminus.

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