---
title: "Ice age"
chunk: 8/9
source: "https://en.wikipedia.org/wiki/Ice_age"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:39:28.674049+00:00"
instance: "kb-cron"
---

The major glacial stages of the current ice age in North America are the Illinoian, Eemian, and Wisconsin glaciation. The use of the Nebraskan, Afton, Kansan, and Yarmouthian stages to subdivide the ice age in North America has been discontinued by Quaternary geologists and geomorphologists. These stages have all been merged into the Pre-Illinoian in the 1980s.
During the most recent North American glaciation, during the latter part of the Last Glacial Maximum (26,000 to 13,300 years ago), ice sheets extended to about 45th parallel north. These sheets were 3 to 4 kilometres (1.9 to 2.5 mi) thick.

This Wisconsin glaciation left widespread impacts on the North American landscape. The Great Lakes and the Finger Lakes were carved by ice deepening old valleys. Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters. The old Teays River drainage system was radically altered and largely reshaped into the Ohio River drainage system. Other rivers were dammed and diverted to new channels, such as Niagara Falls, which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. Another similar waterfall, at the present Clark Reservation State Park near Syracuse, New York, is now dry.
The area from Long Island to Nantucket, Massachusetts was formed from glacial till, and the plethora of lakes on the Canadian Shield in northern Canada can be almost entirely attributed to the action of the ice. As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of loess many dozens of feet thick in the Missouri Valley. Post-glacial rebound continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets.
The Driftless Area, a portion of western and southwestern Wisconsin along with parts of adjacent Minnesota, Iowa, and Illinois, was not covered by glaciers.

== Effects of glaciation ==

Although the last glacial period ended more than 8,000 years ago, its effects can still be felt today. For example, the moving ice carved out the landscape in Canada (See Canadian Arctic Archipelago), Greenland, northern Eurasia and Antarctica. The erratic boulders, till, drumlins, eskers, fjords, kettle lakes, moraines, cirques, horns, etc., are typical features left behind by the glaciers. The weight of the ice sheets was so great that they deformed Earth's crust and mantle. After the ice sheets melted, the ice-covered land rebounded. Due to the high viscosity of Earth's mantle, the flow of mantle rocks which controls the rebound process is very slow—at a rate of about 1 cm/year near the center of rebound area today.
During glaciation, water was taken from the oceans to form the ice at high latitudes, thus global sea level dropped by about 110 meters, exposing the continental shelves and forming land-bridges between land-masses for animals to migrate. During deglaciation, the melted ice-water returned to the oceans, causing sea level to rise. This process can cause sudden shifts in coastlines and hydration systems resulting in newly submerged lands, emerging lands, collapsed ice dams resulting in salination of lakes, new ice dams creating vast areas of freshwater, and a general alteration in regional weather patterns on a large but temporary scale. It can even cause temporary reglaciation. This type of chaotic pattern of rapidly changing land, ice, saltwater and freshwater has been proposed as the likely model for the Baltic and Scandinavian regions, as well as much of central North America at the end of the last glacial maximum, with the present-day coastlines only being achieved in the last few millennia of prehistory. Also, the effect of elevation on Scandinavia submerged a vast continental plain that had existed under much of what is now the North Sea, connecting the British Isles to Continental Europe.
The redistribution of ice-water on the surface of Earth and the flow of mantle rocks causes changes in the gravitational field as well as changes to the distribution of the moment of inertia of Earth. These changes to the moment of inertia result in a change in the angular velocity, axis, and wobble of Earth's rotation.
The weight of the redistributed surface mass loaded the lithosphere, caused it to flex and also induced stress within Earth. The presence of the glaciers generally suppressed the movement of faults below. During deglaciation, the faults experience accelerated slip triggering earthquakes. Earthquakes triggered near the ice margin may in turn accelerate ice calving and may account for the Heinrich events. As more ice is removed near the ice margin, more intraplate earthquakes are induced and this positive feedback may explain the fast collapse of ice sheets.
In Europe, glacial erosion and isostatic sinking from the weight of ice made the Baltic Sea, which before the Ice Age was all land drained by the Eridanos River.

== Future ice ages ==