The Laurentide Ice Sheet (LIS) was a huge ice sheet that covered millions of square miles, including most of Canada and a large part of the Northern United States, many times during the Quaternary period, which began 2.58 million years ago and continues today.
The last major movement of the ice sheet covered most of northern North America between about 95,000 and about 20,000 years ago. This movement changed the shape of the land and created the five Great Lakes and many smaller lakes in the Canadian Shield. These lakes stretch from the eastern Northwest Territories, through much of northern Canada, and into the upper Midwestern United States (Minnesota, Wisconsin, and Michigan). They extend further south through the Finger Lakes region, the Lake Champlain and Lake George areas of New York, across the northern Appalachian Mountains, and into New England and Nova Scotia.
At times, the southern edge of the ice sheet reached areas that are now coastal towns in the Northeastern United States, as well as cities like Boston and New York City. It also reached coastal areas of the Great Lakes as far south as Chicago and St. Louis, Missouri. The ice then followed the path of the modern Missouri River to the northern slopes of the Cypress Hills, where it joined the Cordilleran Ice Sheet. The ice covered areas as far south as 38 degrees latitude in the central part of the continent.
Description
During the Pleistocene epoch in North America, known as the ice age, the Laurentide Ice Sheet was the most important feature. During the Pre-Illinoian Stage, this ice sheet stretched as far south as the Missouri and Ohio River valleys. It was up to 3 kilometers (2 miles) thick in Nunavik, Quebec, Canada, but thinner at its edges, where hilly areas often had exposed rock called nunataks. The ice sheet shaped much of the land in southern Canada and the northern United States, leaving behind valleys carved by glaciers, ridges of rock and soil called moraines, long winding hills of sand and gravel called eskers, and layers of unsorted rock and soil called glacial till. It also changed the shape, size, and water flow of the Great Lakes. For example, near the end of the last ice age, Lake Iroquois covered a much larger area than today’s Lake Ontario and drained into the Atlantic Ocean through the Hudson River.
The growth and melting of the ice sheet had a major effect on global climate. It changed the path of the jet stream, which normally carries warm air from the Pacific Ocean through Montana and Minnesota. This caused the Southwestern United States, a desert, to receive more rain during ice ages, while most other areas became very dry. Similar effects occurred in Europe, where ice sheets may have increased winter rainfall in Afghanistan, parts of Iran, western Pakistan, and North Africa.
When the ice sheet melted, it caused major changes in global climate patterns. Large amounts of fresh water from the Mackenzie River flowed into the Arctic Ocean, possibly disrupting the formation of deep, salty, cold water in the North Atlantic. This interrupted a major ocean current called thermohaline circulation, leading to a short cold period called the Younger Dryas and a temporary return of ice in some areas. The ice sheet did not fully retreat from Nunavik until 6,500 years ago.
After the Younger Dryas, the Laurentide Ice Sheet quickly moved northward, eventually covering only the Canadian Shield before melting completely. The melting of the ice sheet may have indirectly affected agriculture in Europe by raising global sea levels.
Today, Canada’s oldest remaining ice is found in two places: the Barnes Ice Cap and the Penny Ice Cap, which are remnants of the Laurentide Ice Sheet.
Ice centers
During the last ice age, the Laurentide ice sheet stretched from the Rocky Mountains eastward across the Great Lakes, into New England, covering most of Canada east of the Rocky Mountains. Three major ice areas formed in North America: the Labrador, Keewatin, and Cordilleran. The Cordilleran ice covered the region from the Pacific Ocean to the eastern side of the Rocky Mountains. The Labrador and Keewatin areas were part of the Laurentide Ice Sheet. Evidence of many ice lobes and smaller sections of ice exists in central North America. The Keewatin ice covered the central plains of North America from the Mackenzie River to the Missouri River and the upper parts of the Mississippi River. The Labrador ice covered much of eastern Canada and the northeastern United States, meeting the Keewatin ice near the western Great Lakes and Mississippi River.
The Keewatin ice dome had four or five main sections that spread out from a central dome in west-central Keewatin (Kivalliq). Two of these sections met the Labrador and Foxe-Baffin ice domes. The main sections of the Keewatin ice flowed (1) toward Manitoba and Saskatchewan; (2) toward Hudson Bay; (3) toward the Gulf of Boothia; and (4) toward the Beaufort Sea.
The Labrador ice covered all of Maine and extended into the Gulf of St. Lawrence, completely covering the Maritime Provinces. The Appalachian Ice Complex moved from the Gaspé Peninsula across New Brunswick, the Magdalen Shelf, and Nova Scotia. The Labrador ice moved across the mouth of the St. Lawrence River, reaching the Gaspé Peninsula and Chaleur Bay. From the Escuminac area on the Magdalen Shelf, ice flowed onto the Acadian Peninsula of New Brunswick and southeastward to the Gaspé Peninsula, covering the western end of Prince Edward Island and reaching the head of the Bay of Fundy. From the Gaspereau area on the divide crossing New Brunswick, ice flowed into the Bay of Fundy and Chaleur Bay.
In New York, ice that covered Manhattan was about 2,000 feet high before melting began around 16,000 years ago. The ice in this area disappeared by about 10,000 years ago. Since then, the ground in New York has risen more than 150 feet due to the weight of the melted ice being removed.
The Foxe-Baffin ice dome was circular and centered over the Foxe Basin. A major divide across the basin caused ice to flow westward over the Melville Peninsula and eastward over Baffin Island and Southampton Island. In southern Baffin Island, two divides created four additional sections of ice. The Penny Ice Divide split the Cumberland Peninsula, with Pangnirtung causing ice to flow toward Home Bay on the north and Cumberland Sound on the south. The Amadjuak Ice Divide on the Hall Peninsula, where Iqaluit is located, caused ice to flow north into Cumberland Sound and south into the Hudson Strait. A secondary Hall Ice Divide connected to a local ice cap on the Hall Peninsula. The current ice caps on Baffin Island are believed to be leftover from this time, but they were not part of the Baffin ice flow and instead formed independently.
Adjacent ice sheets
The Cordilleran ice sheet covered up to 1.5 million square kilometers (580,000 square miles) during the Last Glacial Maximum. Its eastern edge met the Laurentide ice sheet. The ice sheet was attached to the Coast Mountains of British Columbia and Alberta, extending south into the Cascade Range of Washington. This amount of ice held 1.5 times the water found in Antarctica. Fixed to the mountain range along the west coast, the ice sheet melted away north of the Alaska Range, where the air was too dry to form glaciers. Scientists believe the Cordilleran ice melted quickly, in less than 4,000 years. The melting water formed many proglacial lakes along the edges, such as Lake Missoula, which sometimes caused huge floods, like the Missoula Floods. These floods greatly changed the landforms in Eastern Washington and northern Montana and North Dakota.
The Innuitian ice sheet, centered on the Queen Elizabeth Islands, was connected to the northern part of the Laurentide Ice Sheet.