Pleistocene glaciers once covered >1,200,000 km2, from the continental shelf bordering the North Pacific to the northern foothills of the Brooks Range. Late Wisconsin glaciers occupied 727,800 km2 -- nearly ten times the area of modern glaciers, but only 48% of the state. Alaska's glaciers expanded more than 20 times during the last 3 million years in response to cold and snowy conditions. During the late Wisconsin glaciation, when sea level fell approximately 125 m (approx. 400 ft), the Bering Land Bridge was exposed as a broad tundra plain, and much of the state escaped glaciation due to a cold but dry climate. Deposition and erosion by glaciers in the recent geologic past have greatly influenced Alaska's landscapes and ecosystems.
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Finally, we classified the glacier limits according to "certainty", a measure of confidence relating to both age determination and geographic position. For example, well-dated moraines were ascribed a high level of certainty, comparable to a solid line on traditional glacial geologic maps. Intermediate certainty was encoded for limits without well resolved ice-marginal features, or those with little or no direct geochronologic control (comparable to dashed lines for "uncertain" boundaries on traditional maps). A low level of certainty was attributed to arcs bounding polygons where ice limits are inferred for offshore positions, or for areas lacking detailed air-photo interpretation or significant, field-based glacial-geologic study. The measures of certainty are based on available mapping and publications, and are commensurate with the target scale of 1 to 1,000,000. In many areas of the state, significant uncertainties remain. Research through field-based efforts, remote-sensing techniques, and geochronologic methods are needed to improve the levels of certainty shown here, and at larger scales.