GLACIAL LANDFORMS, ICE SHEETS | Evidence of Glacier and Ice Sheet Extent

Abstract

The most direct evidence of the areal extent of former glaciers is the distribution of erratic boulders, till, or ice-contact stratified deposits. Often the distribution of glacial landforms such as moraines, ice-contact deltas and fans, and drumlins can be identified from aerial photographs. Features formed by subglacial water such as tunnel channels, eskers, hummocky or pitted sand and gravel, and ice-marginal channels also provide evidence of former ice extent. Scoured bedrock surfaces with striations or other small-scale and large-scale abrasion and plucking marks may, in some areas, be the only evidence of former ice cover. The vertical extent, or thickness of ice, is more difficult to determine. Isostatic rebound can be used to calculate former ice thickness over broad areas and within broad limits. Commonly, the areal extent of former glacier ice is easily determined, but in some places there appears to be no till, very few erratics, deep weathering, and little sign of glacial erosion. In these areas, especially where there is also dispute over former ice thickness, the amount of over-consolidation of pre-glacial clay, the distribution of cosmogenic dates on boulders and striated bedrock surfaces, and modeling provide clues to former horizontal and vertical extent of glacier ice.