File:Kelleys Island (Glacial Grooves).jpg|
View of the Glacial Grooves on the north side of Kelley's Island, Ohio, ca. 1930-1960.
A glacier is a mass of flowing ice that forms when temperatures are cold enough for snow to accumulate without melting during the summer. The rate of accumulation is thus greater than the rate of melting. As the thickness of the ice increases, the glacier eventually begins to move down a valley if in a mountainous region, or flow outward if in a continental region. Mountain glaciers form and survive at high elevations where cold conditions prevail. Continental glaciers form when global climate cools sufficiently for ice to accumulate in high northern or southern latitudes and flow into temperate latitudes. Glaciers act as giant bulldozers, scraping and plucking rock and sediment as they move. When the glacier melts, these materials are left behind.