Determining Past Climates
Paragraph 1: Glaciers form over land where temperatures are sufficientlylow so that, during the course of a year, more snow falls than melts. Successive snow accumulations over many years compactthe snow, which slowly recrystallizes into ice. Under the influence of gravity, the ice begins to move, and a glacier is formed. A mere 18,000 years ago, Earth was in the grip of a cold spell, with alpine glaciers extending down river valleys and continental glaciers covering vast areas of North America and Europe.The ice measured several kilometers thick and extended as far south as New York and the Ohio River Valley. Perhaps glaciers advanced ten times during the past two million years, only to retreat. In the warmer periods between glacier advances, average global temperatures were slightly higher than they are at present.
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