Geology Reference
In-Depth Information
Ice carries large boulders long distances and then melts, leaving these boulders in
places where you least expect them. The stranded rocks are called glacial erratics.
Scientists recognize glacial erratics because they're large chunks of bedrock that are dif-
ferent from the bedrock they're deposited on. For example, they may find a large
boulder of metamorphic rock in a landscape with igneous bedrock. (See Chapter 7 for
descriptions of the rock types.) In areas where these rocks dot the otherwise flat, rock-
free landscape, they are often piled up to create fences between farm fields.
If you ever encounter a large boulder than looks as if it must have fallen out of
the sky (for example, there are no rocky cliffs nearby), you've probably found a
glacial erratic.
Where Have All the Glaciers Gone?
Scientists have explained that modern warming in the earth's climate is melting glaciers
and ice caps. Evidence across the landscape tells a story of ice long ago covering contin-
ents and reaching much closer to the equator than it does today. Is modern global
warming responsible for these changes? And what can scientists learn from studying the
glaciated landscape of the past?
In this section, I briefly describe what is known about the history of glaciers and ice
sheets on earth, how the continents respond to disappearing ice, and what the future
may hold as the ice continues to melt.
Filling the erosional gaps
Scientists called glacial geomorphologists study the landscape shaped by glaciers and ice
sheets to understand how much ice covered the continents and when it retreated or
melted. Periods in the past when ice covered much of the continents are called ice ages.
How many ice ages the earth has experienced and the geographic extent of the ice
sheets are just two of the questions scientists try to answer.
Clues left in the deposits help answer these questions like pieces of a puzzle. However,
solving this puzzle is challenging because much of the landscape created by glacial
erosion and deposition has since been eroded further by other geologic agents. This
subsequent erosion creates erosional gaps in the record of ice ages: periods of glacial
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