Geography Reference
In-Depth Information
Figure 7-7:
Ex-
tent of continent-
al glaciers in
North America
during the last
Ice Age.
Generally, as the ice receded across the United States, it left behind a blanket of stones, gravel, and
soil. Melt water issued from the retreating glaciers in innumerable streams that carried and deposited
fine soil particles, which further transformed the post-glacial landscape. Today, thousands of years
later, much of those materials underlie America's agricultural heartland and are important factors in
explaining its productivity.
Making a deposit: Wind
Given sufficient velocity, wind can pick up soil particles and carry them long distances before they
are deposited. This was dramatically demonstrated in the 1930s when portions of the American High
Plains endured the Dust Bowl phenomenon. Wind-related erosion and deposition occurs most com-
monly in arid and semi-arid areas where ample bare ground is exposed to the atmosphere. In such
venues, dust and sand “storms” of varying intensity are not unusual.
Sand dunes
are a classic land-
form that results from the inevitable deposition.
In a few non-arid parts of the world, substantial (that is, meters deep) deposits of wind-blown silt,
called
loess,
are present. Because these are very fertile soils, loessal areas are among the most ag-
riculturally productive to be found anywhere. Large areas of the American Midwest are covered by
