File:Physical and commercial geography; a study of certain controlling conditions of commerce (1910) (14582088608).jpg

Identifier: physicalcommerci01greg (find matches)
Title: Physical and commercial geography; a study of certain controlling conditions of commerce
Year: 1910 (1910s)
Authors: Gregory, Herbert E. (Herbert Ernest), 1869-1952 Keller, Albert Galloway, 1874-1956, joint author Bishop, Avard Longley, 1875-1932, joint author
Subjects: Physical geography Commercial geography
Publisher: Boston, New York, Ginn and company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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formed in the latter way, are widely different inorigin, texture, composition, and productivity from the transportedsoils of southern Texas or the Missouri bottoms, 48. Alluvial plains. A sediment-laden stream deposits its loadwhen its velocity is sufficiently decreased. This is observed in allstreams, but is particularly noticeable where a valley is marked by ab-rupt change from high to low grade, for example, where a precipitousgorge opens upon a plain. This sudden loss of power compels thestream to deposit its burden of rock and silt at the point where thegorge joins the plain. Under normal conditions, in humid regions,the master stream is competent to remove the waste supplied by thetributaries. But if the supply is excessive, as in steep mountain valleys,or the main stream weakened, as in arid regions, an accumulation ofwaste is left at the rivers mouth, — a half-conical pile with its base in ^ Such a surface is also called -a.pene^^lain (that is, almost a plain). TOPOGRAPHY 47
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Fig. 13. Distribution of Population in Utah the valley and its apex at the mouth of the gorge. Such an accumu-lation of debris is an alhruial co7ie ox fan ; or, if of gentle grade, analluvial slope. Many villages in mountain valleys rest on such fansor slopes; and in desert regions fans usually determine the locationof oases, — for water is found here if anywhere, — the texture of thesoil is favorable for agriculture, and the gentle slopes of the fan makeeasy the construction of irrigation ditches. Many cities in Persia and 48 THE NATURAL ENVIRONMENT Turkestan are located on fans, and the distribution of the plateau popu-lation of Bolivia and Peru is, to an important degree, controlled by theseforms. A striking example of this control is seen in Utah, where thepopulation is grouped in settlements, forming a line along the west baseof Wasatch Mountains (fig. 13). In this case the population would beconfined to an even narrower zone if the shifting settlements of themining camp were

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