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The Purpose of this article is to point to some of the relations between structure, climate, and basin land forms in the San Pedro Valley, Arizona and to raise the question of the adequacy of the erosional-cycle concept as a tool in interpreting its geomorphic history. The San Pedro Valley in southeastern Arizona is about 120 miles long. The lower basins of the valley are relatively narrow, the surrounding mountain ranges being higher and closer together north of a constriction known as the Narrows than to the south. Deformation in the basins of the lower San Pedro Valley is indicated by the stratigraphic record and by topography.

The article focuses on the physiographic divisions of the U.S. A number of attempts have been made to divide the U.S. into natural or physical units which should be as nearly as possible homogeneous with respect to certain criteria. Recent demands for such a division of the U.S. have come from various sources. Within four weeks such requests came to the U.S. Geological Survey from the Forestry Bureau, the Division of Pomology and from several geographers. In educational work there is constant need of a scheme of natural divisions which may be described or explained as units.