STRUCTURE, CLIMATE, AND BASIN LAND FORMS IN ARIZONA AND NEW MEXICO1

Abstract

THE geomorphic history of structural basins in Arizona and New Mexico has been studied mainly by Kirk Bryan, his co-workers and students. In 1926 Bryan published his preliminary findings on the San Pedro Valley in Arizona as an abstract.2 Though a short paper and without illustrations, it is important because it contains ideas on geomorphic history that found further and more precise expression in later work on structural basins in New Mexico. Bryan, in his study of the San Pedro Valley, regarded the differential movements that depressed the trough and elevated the bordering mountain blocks as having ended with the post-Gila deformation at the close of the Pliocene period. Subsequent stability allowed the development of a broad erosional surface on the basin fill. Renewed incision by streams and the establishment of successively lower base levels led to the formation of lower erosional surfaces. Bryan did not publish a map or diagram of these erosional levels, and his paper is important chiefly because its suggestion of a correlatable tier of terraces and erosion platforms was used extensively as the fundamental explanation of the geomorphology of basins in New Mexico. Since Bryan's paper, the San Pedro Valley has received little attention from geomorphologists; but in New Mexico the geomorphic history of several basins has been examined