4 - THE ORIGIN OF THE ZAGROS DEFILES

Summary

While the westward-flowing streams of the Zagros mountains provide some of the world's most impressive canyon scenery, they also present an extremely perplexing problem of drainage genesis. Transverse streams that seem unrelated to their geologic environment are rather characteristic of great mountain systems, but seldom are drainage anomalies as pronounced as they are among the great petrified waves of the Zagros, whose scanty vegetation and structural simplicity make every disharmony between structure and surface form conspicuous in the landscape. While the individual mountain ranges of this region are sundered indiscriminately by transverse streams that seem heedless of their presence, the morphology of the ranges in many cases exactly duplicates their tectonic structure. Thus we are presented in the Zagros with an unusual situation: the major land forms are structural, while the drainage lines, both major and minor, appear to disregard the geological environment altogether.

The disharmony between the drainage and the deformational pattern in the Zagros is manifested in the profound gorges, or tangs, which breach range after range in the youngest portion of the mountain system—a zone of powerful but rather simple anticlinal and synclinal deformation. The most typical tangs are extremely constricted slot-like defiles, 1,000 to 5,000 ft in depth, which split anticlinal mountains at any point from their culminations to their plunging extremities. The walls of such gorges rise perpendicularly from the rushing waters to heights of 200 to 1,000 ft or more, before falling back at slightly less precipitous angles; thus they bespeak strong recent incision by the streams flowing westward out of the Zagros.