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. Seldom are drainage anomalies as pronounced as they are among the great petrified waves of the Zagros. 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. The vast majority of the drainage anomalies and the most spectacular tangs in the Zagros are found in the zone of powerful but simple folding along the south-western (outer) margin of the highland. The drainage anomaly in the central Zagros may be resolved into two distinct problems: the courses of the trunk streams, and the behaviour of their tributaries, large and small. Each of these appears to be an independent development.