New evidence is brought forward in a critical discussion of four hypotheses of origin of southeast-flowing streams in the Pennsylvania folded Appalachians. Development of subsequent streams along transverse faults is concluded to be an unsatisfactory explanation because a study of major water gaps revealed no evidence of faulting and because the few recognized important transverse faults are not the sites of water gaps.
The hypothesis of southeast-flowing Permian consequent streams, modified throughout a long erosion history, is not supported by a study of the positions of present streams with respect to geologic structures. Field evidence fails to sustain certain postulated processes of local superposition. The reversal of a considerable amount of Pennsylvania drainage is not explained.
The hypothesis of progressive piracy and local superposition employs a process of water-gap formation which is not accepted because the numerous wind gaps in the Pennsylvania folded belt show that abandonment of gaps, rather than their production, is the normal process now active. The hypothesis does not account for abundant wind gaps, for repeated alignment of wind and water gaps, or for dual water gaps maintained close together in the same ridge.
Additional evidence supporting the hypothesis of regional superposition is the seeming absence of faults in major water gaps and the failure of streams to utilize the few known fault lines breaking major ridges. Accumulated data invalidate various objections which have been urged against the hypothesis.