244 National Geographic Magazine. 



moderate and unequal height over the general plain of the Siluro- 

 Devonian low country, without great strength of relief but quite 

 strong enough to call for obedience from the streams along side 

 of them. And yet near Selin's Grove, for example, in Snyder 

 county, Penn's and Middle creeks depart most distinctly from 

 the strike of the local rocks as they near the Susquehanna, and 

 traverse certain well-marked ridges on their way to the main 

 river. Such aberrant streams cannot be regarded as superimposed 

 at the close of the incomplete Tertiary cycle ; they cannot be 

 explained by any process of spontaneous adjustment yet described, 

 nor can they be regarded as vastly ancient streams of antecedent 

 courses ; I am therefore much tempted to consider them as of 

 superimposed origin, inheriting their present courses from the 

 flood-plain cover of the Susquehanna in the latest stage of the 

 Jura-Cretaceous cycle. With this tentative conclusion in mind 

 as to the final events of Jura-Cretaceous time, we may take up 

 the more deliberate consideration of the work of the Tertiary 

 cycle. 



The chief work of the Tertiary cycle was merely the opening 

 of the valley lowlands ; little opportunity for river adjustment 

 occurred except on a small scale. The most evident cases of 

 adjustment have resulted in the change of water-gaps into wind- 

 gaps, of which several examples can be given, the one best known 

 being the Delaware wind-gap between the Lehigh and Delaware 

 water-gaps in Blue mountain. The wind-gap marks the unfinished 

 notch of some stream that once crossed the ridge here and whose 

 headwaters have since then been diverted, probably to the Lehigh. 

 The difficulty in the case is not at all how the stream that once 

 flowed here was diverted, but how a stream that could be diverted 

 in the Tertiary cycle could have escaped diversion at some earlier 

 date. The relative rarity of wind-gaps indicates that nearly all of 

 the initial lateral streams, which may have crossed the ridges at an 

 early epoch in the history of the rivers, have been beheaded in some 

 cycle earlier than the Tertiary and their gaps thereafter obliterated. 

 Why the Delaware wind-gap stream should have endured into a 

 later cycle does not at present appear. Other wind-gaps of appar- 

 ently similar origin may be found in Blue mountain west of the 

 Schuylkill and east of the Susquehanna. It is noteworthy that 

 if any small streams still persevere in their gaps across a hard 

 ridge, they are not very close to any large river-gap ; hence it is 

 only at the very headwaters of Conedogwinet creek, in the 



