116 A MANUAL OF TOPOGRAPHIC METHODS. 



Every stream tends to extend its drainage area by erosion at its 

 sources mi all sides, necessarily ;it the expense of its neighbors. The stream 

 having the most rapid fall erodes the margin of its basin most rapidly. 

 Hence in their struggle for existence the stream having the most rapid descent 

 succeeds in drawing area from others. But in so doing it diminishes its own 

 rate of fall, so that eventually a state of equilibrium among streams may be 

 reached. This extension of basins is called piracy. It is going on actively 

 in the Appalachian valley, where numerous examples may lie found. 



While under certain circumstances the courses of streams are mutable, 

 under other conditions streams maintain their courses with great pertinacity. 

 Of this, water gaps and canyons across mountain ranges are striking results. 

 Where such a canyon is found, the river flowed before the range or ridge 

 existed. The range may have risen across its course, in which case the 

 river, like a circular saw, maintained its course by corrasion, cutting the can- 

 y< in as the mountain rose. ( )f this action the canyon of Green river through 

 the Uinta range is an example. 



Or, the river, draining a surface of soft or soluble rocks, and eroding 

 this surface down, may have uncovered a ridge of hard rock King across 

 its course. In this case, like the other, the river maintains its course by 

 cutting a canyon through the ridge. The Appalachian valley presents num- 

 berless examples of water gaps formed as above described. Among them 

 may be mentioned Delaware Water gap, through which Delaware river passes 

 Kittatinny mountain, gaps of the Susquehanna and the Juniata, that of the 

 Potomac at Harpers Ferry, and Big Moccasin gap, while Little Moccasin 

 gap is in process of completion. While these are prominent and well known 

 cases, in certain localities, every little ridge is cut into a line of knobs by 

 them, so that, next to the parallelism of its ridges and valleys, the water gaps 

 of the Appalachian valley constitute its most prominent feature. Such of 

 these gaps as can be shown should appear on the map, and when owing to 

 the minuteness of these features it becomes necessary to omit them, one 

 should recognize the fact that the formation in this region is that of parallel 

 ridges and so represent the structure. 



Wind gaps are abandoned water gaps, from which the stream has 

 been drawn away by a more powerful neighbor. These should not be 



