330 ISAIAH BOWMAN 



the profile of the descent is a normal curve, the flattest part being near 

 the mouth and the steepest near the head of the ravine. 



Out on the alluvial fan at the mouth of the ravine the stream 

 finds a much gentler descent than it has previously enjoyed, and so 

 begins to build up its bed, little sand-bars and islands shov^ing the 

 overloaded condition of the stream. 



The heads of the tributary gullies have a characteristic amphi- 

 theatral shape. The slopes all lead to one central point from which 

 the drainage follows a main course to the mouth of the gully. The 

 waste slopes exhibit at present but moderate activity, although six 

 or eight trees, prostrate because of undercutting, bear witness to the 

 steady deepening of the ravine. 



The beheaded valley, Oak Run. — The widths of the flat valley 

 floor of Oak Run valley, expressed in feet, at intervals of 200 feet 

 from the head, are as follows: 26, 22, 17, 16, 15, 10, o. At the next 

 to the last point given. Oak Run begins as a definite stream, and 

 the valley floor gradually disappears, the valley becoming F-shaped, 

 so that, standing at the mouth of Oak Run and looking upstream, one 

 has a view similar to that shown in Fig. 3, and the stream here exhibits 

 the same competency to remove waste, although no prostrate trees 

 speak of great activity. On the map the numbers in the valley of 

 Oak Run speak of abnormal valley descent, the flattest portion of 

 the floored part of the valley being near the head. The steepening 

 of the course continues until halfway down Oak Run, where the normal 

 curve is assumed and maintained for the rest of the way. 



The tributary gullies in the upper part of the valley have the 

 same amphitheatral shape as those of Oak Ravine display, but 

 farther downstream the dendritic pattern is assumed. The gullies 

 have here gashed the upland deeply, and tons of earth are carried 

 down them every year. Those at the head of the valley are long and 

 gently sloping, and are gradually building up the valley floor. Those 

 fiarther down push the weak stream from side to side until its increased 

 volume enables it to pursue a more independent course. The greater 

 part of the old watershed of Oak Run now drains into Oak Ravine, so 

 that immediately after capture had taken place the now greatly dimin- 

 ished Oak Run began aggrading its valley-floor until the grade 

 attained by the stream enabled it to carry all of the waste brought 



