232 



stream will be thrown toward the down-dip side. The channel of the 

 stream will have a more gentle slope on the up-dip side and a more abrupt 

 slope toward the down-dip side. The stream, tending to follow the sur- 

 face of the hard layer in the bottom of the channel, cuts against the 

 bank on the lower side of the inclined bed making that bank more 

 abrupt by under cutting. At the same time the more shallow deposi- 

 tional area of the stream is on the opposite side and its slope is ren- 

 dered more gentle. 



Overhanging Ledges. — Outcrops of rock in inclined strata which 

 contain layers of sufficient induration to project unsupported form on 

 the upper side of the inclined beds overhanging ledges. These ledges 

 occur in layers of hard rock but are more pronounced in outcrops con- 

 taining alternate layers of hard and soft rock. Slight degrees of dip 



j^^n^^jjjio^ 



Fig. i). Notch cut by stream in diiiping strata. Note gentle slope on left and abrupt 

 slope on right. 



may be noted by observing the plane of shadows under these overhang- 

 ing rocks. Frecjuently the direction of dip may be determined by the 

 movement of water on the underside of these ledges. 



Caves. — In limestone regions the position of caves serves as an 

 indication of the direction of dip. Wherever a stream cuts through a 

 thick bed of inclined limestone the valley wall opposite the down-dip 

 side of the stream will have a series of caves which mark the positions 

 of tributaries or of former tributaries of the stream. The opposite side 

 of the valley will contain no caves in its wall. If these caves occur on 

 the west side of a valley trending north and south the direction of the 

 dip of the beds is eastward. 



In the case of a stream heading in an inclined bed of limestone it 

 frequently happens that more than one cave is formed. Frequently 

 one at each terminal of the small tributaries. If these tributaries be 

 close together and approximately parallel one will necessarily be farther 



