233 



down on the inclined slope of the beds than the other. Now since these 

 tributaries are supplied with water draining down the surface of the 

 impervious layer beneath the limestone the tributary farthest down 

 on the slope will receive the greater amount of water. Thus it often 

 happens that there is a lower cave from which a stream of water is 

 issuing and an upper cave that contains little or no running water. 

 In regions of such occurrences the cave on the lower part of the slope 

 is referred to as the "wet cave" and the upper one as the "dry cave." 

 The direction of dip is readily determined by the relative positions of 

 these caves. 



Fig. 6. Shows valley trending at right angles to the dip of inclined strata. 

 and overhanging ledges on left. 



Cave 



Sink Holes. — On moderately to steeply inclined limestone surfaces 

 the shape of the sink holes may be an indication of the direction of dip. 

 As a rule the longer axis of the sink hole will lie parallel to the direc- 

 tion of dip. Erosion produced by water flowing into the sink will be 

 greater on the side opposite the direction of dip. The slope on this side 

 of the sink becomes longer and more gentle. Very frequently there 

 will be one or more short surface streams entering the sink from the 

 side of this gentler slope. 



Length of Tributaries. — In the case of a stream cutting in a direc- 

 tion approximately at right angles to the direction of dip the tribu- 

 taries which follow down the dip will be longer than those which flow 

 up the dip. This would not be true in a rock of uniform hardness 

 devoid of stratification. Such indications are more noticeable in beds 

 containing hard and soft layers of rock. 



Indurated Surfaces. — The surfaces of some porous beds of rock 

 which are exposed on the sides of cuts opposite the direction of dip 

 become indurated by the more or less constant evaporation of water 



