STREAM PIRACY AND NATURAL BRIDGES 



499 



increased gradient of the diverted portion had caused it to intrench 

 itself slightly in the bed of its old course. 



The case illustrates both stream piracy and the formation of 

 natural bridges by lateral erosion. The writer observed two other 

 smaller bridges formed in the same way. In the two latter cases, 

 however, drainage was not diverted. Several cases were noted in 

 which the undercutting was nearly complete. 



Fig. I 



At the time the writer observed the foregoing features all the 

 gullies were dry. One living in a region of loess or badland forma- 

 tions subject to rapid local erosion could doubtless study these 

 features in actual process of formation during any heavy shower. 



Fig. I is a rough sketch, showing the general features described 

 in the foregoing material. 



