74 



V. H. BARNETT 



with almost vertical walls. The rock is a hard, rather blocky, light- 

 blue clay, showing typical bad-land topography and weathering rather 

 rapidly, though not as fast as the softer maroon clays of the Oreodon 



Fig. I. — A natural bridge south of White River below mouth of Porcupine Creek, 

 South Dakota. 



beds. The bridge was formed in the following method : Flowing in 

 the direction indicated by the arrows in Fig. 2, and taking the course 

 of the dotted Hnes, the stream kept cutting in on the narrow ridge at 



