50 RECORDS 



These monuments stand on the edge of the rim rock of an old 

 escarpment nearly 300 feet high. The rim rock of the escarp- 

 ment is a coarse brown sandstone capped by about two feet of 

 thin-bedded dark-brown sandstone containing shark's teeth. 

 The face of the escarpment has recently slipped along a series 

 of joints running approximately parallel to face of escarpment, 

 and in a general direction of S. 30° E. The recesses between 

 slipped blocks can be sounded to a depth of over fifty feet, and 

 are wider at base than top as a rule. 



In the slipping an ancient rock " hogan " 20 feet in diameter 

 has slid 2.5 feet vertically and 8.3 horizontally without displac- 

 ing the rock walls to any serious extent. 



Richard E. Dodge, Arroyo Formation. 



An arroyo is a steep-sided, narrow gulch cut in a previously 

 filled gravel and adobe valley in the arid West. 



The study of process of formation of arroyos, some of which 

 have been under observation for several years, seems to show 

 that the w^ork has changed from aggradation to degradation 

 because of some influence that has caused the focusing of the 

 running water. Such a concentration of water is made possible 

 by overfeeding of the land, which removes the help of roots in 

 holding soil particles, combined with the habit of cattle to move 

 in processions along trails that make natural channels for 

 water. 



The study of the rate of valley filling or erosion is difficult, 

 because of the tendency of arroyos cut in adobe to maintain 

 nearly vertical walls, and because a fallen block of adobe may 

 be sealed over in the next flood, so that it looks in place. This 

 problem is of especial importance, because the adobe deposits 

 in some places contain relics of human occupation to a depth 

 of many feet. The exact or even the approximate antiquity of 

 the deposits cannot be definitely determined, because of the 

 several ways in which the order of ev^ents in such a case may 

 be interpreted. 



Gilbert van Ingen, The Ausable Chasm. 



This paper was a description of the geological and physical 

 features of this celebrated locality, which incorporated the results 



