858 BULLETIN UNITED STATES GEOLOGICAL SUEVEY. 



Stones, if massively bedded, are eroded in the same way, and retain the 

 form of shallow caves for a considerable length of time. 



PRONT RANGE. 



Along the eastern border of the Front Eange many of the sandstones 

 there exposed show interesting results of erosion. Shallow caves have 

 been worn into the yieldingrocks, dependent upon their more or less firmly 

 cemented condition. Within the region containing monuments, such 

 caves are of frequent occurrence. They may be worn into the sandstones 

 by flowing water, or they may be due to gradual disintegration and 

 transportation of certain circumscribed portions. The method producing 

 caves of this character is so simple that it scarcely requires discussion. 

 Frost, rain, or other agents may start a shallow abrasion of the sand- 

 stones, which, in the course of time, will extend toward the interior, form- 

 ing a cave-like excavation. Similar conditions occur wherever sand- 

 stones of the same composition are exposed to fluviatile or other erosion. 

 The shape of such caves is a very simple one, being merely an arched 

 excavation, the plan of which usually resembles either half a circle, or, if 

 very extensive, the segment of a large circle. Modifications of this 

 shape take place in case water finds a free passage through fissures in 

 the rocks into the cave. Hard masses contained within the sandstones, 

 either as impregnations or concretions, remain less disturbed than their 

 surroundings, and form irregular projections on the cave-walls. 



CAVES IN TEACHYTIC CONGLOMEEATES. 



Cave-like excavations are thoroughly characteristic of the trachytic 

 conglomerates. While speaking of monuments, the composition of this 

 deposit has been discussed. It is evident that material of such character 

 will very readily be attacked by both fluviatile and pluvial erosion. Fur- 

 thermore, the results produced will vary according to the local character 

 of the conglomerate. Within the exposures on South River many caves 

 were found. Sometimes they are but slight niches worn into the steep 

 wall, and again they may extend for more than 30 feet into it. From 

 what was there observed, it is certain that frost loosens a large portion 

 of the material which is afterward removed. So far as could be seen, 

 the action is essentially a mechanical one, although decomposition of 

 various feldspars greatly facilitates it. 



Along the western border of the San Juan Mountains, a large mass of 

 conglomerate of this nature lies exposed. Even from a distance it can 

 easily be recognized on account of the rugged and grotesque forms it 

 assumes upon weathering. Dark spots seen before it is reached mostly 

 prove to be more or less shallow caves. JSIear Piedra Falls a number 

 of these were found. It was there noticed that the largest one, about 

 25 feet high, 18 feet wide, and 40 feet deep, owed its existence to the 

 presence of slowly moving water. The opening of this cave, which is 



