I INACCESSIBLE VALLEYS 21 



various grotesque shapes and some of them forty feet high . 

 Hearing the sound of running water at a fissure among 

 some of the rocks, he descended, and found a cavern sup- 

 ported by pillars of the same character as those above, 

 with a small stream, which in the rainy season would 

 become a torrent. Here, then, are ample causes to explain 

 the formation of these great rock- walled branching valleys 

 in the sandstone plateau ; the remaining feature — that 

 the rivers all escape through deep gorges often so narrow 

 or so blocked up with rock-fragments as to be impassable 

 — evidently depends on the fact that the outer escarpment 

 of the plateau is formed of a series of harder rocks, and 

 thus does not wear away laterally. In this respect they 

 resemble those numerous gorges in the Alps which form 

 the only outlet for high valleys of considerable extent, 

 such as those of the Trient, the Reuss, and many others. 



The difficulty as to whither the denuded material has 

 gone, does not seem a great one, when we remember the 

 many millions of years the process of denudation has been 

 going on, with alternating epochs of greater rainfall pro- 

 ducing more rapid-flowing streams and greater floods, by 

 which the bulk of the sandy material would be carried 

 out to sea, while the finer suspended matter would be 

 deposited during wide-spreading floods on the valley 

 bottoms and alluvial plains. The absence of great 

 quantities of rock in the valleys themselves merely indi- 

 cates that the degradation of the cliffs is now so com- 

 paratively slow that the fallen masses are worn down by 

 atmospheric agency at about the same rate as they are 

 reproduced. 



Let us now see how the same general principles and 

 the same denuding agencies will apply under the very 

 different conditions which have prevailed in the district 

 of the Yosemite. These differences are, mainly, the much 

 loftier mountains and the very much greater extremes of 

 climate; the recent occurrence both of glacial and of 

 volcanic action on a large scale ; and, lastly, the whole 

 valley being excavated in granite instead of in sandstone 

 rock. 



