GLACIATION OF THE SIERRA NEVADA. 23 



turing of the earth's surface has been done by the agency of ice. The sub- 

 ject will, it is thought, receive some light from what has been stated in the 

 gravel volume with regard to facts observed in the Sierra Nevada and 

 elsewhere. 



Section II. — Former Gluciaimi of the Sierra Nevada, of the Pacific Coast, and 



of the Cordilleras in yeneral. 



Some account may now be given of the facts which have been collected 

 during the progress of the Geological Survey of California in reference to 

 the former extension of glaciers over portions of the Sierra Eange, and to 

 these will be added such information as is in our possession, obtained from 

 various sources, about the development of the ice-period in regions adjacent 

 to the Pacific Coast, and in the Cordilleras generally. Brief statements of 

 the facts observed in the Sierra Nevada have already been published in the 

 Geology of California, Vol. I. ; and the object at present is to put these 

 and more recent observations into a connected form, so as to open the way 

 for a discussion as to their importance, both in reference to the distribution 

 of surftvce detritus in the Sierra, and to the theories of former ice extension 

 generally. 



The first fact to be noticed is the great ease with which the evidences 

 of former glaciation in the Sierra Nevada are obtained. There is nothing 

 doubtful aljout the matter; once one enters upon a formerly glaciated region 

 thei'e is no possibility of mistaking the origin of the phenomena presented 

 to view. Polished and scratched surfaces, smoothed and rounded ledges 

 [roches mouionnees), transported boulders, and, above all, moraines of various 

 kinds, are all recognized with the greatest ease by the observer, making a 

 combination of occurrences which no one familiar with glacier regions could 

 for a moment hesitate to refer to their true cause. The difficulty has been, 

 however, that a good deal has been published by various persons who 

 were quite unacquainted with glacial work, and who indeed in some cases 

 had not had the advantage of a previous training as observers in any depart- 

 ment of geology ; and these writers have brought confusion into the .subject 

 by referring many phenomena to the effects of ice where there was by no 

 means any sufficient authority for such reference. Some have even gone .so 

 fir as to sprinkle livino- n-laciers all over the Sierra, and others have ascribed 

 to ice the principal share, or even the exclusive agency, in forming and fash- 



