108 CAPPS: AGE OF THE LAST GREAT GLACIATION 



GEOLOGY. — An estimate of the age of the last great glaciation 

 in Alaska. Stephen R. Capps, Geological Survey. 



For many years certain Alaska glaciers have been under 

 observation, and considerable material dealing solely with gla- 

 ciation in Alaska has been published. A large amount of data 

 on glacial conditions throughout the territory has also accu- 

 mulated incident to general geologic investigations, and has 

 been pubHshed in the reports which treat of the geologj^ of the 

 different districts. The pubhshed literature on glaciation in 

 Alaska is, therefore, so extensive that it can not be discussed in 

 this paper. 



It is a generally recognized fact that the present glaciers of 

 Alaska are but the shrunken descendants of vastly greater 

 glaciers which at some former time reached a much more ex- 

 tensive development than they now have. Yet notwithstand- 

 ing the amount of attention which has been given to the sub- 

 ject by many observers, practically no attempt, based on trust- 

 worthy evidence, has been made to date the age of this last 

 great ice advance in Alaska, or to correlate it with any of the 

 glacial stages which have been so well established in the Mis- 

 sissippi Basin. 



Quantitative studies of the variations of certain coastal 

 glaciers of Alaska, some of which have advanced or retreated 

 long distances within a comparatively short term of years, have 

 established a rather general impression that the greater por- 

 tion of the retreat of all Alaskan glaciers from the points of 

 greatest advance to their present positions has taken place 

 rather recently. Few men have been willing tQ hazard a guess 

 as to how many years ago a particular valley was bared of ice; 

 but that the time should be measured in centuries, rather than 

 in thousands of years, would probably have seemed reasonable 

 to most of those who have thought on the subject. The feeling 

 that the time should be measured in hundreds of years, rather 

 than in terms of greater magnitude, has also been strengthened 

 by the small amount of post glacial erosion which can be dis- 

 covered in many strongly glaciated valleys. 



During the progress of a reconnaissance geological survey 



