CAPPS: AGE OF THE LAST GREAT GLACIATION 115 



assuming the rate of 200 years to the foot to be correct, we are 

 brought face to face with the conclusion that a period of at least 

 7800 years or in round numbers 8000 years has been required 

 for the formation of the 39 feet of peat exposed in this section, 

 and that the final retreat of the front of Russell Glacier to within 

 8 miles of its present position had taken place some 8000 years 

 ago. This figure of 8000 years is by no means to be consid- 

 ered as final, but is merely believed to express the proper order 

 of magnitude of the term of years since the disappearance here 

 of the glacial ice. The writer is inclined to believe that this 

 estimate falls considerably short of the time which has actually 

 elapsed. But if the estimate is within 25, or even 50 per cent 

 correct, it gives one a broader basis from which to consider the 

 geologic and physiographic history of Alaska since the with- 

 drawal of the last great glaciers. At the time of its maxunum 

 extension Russell Glacier, according to Brooks, reached as far 

 north as the mouth of Donjek River, about 130 miles beyond its 

 ^present terminus. Its retreat over 90 per cent of this distance 

 had therefore been completed 8000 years ago. 



This retreat of over 90 per cent of the distance must, how- 

 ever, not be considered to represent 90 per cent of the elapsed 

 time since the retreat began. The withdrawal of the ice was 

 probably a comparatively rapid one until the terminus with- 

 drew to somewhere near its present position, and the terminus 

 may have remained nearly stationary for a long while. This 

 glacier was first seen in 1891 and has been visited at intervals 

 since, and during the last 20 years its outer margin has changed 

 but little. 



Various estimates have been made of the length of time since 

 the Wisconsin ice sheet began to retreat. Chamberhn and 

 Salisbury, after reviewing the evidence, place the time as some- 

 where between 20,000 and 60,000 years. The determination 

 made in the White River valley, at a point which was not bared 

 until a large part of the retreat had been completed, is of the 

 same order of magnitude as the figures gained from other widely 

 different sources, and it therefore now for the first time seems 

 safe to say that the last great ice advance in Alaska was con- 

 temporaneous with the Wisconsin continental giaciation. 



