1 36 ALASKA GLACIERS 



This interpretation by Willis seems to me reasonable. 

 The glacial deposits in the southern part of the Puget 

 Sound region are voluminous, and much of their material 

 is of distant origin. This is the place where the ice lobe 

 discharged its load, and it is not probable that in the field 

 of deposition the ice also developed by erosion a system 

 of narrow deep troughs. Regarded as stream valleys, the 

 channels of the sound tell of a pre-glacial base-level at 

 least 500 feet, and probably 1,000 feet or more, below the 

 present sea surface. 



A few features seen at the north might be regarded as 

 confirmatory, but their interpretation is subject to con- 

 siderable doubt. They are apparent exceptions to the 

 general rule that at all low levels the sculpture forms of 

 trough walls are glacial. 



But while the existence of a pre-glacial low base-level 

 is on the whole probable, its precise relation to present 

 base-level and the period of its duration are altogether 

 conjectural. To bring all parts of the deep channels 

 within reach of stream erosion it would need to be 3,000 

 feet below present sea-level in the region of the Alexander 

 Archipelago. Under present climatic conditions, such a 

 change would carry a very large area above snow-line, 

 and would so promote the alimentation of glaciers as to 

 flood the whole district with ice and abolish stream ero- 

 sion. Stream erosion, therefore, could not have been 

 carried, by lowering of base-level, to the lowest parts of 

 the channel system without the aid of important climatic 

 variation. Without doubting the possibility of wide range 

 in independent climatic factors, it seems easier to assume 

 that the lowering of base-level was comparatively mod- 

 erate, and that a considerable part of the down-cutting of 

 the channels was accomplished by Pleistocene glaciers. 



There is equal doubt as to the duration of the low 

 base-level, or the extent of the erosive work it enabled 



