134 ALASKA GLACIERS 



period that the chief pre-glacial erosion of the plateau 

 was accomplished. While the granite tables were being 

 dissected and the harder metamorphics worn into varied 

 and rugged mountains, flat valleys and plains were devel- 

 oped from the broader bodies of weak rocks. 



A Lower Base-level. The degradation of the troughs 

 in which lie the channels, passages and straits among the 

 islands, and the fiords of the mainland, has been carried 

 far below the horizon of the lower peneplains. Close to 

 the Annette Island peneplain Clarence Strait has a depth 

 of 1,675 f eet > anc * 5 miles farther north, where the 

 strait is narrowed by Cleveland Peninsula, a depth of 

 2,100 feet is recorded. Thirty miles inland from Annette 

 Island Behm Canal is 1,800 feet deep; and 60 miles in- 

 land, toward the head of Portland Canal, is a depth of 1,250 

 feet. The greatest recorded depth of Chatham Strait, 60 

 miles from either end and 25 miles inland from the Sitka 

 peneplain, is 2,900 feet; and its northward prolongation, 

 Lynn Canal, has one sounding of 2,475 feet. These fig- 

 ures are selected from charted soundings which indicate 

 great irregularity of bottom configuration; they are 

 maxima, and not averages; but the averages also are im- 

 pressive. In the main part of Lynn Canal (55 miles long) 

 the average sounding along the line of greatest depth is 

 1,300 feet. The similar average for the surveyed part (60 

 miles) of Chatham Strait is 2,000 feet; for Stevens Passage 

 1,000 feet, Frederick Sound 900 feet, Summer Strait 

 1,100 feet, Clarence Strait 1,500 feet, Behm Canal 1,400 

 feet, Portland Canal 1,000 feet. These are all in the 

 region of the Alexander Archipelago. The broad sound 

 east of Queen Charlotte Islands, called Hecate Strait, 

 ranges from 150 to 600 feet, and the narrow passages 

 east of it are somewhat deeper. Queen Charlotte Sound, 

 northeast of Vancouver Island, has a general depth of 

 600 feet and a maximum of 1,140; the Gulf of Georgia a 



