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 and 50 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 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 
