LOW BASE-LEVEL 135 



rough average of 900 feet and a maximum of 1,470; the 

 narrow passage joining the sound and gulf an average of 

 600 feet, with a greatest depth of 1,000. At the extreme 

 south Puget Sound has an average depth of 500 feet and 

 a maximum of 925. 



With the possible exception of Hecate Strait, these 

 water-filled valleys are clearly products of erosion; and 

 it is probable, though not proved, that they are younger 

 than the low peneplains. Whatever the extent to which 

 they were hollowed out by rivers, they were afterward 

 greatly modified by glaciers; and the glaciers are respon- 

 sible for the conspicuous unevenness of their floors. The 

 demonstration of ice work is found in the thorough glaci- 

 ation of all bordering lands, to be presently described, and 

 in the sculpture of rocky islets, which have characteristic 

 moutonnee forms. Much of the unevenness, and espe- 

 cially the deeper basins, must be ascribed to glacial 

 erosion, but a share may also be referred to glacial depo- 

 sition. 



Whatever the extent to which the hollows were deep- 

 ened and enlarged by glaciers, it is probable not only 

 that they were initiated by rivers, but that some of the 

 rivers sunk their beds considerably below present tide- 

 level. The most satisfactory evidence on this point was 

 found at the extreme south. In the Puget Sound region, 

 as shown by Willis, the ice movement was southward, a 

 lobe of the ice-sheet ascending the broad valley of western 

 Washington. This lobe made extensive modification of 

 the face of the country, but chiefly by deposition and only 

 secondarily by erosion. The system of troughs it left 

 behind are regarded as preexistent stream valleys, only 

 moderately scoured and straightened by the ice which 

 overran and occupied them. 1 



1 Drift Phenomena of Puget Sound. By Bailey Willis. Bull. Geol. Soc. 

 Amer., vol. ix, pp. 111-162, 1898. 



