LOW BASE-LEVEL 
*35 
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 x,ooo. 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. 
