Geology of Piujet Sound Basin. — Kimball. 319 
of 2,400 and 3,100 feet* The sub-raarine mouth of the Sacra- 
mento river, according to Mr. Lawson, descends to a depth of 
378 feet, — thus forming the harbor of San Francisco. f 
While those features of the Piiget sound basin which, witli 
subsequent modifications, have adapted it to the present hy- 
drographic system were sculptured by agencies potentially in- 
cidental to a post-Glacial elevation, the varied topography of 
the border region has been produced in a subsidiary way inci- 
dental to still more recent changes of level. Of these the first 
was a post-fluvial subsidence below sea level, whence develop- 
ment of marine inlets from river channels. The partial and 
final re-elevation which followed is indicated by raised beach- 
es and partially base-leveled headlands. Again, while exist- 
ing and pre-existing lakes within the drift or catchment area, 
owe their existence in common with the sound itself primarily 
to specific fluvial erosion, their present condition and re- 
lations are incidental to the same more recent changes of 
level. Thus lake basins have been partially or wholly emptied 
and tideways branching from the sound either drained or cut 
off. and so converted into fresh-water reservoirs or drainage 
channels. 
Throughout the broad expanse of drift filling the Puget 
**ound basin, the present work of erosion and deformation, es- 
pecially along the shores of the sound, is mainly by under- 
cutting, thus giving rise to vertical bluffs. Both sound and 
river bluffs, however, as well as high shores of lakes, existing 
and extinct, have been greatly modified by landslides. The 
extensive development of clay seams throughout the alluvial 
column, and their interception by water-bearing divisions, 
below as well as above tide, combine to produce far greater 
diversity of the surface than can be traced to ordinary effects 
of sub-aerial erosion. Much of the superficial topography 
and terrace-like configuration indicated upon the Seattle sheet 
is due to landslides. Well-known engineering difficulties- from 
the same cause, in this region, as well as recent landslides of 
sound bluffs on the line of the Great Northern railway, near 
point Edmonds, might be instanced for purposes of illustra- 
tion, as also the submerged forest of lake Samamish. Mem- 
*Dana, Manual, 1895, 949. 
t Lawson op. cit., p. 271. 
