120 B. WILLIS—DRIFT PHENOMENA OF PUGET SOUND. 
region of the Puget Sound basin are related in a sequence composed of 
several groups to which successive dates may be assigned. The pro- 
cesses now going on have been continuously in operation since the ice 
last retreated. They define an episode, the latest, but their effects have 
differed locally according to the position of sealevel. It is possible to 
discriminate two incidents, the one related to the present attitude of 
land and sea and the other toa previous attitude of deeper submergence. 
The next antecedent episode is defined by the conditions of the last glacial 
retreat, and preceding that retreat was the epoch of the last glacial ad- 
vance and occupation. These three episodes, which include the history 
of the region from the present time back to the last general episode of 
glaciation, are recorded in details of topographic forms. 
THE PRESENT AND ITS IMMEDIATE ANTECEDENTS 
Method of presentation adopted.—In discussing this portion of the sub- 
ject, it will be convenient to consider in their natural order the effects of 
corrasion by streams, of deposition from streams, and of wave work. 
Ravines and canyons.—Green river is a stream rising in the Cascade 
mountains and flowing westward. It enters the Duwamish valley at the 
same point as White river, a little north of east from Tacoma. Its upper 
course in the mountains is a canyon with very steep but generally not 
precipitous slopes, heavily covered with forest. From this canyon it 
debouches on an extensive gravel plain, across which its course extends 
about 20 miles (82 kilometers) to the Duwamish valley. In slightly less 
than half this distance it sinks below the level of the gravel plain ina 
canyon which is approximately 200 feet (60 meters) deep. In cross- 
section the higher slope of this canyon is banked with gravel terraces. 
The lower part, one-third to one-half the total depth, is cut across tilted 
strata of sandstone and shale which very moderately resist corrasion and 
exfoliation. The lower walls, where of sandstone, are precipitous and 
often overhang. The superficial gravel of the plain is to a large extent 
pebbly till, but the date of the glacier which spread it has not been ex- 
actly determined. If it corresponds to the last general expansion of the 
ice in the Sound basin, as is probable, the canyon represents the extent 
of corrasion since the final retreat. The river drains an area of 250 
square miles or more (64,800 hectares) in the Cascades and is a vigorous 
stream throughout the year. It still falls rapidly through its lower 
canyon. 
Carbon river heads in a glacier on mount Rainier, from which it flows 
in a northwesterly course to the Puyallup valley. Its waters are milky 
with glacial meal, and it sweeps along quantities of finesand. Its volume 
and rapid fall and sufficient abrasive material render it an efficient agent 
