46 
MUSEUM BULLETIN NO. 11. 
The Formation of River Terraces. 
After the glaciers had disappeared erosion began to modify 
the irregular deposits of loose debris left by the glaciers. This 
material was easily moved and transported in great quantities 
into the larger valleys, so that the streams were overloaded and 
began to aggrade their floors. Successive aggradation and 
cutting down then formed the series of terraces found in the 
valley to-day. 
These terraces are described on page 31, and a series of 
them is shown in profile in Figure 3. They may have originated 
in several ways: by successive regional uplifts; by temporary 
obstructions in the valley bottoms, which caused lakes to form 
in which flat deposits of debris were laid down, to be later partly 
cut away and left as a terrace remnant; they may have been 
caused by simple meander of a rather overloaded stream, ac- 
companying gradual downcutting; or they may have been 
caused by climatic changes which caused the rivers to deposit 
their loads at one time and cut into their floors at another. In 
all probability several of these causes operated in their formation. 
We have no definite proof regarding interrupted regional 
uplift after glaciation and do not consider it necessary to the 
formation of the terraces, since the river floor was far above 
the base level of erosion at the end of the glacial period and the 
river would possess potential downcutting energy sufficient to 
account for the amount of such downcutting as it has accom- 
plished since. 
Temporary obstructions in a river channel are undoubtedly 
responsible for the formation of some flat deposits which would 
be partly cut away after the obstruction was removed. Such 
obstructions would be caused by glacial damming or the crossing 
of the valley by a hard formation. The broad terrace at an 
elevation near 2,900 feet, which covers most of the West Fork 
floor near Wallace lake, is probably a glacial lake floor caused 
by the damming of the head of Wallace lake by a moraine. 
An example of a terrace due to rock obstruction is that on the 
floor of Beaver creek opposite Collier lake. Such obstructions 
may be used to account for individual terraces, but they do not 
