Bolson Plains of the Southwest— Tight. 273 
older valley, except in so far as these deposits must con- 
tribute an important factor in determining the character- 
istics of the topography. An important element in this 
definition depends upon the interpretation of the word 
"plain." A bolson is genetically related to other construc- 
tural plains, such as flood plains and terrace plains of river 
valleys. In the flood plain and terrace plain deposits the 
plains are more or less narrow and parallel with the stream, 
which furnishes the major portion of the deposits. On the 
other hand in the bolson, while the deposits are of fluviatile 
origin (or largely so) the plain loses the characters of a 
flood plain or terrace plain in that it is generally much 
broader and the material "is derived from adjacent emin- 
ences," and the plain is of such an extent that the "talus 
hills and fan cones and wash-deposits brought down by 
the mountain freshets" form only the bordering characters 
of the plain, but point unmistakably to the origin of much 
of the deposits forming the floor of the bolson. 
If we look for a moment at the forces involved in the 
formation of a bolson we see that there must exist an ex- 
tensive trough in which the contribution of material from 
the sides of the bordering mountains is much greater than 
the capacity of the pre-existing drainage to remove, or 
that in the structural formation of the valley there is formed 
a closed basin and into this the lateral materials are de- 
posited. In the closed basin type, the basin may or may 
not be the location of a lake. If the older trough has free 
drainage to start with the lateral introduction of debris 
may be such as to divide an otherwise continuous stream 
course into a series of lake basins or to even totally obliter- 
ate the through-flowing stream as a surface feature. 
Mr. Hill says: "These plains or 'basins' as they are 
sometimes called are largely structural in origin," and 
again he uses the terms "structural valleys between moun- 
tains and plateau plains." If it were intended to confine 
the term bolson to those forms which occur in strictly 
structural valleys which have not been subsequently modi- 
fied by erosiorij it wbitld'have a very limited Use indeed, for 
but few valleys of that type are to be found, and the author's 
illustrations do not indicate this' limited use.' I'ut 1 take it 
