GLACIAL PERIOD— EEFILLING OF CAS^ONS. 
229 
the alluvium mounts up on the breasts of the perpendicular scarps. In some 
cases a smooth floor of alluvium extends from side to side of what was 
originally a canon valley. (Plate XXXVII.) The recurrence of a climate 
sufficiently moist to sustain a vigorous perennial stream would probably 
sweep out all this unconsolidated alluvium, and return the valley to its for- 
mer condition of an ordinary canon. 
