PLANT SUCCESSIONS OF MT. ROBSON 213 



successions. The agent of greatest importance is and has been 

 ice. In the retreat of the glaciers since the time of their maximuni 

 extension fresh surfaces have been continually laid bare, and 

 these surfaces have been mainly of two types: bare rock sur- 

 faces, largely cliffs; and moraines. Other physiographic agents 

 began to act upon these surfaces as soon as they were exposed 

 and to modify them profoundly. Thus we find the weathering 

 processes engaged in the task of destroying the smooth rock 

 surfaces. Temperature change, frost, solution by percolating 

 water, and the mechanical and chemical action of plants result 

 in the production of loose fragments of various sizes, which either 

 remain in the spot where they were broken off or are carried to 

 a lower level by gravity, according to the steepness of the slope. 

 In the first case the layer of fragments first formed protects the 

 surface from further extensive weathering; in the second case 

 the removal of the products continually presents fresh surfaces. 

 We thus find that the original rock surface, after a period of 

 exposure, has generated a second type of surface; and there are 

 now two types; areas that still remain bare and relatively smooth, 

 or are kept so by the removal of the weathered material; and 

 those that are covered by weathered fragments, either lying 

 where they were split off, or else brought down from higher 

 levels by gravity. The accumulations of the latter class, termed 

 talus, are very extensive in the Mt. Robson region, as in all 

 glaciated and recently glaciated mountains. The areas of 

 loose fragments thus increase at the expense of the bare rock 

 surfaces, which will finally be entirely eliminated. 



Another agent, running water, is engaged in the destruction 

 of the moraines. The streams issuing from the glaciers, already 

 bearing much fine silt, remove these deposits, often with great 

 rapidity, work them over, and finally deposit them, roughly 

 sorted by sizes, at various places along their valleys, forming 

 shingle and silt flats. Thus we see that the morainic areas also 

 generate a second kind of surface, so that we find both uneroded 

 moraines and shingle flats formed of materials derived mainly 

 from them. The shingle flats are plainly increasing at the ex- 

 pense of the moraines. 



