PLANT SUCCESSIONS OF MT. ROBSON 



225 



side of it. During the retreat from its niaxiiiiuiu recent exten- 

 sion to its present position the ice front paused or advanced 

 shghtlj^ several times, so that there are now five parallel morainic 

 ridges, very distinct along most of their extent, but close to- 

 gether and confused upon the summit of the rock knob. 



To determine the rate of recession of the ice front, measure- 

 ments were made in 1911 and 1913 by members of the Alpine 

 Club of Canada. These show that at the point where the 

 observations were made the total recession for the two years was 

 28.4 feet (8.7 meters). A study of tree ages upon the various 

 moraines has furnished some data that throw light upon the 

 average rate of retreat during the period of recession from the 

 oldest recent deposit. The following table gives a summary of 

 the counts. Large trees were cut with an axe 8 to 12 inches 

 above the ground; very small ones were uprooted and counted 

 at the surface. Allowance must be made for this difference, 

 which will add some years to the large trees, and thus make the 

 recessional rate still slower. 



Counts of tree ages — all Picea Engelmanni 



The small tree on moraine III, which was more than eighty- 

 two years old, is the most important. This was approxi- 

 mately 500 feet from the ice front. It is therefore certain that 

 more — doubtless considerably more than eighty two years have 



