56 GLACIERS OP THE CANADIAN ROCKIES AND SELKIRKS. 



ciently protect the. ice. When this covering becomes too thin, or when it is 

 blown off, or washed down the steep slopes by heavy rains, the ice core becomes 

 exposed and rapid melting ensues, resulting in the destruction of the feature. 

 In case the surface covering is distributed about the base and the pure ice core 

 exposed, the melting does not cease when the general level is reached but con- 

 tinues more rapidly than the surrounding ice to which has been transferred the 

 debris. Instead of a cone, we may now get a basin-shaped depression, which is 

 gradually extended laterally by melting, and into this depression the original 

 material may again slide and be collected at the centre until there is sufficient 

 to prevent further melting. An interesting and instructive experiment, in 

 connection with that suggested upon the dirt wells, is to wash down the gravel, 

 sand or dirt, from a collection of small cones, mark the location, and watch the 

 changes from day to day. 



i. Glacial tables. In the case of a single rock fragment, of sufficient size, 

 resting upon the ice over which surface melting occurs, protection is afforded 

 the ice immediately beneath. As the result of the more rapid melting of the 

 surrounding ice the rock is relatively elevated upon a pedestal of ice and there 

 results what is termed a "glacial table"; as seen in plate xvm, figure i. As the 

 rock is elevated a short and narrow ridge of ice lying to the north of the pedestal 

 (observe the shadow in the figure) is protected from the noonday sun, so that 

 viewed from the east or west the pedestal is unsymmetrical. This lack of 

 symmetry is further emphasized by the undercutting action of the rays of the 

 noonday sun upon the southern side. Some observations were made with a 

 view of discovering the lower limit of the rock fragments that were capable 

 of furnishing the protection necessary to form tables. The following were 

 found forming low tables, or starting to form them. It is obvious that the 

 color and nature of the rock would both have their influence in determining 

 the effect upon the ice. 



Dark gray limestone, 12x12x4.5 inches. 



13 x QX 3 " 



Light " " ii x 6x3.5 " 



Reddish quartzite, 10x8x2.5 

 Rusted limestone, 9 x 4.5 x i 

 Dark limestone, 8x4x2.5 to 3 inches. 



In the case of the last specimen the thicker end was found to be protecting, 

 while the thinner was inducing melting. Owing to the undercutting action 

 of the sun's rays blocks of this size can form only low tables. Larger blocks 

 may rise to a height of three to five or six feet upon the Victoria, the latter 

 heights being unusual. They may persist from one season to another but there 

 is a limit to the height which any particular table may attain, determined mainly 

 by the size and shape of the rock. As the rays undercut, mainly upon the 

 southern side, the block begins to lean to the south and finally topples off in 

 that direction (plate xvm, figure 2). The remnant of the pedestal is removed by 



