ADVANCE AND RETREAT OF CANADIAN GLACIERS 723 



The several factors influencing the rate of melting will be dis- 

 cussed later. The one which it is especially desired to emphasize 

 is the protective effect of a thick covering of debris. 



It is a familiar fact that dust absorbs the sun's heat, causing 

 melting, with the formation of dust wells, while larger fragments 

 shade the ice, and may be left standing on protected pillars, the ice 

 having melted around them. Sand and debris cones are formed 

 in the same way, from the shading of the ice by a pile of material. 

 Obviously, if this process were carried to an extreme, and the "entire 

 surface of the ice covered, the whole would be shaded, and hence 

 protected from melting. 



In regions where glaciers now occur (i. e., regions of high alti- 

 tude or of high latitude) there are great contrasts in temperature 

 between sun and shade. It frequently happens that this contrast 

 involves a range above and below 32°, and that the temperature is 

 below freezing in the shade when it is well above in the sun. A 

 debris covering will protect a glacier from the direct sunlight, and 

 may therefore keep its temperature below 32°, and so prevent melting. 



Moreover, the daily range in temperature at the surface of the 

 soil is much greater than the daily range slightly below the surface. 

 A point below the surface will be shielded from extremes and will 

 be colder during hot, and warmer during cold, hours, than the surface. 

 In the case of glaciers, a debris covering protects the ice from the 

 air of the surface. Hence in times of extreme cold the protected 

 ice will be warmer than the surrounding air; in times of heat it will 

 be colder. Since in regions where glaciers now exist, the yearly 

 temperatures below 32° are greater in number and in amount than 

 those above, this protection from the air will be of more avail to 

 prevent melting than to cause it. 



It is therefore to be expected that protected glaciers should advance 

 farther down their valleys than if not so protected, and that the retreat 

 of protected glaciers would be slower. 



In the mountain region of British Columbia and Alberta there 

 are many hundred existing glaciers. This region is one of especial 

 interest to the glacialist in that it appears to be the center from 

 which came the lobes of a large part of the Cordilleran ice-sheet. 

 The highest parts of the mountains Avere never covered by ice, nor 



