ISO 



PHYSICAL GEOLOGY 



Fig. 135. — Ice pillars protected by slabs 

 of rock. Parker Creek Glacier, California. 

 (After Russell.) 



The portions of a glacier over which dust or thin layers of earth are 

 spread will be melted more rapidly than those not so covered, since 

 the dark dust absorbs heat more rapidly than does ice. In this way 

 dust wells and other irregular hollows several inches in depth are 

 formed, the depth depending upon the diameter of the hollow and the 

 angle at which the sun's rays strike it. The great drifts of snow which 



had to be removed each spring 

 during the construction of a 

 railroad in Norway were scat- 

 tered over with fine dust in 

 order that they might be more 

 quickly melted by the sun. If, 

 however, dust is more than an 

 inch thick it prevents the under- 

 lying ice from melting and 

 forms dirt cones. 



Often the greatest irregulari- 

 ties on mountain glaciers are the long lines of rock debris (surface 

 moraines) which may be of considerable thickness and which usually 

 rest upon high ice ridges formed by the protecting cover of the 

 former. 



The water from the moulins and that which reaches the bottom 

 of the glacier in other ways, as for example that melted from the 

 lower surface of the glacier by friction, that which comes from the 

 springs in the valley through which the glacier is moving, and that 

 which seeps through the cracks of the ice, all emerges from a tunnel 

 in the end of the glacier as a single stream, often of considerable size. 

 These streams flow even throughout the extreme winters of glaciated 

 regions. 



Movement of Glaciers 



The Swiss early had reason to believe that glaciers move, as was 

 shown when two glaciers advanced over fields and meadows, up- 

 setting barns and filling the quarries from which the citizens of Bern 

 obtained their marble. A recent example of this sort occurred in 

 1909 1910, when the advancing Child Glacier in Alaska threatened 

 to destroy a #1,400,000 steel bridge. 



Rate of Movement. — It was not, however, until 1827 that any 

 serious attempts were made to determine the rate at which glaciers 

 move. I n that year 1 lugi built a hut upon the Aar Glacier in Switzer- 



