EVIDENCE FROM GLACIAL PHENOMENA 2t 



as discussed above on page 19. The viscous or plastic habit is more 

 commonly proven in glacial phenomena. The students of glaciers in 

 Switzerland, Greenland, and Alaska have recorded many observations 

 which show the yielding of the ice to obstructions by arching over them 

 or bending around them. The production of roche moutonnee in the 

 beds of glaciers is an illustration of its yielding habit and its adaptation 

 to the irregularities of its bed. The rounding and smoothing of the rock 

 bosses is proof of the polishing action of the glacier, but the existence of 

 the bosses is proof that the abrasion was not sufficient to remove them 

 or to level the bed. The ice could not remove one set of original irreg- 

 ularities in its bed and then produce another set. If the ice could undo 

 its own work, or act as a plane at one time and then in the same place 

 scoop out irregularities and make mounds in its bed, we should find the 

 roche moutonnee form as the common form of glaciated surfaces even 

 in soft materials, whereas the form is characteristic of hard rocks and is 

 doubtless produced by a moderate amount of scouring on weathered 

 rocks. Other writers have recognized that the roche moutonnee form is 

 not consonant with vigorous abrasion or planation by the ice, but, on 

 the contrary, proves relatively slight abrasion. 



Drumlins illustrate on a vastly larger scale than roche moutonnee the 

 same j r i elding habit of the ice. Whatever be the source of the drumlin 

 material, their form is due to overriding by the ice and the rubbing and 

 molding by ice flow. They prove not only failure of erosion at the local- 

 ity of their occurrence, but failure of transporting power. They are large 

 examples of the compliance of the marginal ice of the continental sheet. 



The question might be asked, Could not a very deep glacier, having 

 great pressure on its bed, along with a steep gradient, giving high veloc- 

 ity, rapidly abrade its bed ? The reply is, decidedly no ! The postu- 

 lated conditions can not occur together, except within ineffective limits. 

 Extreme depth with high gradient is an impossibility in rivers and in 

 glaciers. As a matter of fact, the glaciated valleys are not extremely 

 steep for mountain valleys, and the upper and steeper sections often 

 show little erosion (see description of Alpine phenomena, page 31). The 

 great deepening of valleys has been assumed for the lower or ultimate 

 sections (in case of the Finger Lake basins the cutting is assumed where 

 the ice tongues were moving up the valleys). The physical conditions at 

 the bottom of deep glaciers is not known inductively. It is believed that 

 their bottom temperature is constantly at the melting point, which favors 

 fluency and the yielding of the ice, while the slow melting tends to keep 

 the rock debris beneath the ice, where it acts as a buffer. Moreover, in- 

 creasing depth and pressure must tend, other conditions remaining the 

 same, to diminish motion at the bottom. 



