6 ALBRECHT PENCK 



at those places which have been covered by the ice for some time, and 

 then revealed again. Here we observe those very well-known roches 

 moutonnees, polished and striated surfaces, which dip gently in the 

 direction from which the glacier came, but which terminate abruptly 

 in the opposite direction. In general, the rock surface is here limited 

 by joints. It cannot be longer maintained that we have at those 

 places the original surface of the rock before us. We stand rather 

 before a quarry from which rock fragments were broken out and 

 plucked away. This can be proved now and then by observation. 

 At the Hornglacier in the Zillerthal Mountains, for example, I found 

 near a roche moutonnee fragments which had been plucked out there 

 and transported by the ice for some distance, slightly upward. They 

 fitted perfectly into the quarry from which they were taken. Thus 

 the roches moutonnees teach us that glaciers do not exercise a scour- 

 ing action alone on their beds, as generally stated, but that they also 

 effect a quarrying and plucking action, which is not always recog- 

 nized. Therefore we will no longer call the two sides of a roche 

 moutonnee "push side" and "lee side," but we prefer the expressions 

 "scour side" and "pluck side," introduced by Shaler. Plucking 

 forms the most important part of glacial erosion; it is exercised as 

 well on the bottom as on the sides of its bed, and since the glacier 

 can also transport fragments upward, it is enabled to adjust its bed 

 to its mass and its movement. 



The adjustment of its bed to its mass and movement is not con- 

 fined to glaciers; the same occurs in rivers. The difference lies 

 partly in the fact that the glaciers' beds, on account of the slowness 

 of glacier movement, are far more conspicuous than the beds of 

 rivers of equal capacity. The adjustment to which we refer is con- 

 trolled by the necessity that through a given cross-section of a river or 

 of a glacier must annually move the whole quantity of run-off or ice 

 produced in the corresponding catchment basin. There is there- 

 fore a relation between the size of cross-sections (Q) and the mean 

 velocities (V) of the liquids, on the one hand, and of area (.4), pre- 

 cipitation (p), and evaporation or ablation (e), on the other, which 

 may be expressed by the following equation: 



VQ=A(p-e). 



