166 Penck — Climatic Features in the Land Surface. 



arrangement as the grades of fluviatile origin. Glacial action 

 does not impose on the land the continuity of its surface slopes 

 as running water does ; it produces irregularities in its bed 

 which not only show continuous slopes but often opposite and 

 reversed slopes. Wind also produces slopes, but their arrange- 

 ment is still more irregular than those of a glacier bed. There 

 is a sharp limit between the regions of glacial erosion and of 

 glacial deposition ; the forms of eolian erosion and eolian depo- 

 sition also occur in close proximity ; wind does not transport 

 so continually nor so far as running water and glaciers do ; its 

 action is more brief and unsteady. Thus, the arrangement of 

 the slopes of the land surface permits us to recognize fluviatile, 

 glacial and eolian forms. 



Now the action of running water and of glaciers and the 

 display of eolian forces depend on climate ; rivers exist only 

 where a part of the rain runs off superficially on the land ; 

 their morphological action depends, therefore, on the amount 

 of run-off, which is consequently a geomorphological factor of 

 great importance. Glaciers are formed only where the amount 

 of snowy precipitation surpasses that amount which can be 

 melted away by the action of the sun. Wind action finally 

 becomes very visible, and is exclusively performed where 

 neither water nor ice action occur. We see, therefore, that 

 the differences between precipitation and evaporation on one 

 side and between snowfall and ablation on the other, determine 

 the surface features of the land, and hence we can recognize 

 in the surface features of the land certain features of its climate 

 in the same way as in its covering with vegetation. 



The display of river action does not depend alone on climatic 

 conditions. River movement is only possible where there is a 

 slope on which the rivers can flow. Only their existence is 

 due to climate, while the display of their force depends on 

 differences of elevation, and their action consists in the degra- 

 dation of existing heights. Glaciers do not necessarily presup- 

 pose elevations ; ,they can be formed also on low grounds, if 

 the climatic conditions of their formation are given. If more 

 snow falls on a plain than can be melted away, it accumulates 

 and will finally form an ice-cap which radiates from the plain ; 

 if here the accumulation reaches a great thickness, it can then 

 overcome finally by its surface slope existing inequalities of 

 its bed, as is clearly shown by the giaciation of North America 

 during the Great Ice Age, which partly radiated from the low 

 grounds west of Hudson Bay and overflowed in the east the 

 mountains of New England. Whilst river action consists in 

 the destruction of given inequalities of the earth's surface, 

 glacier action can create new inequalities by the erosion of the 

 central floor of an ice-cap formed at a low level and the deposi- 



