Penck — Climatic Features in the Land Surface. 169 



counteracted them the more, the greater the difference between 

 the actual distribution of heights and the ideal ultimate effect 

 of those agencies. In countries with dominant river action the 

 existing elevations are differences between the effects of earth 

 movements and the consequent degradation by river action. 

 Their height is therefore determined by the intensity of crnstal 

 movement and by the time that has since elapsed. There are 

 young mountains, attacked only along certain lines by water, 

 which has only made an approach toward destroying the for- 

 tress ; there are mature mountains which have already lost 

 their unstable annexes and are formed by rocks of greatest 

 resistance ; there are old mountains degraded as far as possible, 

 no longer forming elevations. 



Young, mature and old mountains show very different struc- 

 tures, and we cannot, therefore, connect the idea of a certain 

 class of crustal movement with mountain-making. Mountains 

 are caused by all crustal and volcanic movements which are 

 directed upward, either totally or as a component. But what- 

 ever the character of these movements is and however compli- 

 cated they may be, they only exceptionally interrupt the slopes 

 of fluviatile degradation. This is shown by the facts that even 

 in the regions of most recent elevations the continuity of the 

 fluviatile slopes or grades is only exceptionally interrupted, and 

 nearly all of these interruptions occur in volcanic regions, 

 where sudden outbursts take place. The intensity of crustal 

 movement, therefore, is not so great that it can disturb the 

 continuity of fluviatile slopes ; crustal movement can disturb 

 a former arrangement of slopes, it can produce new slopes, 

 but by their erosion and accumulation rivers maintain the 

 continuity of slopes. 



In desert regions crustal movements are able to interrupt 

 slopes, for they are not counteracted by running water. It is 

 in the deserts of the Far West where we see the best samples 

 of recent faults ; they cut through alluvial fans and form well- 

 marked steps in regions of deposition and erosion. It is also 

 in the deserts where we meet with such an arrangement of 

 mountains that closed basins are formed. These closed basins 

 came into existence because the grading river action was absent, 

 and though formed by crustal movement, they are also climatic 

 features of the earth's surface, for they are only produced 

 under certain climatic conditions, namely, in regions where the 

 amount of evaporation surpasses that of precipitation. 



^Ye see on the earth's surface not only the features of the 

 present climate but also those of a past climate. Very extended 

 areas, formerly covered by ice, are now exposed to river action. 

 The basins formed by the meeting of normal slopes and those 

 reversed slopes which originated under the ice, are filled with 



