170 Penck — Climatic Features in the Land Surface. 



water and form lakes ; the river action has not reached a nor- 

 mal slope ; waterfalls interrupt their courses. These lakes and 

 waterfalls are indications of extreme youth of river action over 

 large areas, and at the same time they are as significant mor- 

 phological evidences of a recent climatic change as moraines 

 and rock striations are in the geological sense. Id mountain 

 regions, as for example in the Alps, the lakes occupy valleys 

 which have the arrangement of river valleys ; they are subor- 

 dinate to an older topography, formed by fluvial action, and 

 we can recognize in the valleys of the Alps not only one 

 climatic change, but a succession of those changes. In the 

 lowlands of the north, however, it is nearly impossible to trace 

 the features of an older topography under the glacial one. 

 This, however, does not show those signs of antiquity which 

 we might expect to find in regions of continual glaciation. 

 Hence we must infer that the glacial features of other northern 

 lowlands are also superposed on an older topography which 

 has been destroyed in its characteristic features. We see on 

 the border regions of the old glaciations moraines which have 

 totally lost their morainic forms, and already possess the surface 

 features of a mature fluviatile topography. These surface 

 features have long ago led to the conclusion that we have to 

 deal here with older moraines, which indicate an older climatic 

 change. Thus regions of river degradation indicate climatic 

 change by their composition as well as glaciated regions indi- 

 cate such change by their forms. 



We meet with proofs of climatic changes also in desert 

 regions. Investigations of American geologists, especially of 

 Gilbert and Russell, have shown that the region of Great Salt 

 Lake was once occupied by a far larger freshwater lake, the 

 shore lines of which can be easily followed and the outlet of 

 which can be recognized in a side valley of Snake river. We 

 can trace in the Sahara very extended river valleys from the 

 Highland of Ahaggar down to the region of the south Algerian 

 Shotts. These valleys cannot be compared with those short 

 wadies in which the water of cloudbursts rushes down. They 

 indicate a former moister climate ; the desert with its surface 

 features extends here over a region of former water action in 

 the same way as, farther north, river action is now displayed 

 on a surface formerly covered by glaciers. 



In the interior of two continents we finally meet with surface 

 features which might be taken for morphological indications 

 of a change of desert climate into a moister one. Three large 

 lake basins interrupt the general grades of the head waters of 

 the principal African rivers. In the region of the Zambesi 

 river we find the very deep Lake Nyanza, in the catchment 

 basin of the Congo the likewise deep lake Tanganyika, and 



