. Elementary physical geography . e folded no more rapidly thanstreams are able to cut their channels, then their growthmust be remarkably moderate. Since there are other possi-ble explanations for these transverse valleys, we must con-sider this explanation as merely an hypothesis. Lakes are verycommon amongmountains, their ori-gin in these placesusually being thefolding of the rocks^which form damsacross the streamcourses. By this ac-tion of rock folding,streams may, in somecases, be transformedinto lakes whichmaintain an outflowin the same directionwhich the river for-merly held; or, insome
. Elementary physical geography . e folded no more rapidly thanstreams are able to cut their channels, then their growthmust be remarkably moderate. Since there are other possi-ble explanations for these transverse valleys, we must con-sider this explanation as merely an hypothesis. Lakes are verycommon amongmountains, their ori-gin in these placesusually being thefolding of the rocks^which form damsacross the streamcourses. By this ac-tion of rock folding,streams may, in somecases, be transformedinto lakes whichmaintain an outflowin the same directionwhich the river for-merly held; or, insome cases, folding ofthe rocks may actu-ally turn the streamfrom its course, andmake it begin to cuta valley at one side. Since the origin of these mountainlakes is that of rock folding, it very often happens thatthey are exceedingly deep. Generally their area is notgreat; but there are some immense basins, the interiorbasins previously described, which have all the charac-teristics of lake basins, but which are prevented from being. SCALE OF MILES 1 I 1 I I I I 0 12 3 4 5 6 Fig. drainage. PLATEAUS AND MOUNTAINS. 367 occupied by lake water because ofthe slight rainfall of the region inwhich they exist. Destruction of Mountains. — It hasbeen said that mountains are the com-bined result of the folding of rocksand denudation. When they aregrowing, the action of folding ex-ceeds that of denudation, and themountains continue to increase inelevation (Fig. 229). With this in-crease, stream action and the actionof weathering have their power in-creased, and the mountains are veryrugged. They are rugged partlybecause they are high, and partlybecause they are deeply carvedby stream erosion. Therefore thehighest and most rugged mountainsin the world are the youngest; andamong such mountains, lakes areusually present; for the recent, orperhaps the present folding of therocks has transformed a part of thestreams into lakes. After the folding has ceased, thereis no longer a tende
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