ORIGIN OF LAKE BASINS. 31 



degradation goes on energetically, and unless the deformation of the 

 surface is comparatively rapid, no pronounced topographic changes result. 

 It is the ratio between the rate of deformation and denudation which de- 

 termines whether basins shall be formed or not. Evidently the most favor- 

 able regions for studying the effects of movements in the earth's crust on 

 the surface relief, are those in Avhich the meteoric and aqueous agencies 

 are least energetic, nameh', in arid regions. 



Basins due to land-slides. — On steep slopes great masses of rocks 

 and earth not infrequently break away, especially after heavy rains, and 

 descend suddenly as land-slides into the adjacent valley. When this 

 occurs, the drainage in the valley may be obstructed so as to cause lakes 

 to form. Avalanches of snow and loose rocks also produce similar results, 

 but of a less pronounced character. 



Small lakes originate in many cases on the surface of land-slides 

 owing to the fact that such surfaces, after the descending mass has come 

 to rest, usually incline toward the cliffs from which they broke away, in 

 such a manner as to enclose basins. At times, a land-slide plows up the 

 floor of the valley into which it plunges and forms a ridge, not uidike a 

 terminal moraine, which may also act as a dam and hold a lake in check. 

 Examples of basins formed in each of these several ways have been 

 examined by the writer in the state of Washington ^ and in other regions, 

 but need not be described at this time. 



Basins due to clieniioal action. — In limestone countries the drainage 

 is often subterranean and finds its way through caverns formed by the 

 solution of the rock. The roofs of such caverns fall in as the general 

 erosion of the region progresses, and ol)struct the drainage channels so as 

 to form lakes. The surface waters reach underground channels through 

 openings termed "sink-holes," or "swallow-holes," Mhicli are eidarged by 

 solution, and frequently become closed so as to hold ponds, in portions 

 of Kentucky and throughout the Great Appalachian valley, where the 

 underlying rock is limestone, circular ponds of this nature are so numer- 

 ous that tliey give character to the landscape. Lakes also occur in the 

 caverns themselves, owing t() various causes, the most fre(pient being the 

 falling of portions of their stalactic roofs, as may be seen in Mammoth 

 and Luray caverns. 



^ "Geological Hfcoinioissaiicf in Central Wasliiiiirtoii." V. S. Geol. Svirv., BuUi'tin No. 108. 



