4 , LAKES OF NORTH AMERICA. 



action, are wind erosion basins or areas of pronounced rock decay, from 

 which glaciers have removed the loosened material without deeply abraid- 

 ing the unweathered rock beneath. The mode of origin of rock-basins 

 is still a matter of controversy, but it seems evident to the writer, not 

 only from reading the various views advanced by others, but also from 

 personal observation in many lake regions, that rock basins have been 

 formed by each of the agencies mentioned as well as by a combination of 

 tlie two. The formation of basins by ice erosion and by chemical solution 

 might be included among the results of atmospheric action, but under 

 the classification here adopted they fall in different categories. 



Atmospheric agencies also lead to the formation of basins by depo- 

 sition ; as for example, when sand is drifted into dunes. Drifting sand 

 frequently travels across the country for scores of miles in the direction 

 of the prevailing winds, and sometimes obstructs valleys so as to cause 

 lakes to form. The best illustration of this occurrence known to the 

 Avriter, is in the central part of the State of Washington. The drainage 

 of one of the deep narrow valleys known locally as " Coulees," which 

 trench the Great Plain of the Columbia, has been obstructed by immense 

 sand dunes, so as to form a dam and retain the water of Moses lake.^ 

 Below the dam of drifted sand there are several springs fed by lake 

 waters percolating through the obstruction. These serve to keep the 

 waters of the lake fresli. The springs below the sand drifts unite to form 

 Alkali creek, which in winter sometimes has sufficient volume to reach 

 the Columbia, but in summer suffers from evaporation, and terminates 

 in a series of allvaline pools. 



Drifting sand may lead to the destruction of a lake as is illustrated 

 by an example in western Nevada. The branch of Truckee river, 

 supplying Winnemucca lake, is partially obstructed by wind-blown sand, 

 and a struggle for supremacy between the river and the encroaching 

 dunes is in progress. Should the sands prevail and a dam be formed, 

 the water supply of Winnemucca lake would be diverted to Pyramid 

 lake, and its basin would soon become desiccated. 



Volcanic dust is carried great distances by air currents, and might 

 accumulate in a valley so as to obstruct its drainage. No lakes, retained 

 by dams of this nature, are known on this continent, although thousands 

 of square miles in the western part of the United States were covered, in 

 Pleistocene and recent times, to a depth of many feet with fine volcanic 



1 I. C. Russell, "Geological Reconnoissance in Central Washington," U. S. Geol. Surv. 

 Bulletin. No. 108. 



