ORIGIN OF LAKE BASINS. 13 



The study both of living glaciers and of the records left l\y ancient 

 glaciers has proven that flowing ice both erodes and deposits, and tlint 

 basins result from each of these processes. 



Whether a glacier shall erode its beds or deposit material upon it, 

 seems to depend largely on its grade, and consequently on its late of 

 flow. In high-grade valleys among mountains formerly occupied by 

 glaciers, the higher and steeper portions of the main avenues of ice 

 drainage, are usually intensely glaciated, and the worn and rounded 

 surfaces are frequently bare of glacial deposits ; but the lower portions of 

 such valleys, especially where they open out on a plain, are almost alwavs 

 heavily covered with morainal material. Not only are moraines deposited 

 in the mouth of the valleys, but sheets of gravel, clay, and boulders are 

 spread over the bottom of the glaciated troughs, showing that the ice- 

 streams in such situations deposited material on the surface over Mdiich 

 they flowed. 



Above the region of most intense glaciation in lofty mountains there is 

 a zone, embracing the higher summits, where polished and scratched 

 surfaces are rare, and where there is but little delnis. This ui)})er 

 region was the site of the neves or snow fields of the glaciers that abraded 

 the rocks at a lower horizon and deposited their loads Avlien the grade 

 decreased and the ice currents were slackened. A similar association of 

 a region of glacial abrasion and an outer zone of glacial deposition, may 

 be recognized in countries formerly covered by continental ive sheets. In 

 the region of most intense glaciation, in the case of both Alpine and 

 continental glaciers, as has been shown by extended observation, there 

 are numerous rock l)asins, the sides and bottoms of which are })()lished and 

 striated. A large number of lakes of this character in the Cordilleran 

 region have been examined b}- the writer, and their stud}- left no doubt 

 that they were due to glacial action. These rock basins are confined to 

 areas of intense glaciation, and are al)sent from adjacent areas where 

 the conditions are essentially the same, except that glacicis have not 

 passed over them. 



It is impossible to point to examples wln'rc living glaciers are actually 

 engaged in wearing out rock basins, since their work of abrasion is neces- 

 sarily concealed; neither is it possible to satisfactorily observe the i)rocess 

 by which glarici's polisli and striate rock surfaces, yet no student of the 

 sul)j('ct doui)ts that tlicsc results arc pioduci'd by moving ice cliar^cd with 

 sand and gravel. The nature of the evidence leading to tlu' conclusion 

 that many rock Ijasins are due to glacial abrasion, is of the same character 



