DEPOSITS BY ICE-SHEETS 527 



show that the ice was several thousand feet deep, only the highest 

 peaks rising through it as nunataks. 



In addition to the great ice-sheets which covered the northern 

 parts of the continent, large local glaciers were developed in the 

 Rocky Mountains, the Sierra Nevada, and other ranges of the 

 western Cordillera. In these mountains almost every valley shows 

 the evidences of former glaciation, both in its scored and polished 

 sides and bed, and in the lines of moraine at its opening. Alaska, 

 strange to say, was not glaciated, except locally, none of the great 

 ice-sheets extending to it. Regarding the Great Plains region 

 there is some difference of interpretation ; by some authorities, 

 especially Dr. G. M. Dawson, it is believed that the northern plains 

 were covered with water and floating ice, while others suppose that 

 the Keewatin and Cordilleran ice-sheets were joined, and buried 

 all the country through northern Montana, Idaho, and Washington 

 to the Pacific. 



Deposits by the Ice-Sheets. — The general name for the materials 

 deposited by the vast ice-sheets and the waters derived from them 

 is Drift, which is both stratified and unstratified. The unstratified 

 drift is that which is made by the action of the ice alone, and 

 assumes several forms. ( 1 ) Moraines, lateral and terminal. These 

 have already been described and need not detain us further. 

 (2) Till is, when typically developed, a sheet of tough clay, which 

 may be more or less sandy, crowded with stones and boulders of 

 various sizes, and with no regularity of arrangement. Most of 

 the stones and boulders show the evident marks of ice action, 

 both in their form and in the scoring and polishing to which they 

 have been subjected. The materials of till are principally derived 

 from the bed rock upon which it rests, or from some spot close 

 at hand, most of these materials having been transported only for 

 short distances. A certain proportion of the stones, however, have 

 been carried for long distances, as may be shown by tracing them 

 to the ledges whence they were derived. Till is supposed to be 

 the ground moraine of the ice- sheet, left behind and overridden 

 when the descending ice reached more level ground, and packed 

 into depressions. This explanation is, to some extent, conjectural, 



