40 UNIVERSITY OF COLORADO STUDIES 



having precipitous walls not covered thickly with neve or ice, and con- 

 sequently abundantly supplied with rock material, build their moraines 

 rapidly and shrink, extend or change the shapes of their fronts quickly 

 in response to fluctuations in the relation of precipitation to temperature, 

 extreme caution is necessary in deducing successive widely separated 

 periods of glacial extension and recession from the interference and 

 crossing of distinct moraines. However, there is evidence of a much 

 more reliable nature indicating at least two such periods near Leadville, ' 

 with a suggestion of a third. ^ It is exceedingly probable that as minute 

 an examination in other parts of the state would bring forth further 

 evidence of a trustworthy character, such as difference in material 

 and amount of weathering of morainal matter, and indications of long 

 periods of erosion between superincumbent and superimposed deposits. 



A visit long ago to some unglaciated portion of the state led a writer 

 in a responsible magazine to deny the existence of any evidence of the 

 former presence of glaciers in the Rocky Mountain region^ — an excellent 

 illustration of the danger of generalization from limited knowledge 

 of a region. Another writer, answering the first, was apparently able 

 to point out but little evidence from the great abundance now known 

 to science.'^ 



It is true that the plains of the eastern portion of the state, the foot- 

 hills and the lower portion of the mountain ranges seem wholly devoid 

 of indisputable evidence of the former existence of glaciers, the ice 

 streams having generally extended but little, if any, below what is now 

 the 8,000-foot line, though whether the altitude was the same during 

 the period of their maximum extension may be doubted. 



There are beds of boulders in the Denver Basin which have been 

 designated " glacio-natant drift" and "upland drift. "^ The explanation 

 of these deposits suggested by those terms does not seem satisfactory, 

 or, at best, other explanations seem just as satisfactory; so their origin 



' Second Annual Repl., U. S. Geol. Sur., p. 229; izlh Mon., U. S. Geol. Sur., p. 30; Jotir. GcoL, Vol., XIII 

 (1905), pp. 285-306. 



* Jour. Geol., Vol. XII (1904), p. 702. 



» Am. Nat., Vol. VI (February, 1872), pp. 73-75. 



* Ibid. (May, 1872), p. 310. 



* Geology of the Denver Basin, 27th Men., U. S. Geol. Sur., pp. 265, 266. 



