36 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



first and the last to permit the supposition that each of them represents a main epoch. 

 Finalljs the oldest terrace does not show sufficient signs of age to warrant the behef that it 

 is as old as the first of the known glacial epochs of the Pleistocene period. It is much worn 

 and eroded almost everywhere, and in many places it has been entirely removed, but its 

 materials are weathered and decayed to only a moderate degree. It seems probable, 

 therefore, as Penck has suggested in regard to those of Asia, that the oldest terrace maj' 

 represent the last glacial epoch, and that the others represent the post-glacial stages, or 

 minor epochs of glacial retreat, of which there is an ever-increasing abundance of evidence, 

 not only in the Alps and Scotland and other parts of Eiu-ope, but in America. Inasmuch 

 as man is known to have existed prior to the last great glacial epoch, the terraces, if our 

 conclusions are correct, preserve the record of a series of chmatic changes which have played 

 a part in shaping human destiny. If the oldest terrace dates back no more than 30,000 

 years, more or less, to the last glacial epoch, the youngest can not be more than 2,000 or 

 3,000 years old at most, and may be much less. 



