CLIMATIC CHANGES 83 



after which a diminution will set in. The interval between two 

 thesial epochs constitutes a short or strophic cycle, while the in- 

 terval from the center of one interstrophe to that of another consti- 

 tutes a grand or climatic cycle. (See diagram. Fig. 15.) 



Indication OF Climatic Changes. 



Topographic Evidence of Change in Climate. Since the 

 angle of slope assumed by waste imder arid conditions is much 

 steeper than that assumed under pluvial climates (see postea Chap- 

 ter XIV) and the material of the former is much coarser than that 

 of the latter, it follows that traces of such conditions in a region 

 now well watered suggest a former greater aridity. This is seen in 

 alluvial fans of great radius and steepness of slope, such as those 

 of the region near San Bernardino with radii of ten, twelve, or 



^ARSIS ^ INTER- STROPHE ---*■• STROPHE --, 



^/^/v^yv/yv>.^-~^* _- ^■■''\y\/yy\y\'^-^^ — 



' THESIS ; 'C'p. 



• ^^ 

 •« CYCLE ---- 





Fig. 15. Diagram illustrating progress of changes of climate during geologic 

 time. (After Huntington.) 



fourteen miles, and an elevation, at the head, of four, six, or seven 

 hundred feet above the frontal margin. Such fans are characteris- 

 tic of arid mountain regions, and their essential features will be 

 recognizable in the topography of a region become more moist. The 

 clogging by waste of deep-cut valleys, formed during a moist cli- 

 matic period, as well as the formation of fans on the much dis- 

 sected mountain slopes, is evidence of change from moist to dry 

 climate. In the same manner the dissection of steeply graded val- 

 ley floors and waste slopes shows a change from dry to moist cli- 

 mate, provided these features do not indicate increased elevation 

 of the region. In lakes of a dry region the water would be low and 

 their shores lined by alluvial fans, against which, on the change of 

 the climate to moister conditions, the waters of the expanded lake 

 would come to lie. This was the case in Lake Bonneville, the wa- 

 ters of which rested against the alluvial fans of the pre-Bonneville 

 dry period. The present dry period is again characterized by the 

 formation of such alluvial fans. The outlets of these lakes are fur- 

 ther marked by the features of topographic youth. Evidences of 



