SUCCESSIVE DEPOSITS IN THE APPALACHIAN REGION 441 



and continuity of the Sierra Neyada and the Cascades cause these moun- 

 tains thoroughly to obstruct the westerly winds in respect of moisture, 

 with the result that great stretches of country east of them are arid 

 wastes. ... In the rain shadow of the Sierra Nevada the mean an- 

 nual rainfall is between 5 and 6 inches and locally as low as 3 inches." 60 

 It is well known that torrential and eolian deposition is most active 

 under conditions of diminished rainfall, where as a result vegetation is 

 scant. With a high Appalachia, then, to intercept the moisture-bearing 

 west winds, we should have abundant vegetation and little deposition on 

 the western slopes, but arid conditions and terrestrial deposits on the 

 eastern. . Thus it is evident that conditions like the present would not be 

 conducive to the formation of such deposits as the Bald Eagle and 

 Juniata. If, however, the winds were reversed; if, in other words, con- 

 ditions were such as to permit winds like the trades to flow from the 

 east across the high ranges of Appalachia, leaving the moisture which 

 they brought from the Atlantic on the eastern slopes, and causing rela- 

 tive aridity on the western, the conditions found in the Bald Eagle and 

 Juniata might readily be accounted for. They might indeed be postu- 

 lated from the given premises. Such conditions now exist on the western 

 slopes of the Andes in South America, which form a barrier for the 

 southern trade winds. West of them the Atacama Desert of Chili ex- 

 tends' from the mountains to the coast, as one of the most barren and 

 forbidding wastes imaginable. If, by way of argument, we postulate the 

 existence of such winds in late Ordovicic time in this part of the conti- 

 nent, we could understand that a moderate elevation of the Appalachian 

 old land would induce heavy torrential deposits on the western slopes of 

 this land-mass, such as are found in the fragment now constituting the 

 Bald Eagle conglomerate. That this is only a fragment, the western- 

 most end of a once much more extensive fan, will presently be shown to 

 be the case. Other fans of this type may have formed to the south, along 

 the western slope of Appalachia and northward in the New England 

 regions. But these have either been entirely removed by erosion or by 

 metamorphisms have been rendered unrecognizable. 



If after the formation of the Bald Eagle fan or fans a furl her local 

 rise of the Appalachian old mountain land occurred, the aridity of the 

 interior or western slope would be increased, and torrential deposits 

 would be replaced largely by eolian deposits, the thorough oxidation o( 

 which would eventually result in the formation of a red series. These 

 oxidized deposits may extend to the margin of the sea and into it, for 



Ibid., p. 120. 



