Axelrod: The Pliocene Verdi Flora of Western Nevada 12:i 



locality over lowland regions. Middle Piocene floras suggest a mild, semiarid 

 climate, often one that was warmer and drier than exists at fossil localities today in 

 areas outside of the present desert region ; in the present desert areas these floras 

 indicate mild, semiarid climate (Axelrod, 1948; 1950d). Although only a few Late 

 Pliocene floras are now known in the far West, they suggest a climate more moist 

 than that which prevailed in the Middle Pliocene. Thus the Middle Pliocene 

 appears to represent the driest part of the Tertiary (Axelrod, 1948). The rela- 

 tively warm, semiarid to subhumid climate of the Verdi area, and the general 

 correspondence of its rainfall to that which might be expected there today after 

 the topographic factor has been taken into account, have been noted above. 

 Furthermore, there are indications that the climate was actually drier than that 

 which might prevail there today if Pliocene paleogeography could be restored. 

 Thus in its general climatic implications the flora appears to be Middle Pliocene 

 (Hemphillian). 



Cliseral relations. — A clisere represents a series of plant formations (or 

 climaxes) that succeed one another in a region in response to secular climatic 

 change. As outlined in table 7, the cliseral relations of Tertiary floras in this region 

 show a shift from tropical forests in the Early Tertiary, to temperate deciduous 

 hardwood forests in the Middle Tertiary, to subhumid and semiarid open forest, 

 woodland, and shrub communities in the Late Tertiary. The following brief state- 

 ments, which summarize some of the details of the cliseral relations of the (a) 

 Arcto-Tertiary and (6) Madro-Tertiary Geofloras in western Nevada during 

 Miocene and Pliocene times, provide additional evidence as to the age of the Verdi 

 flora. 



(1) During the Miocene, the Arcto-Tertiary Geoflora was characterized by mem- 

 bers of the East American and East Asian Elements, groups of fossil plants which 

 resemble deciduous hardwoods and conifers now in the temperate forests of eastern 

 Asia and eastern North America. Species of these elements dominate the Middle 

 Miocene Pyramid flora 35 miles northeast of Verdi ; they are present in moderate 

 numbers in the Late Miocene Buffalo Canyon flora 120 miles southeast ; they are 

 uncommon in the transitional Mio-Pliocene Fallon, Chloropagus, Middlegate, and 

 Aldrich station floras of west-central Nevada; and they are rare in the Early 

 Pliocene Chalk Hills and Lewis Coal Mine floras, both of which occur at low 

 stratigraphic levels in the Coal Valley formation to the southeast. The only Verdi 

 species that can be referred to these eastern elements is Popuhts subwashoensis, the 

 small-leafed derivative of P. washoensis Brown which was in this region in the 

 Miocene and Early Pliocene (Chalk Hills flora). These relationships indicate the 

 post-Early Pliocene age of the Verdi flora, and suggest it is Middle (Hemphillian) 

 rather than late Early Pliocene (Late Clarendonian). 



As deciduous hardwoods of the eastern elements gradually were reduced in 

 numbers, their place was taken chiefly by members of the West American Element, 

 which show relationship to plants now forming the western conifer forests. Mem- 

 bers of this element are rare in the Middle Miocene Pyramid flora ; present in small 

 numbers in the Upper Miocene Buffalo Canyon flora ; common in the Mio-Pliocene 

 Fallon, Chloropagus, Aldrieh Station, and Middlegate floras; and they dominate 

 the Early Pliocene Chalk Hills flora. During the Verdi stage, however, the forest 



