Axclrod: The Pliocene Verdi Flora of Western Nevada 117 



With respect to altitude, we have seen that the Verdi flora finds many of its 

 closest relatives living at elevations near 2,000 feet in woodland areas just below 

 forest on the lower west slope of the Sierra Nevada. But this figure does not give 

 us a valid estimate of the altitude of the Verdi basin for the flora lived during the 

 Middle Pliocene, the driest part of the Tertiary (Axelrod, 1948, p. 132). At that 

 time woodland and forest zones were all displaced up the slopes as compared with 

 their present altitudinal relations. Comparisons of Middle Pliocene floras with 

 present vegetation near the fossil localities have indicated that rainfall was ap- 

 proximately 5 to 8 inches lower than at present. This suggests that the forest zone 

 would have occupied a position at least 500 feet above its present level. Applying 

 this correction factor, it would appear that the Verdi basin had an altitude near 

 2,500 feet. 



Summary 



Paleoecologic relations in the Verdi area, as inferred from geologic evidence, the 

 relative abundance of the fossil species, and the ecologic occurrence of living 

 plants similar to the fossils, may be summarized as follows: 



Geologic setting. — The Coal Valley sediments in the Verdi area were deposited 

 in a basin situated between the Carson Range and Peavine Mountain, both of 

 which appear to have been relatively low areas during deposition. Clastics from 

 the Carson block were derived chiefly from Kate Peak andesite, and those from the 

 Peavine block largely from Alta andesite. Marginal conglomerates graded into 

 tuffaceous sandstones, shales, and diatomite in the central floodplain area, where 

 small ponds, lakes, and swamps were common at the site of deposition, which had 

 an altitude near 2,500 feet. 



Vegetation. — A dense riparian woodland of cottonwood (Populus alexanderi) 

 occupied the stream margins in the Verdi lowlands, together with willow (Salix 

 truckeana). The ponds, lakes, and swamps supported colonies of water lily (Nym- 

 phaeites), pondweed ( Potamogeton) , and stonewort (Chara). 



A forest dominated by fir (Abies concoloroides) and pine (Pinus florissanti, 

 P. prelambertiana), with associates of poplar (Populus subwashoensis) , aspen 

 (P. pliotremuloides) , and willow (Salix boisiensis) on the stream-borders, and 

 with manzanita (Arctostaphylos verdiana), bitter cherry (Prunus moragensis) , 

 and gooseberry (Ribes galeana) on the forest floor, was confined chiefly to moister 

 valleys and slopes bordering the basin ; they appear to have been transported into 

 the lowlands largely at times of flood. Aspen and poplar, however, extended into 

 the lowlands on stream banks in moderate numbers. 



Valley oak (Quercus prelobata) and live oaks (Q. renoana, Q. wislizenoides) 

 formed a mixed woodland and savanna on well-drained, drier slopes and flats, 

 and the more abundant riparian species also ranged into this area. Buckbrush 

 (Ceanothus precuneatus) was scattered in the woodland, and may have contributed 

 to a chaparral together with other shrubs that are still unrecorded in this flora. 

 Closed-cone pine (Pinus pretuberculata) probably occupied rocky, exposed slopes 

 in the woodland-forest ecotone. 



Climate. — Rainfall is estimated to have averaged 18 to 20 inches over the lower 

 part of the basin, increasing to 25 inches at the edge of forest in the bordering 



