168 F, E. PEABODY AND J. M, SAVAGE 



Alternate maps may be presented (Fig. 6) depending on the pre- 

 sumed position of the marine strait, but in both cases demonstrat- 

 ing the barrier at the southern tip of the peninsula. The peninsula 

 persisted until the second peak of Coast Range orogeny in mid- 

 Pleistocene time and the following uplift brought the Clast Range 

 corridor into being. 



Physical changes associated with Coast Range orogeny were not 

 in themselves sufficient to convert the Coast Ranges into a zoogeo- 

 graphic corridor. Entirely coincidental and independent climatic 

 changes were in progress. The work of Chaney (1940) in paleobotany 

 and of Durham in invertebrate paleontology (1950) has mutually 

 documented a continent-wide shift of isotherms southward through- 

 out the Cenozoic, reaching a maximum (with fluctuations) in the 

 Pleistocene epoch. According to Durham, the 18°C marine isotherm 

 was at latitude 35°N, coincident with the Southern Coast Ranges, in 

 the late Pliocene, and shifted 7° southward (over 400 miles) at the 

 peak of Pleistocene glaciation. Here is the climatic stimulus needed 

 to force faunal elements southward into the peninsula, perhaps 

 causing "jamming" or peninsular effects. Future study of fossil 

 mammals in the presumed peninsular area may show such effects 

 to be present. Certainly the marine faunas of the area are extremely 

 provincial in character. However, this is commonly attributed to 

 the many local, shifting basins of deposition attending the Coast 

 Range orogeny. 



Coincidental with the Coast Range orogeny, the vast Sierra 

 Nevada block began to tilt westward, so as to form a high crest 

 running southward from a point near the present Lake Tahoe and 

 curving westward to a junction with the Southern Coast Ranges. 

 Axelrod's masterly use of paleobotanical data (1957) graphically 

 portrays the rise of the Sierra Crest from an average of 3,000 feet in 

 the Miocene to 8,500 feet in the Pleistocene, and the accompanying, 

 drastic, climatic effect on vast inland areas of the continent. Un- 

 doubtedly the formation of the Coast Ranges also contributed to the 

 drying of the interior. The combination of geological and climatic 

 changes in the far west resulted in a southward movement of the 

 Arcto-Tertiary geoflora, especially along the coast, and a northward 

 and northwestward movement of the Madro-Tertiary geoflora from 

 a Mexican center of origin. A Neotropical-Tertiary geoflora re- 

 treated southward along the coast. 



