Tertiary floras from the Rio Grande Rift
— Daniel I. Axelrod

Several small Tertiary floras from the Rio Grande depression occur in sedimentary rocks associated with volcanic sequences that range from late Eocene (~ 40 m.y.) to middle Miocene (~ 14-15 m.y.). The samples are small, yet they represent very different kinds of vegetation, and must therefore have lived under very different climates. Since three of them lived within a time span of only 6 to 8 m.y., from late Eocene to middle Oligocene, the climatic differences they indicate appear to reflect differences in altitude as well. Their analysis can provide information regarding the topographic history of the Rio Grande rift.

 

The nature of the floras, as summarized in Figure 1, reveal a progression from a mixed evergreen-deciduous hardwood forest (Bernalillo flora) that lived under warm temperate climate without frost, to vegetation in the transition from broad-leaved sclerophyll to mixed conifer forest (Red Rock Ranch) that suggests mild temperate climate with light frost, to pure subalpine conifer forests (Hermosa and Hillsboro floras) that represent high montane environments of cold temperate climate with regular frost and snow. This shift toward colder climate appears to reflect increasing altitude, presumably broad doming—no doubt complex—that accompanied thermal expansion as volcanism spread over the region commencing in the late Eocene.