The Horned Toad Formation includes five members that record alluvial fan, fluvial, lake margin, and lacustrine deposition within a relatively small basin just south of the active Garlock fault in the late Miocene-early Pliocene. These sediments experienced northwest-southeast contractional deformation during the Pliocene-Pleistocene accommodated by basement involved reverse faults and associated open folds. Paleomagnetic data suggest that the Horned Toad Hills may have experienced significant clockwise rotation since about 4.5 Ma. Member Two of the Horned Toad Formation has yielded 24 taxa of fossil mammals including a new genus of mouse Repomys (previously described), a new genus of gopher, a new species of the mouse Jacobsomys, and probably a new species of the shrew Cryptotis. Analysis of this fauna suggests affinities with other late Hemphillian faunas from California, Nevada, Nebraska, Texas, and Mexico. The Warren Local Fauna contains five taxa of cricetid rodents including both hypsodont and brachydont cricetines and sigmodontines reflecting the apparently rapid diversification of these mammals during the late Miocene to early Pliocene in the southwestern USA. The Lawlor Tuff has been identified in the Horned Toad Formation and overlies most of the fossil mammal localities. This tuff has been dated elsewhere in California at 4.83 ± 0.04 Ma. Magnetostratigraphy from the Horned Toad Formation is correlated with Chrons 3n.3r, 3n.3n, and 3n.2r suggesting an age range of approximately 5.0 - 4.6 Ma. These constraints suggest an age for the late late Hemphillian Warren Local Fauna of 4.85 - 5.0 Ma. Identification of the Hemphillian/Blancan boundary in the Horned Toad Formation will depend on future identification of definitive Blancan taxa above the Lawlor Tuff.