Volcanic and volcaniclastic rocks in the northern Peruvian Andes (central Cajamarca, 79°10′ W, 6°35′ S) contain a diverse assemblage of permineralized woods known as El Bosque Petrificado Piedra Chamana. The fossil forest and associated paleosol are preserved in ash-fall and lahar deposits of the Huambos Formation. Dating of plagioclase from the ash-fall deposit using 40Ar/39Ar methods yields a middle Eocene age of 39.35 ±0.21 Ma. Accuracy of this age determination is supported by a more robust sanidine age of 39.52 ± 0.11 Ma from an underlying welded ignimbrite. Fossil wood and leaves associated with the ash-fall deposit include vertical trees rooted in the paleosol and buried in situ by the ash. Fossil wood is also present in high abundance and diversity in the overlying lahar. The fossils are significant as a low-latitude assemblage including a diversity of both monocots and dicots and in having fossil leaves occurring in close proximity to fossil woods. Preliminary analyses of wood and leaf characters suggest a megathermal climate with some limitations on plant growth associated with limited (seasonal) moisture availability. The assemblage represents lowland tropical forest that was probably growing near sea level and subsequently uplifted to the current elevation at the site (∼2400-2600 m).