The Chama area straddles the physiographic and structural boundary between the San Juan Basin and the San Juan Mountains. The Mesozoic formations, the Upper Jurassic Entrada and Morrison and the Upper Cretaceous Dakota and Mancos, are separated by a marked angular unconformity from the Oligocene (?) Blanco Basin Formation and the upper Miocene Conejos Quartz Latite and Treasure Mountain Rhyolite. Late Cenozoic eastward tilting and high-angle normal faulting have affected all the bedded rocks. The present topography has been profoundly modified by extensive mass movements and Pleistocene glaciation during the Cerro, Durango, and Wisconsin stages with related outwash terrace deposits. / text