The Upper Triassic Martin Bridge Limestone exposed in Hells Canyon, along the Idaho-Oregon border, is a thick sequence of limestone and dolostone. The allochthonous nature and low-latitude origin of the Martin Bridge and underlying Seven Devils Group, which along with coeval rocks exposed in the Wallowa Mountains, make up the Wallowa terrane, are well established. The Martin Bridge Limestone in Hells Canyon was deposited initially under supratidal and intertidal conditions and then in a shallow subtidal environment. Absence of terrigenous or volcanic sediments indicates that these carbonates were isolated from a source of such sediment. Although potential reef-building organisms are present in the Martin Bridge Limestone, no reef structures are exposed in Hells Canyon. The shelf-edge environment apparently was dominated by relatively coarse grainstone and packstone shoals and banks. Stratigraphic evidence from the Wallowa Mountains and near Riggins, Idaho, indicates that the Martin Bridge carbonate platform was eventually drowned and overlain by fine-grained argillaceous and siliceous clastic sediments. Decreased benthic carbonate production, due to climatic deterioration initiated by northward drift, and increased clastic and volcaniclastic input, from adjacent terranes and the Wallowa terrane itself, facilitated drowning of the carbonate platform.