Ochoco Mountains made of rocks from the Permian, Triassic, and Jurassic periods (300 to 200 million years old) that were transported by the Pacific Plate and accreted in the late Mesozoic era (about 100 million years ago) as part of a vast shallow sea, then slowly uplifted by volcanic eruptions during the Eocene epoch (50 to 37 million years ago) to form the Clarno Formation. From 37 to 17 million years ago, eruptions in the western Cascade Range spread ash across eastern Oregon, forming the John Day Formation. From 17 to 14 million years ago, major volcanic eruptions covered much of the
Geological period example the 'Jurassic System' of rocks was formed during the 'Jurassic Period' (between 201 and 145 million years ago). The following table includes all currently recognized periods. The table omits the time before 2500 million years ago, which is not divided into periods. See Geologic time scale#Proposed Precambrian timeline for another set of periods 4600–541 MYA. In a steady effort ongoing since 1974, the International Commission on Stratigraphy has been working to correlate the world's local stratigraphic record into one uniform planet-wide benchmarked system. American geologists have long considered the Mississippian and Pennsylvanian to be periods in their own right