Jurassic Jurassic The Jurassic (; from Jura Mountains) was a geologic period and system that spanned 56 million years from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period Mya. The Jurassic constitutes the middle period of the Mesozoic Era, also known as the Age of Reptiles. The start of the period was marked by the major Triassic–Jurassic extinction event. Two other extinction events occurred during the period: the Pliensbachian-Toarcian extinction in the Early Jurassic, and the Tithonian event at the end; however, neither event ranks among the "Big Five" mass extinctions. The
John Day Fossil Beds National Monument originated during the late Jurassic and early Cretaceous, about 118 to 93 million years ago. Northeastern Oregon was assembled in large blocks (exotic terranes) of Permian, Triassic, and Jurassic rock shifted by tectonic forces and accreted to what was then the western edge of the North American continent, near the Idaho border. By the beginning of the Cenozoic era, 66 million years ago, the Blue Mountains province was uplifting (that is, was being pushed higher by tectonic forces), and the Pacific Ocean shoreline, formerly near Idaho, had shifted to the west. Volcanic eruptions about 44 million years ago during the