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
The Gap (Sydney) tablet. The Gap is formed from Sydney sandstone making it part of Sydney basin. The cliff was laid as sediment more than 200 million years ago in the Triassic period. During the Jurassic era, 40 million years later, a cataclysmic event resulted in an enormous crack forming within the strata. This fault allowed erosion from wave action to create the cliff line around Sydney. Tessellated basalt rock platforms lie at the base of the cliff because The Gap is bordered to the south and west by an older sequence of largely low-grade metamorphic and granitic rocks of the Lachlan Fold