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
Swithland Wood and The Brand high mountain range, the processes of erosion began. By the Triassic Period, (240 million years ago) the mountains were eroded down to something like their current height. There then began a new deposition, this time of desert sand and fine dust, that produced a new layer of soft red marl, creating the gentler rolling appearance of the land, with just the tips of the older rocks reaching the surface. Over most of Swithland Wood the marl is the surface rock type (although the slate beds will occur underneath that). Finally, during the ice age, glacial erosion stripped back the Triassic