The Picture Gorge Basalts (PGBs) are found all throughout the Sheep Rock Unit of the monument, seen close to the tops of the John Day Valley's peaks. These basalts were emplaced 16 million years ago (Mya) by a series of 61 sporadic lava flows that erupted, on average, every 15,000 year from fissures in the Earth’s crust. Geologists have proposed that approximately 17 Mya, a piece of the Farallon plate (a tectonic plate) broke off as it was subducted (one plate moves under another as it melts back into the Earths mantle). This resulted in slab tear (portion of plate "breaking" off) and upwelled subducted oceanic mantle that lead to a 560 mile long rupture in eastern Oregon and northern Nevada.
Floods of Fire...
The PGBs covered approximately 2,500 square miles in basaltic lava, with some flows being over 50 feet deep. All together the basalts in this area are over 1,300 feet deep. Picture Gorge Basalts are a subgroup of the Columbia River Basalts, which include hundreds of lava flows in the Pacific Northwest.
Though these flood basalts incinerated the prior savannah landscape, life re-established itself in the area over a period of hundreds or thousands of years. After the flood of lava cooled, insects and spiders were the first arrivals to the newly barren landscape, followed by windblown sediments and seedlings. Trees and grasses eventually followed and took hold on the newly formed soils, with animals arriving last. Life in the area was then consumed by the next basaltic lava flow, but not without leaving evidence of its existence, sometimes preserved in the baked paleosols found between some layers of basalts.