_Picture Gorge._ Visible to the left of Picture Gorge, from north around
to west, in order of their deposition and geologic age from oldest to
youngest, are the Picture Gorge Basalt, Mascall Formation, and
Rattlesnake Formation (Fig. 5). The Picture Gorge Basalt flows and ashy
beds of the Mascall Formation were tilted southward together and eroded
before the Rattlesnake Formation was laid down horizontally across them.
Picture Gorge and the present valley were then cut by the John Day River
after the Rattlesnake Formation had been tilted in its turn. The five
benches or terraces on the basalt just east of Picture Gorge mark
temporary halts in downcutting of the John Day River.