_Holliday Rest Area._ The main John Day fault, which here is buried
under river gravel, is believed to be just south of the rest area. A
parallel step fault (Fig. 2) cuts from left to right across the southern
slope of Mt. Vernon Butte about where the juniper trees thin out. Flows
of Picture Gorge Basalt in the upper part of the butte slope down to
valley level on your left. Rocks in the foreground and to the right are
rudely bedded volcanic breccias of the Clarno Formation. The Clarno
Formation has been raised as much as 250 feet by movement along the
fault to position it against the basalt as indicated in the diagram. The
fault can be seen best in afternoon light in the ravine to the right,
just south of some brick-red layers in the basalt.
Movement on the main John Day fault to the south appears to have raised
the rocks at least 1,000 feet, so the two faults have stairstepped the
rock layers. The main John Day fault has been traced about 80 miles.
[Illustration: Fig. 2.—View of Mount Vernon Butte and diagram of
faulting along its south slope.]
[Illustration: ]
[Illustration: Fig. 3.—View of the John Day fault in Mascall Formation
at the mouth of Fields Creek, and its relation to the structure of the
John Day River valley.]