Geology Reference
In-Depth Information
sediments. Miocene basalts making up Mt. Hebo and
Neahkahnie Mountain are also part of the same
coarse-grained Columbia River series. North of Neha-
lem Bay, the thickness of the layers making up Neah-
kahnie Mountain suggest that lavas must have ponded
up in a bay or similar low spot prior to cooling.
In contrast Marys Peak, the highest elevation
in the Coast Range, is capped by a resistant 1,000-foot
thick, flat-lying sill of medium-grained gabbro that
intruded Eocene sandstones of the Flournoy Forma-
tion. Oligocene intrusions of this type are common in
the central part of the range, and their emplacement
took place during the same time as coastal rotation
about 30 million years ago. An easy drive to the crest
of Marys Peak yields scenes of grassy alpine meadows
and a magnificent view. To the east the Willamette
Valley and Cascade volcanoes form the skyline while to
the south prominent peaks are Flat Mountain and
Green Peak, both intruded sills. Westward the Coast
Range and Pacific Ocean stretch to the horizon.
Marys Peak looking south. The Kings Valley fault trending north-south cuts directly across this
promontory (photo courtesy U.S. Forest Service).
