Geology Reference
In-Depth Information
Landslides into the sea just south of Humbug Mountain (photo courtesy of State Foresty Service)
McVay Rock, an impressive stack also stranded ashore
by a lowering sea level, has had its size reduced signifi-
cantly by quarrying. Offshore here, Goat Island, with an
area of 21 acres and a height of 184 feet, is Oregon's
largest coastal island. All three structures are com-
posed of 150 million year old dark gray sandstones,
siltstones, and volcanics of the upper Jurassic Dothan
Formation.
Near Crook Point, offshore rocks making up
Mack Reef and Yellow Rock are a melange of resistant
rocks that are part of the Jurassic Otter Point Forma-
tion. Mack Reef is a complex of parallel stacks running
to the south and lined up with Crook Point. Offshore,
the 325 foot high monolith, Mack Arch, is the remain-
ing section of a reef which has had a tunnel cut
through by persistent wave action, similar to Arch
Rock Point in nearby Boardman State Park and
Natural Bridge slightly north. From Mack Reef north
to The Heads at Port Orford, most of the reefs and
stacks scattered along the shoreline in a variety of
extraordinary shapes are made up of mudstones,
sandstones, conglomerate, and volcanic rock of this
same heterogenous formation. An exception is found at
Cape Sebastian, where the large Hunters Island has
been eroded from the same massive Cretaceous sand-
stones that make up the cape.
