The present Olympic Mountains were born between 12 and 20 million years
ago when western Washington was pushed up into a great range that
extended from Cape Flattery southeastward to the eastern part of the
State. At the same time, the land to the north and south was depressed
and remains depressed today as Juan de Fuca Strait and Chehalis Valley,
respectively. The Olympics were further elevated about 5 million years
ago. This coincided with the building of the Cascade Mountains and the
down-folding of the land between to form the Puget Sound trough. The
Olympics were now isolated, having lowland on all sides.
Olympic rocks formed in shallow seas that at least five times have
covered western Washington. Sediments washed from adjacent land areas
and accumulated on the sea bottom. Muds became shales and sands were
cemented into sandstones. Molten lava erupted through these beds and was
quickly cooled by the water. Thousands of feet of rock material formed
in this way.
[Illustration: Moss-covered tree]
When earth forces lifted the sea floor, the sea disappeared, and for
long periods there were mountains where the sea had been.
Pressure and heat changed the rocks, especially the sedimentary rocks,
which became harder and tougher. Shale changed progressively into slate
and phyllite. All of these rocks are found in the Olympic Mountains. The
sedimentary rocks and lava flows, originally horizontal on the sea
floor, were tilted and folded when uplifted and this is how we see them
today.
Long periods of erosion have removed thousands of feet of rock and
remolded the Olympics into magnificently rugged mountains. Thus, earth
forces build mountains and water slowly carries them back to the sea. So
it has been since the first rains fell upon the cooling earth.
Today only the oldest rocks remain, for these were the bottom layers.
The greater part of the Olympic Mountains are made up of these rocks,
now mostly slates and hardened sandstones. This includes all the rock
inside a horseshoe-shaped line running from the village of Sappho east
to Lake Crescent, Lake Mills, and Deer Park, then south to the west side
of Mount Constance and the north end of Lake Cushman and then west to
Lake Quinault. The horseshoe-shaped rim of the mountains outside this
line is mostly basaltic lava.
[Illustration: MOUNT ANGELES SHOWS TILTED ROCKS UPHEAVED FROM THE
SEA BOTTOM.]
Because fossils are scarce in the oldest rocks, geologists are not
certain about their age, but they are thought to be about 120 million
years old. The rocks in the outer rim of the Olympic Mountains contain
more fossils. These have been found in the sandstones, shales, and
limestones interbedded with the thick volcanic rocks. Fish teeth, marine
clams, snails, algae, wood fragments, and microscopic shells found here
represent forms of life that existed 50 to 60 million years ago.