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The Geologic History of
Washington State & Kittitas
County
Jana Jones Mabry
Geologic History of Washington
State
 Overview
of the processes
responsible for the underlying
geology
 Overview of different geologic
provinces of the state
 Overview of the geology specific to
the area of Snoqualmie Pass and
Lake Easton State Park
The Geologic Processes responsible for the
landscape features seen in Washington
Plate Tectonics
– Volcanism
– lava flows
 Glaciation

– Alpine Glaciers
– Massive Flood Events

Weathering and Erosion
Plate Tectonics



The theory and study of the
earth that describes the
processes of continental
formation and movements.
It proposes explanations for
how, why, and when
continents are built and how
they move.
It is the process responsible
for the physical conveyance
of materials that form the
bedrock geology of
Washington State.
Volcanism

Three types of volcanic processes are
associated with Washington Geology
– Batholiths and plutons
–
Magma trapped and cooled under the surface.
– Volcanoes

–
Magma that escapes through a vent in an explosive event
or the repeated rise of magma to the surface.
Plateau Basalt Provinces
Glaciation


The accumulation and movement of flowing ice.
In Washington the most significant glaciers occurred
between 1.6 million years ago to about 10,000 years
ago.
– The glaciers carved sharp features in the high peaks
– created glacial troughs, lakes, and ice dams that led to massive
flood events
– deposited glacial debris called till and loess along their paths
Overview of the Geologic History
of Washington State
The oldest rocks in Washington State are part
of the 1 billion year old Belt Supergroup
The North American
Continental Coastline
was located
approximately 30
miles west of the the
present day border of
Idaho and
Washington
The continued addition of material to the North
American Continent was through the process of
Plate Tectonics

Plate tectonics
conveyed small
volcanic islands and
fragments of other
continents called
terranes onto the
North American
Continent and accreted
them in the area of the
existing coastline.
The first terrane to arrive was the Okanogan Microcontinent at 190 to 160 million years ago

This micro-continent
crushed the coastal plain
that had been building for
600 million years (800 to
200 myBP) between the
North American Continent
and the Okanogan
creating an area of
displaced marine
sedimentary materials
called the Kootenay Arc
The next major addition occurred 90 to 100 million
years ago with the arrival of the North Cascade
Terranes



The North Cascade
Terranes are believed to
be made up of a group
of 6 different islands.
Each individual terrane
can be distinguished by
its unique rock type.
These terranes were
covered later by
volcanic materials and
complicated their
interpretation.
The Insular Terrane arrived behind the North
Cascade Terranes at about 100 million years ago

This terrane is
believed to have
been two large
pieces that make
up the basement
rocks under the
San Juan Islands
and the Blue
Mountains
The last event to add to the Washington coastline was
approximately 25 million years ago is called the Crescent
Terrane located in the areas of the Puget Lowlands, Olympic
Peninsula and the Willapa Hills

The Crescent Terrane is believed
to be a portion of ocean crust
that was stranded between an
extinct trench and the present
day trench. Some how more
buoyant rocks were carried
underneath the continental crust,
then escaped pushing the
overlying rocks as much as 2
miles above the ocean crust
More Changes occurred to the landscape with
the eruptions of the Columbia River Basalt
Group between 17.5 to 6 million years ago


These basalt lava flows
originated from dike
swarms and cracks in the
earth called fissures in the
area where Washington,
Oregon and Idaho come
together
Lava flowed out onto the
surface in 120 individual
flows covering 100s of
square miles of this region
Pleistocene ice sheets and glaciers that occurred from
2 million to 500 years ago


The area we are in was
carved by the glaciers and
the sharp features on the
high peaks are called
aretes, horns, cirques and
hanging valleys.
Many of the lakes were
created by the glaciers and
glacial outwash called till
and loess blanket the
region.
The last significant volcanic events to shape the
Washington landscape were the birth of the Cascade
Volcanoes between 1 million and 75,000 years ago


Creation of the Cascade
Volcanoes was a direct
result of plate tectonics and
the subduction of the
Fallaron Plate under the
western coast of North
America.
The volcanoes were created
in a progressively northern
direction beginning in
Northern California and now
extending into Canada.
The Cascade Volcanoes of Washington were the
icing on the cake that make up the geology of
the state


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Plate tectonics and the resulting volcanism remain
the most active and significant geologic processes
that affect geology of Washington.
The Juan de Fuca Plate, is still subducting under the
northern-most portion of the North American
Continent.
It is responsible for the earthquakes we experience,
the changes to the topography, and controls the
energy flow and climatic conditions of the region.
The Ice Sheet and Glaciers that have occurred over the past 2million years
have had a significant impact on the topography of this area.
The Yakima Glacier flowed from the Snoqualmie Summit to Thorp The
three depressions now holding Keechelus, Kachess and Cle Elum Lakes
held glaciers that fed into the main stream Yakima Glacier.It created the Ushaped valley in which the present day Yakima River flows and covered the
bedrock geology of the whole area with glacial debris and alluvial (water
deposited) materials.
The Glaciers in this region carved the paths
and natural depressions now used as
reservoirs for the south-central region
Lake Easton is a man made lake created where the
Yakima and Kachess Rivers come together. This reservoir was
built 1928 through excavation and removal of the glacial till.
Its primary function is to supply water for the agriculture of
south-central Washington and a variety of recreational
activities to the central region.
The view to the east of the park provides a glimpse into
the rich geologic history of the region
The Rocks of Central
Washington and Lake Easton

Volcanic (Igneous)
– Intrusive (cooled
underground as
batholiths) Granite


Mounts Stewart and Si
Metamorphic (cooked)
– Ingalls Tectonic Complex


Easton Schist
Sedimentary
– Glacial Debris
– River Deposits