USGS/Cascades Volcano Observatory, Vancouver, Washington
- Olympic Mountains
- Historical Information
- Geologic History
- Ice Sheets and Glaciations
From: Olympic National Park Website, March 1999
Consider the Olympic Mountains to be an ancient
sea floor that has been
wedged into the North American Continent and uplifted into the
atmosphere by surrounding geologic pressures.
As the topmost layer of the sea floor is scraped off
and crumpled on to the North American Plate, the
lower branch dives beneath Olympic creating heat
and pressure that eventually form the
volcanoes of the Cascades.
To the north and east of the Olympic Peninsula lie
the Strait of Juan de Fuca and Hood Canal.
These bodies of water along with Puget Sound
were carved by
ice sheets over 13,000 years ago.
The Olympic Mountains are not very high --
Mount Olympus, the highest, is just under 8,000 feet --
but they rise almost from the water's edge
and intercept moisture-rich air masses that move
in from the Pacific. As this air is forced over the
mountains, it cools and releases moisture in the
form of rain or snow. At lower elevations rain
nurtures the forests while at higher elevations
snow adds to glacial masses that relentlessly
carve the landscape. The mountains wring
precipitation out of the air so effectively that
areas on the northeast corner of the peninsula experience a
rain shadow and get very little rain. The town of Sequim gets
only 17 inches a year, while less than 30 miles away
Mount Olympus receives over 220 inches falling mostly as snow.
There are about 266 glaciers crowning the Olympics peaks; most of
them are quite small in contrast to the great rivers of ice in Alaska. The
prominent glaciers are those on Mount Olympus
covering approximately ten square miles. Beyond
the Olympic complex are the glaciers of
the Bailey Range,
Mount Christie, and
Vancouver's Discovery of Puget Sound, Portraits and
Biographies of the Men Honored in the Naming of
Geographic Features of Northwestern America, by
Edmond S. Meany, Professor of History, University
of Washington, Secretary of the Washington
University State Historical Society, The Macmillan
Company, New York, 1907. from:
Library of Congress/American Memories Website,
For a period of thirty-seven years, from 1774 to 1811,
Nootka Sound was the best-known and most-frequented harbor
on the Northwest Coast of America. The first date is
given is that of the harbor's discovery by the Spaniard,
Juan Perez, and the second that of the founding of
Astoria at the mouth of the Columbia River. ...
"On proceeding south, on August 10, 1774, Perez saw in
latitude 48 degrees 10 minutes, a beautiful snow-white
mountain, which he named Santa Rosalia. Fourteen
years later it was rechristened by John Mears, the English
captain, and from that time it has been known as Mount
U.S. National Park Service,
Olympic National Park Website, 2000
These mountains have arisen from the sea. For eons, wind and rain
washed sediments from the land into the ocean.
Over time these sediments
were compressed into shale and sandstone. Meanwhile, vents and
fissures opened under the water and lava flowed forth, creating huge
underwater mountains and ranges called seamounts. The plate(s)
that formed the ocean floor inched toward
North America about 35 million years ago and most of the sea floor
went beneath the continental land mass.
Some of the sea floor, however, was scraped off and jammed
against the mainland, creating the dome that was the forerunner of today's Olympics.
Powerful forces fractured, folded, and over-turned rock
formations, which helps explain the jumbled appearance of the Olympics.
Radiating out from the center of the dome, streams, and later a
series of glaciers, carved peaks and valleys, creating the beautiful,
craggy landscape we know today.
Ice Age glacial sheets from the north
carved out the Strait of Juan Fuca and Puget Sound, isolating the
Olympics from nearby landmasses.
Ice Sheets and Glaciations
U.S. National Park Service, Olympic National Park Website, 2002
In the past, a vast
continental ice sheet,
Alaska, south through British Columbia to the Olympics.
The ice split into the Juan de Fuca and Puget ice lobes,
as they encountered the resistant
A glacial outwash stream surged around the southern end
of the peninsula to the Pacific Ocean. This isolated the
Olympic Peninsula from the nearby Cascade Mountains
and limited species from entering and exiting the
When the ice sheet reached the Peninsula, large areas
of the continental shelf were also exposed by the lower
sea levels since so much water was trapped as ice. This
created a coastal refuge. The distance from Mount
Olympus to the Pacific Ocean may have been double
that of today.
U.S. National Park Service, Olympic National Park Website, 2001,
Mountain Goats in Olympic National Park, 1994,
Biology and Management of Introduced Species,
Scientific Monograph NPS/NROLYM/NRSM-94/25
The Fraser glaciation lasted about 10,000 years
and consisted of 3 stades (periods of ice expansion)
and 2 interstades (ice recession).
The Fraser ice advance in the Olympics began with
expansion of alpine glaciers --
the Evans Creek stade.
Although poorly dated on the Olympic Peninsula, at maximum advance --
about 21,000-19,000 B.P.(before present) --
glaciers extended down west-side valleys. Glaciers in
east-side drainages were smaller and were
restricted to upper valley areas or headwalls. The
Evans Creek advance coincided with the
beginnings of an enormous
ice buildup in the mountains of British Columbia.
- Alpine glaciers retreated to undetermined
positions up valleys following the
Evans Creek stade.
This brief interstade was followed by
advance of Cordilleran ice sheet from British Columbia
into the Puget Sound area --
the Vashon stade.
The ice reached its maximum extent
around 15,000 B.P., splitting into the
Juan de Fuca and Puget ice lobes
as it encountered the Olympic Mountains. Ice at the northeast corner of
the Olympics was at least 3,800 feet thick at
maximum advance. The Vashon ice produced
glacial lakes behind massive ice dams that
formed in the northern and northeastern river
valleys. The ice sheet apparently did not contact
the remaining alpine glaciers. The spatial and
temporal relations between ice sheets and alpine
glaciers have important implications for the
biogeography of endemic taxa; suitable habitat for
alpine plants evidently persisted in or near the
Olympic Mountains during both alpine and ice
sheet advances of the Fraser glaciation.
The Vashon advance was short-lived; by 13,600 B.P.,
the two lobes had receded into a single lobe
located in the northern Puget lowlands. A minor
readvance (the Sumas stade) occurred about
11,500 B.P., but the extent and climate
significance of this stade has been questioned.
The Fraser glaciation ended about 10,000 B.P.
when major climatic changes occurred.
MORE about Ice Sheets and Glaciations - Menu
[Olympic Mountains Menu] ...
[Washington State Volcanoes and Volcanics Menu] ...
[Plate Tectonics Menu] ...
URL for CVO HomePage is:
URL for this page is:
If you have questions or comments please contact:
07/23/02, Lyn Topinka