Chapter 2 Fire and Ice – Washington’s Geology
A Changing Land
The movement of large land masses is called a continental drift. Continental drift gives shape to land
masses and directs how the continents are linked. Geologists study rocks, land formations, and fossils to
learn the history of the land.
Tectonic forces are strong forces that in the earth that cause landmasses to move. The landmasses, or
plates, hold, or carry, both continents and pieces of the ocean floor. Sometimes plates spread apart and
sometimes they collide. A shift in one of these landmasses causes an earthquake.
Forming Our Mountains
Mountains are formed by the force of the tectonic plates colliding and land ridges being forced upwards.
Others are formed by volcanic activity. The great Ring of Fire is a circle of volcanoes that include the
Cascade and Olympic Mountains as well as mountain ranges in Indonesia, the Philippines, Japan, Alaska,
and the west coast of North and South America. The Cascade Mountain Range includes Washington’s
five “sleeping giants.” These are dormant volcanic mountains and include Mt. Baker, Glacier Peak, Mt.
Rainier, Mt. Adams, and Mt. St. Helens.
After mountains are formed, erosion by wind, water, and ice shape them. Older mountains are more
rounded and newer mountains are more jagged. The Rocky Mountains are the oldest mountains in the
West. The Olympic Mountains, once covered by water, were once islands.
Forming the Columbia Plateau
Volcanic activity also formed the Columbia Plateau. Repeated volcanic eruptions formed long fissures,
long cracks in the earth’s crust, and covered the plateau. After the lava flow stopped, rich soil was
formed by deposits of glacial loess, a mixture of fine volcanic ash and dust that was carried by the wind.
Soil up to 150 feet deep gradually covered what is today the Palouse. Today, this is Washington’s wheatgrowing region.
Ice Age Washington
Long after the lava flows stopped, the air got much colder, snow and ice accumulated and the polar ice
cap moved southward. This continental ice sheet moved into what is now Canada and the Northern
United States. Cooling and warming repeated itself; we call the last glacial period the Ice Age and it
happened about one million to ten thousand years ago.
The moving glaciers formed the Puget Sound lowlands. The ice sheet over what is now Bellingham was
over 5,000 feet thick! The ice was over 4,000 feet thick over what is now Seattle and 1,800 feet thick
over Tacoma. The Ice Age glaciers ended south of what is Olympia.
Floods caused from melting ice also played a huge role in shaping Washington’s landscape. The Grand
Coulee is one of Washington’s amazing geologic features. (A coulee is a dry streambed.) Erosion from
flowing water created by glacial floods shaped much of Washington’s landscape.
Ice Age Animals & Land Bridge
The Columbian Mammoth lived in North America during and after the Ice Age. Sloths, saber tooth tigers,
camels, and giant bison also lived in regions of North America including the Pacific Northwest. Many
scientists believe that Ice Age glaciers made it possible for groups of people to travel from Asia to the
Americas about 12,000 years ago. The ice created a land bridge called the Bering Land Bridge.