Coulee City to Grand Coulee 2

Lower Grand Coulee, Talus Slopes and
Steamboat Rock
Continued

[131.1 MM 10]
We continue up toward the town of Grand Coulee. The Grand Coulee begins to widen and soon we see a large flat island on our left. The map at right shows that we are near the end of this leg of our journey. Click on the image or here for a larger map.
(WA State Parks map)

[136.0 MM 15]
Steamboat Rock is a huge island of layered basalt in the channel of the eastern end of Lower Grand Coulee. There is a state park around the island.

Steamboat Rock.

"Water diverted south by the Okanogan ice lobe (during the last ice age) on this part of the Columbia Plateau carved the immense channel known as the Grand Coulee. Floodwaters here, at peak stage, reached nearly to the rim of the canyon for a flood depth of 500 feet. Geologic evidence shows that some floods overtopped Steamboat Rock (there are granitic boulders on the summit, deposited as ice rafts became stranded and melted!)" [Buchanan]

When an ice dam blocked the Columbia River here, Glacial Lake Columbia was formed. This is where Grand Coulee Dam was built, also. When the ice dam holding Glacial Lake Missoula broke, floodwaters could not get past this ice dam and flowed south creating the Grand Coulee. Banks Lake reservoir partially fills the coulee. This USBR aerial view of the area shows where the ice age floodwaters traveled to create the Grand Coulee. Steamboat Rock (upper left of the photo) divides the channel.

For more on these huge floods see the Field Trip to Mars Catalog The Great Floods Video which includes animations of the floods and aerial views of the Channeled Scablands.

[137.4 MM 17]
Beneath the Columbia Plateau basalt lies granite, schist and gneiss as seen in the photo below. Near the center of the picture is the lighter gray granite. The layer on top of that is schist, a metamorphic rock with light and dark layers. The brownish rock on top of that is gneiss, a flaky, grainy metamorphic rock. Note the basalt talus in the lower left corner and the basalt columns in the shadows behind the granite.

Granite, schist and gneiss beneath basalt columns.

[139.2 MM 19]
More granite and gneiss can be seen exposed in this road cut. The dark "v" shape is about 3 feet high and 2 feet wide. Note the veins and stripes that formed as the rock recrystallized under heat and stress from pressure. Click photo or here for a larger image.

[139.7 MM 19]
Look for the exposed lakebed that formed after the floods in the wave cut cliff to our left.

Exposed lakebed in wave cut cliff.

[142.7 MM 22]
Looking to the right we see a whole valley filled with basalt. The edges of the valley are gray granite and you can see the familiar columns of basalt on top.

[145.2 MM 25]
We arrive at Electric City and continue on to the town Grand Coulee.

Geology Terms

Here are some basic terms used on this page. Find more geology terms in the Glossary.

Basalt

Volcanic rock caused by partial melting of the Earth's crust.

Channeled Scabland

Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J Harlan Bretz during the 1920's in various publications. See also Channeled Scabland and Glacial Lake Missoula.

Coulee

Long winding channel cut through lava formations. A term primarily used in the northwestern United States.

Erosion

Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.

Gneiss

A type of metamorphic rock formed as older rocks recrystallize under heat and stress from pressure chactorized by a layering of light and dark bands or streaks.

Granite

Coarse-grained igneous rock usually without obvious bands or markings.

Schist

A type of metamorphic rock formed as older rocks recrystallize under heat and stress from pressure charactorized by a flaky layering of light and dark areas.

Talus

Rock debris that has fallen from the sides of a cliff or steep slope.

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