The rock along the shoreline, in the foreground of the photo, is granite. Granite forms by crystallizing within the earth's crust, deep beneath the surface. Steamboat Rock is in the background. Steamboat Rock consists of basalt flows of the Columbia River group stacked like layers in a cake. Basalt originates as flows of red-hot, liquid lava on the surface of the earth. Steamboat Rock is located in Grand Coulee, the largest coulee in eastern Washington. The geologic relationships visible in the photo raise some questions about Steamboat Rock:

What is the relationship of the granite bedrock to the basalt?

How did Grand Coulee form?

How did Steamboat Rock form?

A climb to the top of Steamboat Rock leads to more questions including, what is the source of the granite boulders that sit on top of everything else? The answers to the mysteries of Steamboat Rock have to do with old landscapes being covered by younger lavas, which were subsequently reshaped by ice sheets and floods, creating a new landscape.

Northup Canyon

The foreground of the photo shows granite and the background shows a colonnade, the lower layer of a Columbia River basalt flow. This photo was taken in Northup Canyon, on the side of the Grand Coulee east of Steamboat Rock. The bottom basalt flow rests directly on the granitic bedrock here on the side of Northup Canyon, just as it does at the base of Steamboat Rock. Select the image to see a larger view. Use your browser's back button to return to this page.

Flood Deposits

This is the wall of a gravel quarry that was excavated into loose sediments in the Grand Coulee. It is located at the mouth of Northup Canyon near Steamboat Rock. The cross beds and mixed sand and gravel layers indicate that the sediments were deposited by high-energy water surging through the canyon and coulee. The fact that these sediments were left as deposits in the coulee indicates that the coulee had to have been there before the sediments were deposited. Select the image to see a larger view. Use your browser's back button to return to this page.

View from Top of Steamboat Rock

Part of the top of Steamboat Rock is in the foreground of this photograph. It is incised by a shallow coulee. The presence of a coulee running across the top of steamboat rock suggests the likelihood that before the Grand Coulee formed, some other, smaller coulees had been eroded across the area and a remnant of one of them is preserved on top of Steamboat Rock. Select the image to see a larger view. Use your browser's back button to return to this page.

Granite Boulder

The split boulder in the photograph is made of granite and sits on top of Steamboat Rock. What is a loose granite boulder doing at one of the highest points on Steamboat Rock, when layer after layer of basalt flows is underneath it? Select the image to see a larger view. Use your browser's back button to return to this page.

Landscape on Top of Steamboat Rock

This photograph shows the landscape in which the boulder of the previous photograph sits-an area of small mounds with granite boulders scattered here and there. It seems likely that the mounds and boulders are part of a terminal or recessional moraine of an ice sheet. If you trace these mounds of loose debris and granite boulders they seem to partly drape over the coulee that cuts through the top of Steamboat Rock. If so, that would mean that the edge of the glacier came to this vicinity after the small coulee formed. Select the image to see a larger view. Use your browser's back button to return to this page.

Glossary terms that appear on this page: granite; basalt; colonnade; cross beds; sand; gravel; coulee; terminal moraine