This view looks northwest from Highway 243 to Sentinel Gap, where the Columbia River cuts through the Saddle Mountains. In the channel at the base of the cliff, the Columbia River flows from right to left.

The Saddle Mountains are a major anticline ridge in the Yakima Fold Belt, and the northern flank of the Saddle Mountains overrides a thrust fault. These two related structures -anticline and thrust fault -are due to compression of the crust. The orientation of the structures shows that the crust was pushed toward the north.

The channel of the Columbia River has been diverted tens of miles to the east by growth of the Yakima Fold Belt. Old channels and channel deposits show that the Columbia River once flowed in the vicinity of Yakima. Gradually, the northward compression forced the river to change its course as it raised anticline ridges and thrust-fault hanging walls in the path of the river.

Now, south of Wenatchee, the Columbia turns eastward. It flows through the water gaps it has cut into the eastern ends of the uplifted ridges of the Yakima Fold Belt. After flowing through the Tri-Cities area in south central Washington, the Columbia River turns west at the Oregon border and flows toward the gorge it cut through the Cascades Mountains as they uplifted.

Prior to development of the Yakima Fold Belt, the course of the Columbia River had been pushed out to the margins of the Columbia River Basalts by the accumulating pile of basalt flows. It still flows near the margins of the basalt from Lake Roosevelt (behind Grand Coulee Dam) to Wenatchee, making a big loop around the northern half of the Columbia Plateau.

Saddle Mountain Dip Slope

This image shows the east wall of Sentinel Gap. Here you can see the beds of Columbia River basalt tilting upward toward the north. Geologists would say that the beds dip toward the south. Look closely to see the basalt beds sloping down to the right and toward the foreground. Here the layers of Columbia River Basalt have been ramped up (tilted) along the Saddle Mountains thrust fault. The thrust fault lies beneath the basalt layers that precipitously come to an end, on the steeper, far side of the ridge. Select the image to see a larger view. Use your browser's back button to return to this page.

The light-colored splotch on the hillside, between basalt flows and about halfway up the hill above the sprinkler, is a layer of sediments laden with volcanic ash. This is part of the Ellensburg Formation, described below.

Saddle Mountain Flows and Ash Interbeds of the Ellensburg Formation

This photo of the east side of Sentinel Gap shows more of the layers of the Ellensburg Formation, which are white because they contain so much felsic volcanic ash, which comes from explosive volcanic eruptions. Composite cones are prone to venting such explosive, widely distributed eruptions when their magma is felsic and oversaturated with gas. Select the image to see a larger view. Use your browser's back button to return to this page.

A closer look at the Ellensburg Formation would reveal that most of its beds are actually reworked and deposited by streams that flowed down slopes strewn with volcanic debris. Other beds in the Ellensburg Formation appear to be volcanic mudflows (lahars), and some appear to be ash flows and ash falls that were not re-deposited by water but fell directly from the volcanoes. Geologists generally infer these volcanic ashes to have erupted from the central Cascade Mountains in what is now Washington State. This means that even as the huge eruptions of flood basalt were spreading across Eastern Washington from volcanic fissures near the junction of the borders of Oregon, Idaho, and Washington, composite cone volcanoes were growing and erupting violently in the Cascade volcanic arc to the west.

Basalt Columns

This photo shows natural columns of basalt in the Columbia River Basalts 10 miles north of Sentinel Gap, along Highway 243. Recent human visitors have added a little bit of unnatural graffiti. Select the image to see a larger view. Use your browser's back button to return to this page.

Vesicles

This image shows small holes known as vesicles in Columbia River basalt, 10 miles north of Sentinel Gap, along Highway 243. Bubbles of gas escaping from liquid lava form vesicles. As the lava solidifies, the bubbles become frozen in place. Because vesicles tend to concentrate toward the tops of lava flows, their presence can be helpful in distinguishing individual lava flows within a stack of flows. 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: anticline; thrust fault; compression; hanging wall; basalt; dip; volcanic ash; felsic; composite cone; magma; lahar; ash flow; ash fall; flood basalt; volcanic fissure; volcanic arc; vesicle