Photos of Pacific Coast, Cascades, Columbia Plateau
Geology of the Pacific Northwest

Virtual Field Site

Frenchman Hills Anticline photo

If you drive to where Interstate 90 crosses the Columbia River, you come to the town of Vantage, Washington. The Vantage area offers glimpses of the depth to which the Columbia River flood basalts accumulated, and the other events that took place in the area during the long intervals between the flood basalt eruptions. During the Miocene Epoch, the plants and animals that lived in the Vantage area were different and so was the climate. Since that time, probably starting in the Late Miocene, the layers of basalt in the area have undergone folding and faulting. More recently, actual floods that burst out of Glacial Lake Missoula swept across the area several times, excavating coulees and depositing sand and gravel.

This picture shows layer upon layer of Columbia River Basalt, as seen above the east side of the Columbia River from the town of Vantage. After they solidified, the layers of basalt were folded on a large scale.

The picture shows a naturally eroded cross-section of the northern limb of the Frenchman Hills Anticline, which is one of the major folds in the Yakima Fold Belt.

Most of the Columbia River Basalt eruptions took place between 17 and 14 million years ago, including the ones visible here. It can be inferred from the principles of relative dating, in particular the principle of original horizontality, that the folding must have taken place since the basalt layers erupted and solidified.

Basalt Pillow

Basalt pillow photo

In this picture a rock hammer rests on a basalt pillow, part of a pillow basalt delta exposed along Highway 26 where it turns east from the Columbia River, two miles southeast of Vantage. Mafic lava that erupts into water forms basalt pillows, the outer part of each lava extrusion hardening quickly underwater into a rind while the still-molten inner part keeps flowing until it is pinched off from the eruption source. The orange-yellow material around the basalt pillow is called palagonite, which is a generic term for the clay-plus-iron-oxide material that tends to form around basalt pillows. Palagonite forms from altered shards of volcanic glass that have reacted with water and oxygen during and after the eruption and solidification of the basalt pillows. Select the image to see a larger view. Use your browser's back button to return to this page.

Petrified Wood

Petrified wood photo

This photo shows a person examining a large piece of petrified wood, one of many on display at the Gingko Petrified Forest State Park visitor center in Vantage. The petrified wood comes from a layer of sediment that was deposited between two of the Columbia River basalt flows. More specifically, the petrified wood comes from the base of a basalt flow, which inundated a wet, muddy terrain that was strewn with logs and stumps, possibly a landslide or mudflow deposit where many logs from the surrounding, forested Miocene landscape were swept together. Today, sagebrush, rabbit brush and grass cover the hills around Vantage. Select the image to see a larger view. Use your browser's back button to return to this page.

Petrified Log

Petrified log photo

This image offers a close-up view of a petrified log at Gingko Petrified Forest State Park visitor center in Vantage. The log was petrified after it was buried beneath the ground to below the water table. Water percolating through the wood carried dissolved silica, which gradually precipitated into the interiors of the cells of the wood. This replaced the original plant material with silica and preserved the shape of the wood down to the level of the shapes of the cells. This kind of preservation allows identification of plant species and study of their growth patterns. The plants preserved as petrified wood near Vantage suggest a climate that was wetter, and warmer in the winter, than the climate there today. It may be that the Cascade Mountains had not yet grown to their present high, continuous form. In that case, there would not have been a rain shadow in the area at the time, so that the moist, temperate, marine air moving westward from the Pacific Ocean would have had a much stronger effect on the climate. Or, it may be that climates in the area were warmer (in winter) and moister in general, despite the Cascades Mountains. Select the image to see a larger view. Use your browser's back button to return to this page.

Diatomite Deposit

Diatomite photo

This photo shows part of a deposit of diatomaceous earth, or diatomite, located seven miles northeast of Vantage and about one mile from Interstate 90, near Silica Road. If you have ever attended a concert at "The Gorge" amphitheater near George, Washington, you have driven by these diatomite deposits. The diatomite deposits sit on top of the Columbia River Basalts that cover the region. Select the image to see a larger view. Use your browser's back button to return to this page.

Diatoms are microscopic, unicellular algae that perform photosynthesis and have a hard exoskeleton made of silica. In pure, freshwater lakes perched on volcanic rocks that supply the water with dissolved silica and in which diatoms can flourish, diatom exoskeletons may accumulate on the bottom, forming a deposit of virtually pure diatomite. The intricately perforated exoskeletons of diatoms, and the pure silica composition, make diatomite an excellent material for filtering and purifying liquids, including beer, vodka and syrup. That is why the diatomite deposits northeast of Quincy have been mined commercially for years.

Location Map

Vantage Location Map


Vantage Stratigraphy

Glossary terms that appear on this page: flood basalt; coulee; anticline; pillow basalt; diatomite

Geology of the Pacific Northwest
Virtual Field Site--Vantage
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated 6/20/13