View to the west down Frenchman Coulee, to where the coulee opens up into the gorge of the Columbia River. The coulee is eroded into Columbia River basalt flows. There is no stream at the bottom of the coulee, only a dirt road. There is no sign of any stream having flowed along the bottom of the coulee since it was formed.

Coulee Wall

View to the north across Frenchman Coulee. This image shows several Columbia River basalt flows, one on top of the other. Most flows have two major layers: a vertical set of regularly jointed columns - the colonnade - forms the lower layer, and an irregularly jointed upper layer called the entablature. In some flows, the upper part of the entablature straightens up into vertical, regularly spaced columns - an upper colonnade. Select the image to see a larger view. Use your browser's back button to return to this page.

 

Roza Flow Colonnade

View southwest of large basalt columns on the south rim of Frenchman Coulee. These basalt columns are part of the Roza Flow of the Columbia River Basalt, which is one of the world's great flood basalt provinces. This image shows part of a colonnade. The rest of the colonnade has been eroded away, mainly as a result of undermining and plucking by the high-energy glacial lake outburst floods that surged through and created Frenchman Coulee. These unusually tall and well-exposed basalt columns are popular with rock climbers. The age of the Roza Flow, using K-Ar isotope measurements, is 15 Ma (15 million years old, rounded to the nearest million years.) Select the image to see a larger view. Use your browser's back button to return to this page.

Olivine Phenocrysts

Plagioclase phenocrysts in a Columbia River basalt flow at Frenchman Coulee, with one of them pointed out at the end of a finger. Select the image to see a larger view. Use your browser's back button to return to this page.

Geologists have used a variety of properties of basalt flows to tell one apart from the other, as they mapped out the extent of individual basalt flows across the Columbia Plateau and, for some flows, down the Columbia River to the Pacific Coast. One property that distinguishes a flow is the types of phenocrysts it contains. A few Columbia River basalt flows contain olivine phenocrysts. Others contain only plagioclase phenocrysts. Some contain both. A few contain pyroxene phenocrysts. Some flows contain no phenocrysts.

The picture shows a finger pointing at phenocrysts of the mineral plagioclase in the Roza flow of the Columbia River Basalt group. The Roza flow is distinctive in several ways, including the fact that it contains phenocrysts of plagioclase and little groups of several plagioclase crystals clumped together.

Another property used to tell basalt flows apart is their chemical composition in detail, down to what are called trace elements. Each flow has, in its trace elements, a distinctive chemical fingerprint.

Another important property is the paleomagnetism of the flow. Whether the flow has normal or reverse paleomagnetism, pointing to the north magnetic pole as it is today or pointing approximately in the opposite direction, is a distinctive characteristic. The age sequence of the flows has been pinned down by studying the paleomagnetism of all of them, and correlating each flow's paleomagnetism with the worldwide magnetic polarity timeline. The absolute age of many of the Columbia River flows has been determined with radiometric isotope analyses, yielding more precise age measurements than paleomagnetism those derived from the paleomagnetism.

Base of Flow and Opal

This is the base of a Columbia River Basalt flow at Frenchman Coulee. A layer of soil that developed on top of the previous flow lies beneath the base of the basalt columns, showing that enough time passed between the two flows for a soil to develop. Select the image to see a larger view. Use your browser's back button to return to this page.

If you look closely, you may be able to see a thin layer of white material along the very base of the basalt flow. This is low-grade opal, which is produced by hot, molten basalt lava encountering moist soil. Opal is a combination of silica and water.

Location Map

Stratigraphy

Glossary terms that appear on this page: basalt; coulee; joints; colonnade; entablature; talus; olivine; phenocryst; plagioclase; pyroxene; paleomagnetism; radiometric; isotope

---