Fall 2007 Columbia Basin College Fieldtrip: The Channeled Scablands of Eastern Washington

Physical geology students explored Eastern Washington’s famed Channeled Scablands and Columbia River Basalt province on a rainy Sunday. Following the route the catastrophic Missoula floodwaters carved in reverse, we worked our way ‘upstream’ from the Pasco Basin north to Sun Lakes State park. Early in the day, fog hindered our views. Our first stop overlooked the western reaches of the Hanford Nuclear Reservation. Students were asked to imagine themselves in the Pleistocene, standing under a deep, widespread lake (Lake Lewis) born of the Missoula floodwaters that covered the Pasco Basin, including all of Hanford. Not a difficult chore, since much of the Hanford site was hidden beneath the fog. From our viewpoint at the Vernita Brige along highway 210, the looming 300 ft high, 10 mile-wide Priest Rapids gravel bar that marks Hanford’s northern border was only a ‘suggestion’.

As we drove north, the fog began to lift, and students were not only able to see the rocks and landforms, but also explore. At Sentinel Gap, floodwaters were temporarily blocked by the rise of the Saddle Mountains, one of the Yakima Fold and Thrust Belt anticlines. Floodwaters punched through here, widened the Gap, and continued through towards the Pasco Basin. At Sentinel Gap, students were also able to map out several individual flows of Columbia River Basalt. The Columbia River Basalt Group (CRBG) represents the largest flows of flood basalt known on Earth. These Miocene-age basalts are a result of hot spot activity near the present day Oregon/Idaho/Washington border. When that portion of the North American plate sat above the hot spot, magma migrated up through the thinned crust and poured from large northwest-southeast trending fissures. This flood lava copiously poured from the fissures for ~12 million years (from ~17 million years ago to 5 million years). This predates the Missoula flood catastrophe. Flow after flow of CRBG covered large portions of Eastern Washington, parts of Oregon and Idaho, and also followed the path of ancestral Columbia River towards the ocean, filling the Columbia River Gorge with molten rock. Here in Sentinel Gap students could not only pick out individual lava flow layers, but also see how these layers are now tilted. The CRBGs have been deformed into a series of folds (anticlines and synclines) as part of the Yakima Fold and Thrust Belt.

The next stop was Gingko Petrified Forest State Park. This park has two parts: a museum perched atop a cliff along the Columbia River and a wilderness area for hiking. The park preserves an important part of Washington’s paleoenvironment: the remains of a Miocene forest! This indicates that Eastern Washington was not always semi-arid. Once, this area was lushly forested with deciduous trees and was home to numerous ponds, lakes and even perhaps swamps. Preserved through replacement (silica replacing original wood material), the trees of this ancient forest were petrified and encapsulated in one of the CRBG flows. Uplift and erosion have exposed these entombed trees, which include many species not native to North America today, like the Gingko (for which the park was named.) Because of their preservation condition, including their relative position (not upright), geologists believe they were originally downed logs floating in a body of water. Evidence of a lahar (giant volcanic-induced mudslide) found by Dr. Steve Reidel of Washington State University near Gingko State Park suggests that these trees may have in the path of a lahar originating from one of the Cascade volcanoes east of the area. The lahar may have dammed up the Columbia River, creating a lake. The trees downed by the lahar may have been dumped into the lake, and this is how the next flow of CRBG encountered them. On this misty Sunday in November, millions of years after the trees’ death, students hiked along trails through sagebrush on the lookout for petrified logs (conveniently labeled along the trails) and glacial erratics. Erratics are rocks that were carried along with the Missoula floodwaters on their path to the Columbia Gorge, probably as cargo on floating ice, and were deposited many miles from their original location. Erratics are a common sight anywhere the Missoula floodwaters flowed. Small pieces of petrified wood are very common throughout the park. Students wanting a piece of the wood for themselves traipsed on down the road to the Gingko Rock shop, where wood collected off private and BLM land is for sale.

Northward we drove after leaving Gingko, heading towards Grand Coulee and Sun Lakes State Park. Sun Lakes State Park is home to Dry Falls, a cataract that during the Missoula floods was a waterfall 4x the size of Niagra Falls! We explored the basalt boulder-strewn lands at the foot of Dry Falls, and then drove up top to appreciate the aerial extent of the Falls. One look at the size of the boulders tossed about by flood waters, and no student underestimated the energy of this catastrophe! The rain started to fall substantially at this time, so we cut the trip short and headed back towards Pasco and CBC. On the way home, we stopped at the Lake Lenore Caves. These caves are not true caves, in the sense that they are shallow and light easily penetrates throughout the opening. In addition, these caves are more exotic than our more common dissolution-derived caves (karst), in that they were created by the torrential Missoula flood water. As flood waters tore through Lower Grand Coulee, the colonnade entablature of the CRBG was susceptible to the plucking force of the water. Columns were torn from the coulee wall, leaving behind shallow ‘caves’. In some cases, whirling vortexes of water became focused in an area, and tunneled through the rock leaving potholes, some tiny (1” diameter), some enormous!

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