OCEANOGRAPHY FIELD TRIPS IN THE DESERT

Although much of the Columbia Plateau of eastern Washington and Oregon gets less than 20 cm mean annual precipitation, the area abounds in features to demonstrate the geology and, to a limited extent, the biology of ocean floors and margins. The Columbia River basalt has, in places, a thickness comparable to that of oceanic crust. Although most of this basalt is subaerial, in places it is pillow lava, for the Miocene flows disrupted the preexisting drainage system. In scattered areas across the region, the Pleistocene Touchet Beds have much in common with turbidities. Deposited by jökulhlaups from glacial Lake Missoula, these rhythmites are graded beds with load casts, rip-up clasts, and other sedimentary structures like those in sediments deposited by turbidity currents at the base of the continental slope. Erratics transported by icebergs riding the Missoula floods are akin to dropstones in glaciomarine drift deposited from ice shelves and icebergs. Sheeted clastic dikes within the Touchet Beds are in some ways like dike swarms at spreading ridges. The edges of reservoirs impounded along the Columbia and Snake Rivers have physical, geological, and biological characteristics similar to those along coasts. The weekly and annual fluctuations of reservoir levels, although not on the same time scale, are not unlike the rise and fall of the tides. Mud flats exposed when reservoirs are low are similar to estuarine mud flats. Tributaries to the Columbia Snake River, such as the Yakima and Walla Walla Rivers, are building deltas out into the reservoirs. These deltas have morphologies and sediments similar to those of larger deltas along coasts. The deltas, and places where reservoirs are very shallow, such as at McNary National Wildlife Refuge, have freshwater wetlands with plant and animal species similar to (e.g., grasses) or identical to (e.g., bald eagles) those in coastal wetlands. On these field trips students supplement what is learned in class, for example seeing beaches with waves, and imagining an alluvial fan as a deep-sea fan. They compare and contrast continental and oceanic earth materials: caliche with marine carbonates, diatomite with siliceous ooze, loess with eolian marine sediment, Mazama ash with marine tephra. Our desert is a good substitute for the real ocean, which is more than 500 km from the classroom.