Most of central Oregon and Washington is buried by thousands of feet of lava rock known as basalt. As the layers of dark black ooze cool, they begin to shrink. The resulting fractures join, creating polygonal columns which can be seen in the layer crossing the middle of the photo. Non-uniform cooling results in distorted columns which can be seen at the top of the photograph. The rate of cooling determines the size of the column, the ones pictured here being relatively large. Use the semi and train cars for scale. The same fracture patterns resulting from shrinkage can also be seen in mud cracks, pottery glazes, and tundra ice wedge polygons.
The reason we are able to observe the results of this geologic phenomenon from the deck of the Sea Bird is because of the deeply cut Columbia River Gorge. Much of this carving took place between 12 and 16 thousand years ago when a glacier in northern Idaho backed up an enormous lake covering most of western Montana. When the depth of the water at the ice dam reached two thousand feet the glacier began to float. This led to a catastrophic failure of the dam, sending a 600-foot wall of water across eastern Washington. It is estimated that this occurred forty times over a period of four or five thousand years and that where the flow became constricted it was ten times the flow of all the rivers in the world combined! With this amount of energy at work it is easy to understand how this extremely hard rock was so easily cut into one of the most spectacular canyons in the world.
