Thanks Jim - it's more of a divide than we used to think. Geologists from Oregon State University have discovered several distinct layers on the Oregon side of the Columbia that were once the Columbia river beds. The south side appears to be lifting upwards as are no corresponding layers on the Washington side which strongly suggests that there is a fault line under the river. The old river bed layers are amongst the Columbia Basalt flows which dates those uplifts to roughly 15-17 million years ago when the basalt layers were extruded and flowed down what is now the Gorge.
