for a large percentage of the year and very difficult to get over. There are only a couple of passes suitable for railroads, like Stevens Pass for example, and really only one river crosses the Cascades, north of the Klamath River, only one river crosses the Cascades from east to west and that’s the Columbia. So that barrier to transportation and the
9 concentration of the transportation along the route of the Columbia River are two additional element s of the significance of the Cascades in Northwest history. Let’s go over to the east side now. [Slide: map showing where video is being taken] Well we’ve made it over here to the east side of the Cascades. Notice how much drier this landscape is – browns and grays are the colors that dominate, rather than green. We’re at Dry Falls near the south end of Grand Coulee an awesome spectacle. Notice the rock cliffs that you see around you are in layers. This landscape, what we’re going to talk about here is the geology and the geography of this landscape on the east side and what you’re looking at in the layers are basalt flows, flows of molten basalt rock coming up out of the earth and that sort of underlies the entire Columbia basin plateau. We’ll talk about t hat for a few minutes here. Sometime about 17 million years ago, going to about 7 million years ago this entire landscape here in central Washington and parts beyond that, experienced a series of lava flows, molten basaltic rock coming out of fissures in t he Earth’s crust. This is particularly fluid form of basalt that flowed very rapidly and filled entire lowlands, they call it flood basalt for that reason and you can see the layer upon layer upon layer and there are as many as 50 layers of basalt that flowed out at different times that flowed out one on top of the other that created this layer cake effect. This basalt that you see here underlies a huge part of the Northwest, this interior plateau region of the Northwest, basically from the Rocky Mountains to the Cascades from British Columbia down into Oregon. It covers an area of 50,000 square miles, this is a tremendous basalt flow, and in fact it’s one of the three or four largest basalt flows on the planet. To give you another idea of the extent of these flows, there were a couple of vents in Eastern Washington, cracks in the crust that this basalt flowed out of and one particular one individual, single basalt flow called the Roza flow [slide: map showing the Roza Flow] covered 9,000 square miles, this lava flowing up out of fissures and covering 9,000 square miles, a single flow. The lava sometimes flowed at speeds of 25 to 30 miles per hour and it burned and buried everything in its path. Now when that lava was flowing out the Cascade Mountains were not here, and the continent was a little bit farther south it was warmer and much more moist since that Pacific air was coming straight off without hitting the Cascades and dropping it’s load of moisture [slide: picture of what the continent may have looked like] .
- Fall '19
- Columbia River, Cascade Mountains