We arrived several minutes behind the caravan to our first stop, a red-bed exposure in a twenty-plus meter high road cut along Highway 26. A road-cut is a geology instructor's show-case. I do believe the American highway system has done more to advance the geology classroom than most of the books combined. Melinda constantly reminds the geo-scholars to, "watch the road,look out for the traffic!"
This steep road embankment is a red sedimentary deposit. The dominant rock is arkose sandstone which is characterized by fine grains of white plagioclase feldspar and quartz. Other rocks present are igneous boulders. The surface of the slope is a crumbly mass of sandy clay from the weathered rock. Weathering will penetrate slowly into this formation and the surface will degrade and crumble slowly over the years so the steep slope is relatively stable next to the highway.
The red color is oxidized iron from exposure to oxygen and water. This is chemical weathering. The most weather resistant rock to erosion is the igneous rocks which chemically degrade to clay.
The large igneous boulders have weathered, physically rounding into spheres. Their edges and corners allow water and oxygen to penetrate faster than a round shape so the rough surfaces are diminished faster. Shattering the igneous rocks to expose their unaltered centers shows their composition to be closer to andesite than basalt. They are possibly from the Columbia River Flood Basalts or Western Cascade igneous origins.
The deposit is riverine and was probably a very strong river estimating the size of current necessary to move many of the larger rocks. This was perhaps part of a delta or river mouth
Highway 26 tunnels through one of the ridges of the Coast Range between Stop #1 and Stop #2. The weathering of the country rock above this tunnel caused trouble last year when it collapsed and killed a highway repairman examining where water was coming in through the roof.