After looking at the cool rocks in Silver Falls State Park, I went back to Gopher Valley, on the other side of the Willamette Valley, and took some photos of the rocks in our hillside. I have been puzzling over these since we arrived.
I grilled our instructor, Lockwood about these rocks. Then I sent him photos. He promptly returned my email saying they were examples of SPHEROIDAL WEATHERING.
I was having a hard time visualizing how this happens. I know that there are different kinds of weathering – the one I automatically think of is MECHANICAL or PHYSICAL weathering, like these rocks from a beach in Scotland. They’ve been in the geological rock tumbler since the continents formed, and they are still hard as well, rocks.
But CHEMICAL weathering is just as prevalent, and that’s what is going on with the rocks in Gopher Valley. These rocks were extruded 17 to 6 million years ago (so young!), probably as one of the Columbia basalt lava flows. They have been weathering in place ever since, undergoing chemical changes.
Chemicals dissolve and change the rock, water expands and contracts inside cracks, creating that cool onion skin flaking. If you start with an angular shape, the corners and edges sticking out start to weather first, more than the flat sides. Eventually, the corners are gone and these spheroidal shapes develop just by the action of chemicals rather than abrasion. Word for the day: spheroidal weathering!
So, why are these particular rocks so crumbly and coarse and those 2.8 billion year old ones are hard and dense (and also still around??) That is a chemistry question, and I’m still working on understanding how geologists use nomenclature to describe what material rocks are made of, the chemicals in them, and the texture. When I do that I will be able to say that geology is not complicated or confusing anymore. And I will be able to explain it.