At some point, I'll copy over some tweets and retweets from the past couple of hours, but for the time being, science people on Twitter are being like this. And I'm one of them. I'm not sure that anything else will be happening today. Thanks, ESA!
Above is a closer shot of the same general area on the petrified log at the Cascade Locks Visitor Center as in the previous post. Looking even more closely in a crop from this photo, there's some very pretty agate with a botryoidal (grape-like) texture.
I've been meaning to discuss just why these ash-rich sedimentary environments tend to have such rich and well-preserved fossils. This is not just a feature of the Cascades; many renowned fossil localities are in ash-rich sediments, including Petrified Forest National Park in Arizona. Volcanic ash is, in large part, simply broken glass. Glass is geologically unstable- it would "rather" be in the form of quartz and feldspars. In other words, those minerals are more stable. If there is organic material nearby, SiO2 (silica) will dissolve from the glass and re-precipitate as quartz in and on the organic material. I'm not familiar enough with the geochemistry of silica and organic matter to say why the latter is a preferred nucleation site for the former, but my petrology professor said that silica is more soluble in bases, while organic matter tend to decay to acidic end products. This would mean silica tends to dissolve in areas away from the organic material, and precipitate nearby. I can say from experience that in ash-rich environments, preservation appears to be rapid, thorough, and with exquisite retention of cellular-level detail.
No, not a redwood, but there's a fair chance this was a dawn redwood, or Metasequoia. Peavy Hall, the Forestry building at Oregon State, has a living dawn redwood near the north entrance, and two petrified logs of it, one near the east entry, and another in the atrium. Both were from the Eagle Creek Formation, as I suspect the above is. Not a terribly informative photo, but I love the color, and the contrast with the fern below.
I'll spare you the bulk of the photos I took of this beautiful petrified log we found at the visitors' center at Cascade Locks. I'm pretty confident this is from The Eagle Creek Formation, which contains a lot of petrified wood and leaf fossils, and has been dated as early Miocene in age. So it's younger than similar-looking plant fossils in the Sweet Home-Quartzville area, which are Oligocene. I haven't found a good on-line reference to The Eagle Creek Formation, but Interzone's wi-fi is having another difficult day, so I haven't been able to review some possibilities. I will post another shot of this log tomorrow, and I'm hoping by then I can find some more useful information.