Change is the only constant, and that's true of the Rogue Valley's climate, which hasn't always been the same.
Climate-change clues are evident in fossilized (petrified and carbonized) wood, which is found exclusively in rocks on the east side of the valley and eastward to Lost Creek Reservoir.
Petrified wood becomes younger as you move eastward — as do the rocks that contain it — and records a change in climate from tropical to temperate. Lack of significant tree-growth rings is typical of tropical to subtropical environments, where yearly growing conditions are relatively constant. Furthermore, the vascular structure of tropical flowering plants (dicotyledons) is generally constant, while the vessel size of many temperate dicot trees changes with the seasons.
Tropical to subtropical petrified wood such as Oregon tree ferns, tropical hardwoods, palm trees and low-grade coal (Coal Mine Road), which are found low on the east slopes of the valley as well as in Sams Valley, are between 35 and 45 million years old (Ma).
Younger, distinctive, carbonized leaf fragments of metasequoia, a deciduous "living fossil tree" found now in China, are present in tuff (solidified volcanic ash) not far north of the Oregon-California border and at the ridge top of our valley. In both areas, growth rings in fossil subtropical to temperate climate dicots indicate increasing seasonality.
The abundance of specimens with distinct growth rings northeast of Eagle Point signifies a change to seasonal temperate climates by about 24 million years ago. Forest ecology (and climate) similar to the present-day was established by about 15 Ma.
The Siskiyou Summit road cut is an easily accessible place to observe petrified wood. Two white tuff layers (hot volcanic ash flows analogous to those of Mount Saint Helens) contain carbonized wood and leaf fragments as well as harder silicified wood. The latter has changed to solidified colloidal silica called chalcedony, or agate, produced through a replacement process called permineralization. Dark flattened shapes parallel to the layering (most obvious in the upper ash layer) are ancient trees blown down by violent volcanic eruptions then, like shanghaied sailors, whisked down an ancient slope.
For those less willing to breathe truck fumes, visit the world-class display at the Crater Rock Museum in Central Point (2002 Scenic Drive; 541-664-6081). Grateful appreciation for information is extended to Doug Foster, associated with the museum, and Bill Elliott, geology professor of the former SOU Geology Department, currently at the University of Southern Indiana.
Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at firstname.lastname@example.org.