The Rogue Valley region in SW Oregon: displaying 300 million years of geologic time
April 19, 2020
Typically, I post blogs when traveling away from my home in Ashland, Oregon. But with COVID-19 keeping us all at home, it seems a good time to investigate the landscape of my local region. Ashland is a tourist town best known as the home of the Oregon Shakespeare Festival (OSF), an eight-month season of 11 plays—four by Shakespeare and seven by classic and contemporary playwrights—in rotating repertory in three theaters.
Ashland is also known for its scenic beauty; its valley location between steep mountain ranges provides stunning vistas and many opportunities for outdoor activities such as hiking and skiing. You will not be surprised to learn that the geologic makeup of the region is responsible for this natural grandeur. The valley where Ashland is located is the Bear Creek Valley, which feeds into the Rogue Valley farther north near Medford. The geologic setting is the same, and for simplicity I will refer to the whole region as the Rogue Valley region.
There is certainly geologic complexity in the region, but the basic distribution of rock units is straightforward and clearly displays nearly 300 million years of geologic history. The map below shows the distribution of geologic units as viewed from above, as if you were in a plane surveying the landscape. Each rock unit is denoted by a different color. The cross section below it shows the distribution of geologic units in a vertical profile—imagine using a huge knife to cut through the surface layers to expose what they look like beneath Earth’s surface.
There is a lot of complexity in the western (oldest) part of the region but we will dispense with it quickly in this post! The Klamath terrane is all of the area between the Rogue Valley and the Pacific Ocean. [Note that for geologists the Klamaths are specifically the colored area that extends from Redding to Roseburg on the map below. In contrast, the Siskiyous are a broader geographic region that also includes younger rocks of the Western Cascades that are not shown on the map below.]
We need to examine the basics of an important rock type in the region—igneous rocks, which are a primary rock type because they form from liquid rock (called magma) that rises up through Earth’s crust (see diagram below). The other two rock types are mostly secondary. Metamorphic rocks form when pre-existing rocks are altered through increases in applied temperature and/or pressure. Most sedimentary rocks form from erosion of pre-existing rocks, although a few are primary (e.g., salt deposits).
Referring back to the geologic map and cross section above, recall that the rocks in our region are tilted toward the NE, which means the oldest rocks are to the SW and the youngest rocks are to the NE. Why are the rocks tilted? Because they are one limb of a very large-scale fold. To learn about folds, go to the Geology Page site: http://www.geologypage.com/2015/12/geological-folds.html. You can search this site to find basic information about many geologic processes.
In the next post, we will examine the reasons for topographic differences: why is the valley a low part of the landscape and why are the ridges high parts of the landscape, even though they are all tilted in the same direction.
Wiley, T.J., et al., 2011, Geologic database and generalized geologic map of Bear Creek Valley, Jackson County, Oregon: DOGAMI Open-file report 0-11-11. The full version of the map (partially shown above) and the report can be accessed at: https://www.oregongeology.org/pubs/ofr/p-O-11-11.htm
Miller, Marli B., 2014, Roadside geology of Oregon, 2nd edition: Mountain Press Publishing Company. Source of time scale and Klamath terrane map. Marli also maintains a web site with her fabulous geology photos: https://geologypics.com/