Why is the Rogue Valley a valley?

In the April 19 post, we examined the geologic units that are exposed in the Rogue Valley region, and saw that the units are all tilted toward the NE, with the oldest units located to the SW (between the valley and the coast), and the younger units located toward the NE (toward the active volcanoes of the High Cascades like Mt. McLoughlin). You may wish to review the April 19 post before reading this post, which focuses on the topographic variations that make the region so scenic—a verdant valley flanked by magnificent mountain ranges.

This view, from the top of Wagner Butte, is looking SE from the granitic rock of the Ashland pluton across the Rogue Valley (Ashland) to Grizzly Peak and the active volcano of Mt. McLoughlin on the skyline. On the right side of the photo are Mt. Ashland and the active volcano of Mt. Shasta in the distance. You may even be able to pick out the volcanic remnant of Pilot Peak beneath the horizon.

There are many reasons why parts of Earth’s surface are higher in elevation that others. Some reasons are obvious; for example, Mt. McLoughlin and Mt. Shasta are high features because they are active volcanoes built up as lava and other volcanic products have been extruded from underground up onto the surface. Another example is the Sierra Nevada in California—it’s a mountain range because a large fault along its eastern edge is causing the Sierras to be uplifted to high elevations relative to the valley farther east.

But we must look to another reason for topographic differences in the Rogue Valley region. The main reason is that some of the rock types are very resistant to the processes of weathering and erosion, whereas others are not. If the elevation is high, the underlying rocks are “tough guys” (i.e., resistant to erosion), but if the elevation is low, the underlying rocks are weak (i.e., not very resistant).

When looking SW across the Bear Creek / Rogue Valley to Mt. Ashland (with snow), you might speculate that the granite of the Ashland pluton is a tough (i.e., resistant) rock type and that the sediments of the Hornbrook Formation are weak. And you would be correct! When walking uphill from downtown, you can tell when you’ve reached the granite by the increased steepness of the slope. The granite rock is the reason for the high topography of the watershed and the wonderful recreational area of Mt Ashland. The sedimentary Hornbrook Formation is much weaker and underlies the valley.

When we see the Ashland pluton granite at the surface, however, we might not think it is such a tough rock. That is because the top meter or so of the rock (top 3–6 feet) is exposed to the effects of physical (e.g., rain and wind) and chemical (e.g., oxidation) weathering processes that cause the minerals to separate and fall apart to form the DG (decomposed granite) soils that many of us must attempt to grow plants on. But underneath this weathered layer, there is very tough rock that does not easily release its minerals to make sediment and soil.

This exposure of granite, along Glenview Drive on the south side of Lithia Park, remains in place but is highly weathered and would turn into loose sediment simply by scraping the exposure with a rake. The brown color is from oxidation of iron-bearing minerals—in granite it’s usually biotite, a type of mica. Notice that the upper 30 cm (12 inches) of the granite have turned into a thin soil. Roots have been able to grow farther down, below the soil, because the granite is so highly weathered. That’s poison oak for scale!
This is a photograph of minerals in a granitic rock viewed through a microscope—the width of view is about 2.5 cm (1 inch). Notice that the minerals (white–to–black-colored quartz and feldspar and brightly-colored mica) are angular and interlocking—this is because they formed by cooling from liquid magma and grew together as they crystallized. This is an example of unweathered granite—with its interlocking, high-silica minerals, it makes a very resistant (i.e., durable) rock. This igneous rock type makes up the highest-elevation parts of the Rogue Valley region (e.g., Mt. Ashland). (image from an educational website)

On the other hand, sedimentary rocks tend to be less resistant to the effects of weathering and erosion. Most sedimentary rocks are made of pieces of pre-existing rock that have been eroded and carried for a short or long distance, and deposited in rivers or lakes or the ocean, then buried by younger sediments and turned into rock. Think of the sediment in Bear Valley Creek, which originated on the slopes of the valley and was carried down-slope into the stream. The stream will then carry the sediments northward to the Rogue River and eventually to the Pacific Ocean.

This is a photograph of minerals in a sedimentary sandstone viewed through a microscope—the width of view is a little less than 2.5 cm (1 inch). Notice that the sand grains (mostly quartz) are rounded—this is because they were eroded from pre-existing rocks and then rounded from flowing in a stream and/or rolling in ocean waves. The bright-colored mineral is calcite that is “cementing” the grains together. (image from an educational website)

There is considerable variation in the resistance of sedimentary rocks. Sandstone (consolidated sand) and conglomerate (consolidated gravel) tend to be more resistant than mudstone (consolidated mud) if the grains have a strong cement (usually calcite or quartz minerals) that holds the grains together. Think about sand on the beach, where grains are completely unattached. If that sand is buried, fluids moving through the sand can precipitate minerals between the grains that act as glue or cement. The stronger the cement, the stronger the rock. On the other hand, mudstone is almost always very weak. The mud grains have very small sizes and flat shapes and they pack together so tightly that fluids, with cement-forming minerals, cannot enter.

This is a photograph of the Payne Cliffs Formation (Tpc) that is exposed around the edge of Emigrant Lake. The layer of sandstone is more resistant than the underlying mudstone (with hammer for scale) and so it sticks out of the slope whereas the mudstone is more recessive.
This photo is also around Emigrant Lake. The view is toward the north, looking across the narrow southern arm of the lake. In the foreground is mudstone of the Hornbrook Formation (Kh). The contact with the overlying Payne Cliffs Formation (Tpc) is where the slope increases in steepness—this indicates an increased resistance to erosion. The Tpc unit consists of sandstone and mudstone. The sandstone underlies the three hills—the one labeled Tpc and two more that extend to the left in the photo. The mudstone underlies the low areas between the hills. Tv=volcanic rocks on Grizzly Peak.

So what about those mountains (e.g., Grizzly Peak) on the NE side of the valley? As you might guess, this is also an area of high topography because the underlying rocks are more resistant.

This view is from the valley looking NE-ward to Grizzly Peak (labeled Tv). The Hornbrook Formation (Kh) is mostly mudstone and underlies the valley. The overlying Payne Cliffs Formation (Tpc) is a mixture of conglomerate, sandstone and mudstone. The sandstone and conglomerate layers are more resistant and form hills such as Pompadour Cliffs (the outcrop above the right-side Tpc lettering). The unit is named after the Payne Cliffs that are located farther north, near Medford. The Western Cascades volcanic rocks (Tv) include a variety of igneous rock types, but the lava is, like granite, an igneous rock that crystallized from magma. The interlocking crystals make this another tough, resistant rock type.

In the valley there are other hills that are high because of igneous intrusions (Ti) that are associated with the volcanic rocks (Tv) that make up Grizzly Peak and its slope.

This view is looking SE within the valley. In this view, it is possible to see the NE tilt of the layers—notice the slope on the left side of the hill labeled (Ti/Tpc). This hill, made of the Payne Cliffs Formation (Tpc), is prominent because magma intruded into the sedimentary layers to produce crystalline igneous rocks that are very tough (i.e., resistant). These igneous intrusives (Ti) cool from magma beneath Earth’s surface but are not large bodies like plutons; rather, they are small linear bodies that intrude into fractures (cracks) in the pre-existing rocks. These bodies, called dikes or sills, intruded into the older sedimentary rocks when the Western Cascades volcanoes were active ~35–10 million years ago. If we could go back in time, we would see that Grizzly Peak was then an active volcano!
This geologic map, of the entire Bear Creek Valley watershed, is impossible to read, but please focus on the colors (also see map in the April 19 post). The orange area (Ti) just north of Emigrant Lake (blue area labeled lake) is the hill shown on photo above. (Wiley et al., 2011)

To review the units in the region: (1) To the SW, the Klamath terrane metamorphic rocks and the Ashland granite are resistant rock types that form the mountains between the Rogue Valley and the coast. (2) In the valley, the Hornbrook Formation (Kh, green colors) is mostly mudstone that is not resistant and has been eroded to form the valley. Notice that the valley gets wider to the NW—this is because the area underlain by the Kh mudstone is wider. The darker green color (Kh) indicates sandstone/conglomerate that forms hills within the valley—for example, the hill on which the Rogue Valley Manor (RVM) is located. (3) To the NE, parts of the Payne Cliffs Formation (Tpc) form hills (sandstone) and other parts form low areas (mudstone). Overlying the Tpc is a thick sequence of volcanic rocks that are resistant to erosion and form the ridge that includes Grizzly Peak.

There are two other prominent features in the valley that residents and visitors are likely notice. At the north end of Ashland is an area labeled “ls” (Quaternary landslide). This is a large flat surface that extends from high up on the slope all of the way down to the valley. This is a compound landslide that has been active at various times during the past 100,000 years or so. When walking on this surface, one encounters large pieces of volcanic rock that came tumbling down the slope in debris-type flows. See photo below.

Another feature is Table Rocks, located at the north end of the map adjacent to the Rogue River. These flat-topped land-forms are high because lava from modern-day volcanoes flowed down the Rogue River valley from the east about 7 million years ago. The lava is more resistant than the underlying Tpc rocks and forms the flat tops. Where the lava cap has been removed, however, the underlying Tpc sedimentary rocks erode quickly. One of the cross sections in the lower right corner of the map shows the lava cap and underlying Tpc.
This photo is looking NE across the valley and downtown Ashland. The large-scale landslide is the surface above the “ls” labels. It extends from the valley upward toward the Grizzly Peak volcanic rocks (Tv) that are the source of landslide materials. Also labeled is Payne Cliffs, the rocky outcropping after which the Payne Cliffs Formation was named. These sedimentary rocks underlie the landslide but at the surface what is visible is the volcanic pieces that have been transported from farther up-slope.

In summary, the mountains to the NE and SW of the Bear Creek/Rogue Valley are high because they are made of igneous rocks that are resistant to erosion. The valley is underlain by sedimentary rocks that are less resistant. The small hills within the valley occur where there are more resistant rocks—either igneous intrusions or tougher sediments such as sandstone and/or conglomerate.

In the next post, we will tell the story of how the Rogue Valley region has changed with time based on evidence in the rock units Kh, Tpc, Tv.

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  1. Mark on April 26, 2020 at 2:09 am

    Of particular importance for wine growing is the texture and water holding capacity of soils, and with such variability in the hardness of parent materials there must be a lot of diversity in the water holding characteristics of the soils in the valley. This will potentially lead to a wide range of different wine growing “terroirs”… which will lead to a wide range of different wine styles… which then must all be drunk to be fully appreciated. Darn. Cheers to geology!

    • Landscapes Revealed on April 26, 2020 at 12:33 pm

      I love how you point out the practical aspects of geology! Those who grow any kind of plants notice these soil type differences. Gardening on the granite means finding plants that like well-drained, nutrient-poor soils. In the valley, gardeners often have to deal with an excess of poorly-draining clay, from the mudstone.

      • Janie Burcart on April 28, 2020 at 2:54 pm

        Yes, even as an amateur gardener I appreciate all this information that you so interestingly explain…your excitement in the subject matter is contagious! My Quiet Village garden is partly tough, hard clay and partly nutrient-poor, dry, sandy soil that is FULL of rocks! Ive been able to build 2 rock gardens & place boulders under & along all 3 fences. Oh for some loamy rich soil!!!

        • Landscapes Revealed on April 28, 2020 at 3:25 pm

          Yes—very important for gardeners! You have clay because of mudstone in the underlying Hornbrook Formation, but alluvium flowing from the Ashland pluton also gives you the sandy soil with rocks. Thanks for the observations!

  2. Karen S. Smith on April 26, 2020 at 8:26 am

    Absolutely fascinating, Karen!!! As usual! You’ve added SO much to my understanding of our valley with this articulate, scientific post. Many thanks! You know I love it when you settle in to explain the terrain!
    xoxoxo,karen sue

    • Landscapes Revealed on April 26, 2020 at 12:20 pm

      It warms my heart to have appreciative readers like you—thanks so much for the comment.

  3. Mary O’Kief on April 27, 2020 at 1:18 pm

    I am getting a lot out of this class. Glad I took your in-person class. These blogs help clarify basic concepts which is helpful because I have very little science background. Thanks Karen!

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