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.
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 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.
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.
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.
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.
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.
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.
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.
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!
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.
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!!!
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!
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
It warms my heart to have appreciative readers like you—thanks so much for the comment.
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!
So glad it’s helpful. Thanks Mary!