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.
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.