What's the Story with the Coastal Mountains?

January 2015. So, I'm sitting in my rented house back East, waiting out the blasted winter so I can drive the family car across the continent to Coos Bay, and I'm wondering what the mountains there will be like. I was sort of expecting a low coastal range, with a great, flat-bottomed north-south valley behind that, and a range of great mountains behind that. The maps, however, are showing me that, while this may be true of northern Oregon and much of California, it just isn't so for Southern Oregon. The picture there is much more complicated. So I made the map you see on the left to help me figure it out. If it seems a bit chaotic, bear with me. It'll make sense in a little bit.

When web sources talk about the coastal mountains at all they mostly say why you don't want to spend much time in them. In this they cite the large timber industry and the low elevation of the peaks. The large timber industry actually makes some sense; neither recreationists nor foresters particularly want to get in each others way, and working people have the right of way over playing people. However, logging is about as low intensity as it gets, and this leaves plenty of room for the two groups to stay out of each others way. And the claim that these mountains are too small can't be right. Compare the Coos Bay mountains to some other mountains of the same height. Mountains that make up the Amish areas of Pennsylvania or the Rocky Mountain Front just west of Boulder aren't any higher than the ones around Coos Bay; although they may have higher peaks, their valley floors are higher as well. The problem here is more subtle than industry or height.

Let's look at the mountains stripped of all detail except relief. Here's a 32 square mile section directly east of Coos Bay, followed by exactly the same sized sections from the Amish Country in Pennsylvania and the Rocky Mountain Front at Boulder. It's pretty striking, isn't it? The Coos Bay mountains are just a jumble, with no apparant pattern. The Pennsylvania mountains, however, have a very distinct pattern; this makes them look much higher, but they are actually lower. The Rocky Mountain Front also has a distinct pattern. They look huge and draw a lot of tourists, but in reality they are only a few hundred feet higher than the Coos Bay mountains. It's not just the maps, either, When you visit either of these mountain ranges the pattern gives you something to focus on, something to look for and wrap your mind around. It's the jumbled non-pattern that makes the Oregon Coast Range hard to grasp. So let's organize it a bit.

Geologists don't really deal much with the way mountains are shaped; like Gollum, they want to penetrate deep underneath looking for hidden secrets. Here I'll deal solely with what the mountains look like, a much simpler question. The most notable things here are the softish rocks of the Coast Range, and the strongly defined valley behind it to its east. That valley is filled with alluvium to quite a height, so that its floor is higher than that of the rivers that drain westward through the mountains into the Pacific. A third factor would be whatever rivers were flowing through the area before the mountains uplifted. A river that cuts downward faster than the mountains rise upward will slice a gorge through the mountains rather than be deflected.

Here's a diagram of the Coast Range right around Coos Bay. In it I've traced out the main ridgeline and color coded it for height, where light tan is up to 2,000 feet, the darker tans get you to 4,000 feet, and the charcoal gray goes from 4,000 to nearly 6,000 feet in spots. You can see that this ridgeline runs fairly straight north-south at a distance of 35 to 40 miles inland, and gets steadily lower as it goes north. Between the ridge and the sea smaller streams cut out the tree-like drainage patterns called "dendritic", typcial of normal erosion. On the eastern side, however, the mountains drop quickly away into a series of valleys that line up with that of the Willamette River (further north).

The odd thing here is how the Umpqua and the Rogue Rivers break right through the mountains. As a river can hardly bulldoze its way through solid rock these two rivers must have found another way through. One way is to slowly grind away at its headwaters until it encroaches on another stream basin that happens to be at a higher lever. The Umpqua, to the north of Coos Bay, looks like it may have done this to the uppermost part of the Willamette River, stealing its headwaters. This "stream piracy" is found throughout the coastal mountains; look for streams running right through mountain passes (such as Pass Creek west of Cottage Grove) to spot the ones currently in action.

The Rogue River, on the other hand, looks like it has carved its canyon straight through mountains, and at their highest part at that. It could have done this if it had existed before the mountains started forming, and cut its way downward faster than the mountains rose upward. Here you find the highest peaks, reaching 4,300 feet on the north side (Mount Bolivar) and 5,300 feet on the south (Brandy Peak). As a result it has formed a remarkable gorge, beloved for its whitewater and wild scenery, the one portion of the coastal range more given to recreation than logging. It's a congressionally-declared Wild and Scenic River, as is its major tributary the Illinois river. The western half of its gorge is protected as the Wild Rogue Wilderness.

The Coast Range continues southward into California to the Klamath River, 150 miles south of Coos Bay, and northward 200 miles to Astoria. The southern stretch — often treated as a separate mountain region called the Klamath Range — remains mighty, mostly staying above 4,000 feet with peaks exceeding 6,000 feet. North of the Umpqua, however, the ridge struggles to reach 2,000 feet for a distance of 70 miles, and won't see 4,000 feet again. Indeed, the first 20 miles north of the Umpqua are a crazy quilt of stream piracy — yet none of these streams has managed to cut all the way through to the Willamette.

There is, of course, a second coastal range farther inland and much mightier — the Cascades, whose glaciated peaks can reach 10,000 feet and higher. In Oregon this range runs almost in a perfect north-south line 105 miles inland, its ridges and peaks lying mostly between 5,000 and 9,000 feet above sea level. While Wikipedia claims that the Cascades continue south to Lassen Vocano to merge with the Sierra Nevadas, my ridge-tracing tells a different story. On the map to the upper left you can see the Cascades ridgeline being eaten westward by the Klamath River, then regaining its old line at the Trinity Alps, to come to a sudden end at 6,100 foot Goat Mountain just south of California's Snow Mountain Wilderness. The Sierras, in this reading, run 50 miles farther inland and disappear under the massive lava flows of Mount Shasta and its many siblings.

So much for speculation, fueled by my poring over maps. In a few months I'll be able to see for myself.