This is an ambitious side trip that you might try on your next trip north to Portland. At Redding, you turn left off the freeway and instead of the 70-mile burn up I-5 to Weed, you take about 120 miles and 3 or 4 hours extra to meander through some of California’s most unusual country: the peridotite lands of the eastern Klamath Mountains.
Peridotite (“per-RID-atight”) is the rock type that exists beneath the Earth’s crust in the mantle. It’s a dense rock made of iron-rich minerals that is rarely seen. It’s generally altered into serpentine rock and not much escapes. California has the lion’s share of America’s peridotite, and the biggest piece of it is in the eastern Klamaths along State Route 3.
The Klamaths are an outlier of the Sierra Nevada that somehow ended up about a hundred miles west. How that happened is still a puzzle to geologists, one that won’t affect your enjoyment. It means that the ancient rocks similar to those that were invaded and disrupted by Sierran granites are exposed in somewhat better shape in the Klamaths. But the range is still a geological scrapple, a collage of many different rocks blanketed by roadless rain forest. Route 3 provides a lucky glimpse of its most distinctive component. Here’s the road route as shown on Google Maps.
And here’s how the route fits the geological underlayment, as shown on the interactive state geologic map.
If you know your traditional geologic map colors, you’ll see at a glance that the young volcanic rocks of the Cascade Range take up the eastern side of the map and Redding sits in the young sediments of the northernmost Great Valley. Considerably older metamorphic rocks, studded with granite upwellings (pink), take up the central and western part of the map area. The large area of purple is the cool stuff, punctuated with violet bodies of gabbro, another deep-seated iron-rich rock type.
As you take State Route 299 west from Redding, watch the roadside rocks change from the gravelly sedimentary rocks of the Great Valley to the easily weathered granite . ..
. . . and to the older metamorphics, shot through with veins during their recent uplift into today’s mountains.
The Klamaths were gold country like the Sierra, but some of the early prospectors found platinum instead, a metal that was worth little at the time. So keep your eyes open.
At Weaverville turn right on Route 3 up the Trinity River valley. The Trinity River has not been treated well, especially since hydraulic mining was allowed on it until the 1960s, and then it was dammed. (The Trinity River Restoration Program is starting to undo the damage.) This stretch of road is in the river’s headwaters, part of the Trinity Heritage National Scenic Byway, featuring views like this one of Billy’s Peak.
Its dark rock is part of the peridotite terrane. Keep an eye out on the roadcuts for the dark outcrops of peridotite. Much of it is studded with greenish-black crystals of pyroxene, and many parts are altered to beautiful serpentinite.
At 56.5 miles after the turnoff from 299 to 3, turn right on Stewart Springs Road, staying in the Trinity River Valley. This is a well-graded gravel road that goes over a pass at 6800 feet. If the weather is threatening or the season is iffy, check ahead or stick to Route 3. In this reach of the Trinity River, the streambed rocks are different from anything you’ll ever see. Pull over and study them. This is Forest Service land and you can take a stone or two. Also, look around at the thin forest that inhabits this nutrient-poor rock.
At the top of the pass, the road intersects the Pacific Crest Trail. Take a break and examine the peridotite boulders there. This one shows how invading hot water starts to convert the gray-green peridotite into serpentine minerals, including thin veins of fibrous chrysotile.
How do pieces of the mantle end up on the continents, anyway? The answer is that plate tectonics occasionally does that; the packages of rocks, of which peridotite is just the bottom layer, are called ophiolites. The one on this drive is the Trinity Ophiolite.
The long downhill drive that follows will deliver you to I-5 just north of the town of Weed. Well before that, though, you will be arrested by the spectacle of Mount Shasta.
That’s another story entirely.