You Just Can't Take Sonora Pass for Granite: Travels Around the Sierra Nevada High Country

Sonora Pass in the Sierra Nevada is a sort of forgotten step-sister of the more famous Tioga Pass in Yosemite National Park. A paved road, State Highway 108, crosses the crest of the Sierra about forty miles north of Tioga, cresting out at 9,624 feet, but it sees far less traffic. There are, for instance, no national parks, no major ski areas (Dodge Ridge is thirty miles to the west), and no major resorts, and snowdrifts keep the pass closed for much of the year. But Sonora Pass is an incredible place to visit if you ever get a chance, and it makes for a nice loop trip out of the Central Valley, including Tioga on the way back.

The road is steep and winding towards the top, but the views are extraordinary. We are in the headwaters of the Stanislaus River, and the region was heavily glaciated during the ice ages, making for spectacular alpine scenery. But one thing seems to stand out as one approaches the summit. You can't take this pass for granite. At least not all of it. This bad pun can be understood in two ways: there is volcanic rock at the top of the pass, not granite, and what "granite" there is along the lower reaches isn't actually granite.

First off, there are lots of fine exposures of granitic rock below the pass, and along the highway for a considerable distance downstream to the west. But it is not technically granite. If one has had a basic earth science course, a person may have learned that there is granite, diorite and gabbro as the plutonic igneous rocks (light, medium, and dark). But if one takes physical geology or an upper division petrology course, one finds there are other rocks intermediate between granite and diorite. The differences come from the proportions of potassium feldspar, plagioclase feldspar, and quartz. As such, true "granite" is actually somewhat rare in the Sierra Nevada, perhaps accounting for 10% of the total. The most common rock is granodiorite, a granite-looking rock rich in plagioclase, but lacking in potassium feldspar.

I'm the first to admit that the difference to a layperson may seem trivial, but recognizing these differences allows researchers to draw many conclusions about the circumstances and timing of the intrusions that formed the backbone of the Sierra Nevada. And some of the other
"granitics" are truly beautiful rocks. At Sonora Pass, the common granitic rock is called the granodiorite of Topaz Lake, and it is one of the younger intrusions in the Sierra Nevada, having cooled about 89-93 million years ago. It has massively large crystals of potassium feldspar, 2-3 inches long, embedded in a groundmass (matrix) of smaller crystals of quartz and plagioclase. Such rocks with two distinct grain sizes are described as porphyritic. Glaciers have smoothed off the rock in many places ('glacial polish'), almost providing the effect of a polished countertop.

The other way that Sonora Pass cannot be taken for granite is the fact that the rocks at the pass are actually volcanic in nature. Between 11 and 9 million years ago, this area was an extensive volcanic center broadly similar to the one that exists today at Lassen Volcanic National Park, with andesitic stratovolcanoes, lava domes, and a modest caldera complex.

The volcanoes are long extinct, and glacial erosion has greatly modified their original outline. The mountains are striking if for no other reason than that they are different from many of the light-colored peaks of granitic rocks seen elsewhere. In a sense, they almost seem somber and dark. John Muir would no doubt have seen their beauty, but probably wouldn't have been inspired to call them the "Range of Light".

If you ever want to brag about your hiking exploits, but just don't have the energy, you can go to Sonora Pass, and wander a few yards up the trail that crosses the pass going north and south, perpendicular to the highway. That way, when people ask what you did last summer, you can truthfully say something like "I spent a bit of time on the Pacific Crest Trail"!

If you long to spend time in the Sierra Nevada, and just can't fathom the thought of fighting the crowds at Yosemite Valley and other parts of the national park, give Sonora Pass a look. I'm pretty sure you won't be disappointed. If you need a guide, my colleague Noah Hughes and I edited a field trip guide for the region for a meeting of the National Association of Geoscience Teachers in 2012, and the book we published is for sale (details are here; all proceeds support the scholarship program of the NAGT-Far Western Section).

The Other California: Highway 108 and Donnell Vista - like Yosemite, only with Volcanoes!

Those of you who've followed my blog over the years may remember that I produced a blog series called "The Other California" which existed long before any Toyota commercials with a similar slogan. I come back to it time and again when I find (or rediscover) those places that don't always show up on the tourist postcards, but which have incredible geology and are scenic to boot.

Today we are exploring a few corners of the Upper Stanislaus River drainage in the country north of Yosemite National Park. People from outside the region can be forgiven if they think that the Sierra Nevada is just Yosemite Valley, Sequoia National Park, and Lake Tahoe. The range is 400 miles long, and pretty much all of it is spectacular, except Yosemite and Sequoia are just a little more so. I cannot doubt that if Yosemite Valley didn't exist, the upper Stanislaus would have been one of California's national parks. In some ways, Sonora Pass and the upper Stanislaus are even more interesting in the geologic sense. The reason? Volcanoes.

Donnell Vista Point on Highway 108 about 18 miles above Pinecrest Lake and Strawberry is the site of today's exploration. The parking lot and quarter-mile trail to the viewpoint were recently renovated with funds provided by the America Recovery and Reinvestment Act. The old worn-out trail was resurfaced, and a new ADA compliant trail winds along the western part of the slope, providing some new views to the west.

The first thing one notices from the overlook at Donnell Vista is the deep U-shaped canyon occupied by the lake behind Donnell Reservoir. During the Pleistocene Ice Ages, rivers of ice repeatedly scoured the gorge below, with the last glacial stage ending only about 12,000 years ago. The ice exposed the underlying granite, forming steep cliffs and numerous small rounded asymmetical domes called roches moutonnées. Some nice examples of glacial polish can be found in the region.

The ice in most places removed a cover of volcanic lava flows and any soils were stripped away as well. As a result, wide areas of granitic rock are exposed, and the forest grows only in fractures and joints where bits of soil can accumulate.

Glaciers also scour out basins where lakes can subsequently form, but the lake below the view point is clearly not natural. It is Donnell Reservoir, constructed in 1957 by the Oakdale-South San Joaquin Irrigation district for storage of water for agricultural irrigation and power generation. It stores a bit more than 60,000 acre-feet of water which today is only used to generate electrical power. Storage for irrigation now takes place downstream in the much larger (and more controversial) New Melones Reservoir.

The high peaks to the north and east of Donnell Vista preserve the evidence of the volcanic activity that took place here 10-12 million years ago. The few remnants not stripped away by the ice form the mesa-like peaks on the skyline. When the volcanoes were active, the region would have looked a great deal like the Mt. Lassen volcanic center, which remains active today.

The lava flows filled a fault basin near the present-day Sierra Crest in the vicinity of Sonora Pass, and at times spilled over into the adjacent river valleys. One lava flow traveled nearly sixty miles to Knights Ferry in the Sierra foothills above Oakdale. The landscape surrounding the lava flow eroded away during the uplift of the Sierra Nevada block, leaving the lava flow as a prominent ridge called an inverted stream.

The upper drainage and headwaters of the Stanislaus River up to and over Sonora Pass offers up some fascinating geology. I helped edit a field guide of the geology of the region for a meeting of the National Association of Geoscience Teachers in 2012. It's for sale through Sunbelt Publications and the proceeds support scholarships for geology majors in California, Nevada and Hawaii. Get more information about the book here.

PS: Ron Schott has a great gigapan shot of the view from Donnell Vista at  http://www.gigapan.com/gigapans/114032

The Other California: The Other Side of the Sierra, Down The West Walker River

Ask most people their image of the Sierra, and you will hear about Yosemite Valley, Sequoia, granite, and the Mother Lode. Ask them about crossing the Sierra, and they may mention Tioga Pass or Donner Summit. There is a lot more to the Sierra, and this week I am going to highlight some of those places. The "Other California" series is about these kinds of places, the lesser-known spots of geological interest.

There are a total of four paved crossing of the Sierra Nevada between Yosemite's Tioga Pass and Donner Summit. Sonora Pass is crossed by Highway 108 out of Sonora and Pinecrest, and reaches an elevation of 9,624 feet, making it the second highest crossing of the range, just short of Tioga's 9,950 feet. The road drops into the drainage of one of California's little treasures, the West Walker River.

The West Walker is one of the longest rivers in the eastern Sierra Nevada. Most east-side rivers drop steeply and within a few miles disappear into the desert of the Owens Valley or Mono Basin. Because of a structural glitch in the range, the Walker runs mostly north through a relatively unbroken stretch of mountainous terrain before flowing into the Antelope and Smith Valleys in Nevada, and finally ending at Walker Lake, a bit more than 100 miles downstream.

The upper reaches of the West Walker are a bit different than other parts of the Sierra Nevada. The mountains are not composed of granite, but instead are somber red and gray volcanic rocks extruded in Miocene time, around 14-6 million years ago. They are assigned to the Relief Peak formation and the Stanislaus Group. They were originally mapped in the 1950's (by my thesis advisor for one), and are the object of ongoing research today (see below for an example).

Near the summit of Sonora Pass there is an interesting outcrop where the volcanic rocks lie in direct contact with the Sierra granites. There were canyons here that were filled when the volcanoes began erupting. Considering that granite forms several miles deep in the crust, that surface between volcanic and granitic rock represents the disappearance by way of erosion of perhaps four miles of rock between the late Mesozoic around 80 million years ago and the Miocene, about 14 million years.

Evidence of glaciation abounds in the high country in and around Sonora Pass. The West Walker River had the longest glaciers of any east-side canyon, with the longest reaching around 20 miles downstream. The Leavitt Valley, below, is a beautiful glacial trough. The main West Walker has its headwaters several miles above the point where the Highway 108 winds down a steep canyon wall.

The smaller glacier coming from Sonora Pass could not carve as deep a valley as the main glacier, so it forms a hanging valley, with a pretty cascade called Leavitt Falls visible from a pullout. It's a good spot to stop and let your brakes cool off. As long as I mention brakes, the Sonora Pass highway is not for the faint of heart in its highest reaches. Most cars do just fine, but a big RV would be a nightmare, not just for the driver, but for everyone else who has to make their way around it. In other words, this is one of my favorite paved roads in the state!

Highway 108 passes the Marine Base at Pickle Meadows and ends at Highway 395. At this point the river has a distinct change of personality, and that will be the subject of my next post.

Busby, C.J., et al., 2008, The ancestral Cascades arc: Cenozoic evolution of the central Sierra Nevada (California) and the birth of the new plate boundary, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 331-378. (Forrest, I assume this is your father?)