The Sierra Beyond Yosemite: Donnell Vista and Sonora Pass

It's gloomy and foggy, and I haven't seen the sun for days. It's a few more weeks before field season (Death Valley!), but I can't help exploring the sunnier places from warmer times. I'm looking back at the pics from our fall semester where we explored a lot of places in the Sierra Nevada that aren't Yosemite.

That's the thing. Say "Sierra Nevada", and a lot of people will immediately think of the beautiful valley of the Ah-wah-nee, John Muir's favorite place on the planet, and in many ways mine as well. But the floor of Yosemite Valley is about 7 square miles. The national park covers 1,190 square miles (3,081 square kilometers). But the Sierra Nevada? It covers 39,612 sq miles (102,594 km²).  You could hide more than 30 Yosemite parks in the rest of the range. It is in fact the largest single range in the lower 48 states (large mountain systems like the Rockies and Appalachians are made up of numerous smaller sub-ranges).

So we are off onto a short exploration of some of the wonderful corners of the Sierra Nevada that aren't Yosemite Valley. We are following a week's worth of our trips last fall that took us over the range at Sonora Pass and down the east side of the range as far as Lone Pine and Mt. Whitney. We'll also explore the other national parks of the range, Sequoia and Kings Canyon, which we visited on a second trip.

We began our journey in some serious smoke from a series of fires burning through the western slopes of the Sierra Nevada. The drought and the fires have been catastrophic. We made a stop at the Twain Harte Lake exfoliation site. I wrote about it back then, and it was picked up on Reddit and IFLS, links that led to the post being the most read ever on Geotripper (12,400 hits and counting). We emerged from the smoke and climbed the upper reaches of the Stanislaus River, approaching Sonora Pass, which after Tioga is the highest paved highway over the Sierra Nevada at 9,624 feet (2,933 meters). Tioga Pass in Yosemite is 9,943 ft. (3,031 m.).

In a series of ice ages, glaciers covered about 30% of the range, reaching as low as 3,000 feet or so in some of the deeper canyons. Yosemite is simply the most famous of the glacially carved gorges, and many others are of incredible and spectacular beauty. This was not always fully appreciated, and some of these wonderful wild canyons were dammed for irrigation storage and domestic use. Hetch Hetchy is the most familiar, but the canyon below Donnell Vista on Highway 108 has also been inundated. Still, the glacial heritage of Middle Fork of the Stanislaus is evident from the viewpoint. The steep canyon walls of granitic rock and the overall U-shape of the valleys are the result first of ancient river erosion and then modification by thick rivers of ice.

If you look at the second picture above, you can see some unusual looking mountain peaks. Their blocky flat aspect indicates they are composed of something different than the "expected" granitic rock. They are the remains of lava flows, ash flows and volcanic cones that once covered this part of the Sierra Nevada. Indeed, until 9 or 10 million years ago, the Sierra looked far more like today's Cascades Range than the lofty glacial peaks we see today. There were a number of snow-covered stratovolcanoes, but much of the remainder of the range was composed of lower hills. The upper reaches of Highway 108 where it crosses Sonora Pass cut through some of the volcanic rocks.

We stopped a mile or two short of the pass to get a detailed look at the granitic rocks. Depending on the relative proportions of plagioclase and orthoclase feldspar and quartz, rocks may be identified as granite, granodiorite, tonalite, diorite, or monzonite. The rock exposed just below the pass is called the granodiorite of Topaz Lake, dated about 89 to 83 million years ago, during the Cretaceous era. It was intruded in the deep crust about 4 or 5 miles down where it cooled slowly, forming visible crystals of feldspar, quartz and dark minerals like biotite mica and hornblende.

Glaciers scoured the surface of the granodiorite, polishing it and providing a nice view of the structure of the rocks. Some of the orthoclase (potassium feldspar) has formed huge blocky crystals easily visible in the shot below. Even better, during the intrusion process, blocks of the surrounding rock broke off and sank into the magma. Composed of minerals that had higher melting points, it didn't melt, but instead persisted as an alien mass in the granitic rock. Such inclusions are called xenoliths. They provide a peek at what existed here before the intrusion of the magmas.

We drove over Sonora Pass and headed into the barren lands beyond. Our destination was the site of a gold rush, but not the one that Californians are familiar with.

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