A Day in the Field...Day Backwards On Fun Having, Part Three. Stanislaus Table Mountain from above

The conclusion of my backwards mini-series on the Stanislaus Table Mountain starts with an earlier trip that will serve to explain the origin of the backwards geology of our hiking destination from last weekend (parts one and two here and here). The picture above reveals our path from a slightly different point of view, mainly from several thousand feet above. Back in 2002, the father of one of my students called me early one morning and said "Let's fly my plane. Where do you want to go?" I immediately thought of Table Mountain, because it is best seen and appreciated from the air.

I've been saying "backwards" a lot in these posts, but the actual term we want to use here is "inverted", as in "inverted stream". Our hike was to the top of a lava flow that sits hundreds of feet above the surrounding landscape. On first glance, this may not seem to make much sense. The key to understanding this oddity is to realize that the lava flow did not start out on top of a ridge.

Around 6 to 20 million years ago, volcanic activity was producing ash and lava flows in the summit region of the growing Sierra Nevada in the vicinity of Sonora Pass and the Dardanelles. The earliest volcanic activity included violent explosions of light colored rhyolite ash that coated the Sierra foothills (the Valley Springs Formation), but later eruptions mixed with water and snow to form volcanic mudflows (called lahars). These mudflows and river deposits coated the foothills region hundreds of feet deep, a series of layers called the Mehrten Formation. In the picture above, the layers form contours on the sides of the hills. For those familiar with the Knights Ferry area, the Mehrten forms the cliffs of Lovers Leap.

In the midst of this activity, about 10 million years ago, a single flow of latite (a sort of orthoclase bearing andesite) traveled more than forty miles down the western slope of the rising mountain range. Having flowed far beyond any volcanic slopes, the lava followed a riverbed, an ancestral path of the Stanislaus River. The picture below reveals the winding path the lava followed (our hike was located right in the center of the photograph). The surrounding rock was easier to erode than the latite, so when the mountains continued to rise, the lava flow remained while the other rocks were carried away. The bottom of the river became the top of the ridge: a textbook example of an inverted stream.

Farther "downstream" the lava flow widened considerably, forming a more plateau-like landscape. The Stanislaus River ended up carving through and around the lava flows. Some of the steep gorges became idea sites for modern-day reservoirs, like New Melones and Tulloch, seen below. The Table Mountain lava flow disappears beneath the slopes around Knights Ferry.

I mentioned in the previous post looking over towards the Harvard Mine from the summit ridge of Table Mountain. The open-pit mine was active from 1986 to 1994, producing about 660,000 ounces of gold from about 17 million short tons of rock. The lake in the pit is about 300 feet deep. Just prior to closing down, the miners recovered a huge mass of crystallized gold, weighing more than 40 pounds. The gold is on display at the Ironstone Winery in Murphys.

It was a lot of rock that was removed and piled up to get at the gold. So not only is the local landscape inverted, it has also been turned inside out.

I hope you've enjoyed these vignettes of my pair of journeys to a backwards landscape! The flight was made possible by Ken Iwahashi of KKI Corporation in Modesto. I have a web page with additional pictures and descriptions of the flight at Geotripperimages.com.

For more on the volcanism of the Central Sierra Nevada, check out:

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.

A Day in the Field...Day Backwards On Fun Having, Part Two

Continuing our previous blog entry about exploring part of the California Mother Lode, we reach the summit of Table Mountain, near New Melones Reservoir. We had made our way across some grassy meadows and oak woodlands, and then climbed up a steep talus slope through brushy thickets to reach the edge of the summit. We walked into a strange forbidding landscape. The summit of Table Mountain at first looked barren and practically lifeless, but it took only a moment to realize it was full of life, especially on this particularly strange day of rain and storms.

I walked quickly over to the other side of the ridge and looked down on Highway 108 and the Jamestown area, including the mothballed Harvard Gold Mine. The ridge is long (miles long) and winding, and flat. But it is only a few hundred feet wide in most places. Boulders of lava were scattered all across the surface. The 'backwards' theme of these post has the most to do with the strange location of the lava flow at the top of the ridge. I'm going to fully explain that in the next post, but first I wanted to mention something outside my expertise: the biology...

As I wandered across the summit plateau, I realized that something was missing: fields of waving grass. It was striking. In the meadows below, exotic grasses were growing profusely everywhere. Up on top, grass was present, but as can be seen below, it is dominated by other plants; especially small flowers and mosses. Clearly the soils were very thin due to the nature of the lava, but these non-grass plant species were doing just fine, from the looks of things.

From what I could understand from some of the environmental studies of the volcanic tablelands, the exotic grasses have never really established themselves on the plateau the way they have in the pasture lands and meadows below. It's too tough of an environment. Apparently, most of the plants up here are natives.

It is a mostly dry and hot environment for much of the year, but for a few weeks or months, rain fills a number of low areas called vernal swales, which provide a unique growing environment for endemic species. Invasive exotic species cannot tolerate the extreme conditions. The vernal swales are broadly similar to the vernal pools found on the floor of the Central Valley (in the few undeveloped areas), but are more rocky with less clay. I saw quite a few unfamiliar flower species.

As I got closer (to the ground, that is), more and more species of plants appeared. I had to step carefully, ponds were everywhere! Most of the flowers were very small, reminding me of the toy flowers a child might use to decorate the outside of dollhouse. A garden of miniatures....

The colors were vivid, despite the grayness of the skies. Actually, as I looked around the skies were not just gray, but threatening. Rain was falling all around us, and a bitter cold wind was sweeping across the summit plateau. I wasn't quite ready to retreat just yet.

I got on my hands and knees and realized I was looking at an entire ecosystem on the surface of a single boulder (below). It was an incredibly colorful palette of living things, and a wonderful confluence of geology and biology.

I generally don't make a practice of quoting long passages of someone else's work if I can do a decent job of paraphrasing. but I'm not much of a biologist, and I have a feeling some of my biology-loving readers might want some of this info. It was buried deep in an environmental impact report by the Bureau of Reclamation about the Table Mountain flora and vernal swales (found here, starting at page 4-29):

"Vernal pools are an ephemeral wetland vegetative community with predominantly low-growing, ephemeral herbs. Germination and early growth occur in winter and early spring, often while plants are submerged, and pools dry out by summer. Flowering is often in bands at the margins of the pools. This community type occurs in shallow depressions, ranging from a few meters to tens of meters in diameter. Characteristic plant species found in vernal pools are Pacific foxtail (Alopecurus saccatus), common blennosperma (Blennosperma nanum), Cleveland’s shooting star (Dodecatheon clevelandii var. patulum), toothed downingia (Downingia cuspidata), spinysepaled button-celery (Eryngium spinosepalum), hedge-hyssop (Gratiola ebracteata), Fremont’s goldfields (Lasthenia fremontii), Douglas’ meadowfoam (Limnanthus douglasii var. rosea), white-headed navarretia (Navarretia leucocephala ssp. leucocephala), adobe popcorn flower(Plagiobothrys acanthocarpus), miniature popcorn flower (Plagiobothrys stipitatus var. micranthus), Sacramento pogogyne (Pogogyne zizyphoroides), Delta woolly marbles(Psilocarphus brivissimus var. multiflorus), greater duckmeat (Spirodela polyrrhiza), and Wildenov’s clover (Trifolium willdenovii) (Stone et al. 1993 in Reclamation 1995). Special status plant species that may grow in the planning area vernal pools include Sacramento orcutt grass (Orcuttia viscida), slender orcutt grass (Orcuttia tenuis), Bogg’s Lake hedge-hyssop(Gratiola hetersepala), and legenere (Legenere limosa).

Within the planning area, intermittently-formed pools appear after rainfall or snowmelt on top of Table Mountain between 1,200 feet in elevation in the south and 2,600 feet in the north. Although these pools share some of the characteristics of some vernal pools in the Central Valley, they are not true vernal pools in that they do not have a clay underlayer that prevents percolation. Instead, they form in swales in the rocky surface of Table Mountain. The soil is poorly drained and the parent material on Table Mountain is a Pliocene lava flow (andesite). Intermittent pools occur on Table Mountain in seasonally wet to saturated rocky meadows that have slight soil development (Evens et al. 2004). They are interspersed within the annual grassland (Reclamation 2006b). Intermittent pools at Table Mountain do not support the range of species found in vernal pools in the Central Valley, possibly due to differences in substrate(primarily shallow, rocky substrate versus clay substrate in valley vernal pools). Although vernal pool habitats are very delicate and easily disturbed in general, this is even more pronounced on Table Mountain where soils are poor, shallow, and loose.

To date, vernal pools have resisted invasion by exotic plant species, probably due to their ephemeral nature (USACE 1997). However, the scientific community is concerned that exotic plants may colonize vernal pool communities, possibly displacing the highly specialized native vernal pool species (USACE 1997). Despite these concerns, there is no supporting evidence that this change is occurring in vernal swales found on Table Mountain (USACE 1997).

So much of the time I am showing people a place that is new to them. It is always fun to show someone the treasures that exist practically in their own backyards. On this hike, I got that experience of seeing a place for the first time. I loved every moment!

In the final post of this mini-series, I will take a look at Table Mountain from a slightly different point of view...

A Day in the Field...Day Backwards On Fun Having, Part One

Another day in the field...today it was our Geology Club. It's not enough that I make my students work hard in the classroom and lab, and drag them on field trip courses. The club members raised the funds to go out on their own and see something new and different. Once again the students led the teacher; I even rode in the back of the van! Yup, backwards day it was, in many different senses.

Strange as it may seem, I had, after twenty-two years in this region, never been to the top of what may arguably be the most famous geologic feature in our immediate vicinity. I've flown over it (a near-future blogpost), driven beneath it countless times, driven over it through a couple of erosional breaches, but I have never stood on top of it. So today I got to check off one more item on my life list: climbing the Stanislaus Table Mountain.

Things were backwards in a number of ways. We found the sweet spot between two powerful storms that are battering California this weekend. Yesterday there was so much snow that Highway 80 had a 45-car chain reaction accident and got closed in both directions. Right now the winds outside are gusting to frightening levels. But for four hours this morning the rain in the foothills let up, and we had a pleasant hike, aside from a bitter cold wind at the top. But instead of a normally hot and arid landscape, we had a muddy trail, rivulets of water everywhere, and waterfalls where waterfalls pretty much never happen (see above).

The trail to the summit starts at the end of Shell Road off of Rawhide Flat Road near the gold rush town of Columbia. "Summit" is not the best term though because Table Mountain is not a single peak at all. It is a flat-topped ridge that winds for many miles through the Sierra foothills. It is composed of a basalt-like volcanic rock called latite, and the lava flow displays some interesting columnar jointing in places (below). The flow is 9 or 10 million years old. How did it get to the top of the ridge? That question will be dealt with at a later time.

The trail crosses gently sloping meadows and oak woodlands for a mile, and then turns and climbs steeply for a half mile to the summit ridge. The ecosystem changes drastically as the trail climbs the north-facing slope. The path is shaded by numerous trees and shrubs, with some unexpected species for a region that regularly reaches 100 degrees in the summertime.

It's a thicket, really, and I'm glad we had a trail to follow instead of bushwacking. The poison oak would have been a real problem! In the picture below one can see the edge of the barren summit ridge.

We passed a glen full of ferns opening up for the first time among the brown husks of last year's crop.

The slope we were climbing was composed of large fallen boulders from the lava flow, a feature called a talus slope. The openings between the boulders can extend hundreds of feet into the slope, and cool moist air will seep out of the slopes all year long. Mosses cover the boulders.

I hope my flower-loving friends will identify the flower for me...I haven't noticed this species before (mind you, I'm a geologist, and flowers are as much a mystery to me as minerals are to botanists). It was fairly common and obvious on the shaded slopes.

We were nearly to the top, and had a close up view of yet another series of small ephemeral waterfalls. A few more switchbacks and we would reach the summit, the "top" of Table Mountain. Our backwards story will continue in the next post!