Miwok 100K: A geological tour

by Melanie Michalak

The 17th running of the Miwok 100k was characterized by a course change, which turned out to make the already difficult race even more challenging. Throughout the day, it was clear that time goals were to be reevaluated, as those aid stations took longer and longer to get to, even for the front-of-the-packers. After Miwok, the Facebook page, twitter feeds, blogs and online articles lit up with adjectives describing the new course, “monster,” “brutal,” “carnage,” “25% DNF rate.”

The added difficultly wasn’t solely the extra couple thousand feet of climbing, it was how and where that extra elevation gain was distributed. Further, the type of terrain the extra climbing was done on played a role in slower travel times.

If you ran the 2012 Miwok 100k, I don’t need to tell you this. This report is not a typical race report but a tour of the fantastic geologic story that underlies how, why and when the incredible topographical relief the 2012 Miwok runners traversed over formed.

We began the day under a full moon, in Stinson Beach, CA. Stinson Beach is located on the largest fault zone in the Western United States, the San Andreas Fault System (SAF). It is a strike-slip (or transform) boundary between two tectonic plates- the North American plate and the Pacific plate. This means that the plates are sliding past each other, where the North American plate is moving south relative to the Pacific plate.

The dashed yellow line denotes the rough plate boundary between the North American and Pacific plates (which is the San Andreas fault system). The red squiggly line is near and dear to our hearts- the 2012 Miwok course! Source of .kmz of course: Dalius Kumpa.

The SAF continues north toward Mendocino County, and south toward Baja Mexico. Sometimes the SAF curves a little bit, and a certain type of curve in the fault, called a restraining bend, can generate some very high topography. The San Gabriel Mountains of Southern California are the best example of this, as many of the Southern California runners know – these mountains are host to some seriously steep and epic trails. Another example is the Santa Cruz Mountains in the San Francisco Bay Area. Both of these mountain ranges were produced by restraining bends in the SAF system.
Here you can see the GIANT bend in the SAF. There is a smaller bend by the Monterey Bay (has produced the Santa Cruz Mountains), but in the headlands area, the SAF is quite straight.

While we climbed out of Stinson Beach up Willow Trail to Bolinas Ridge on race day, I had been thinking that the Bolinas ridge must have been produced in a similar way. However, as you can see from the map, the SAF is quite straight here, and in fact, there is no restraining bend responsible for the high relief of the ridge.

As it turns out, the geologic community actually doesn’t quite know what has produced the high relief of the Bolinas ridge and adjacent Mt. Tam, but the best theory is that there is actually another type of fault, called a blind thrust fault, that has generated the local mountains (Kirby et al., 2007; Journal of Geophysical Research). A blind thrust is “blind” because the trace of the fault is not visible on the Earth’s surface. This kind of fault is especially dangerous to cities, because geologists often have no idea that they are there, but they can be capable of nasty earthquakes. The Northridge quake of 1994, for example, was caused by a blind thrust fault.

Illustration showing a schematic blind thrust fault. The fault trace does not outcrop at the surface. Source: USGS

As we directly experienced, that first climb was a heck of a lot harder than the second climb out of the Randall Trail aid station, agreed? And that is because the blind thrust is proposed to be moving south, generating higher mountains in the southern portion, thus the high topography fades toward the north.

After descending back into Stinson Beach, we climbed southeast, up the Dipsea trail and crossed a minor fault oriented east-west, which separates the Franciscan sedimentary rocks from the Franciscan mélange. Franciscan is just a name given to the whole suite of rocks that are of a similar geologic age and happen to form the majority of the Marin Headlands area, and almost all of the Miwok 100k course traverses over a variety of Franciscan rocks, often called a “complex”. Anyway, here is where it gets interesting. The Franciscan complex is a mad jumble of rocks that started forming about 200 million years ago, when the tectonics of the Bay Area looked quite different. At that time the location where the Bay Area is now was the ocean floor, and there was a tectonic plate called the Farallon plate that was subducting (or being pushed beneath) the North American plate.

Cross sectional view within the Earth of the Farallon plate subducting beneath the North American Plate. This geologic setting continued from several hundred million years ago to about 15 million years ago, when the plate boundary changed and the San Andreas strike-slip boundary developed. Source:National Park Service

Subduction is what is happening beneath the states of Washington and Oregon today. California was a subduction zone until the Farallon plate “ran out” underneath North America and the Pacific plate came along, moving north relative to the North American continent, and now California is a strike-slip boundary. While the San Andreas strike-slip system is rather young (~10-20 million years old), the rocks that compose the Marin Headlands are actually quite old (200-50 million years old), and formed several kilometers deep in the Earth’s crust. You can see in the image above, the “Franciscan complex” is a triangle-shaped chunk of crust – think of it as a garbage can for rocks. Rocks get so jumbled, squeezed, folded, metamorphosed and faulted in this triangle that it is difficult to identify them. So geologists call this “mélange”. Melange consists of basalt, blueschist, greenschist, sandstone chunks, chert chunks and serpentine (basically a potpourri of rocks). From the climb out of Stinson to Tennessee Valley, we ran over mélange. You may have noticed chunks of big gray rocks sitting out on the landscape, and these are big boulders of either blueschist or basalt (sometimes I take time out of my runs to confirm the rock type, but the cut-off times were scary enough to quench my curiosity).

Photo credit: Glenn Tachiyama. Giant block of rock in melange. Runners for scale.

How did these rocks that were once on the ocean floor end up making the rolling hills that we ran on for 9,12,15,17 hours? Well, the simple answer is: fault activity. The whole San Andreas system and other minor local faults have worked together to uplift and expose all of these rocks that were once on the bottom of the ocean. Here is the geologic map of the whole course, where each color represents a different rock type outcropping on the surface, and the thick black lines are faults.

Geologic map (from USGS) overlay on GoogleEarth. As discussed previously, dark green unit is Franciscan melange. Light green unit (outcropping in the Bolinas ridge) is sandstone. Light yellow units are stream deposits. Red units are chert. Purple units are volcanic rocks, including basalt.

Continuing along the course, shown above in red, we had traversed the out-and-back along Bolinas Ridge (light green unit), climbed over mélange (dark green unit), and then the course took us down into Muir Beach. You may have been delighted to descend down into Muir Valley and actually run along flats for a while. I know I was! A few hundred thousand years of water pouring off the hillslopes into tributaries and into a main trunk stream have carved out this flat portion. Luckily, the trail goes parallel to the stream and we got to enjoy this beautiful (and flat!) section. On the geologic map you can see the yellow color that follows the course. This light yellow represents a veneer of sediment which is the eroded sand and material from the surrounding hills, deposited in this valley.

After leaving Muir Beach and saying “goodbye” to the glorious flat part, we again headed up into rolling hills, traversing over more mélange. Descending  into Tennessee Valley, we left the mélange and after a quick stop to eat, drink and perhaps receive a few words of encouragement from our crew, we did the ~12-mile loop that many people were looking forward to, perhaps because of the glorious view of the Golden Gate Bridge and the city of San Francisco awaiting us, glittery in the distance. I was giddy with excitement because I had been looking forward all day to running along the familiar, and quite famous (among the geology crowd) Franciscan chert. This is one of the largest sequences of marine chert in the world, at 80-100m thick, and displays a beautiful reddish-orange color and many folded sections.

Folded, reddish radiolarian chert. Photo taken along Conzelman road.

Chert forms on the ocean floor, in a warm, shallow ocean environment, where there are little organisms called diatoms and radiolarians living in the water. These organisms have a glassy skeleton made of silicon oxide. When they die, their skeletons sink to the bottom of the ocean, and tens of millions of years of uninterrupted radiolarian graveyards produce this fantastic rock – chert. Chert is extremely resistant to erosion, and will break up into chunks, but does not like to weather or erode, which prevents formation of a nice soil cover like other, weaker rocks. You may have noticed when we climbed out of Rodeo Beach how crappy the trail was; broken debris of chert, instead of a nice soft trail like other parts of the course. This is because the chert is simply too strong to be physically broken down and chemically altered into dirt.

On left: zoomed in image of chert. On right: images of microscopic diatoms and radiolarian skeletons that make up the rock. Source: National Park Service.

This climb was after the windy jaunt across the sand of Rodeo Beach. Notice how grayish and coarse the sand was? This is because unlike many beaches in California, which are white sand (derived from sandstone-type rocks that characterize much of California’s coast), Rodeo Beach is perched on and below massive sequences of chert (red and sometimes green), basalt (grayish) and blueschist (blueish), which gives the beach its coarse, grayish sand. If you spend sometime walking along the beach you’ll also notice that closer to the tide-line are colorful pebbles – black, red, and green mostly, and these are smoothly polished (by the water) chunks of the quite rare and glorious rocks surrounding Rodeo Beach. Stinson Beach on the other hand, is a white-sand beach, which is because the surrounding rocks from which the sand is derived are largely sandstone.

On the geologic map you will notice the rows of red units cut up by black lines – those black lines are thrust faults that serve to fold and fault and jumble the rocks further. I was both sad and happy to leave one of my favorite geologic places in the area, but we charged on, many of us now with a pacer at our side, back into the Franciscan mélange and down into Muir Beach, visited the flat section again (yee-haw!), climbed back up some more mélange and finally, finally, descended the stairs into Stinson Beach.

We traversed old rocks that were formed underwater, in a subduction zone, that would have never seen the light of day if it weren’t for the San Andreas Fault system that came through and uplifted and exposed them, for us to run on, and for all to enjoy.

I had a great time running the Miwok 2012. I couldn’t have done it without my Santa Cruz Track Club teammates, parents, friends and of course – the inspiring terrain of the Miwok (I love rocks!)

Shout-outs to my Santa Cruz Track Club teammates. My boyfriend Chris Wehan had a phenomenal race, placing fourth overall. He is virtually an unknown runner in the whole “scene” at this time, but after 2012 results of seventh, 10th and fourth at WTC, AR50 and Miwok 100K, respectively, that will change. He not only has had success at 50k distances but has clearly demonstrated talent at the longer distances. Watch out, fast people. Mike Wright had a great race in 13:29. He is one of the toughest and most consistent runners I know, and has taught me how to properly refuel after runs (beer). My running buddy Jon Beard ran with a broken hand and in desperate need of stitches from two nasty falls early in the day. Flirting with the cut-off time at mile 38 he pushed on hard through the Rodeo Valley loop, and made it to mile 50 under the cut-off time. Then he went to the ER, came back that night with a cast, and in the morning was making plans for next year. In ultraracing, you can’t control much – except your attitude.

I will be back, Miwok. Maybe next time I will pace, or crew, or simply bring out my hand lens and look at rocks and cheer all day. The Headlands are a special place and I enjoy every race I do up there, but Miwok is a special beast.

If there are any questions or corrections please email me at melanie(at)ucsc.edu.