Mickey’s Beach TRBTC Script
So normally when I think about an outdoor rock climbing crag, I have it in my mind that the routes and especially the walls and formations will be the same every time I go back. The geology and weathering of most landforms are processes that happen over millions of years, so I can count on rocks remaining relatively unchanged each time I return back aside from maybe a chipped hold or a slightly smoother edge.
Now this adage is true for a large number of rock climbing spots, but at Mickey’s Beach you can have different climbs on routes that didn’t previously exist or on rocks that weren’t previously present. Things change drastically out there not on the scale of millions of years, but rather on the tens of years. In fact you could actually have a completely different experience on completely different routes just based on the season or even the time of day that you show up. On this episode I explore how this is possible. Welcome back baby.
Hello and welcome back to the Rock Behind the Climb. I am your host “The Jazzhammer” Quinn Todzo, and my goodness it feels good to be back on the pod. As a quick note before I get into it, I am going to upload this script along with my reference photos onto the blog, in case you want to go back and reference something that I talk about. That will be complete with of course all the sources that I used to write this episode. The link to the blog as well as my social media and email can be found in the episode description, so check all of that out and say hello while you’re at it.
Alright, so this episode we are travelling just north across the golden gate bridge from San Francisco, California to Mickey’s Beach. Actually, Mickey’s Beach is only a few miles away from the site of another Rock Behind the Climb episode that I did on Mt. Tamalpais. For those who didn’t listen to that episode, Mt. Tam is this marquee vista point that gives a 360 degree view of San Francisco, Oakland, the bay, and the pacific ocean. Mickey’s Beach is a stretch of rocky coastline that is pretty much directly underneath Mt. Tam.
Located between the very popular Bay Area weekend getaway town of Stinson Beach and another popular destination Muir Beach, Mickey’s is this stretch of coastline dominated by shear menacing cliffs, lively tidepools, and humongous nearly 80 foot tall boulders that feature some of the bay areas hardest and most notorious sport climbing routes. Also there are many not so humongous boulders that provide a wide range of grades for ropeless bouldering.
However, I am going to be honest with you guys, I was not sure when I started working on this podcast last year if I would actually do an episode on Mickey’s beach. For starters, most of the coolest, most notorious climbs are ones that I don’t know if I’ll ever be able to do since they average like a 5.12 difficulty rating. Also, personally I have struggled with the bouldering there, which at first took away from my enthusiasm with the place. On top of all that, while really cool, the geology in the area is very confusing, and I doubted whether or not I would be able to do it any justice
But, I recently was made aware that the prophet of bay area climbing himself Jim Thornburg had put up new more moderately graded sport climbs in a new sub area called Kim’s Gym which he has included in his new guide book. In checking out and subsequently loving the new routes, I was able to give more appreciation to everything concerning the location, from the absolutely spectacular scenery, to the actual bouldering and roped climbing. Thus, I have accepted the challenge of trying to describe the geology of this place, because after learning some of the recent history of how this crag has evolved over time like I alluded to in the intro, I just knew that this place was worth discussing. So, with that being said let’s dive into what has created and caused all of these massive chunks of rock to land on this stretch of coastline.
To make things simple, I am going to break down how these giant boulders and rocks originally formed, then what then caused them to end up right on the shoreline, and finally how they continue to be shaped and altered to this day.
But first let’s talk a little more about the setting. So just above the boulders to the east on Mickey’s beach is this huge steep cliff that rises probably around 1500 feet up into the area known as the Marin headlands. It is important to keep in mind that these hills are really steep, especially close to the coast where these hills turn into a 200 foot shear drop into the ocean. To the west of course you have the vast Pacific ocean, but also there are these dark colored rocky protrusions sticking up out of the water just off the coast called sea stacks, typically slathered in bird poop. As a quick aside, the sea stacks are different from the giant boulders you find on the beach because the sea stacks are still technically part of the entire rock formation complex while the boulders that you climb on are detached from the ground at large. And then finally to the north, you can see the flat sand bar that is Stinson Beach in front of this narrow inlet called the Bolinas Lagoon.
Now if you have time, I really suggest you take a look at the
episode notes for the google earth screen shot and photo of what this lagoon
channel looks like just north of Stinson Beach, because it marks the exact
trace of the San Andreas fault. If you follow the trajectory of this fault line
from the photo that I posted, you can see that at mickey’s beach the fault line
is no more than 50 feet off shore, putting it right on the edge of the continent.
Furthermore, some of my friends asked me about why there are hot springs at
Mickey’s beach, and in short it is because there is a trench that more or less
leads down to the center of the earth right there, locally heating up the cold
ocean water.
Just like with most of the crags I’ve talked about in the bay area, the juxtaposition of this fault line has a lot to do with how the rocks were formed and in the case of Mickey’s Beach why they appear as giant boulders on the beach that are constantly in a state of flux.
So to reiterate, we’ve got the steep cliffs and hills to the east, ocean to the west, and a major tectonic fault running right by.
So how did these rocks form in the first place? I should mention that unlike most rock climbing crags that I am aware of, Mickey’s beach features a huge amalgamation of different rock types, rather than just one. In fact on the rock known as the Old Man Boulder you can start on a section of graywacke, pass over a section of shale, and top out on greenstone. On almost every boulder and sport area I’ve seen out there is some combination of different rock types, but the two most common that you’ll actually climb on are called greenstone and graywacke.
Photo of the Old Man Boulder taken from Mountain Project user "Luke Moore". The dark patches are sections of greenstone and the lighter patches ore the graywacke
The rock most people refer to as greenstone is actually a type of basalt, or lava rock, that formed underwater. It formed in an area where two oceanic tectonic plates were pulling apart from one another thus allowing for magma from the Earth’s mantle to seep up into the earth’s crust and slowly spill over onto the sea floor. Sometime’s you will hear this referred to as pillow basalt because when multiple layers of this basalt with varying viscosities spill over themselves, they form structures that kind of look like pillows. However, these pillow like structures are not always discernable at Mickey’s Beach, but if you are interested in what these look like you can check out the source titled Franciscan Complex Rocks, which goes into more depth about how all of these rocks formed
A better way of picking out the greenstone sections of rocks is to look more to the minerals which are fine grained and either very dark gray to black, green, or reddish orange. The black and green colors come from the heavy metal rich minerals that were quenched from the molten lava, and the orangish red color comes from those minerals oxidizing or rusting over time. I do want to mention that this rock does bear a lot of resemblance to Serpentonite which I talked about in the Mt. Tamalpais episode and is also found at Mickey’s Beach. It can actually be quite difficult to tell the two apart in a lot of situations, but just know that the very dark rock, with a hint of green, that potentially contains some bulbous looking pillow structures is most likely greenstone.
In terms of it’s formation, the one characteristic that really carries over into the aspect of the actual rock climbing is that the rock started out quite strong because this it is a bunch of minerals fused together. This initial formational rock strength then got put to the test when eventually the tectonic plate that the greenstone formed upon, called the Farralon plate, subducted underneath the North American plate about 100-200 million years ago. While a lot of this tectonic plate ended up below the earth’s crust never to be seen again, this mound of basalt that had originally formed at a spreading center scraped off of the Farallon plate and onto the North American plate. The act of this rock getting smushed up onto the North American plate is one of the reasons it ended up highly fractured and jointed. Then millions of years later when the Farallon plate got fully swallowed underneath North America and subsequently gave way to the transverse moving San Andreas Fault, the rocks continued to get fractured.
On the rock titled the Main Bouldering Rock (which from my perspective
is pretty much all greenstone), you can see many criss crossing cracks and
joints of various sizes creating all kinds of sharp holds. On these boulders
you can get a lot of variety with the hold sizes and route types because of how
random this jointing can be. This also plays into why this crag changes all the
time, but more on that in a bit.
First I wanted to get into the second major rock type that I noticed out there, which is called graywacke, which is a type of sandstone. This rock formed during the subduction and subsequent accretion of the Farallon plate onto the North American. From what I understand it is a result of basically all of these different rocks getting crunched together thus causing these huge submarine landslides of a bunch of different sediments comprised of the rocks getting accreted, making it this weird mix of sands, clays, and volcanic rock fragments. So, while a lot of sandstones are comprised of mostly subrounded quartz grains in a relatively narrow range in sand grain sizes, graywackes have a larger range of grain sizes and variety of grain types that are highly angular.
The large range in grain sizes makes the graywacke more compacted because it has smaller grains to fill in the gaps between larger ones, which makes it generally stronger and less permeable than other sandstones. Also, the higher angularity of the grains gives an increase to the friction of the rock. I particularly noticed this while climbing at Kim’s Gym which is entirely graywacke. I was astonished by how grippy and safe the rock felt because of this grain angularity, even with a heavy amount of jointing, caused by the same tectonics I talked about with the greenstone.
The one last thing I will note about Graywacke is that you will
find this surficial feature on it called tufoni. Tufoni are these small scoop
like structures embedded in the rock which make for nice finger pockets and holds.
Interestingly, no one actually has a definitive answer on how tufoni actually
forms on rocks, but you see it a lot on rocks right on the coast. I’m going to
leave the discussion of tufoni structures at that for now, but I may come back
to it in a later episode, because they are these very striking features and
certainly an integral part to many seaside climbing areas.
Close up of a section of graywacke with surficial tufoni structures
OK, so we now have the way that the rocks formed and ended up in a compressed pile on North America, but how does that translate into giant nearly 80 foot tall boulders on the beach below the cliff? Well the short answer is giant landslides, but why do these giant landslides occur?
For that, we start by turning to the west and the great pacific ocean, and the giant waves it tends to produce. Now the giant wave potential might not be evident on the day that you go out and climb, actually hopefully it isn’t evident when you go out and climb, but any surfer in the area will tell you about the massive swell potential in Northern California.
Now when those giant waves smash into the heavily fractured and therefore unstable hillside, it starts to deteriorate and break from the bottom up over time creating that shear cliff. When you make your way over to Kim’s Gym, which is actually a part of the cliff and not a boulder like most of the other Mickey’s Beach locations, you can see the giant fractuared cliff that looks like it could easily give way to rock fall with the force of a powerful wave.
On top of that, literally, the deterioration of the hillside at the bottom makes the upper hillside more unstable. This is because the lower portion of the hillside that used to exist is what was holding the upper hillside in place, thus creating a buttress. Take away that buttress, and boom now a much larger chunk of the hillside is susceptible to landslide.
Then you add in an event like a really rainy and stormy winter that allows for water to seep into the ground giving the hillside the final push to destabilize and slide catastrophically. Other factors that increase the likelihood of a landslide include places where a landslide has previously occurred because now there is a sheared slick surface for even more material to slide down.
And if you look at a birds eye view or google earth image of this
area, you can really see how this all happens. Attached in the episode notes is
a google earth image of the emperor boulder, which is this huge majestic oval
shaped boulder sitting right on the edge of the land. You can clearly see
though where about 1000 feet up from the water there is a landslide scarp and
subsequent flow path where presumably the emperor boulder got dislodged from
the bedrock and rolled down to its current resting place.
And its not like these landslides occurred super long ago. According to the users on the Mountain Project, some of the most classic boulders were created after a giant storm in 1986. And honestly it isn’t that surprising, because as humans we are actually speeding up this process, and I’m not just talking about climate change and its effects on coastal erosion. No I’m talking about the road right above the cliff, the notorious highway 1 which is cut right into the hillside. When you make a road cut like this and take out a chunk of the hillside, it has a similar effect in that it further destabilizes the hillside. I mean people always ask me why highway 1 is always experiencing landslides. Well the very nature of highway 1 being cut into the slope is in part what destabilizes the slope and causes the road to be shut down all the time.
Even after forming, getting shoved up onto north America, getting churned
by the San Andreas fault, and then falling or rolling down the hillside to the
foot of the ocean, the shaping of this rock doesn’t stop. You see, these forces
of tectonics and ocean waves continue to affect the rocks even after they reach
the oceanside. All of the joints and cracks that formed in the rocks allow for
big chunks of the rock to give way and break off. For instance there was once a
classic climb on the Main Rock called the Mickey’s Beach Crack, which was once featured
on the cover of the Rock and Ice magazine. That climb no longer exists because
the right lobe of the crack fell into the ocean after a huge storm in 1993, and
to this day you hear about smaller chunks of rock coming off altering the
routes further. I noticed back at Kim’s Gym on the first two 5.8 rated routes with
the way the joints were oriented in the graywacke that at some point a giant
shelf of rock towards the top is going to slide off at some point. While these rock
falls erase climbs that used to be there, they also give way to new routes for
people to rebolt and send.
Daylighting joints at Kim's Gym which will one day slide off
Furthermore, wave action also plays a role in changing the crags. For instance, in 2017 a huge wave came through and rocked the orange buddah boulder tilting it such that all of the bouldering routes on it completely changed because of the new orientation of the rock.
Now as a disclaimer I don’t mean to give any indication as to the stability of any of the routes on a given time. In my experience everything has felt fine on the days that I’ve been out. However, maybe don’t go out during the middle of a torrential downpour or huge ocean swell, if studying the history has told me anything.
So I’ve talked about how there are constant landslides causing these boulders to roll downhill, and how further weather and swell conditions can alter and shape these boulders further, but still there are more factors that can change your experience out there depending on when you go out.
First of all the tide and sand bar heights can fluctuate as much as like 5-8 feet meaning that parts of climbs might be accessible at a low tide or sandbar that aren’t at a higher tide or sandbar. Also the classic bay area fog can definitely play a role on especially the greenstone portions, from my observation, in making the holds more slippery thus changing how the routes climb.
All of this is to say that with such a dynamic environment, you are bound to have a different experience every time you head out there, because you never know what things are going to look like. One thing is for certain though, with the epic backdrop of the pacific ocean, the liveliness of the tide pools, and some of the craziest geology that the world has to offer, your trip to Mickey’s Beach is bound to be epic.
Thank you so much for tuning into this episode of The Rock Behind
the Climb. If you have a chance, I highly suggest you check out the
accompanying blog post that goes along with this episode which is complete with
the episode script, photos, and sources. Also always feel free to reach out to
me via email, Instagram, or mountain project and let me know what you think.
With that, happy sending and I’ll catch you all on the next one. Jazzhammer
out.
https://www.youtube.com/watch?v=XmV67v8TP_c - Video on some of the geologic aspects of the area
https://pubs.usgs.gov/wsp/0338/report.pdf - hot springs
https://pubs.usgs.gov/bul/b2188%202/b2188ch3.pdf - franciscan complex rocks (referenced in episode)
https://www.nps.gov/goga/learn/education/pillow-basalt-faq.htm - greenstone
https://petrowiki.spe.org/Rock_type_influence_on_permeability - graywacke permeability
https://www.britannica.com/science/sedimentary-rock/Classification-of-sandstones - types of sandstones
https://www.carleton.edu/departments/GEOL/Resources/comps/CompsPDFfiles/2007/Schnur2007.pdf - pillow basalt formation
https://www.nps.gov/goga/learn/education/graywacke-sandstone-faq.htm - graywacke deposition
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/tect.20078 - more in depth on graywacke
https://www.colby.edu/geology/GE254/Rodriguez-Navarro.pdf - tufoni weathering
http://archives.njit.edu/
