Episode Link:
Episode Transcript:
I want to climb
Somewhere Devine
I’ve been I miner for a crag of gold
Tectonic compressions
And volcanic slime
Keep me climbing at
A crag of golddddd
And im getting bold
Hello and welcome back to the Rock Behind the Climb, the podcast about the geology of different rock climbing spots. I am your host Quinn “The Jazzhammer” Todzo bringing some rare auriferous luster back to your podcast feed. This episode I am taking you to gold country in the Sierra foothills of California, to discuss the formation of giant stone columns, perfectly vertical cracks, and the great wall of California. I am speaking of course about The Grotto in Table Mountain.
I have wanted to cover this spot for a long time. Just looking at photos of these 100 foot colossal stone columns separated by perfectly vertical cracks that tower over this subterranean pit will give you a sense of FOMO. And then, you take a step back and look at the formation at large: this giant long and skinny plateau of rock that snakes around the area and looks like the great wall of china, and you can’t help but wonder what the heck created it.
Usually when I go out rock climbing, I have to be the one that brings up the geologic significance of the area to the group, but this time I had people asking me about the rock formation literally as soon as we arrived so lets get into it shall we.
Located in the foothills of the Sierra Nevada right near Sonora, California “The Grotto” is a climbing area on Table Mountain, the great wall of china looking thing I mentioned earlier. Honestly I think calling it a mountain is really a misnomer because rather than a defined pinnacle like most other mountains it has a flat ridge that is about as wide as a football field and snakes around the area for a length of nearly 15 miles. I guess if they were trying to be technical the namers could have called it Table Inverted Valley (For reasons that I will get into), but I guess that just doesn’t roll off the tongue as well.
Table Mountain is well known for a number of different climbing crags, but I am going to focus this episode on the specific area known as “The Grotto”, which exhibits the marquee stone columns.
So we’ve got this giant 15 mile long great wall of china looking feature held up by giant vertical columns that seem as though they could be supporting the parthenon. So what in the heck is going on here, and how on earth did it all happen naturally.
Table Mountain, Sonora on the way to the Grotto (left), Aerial View (right). Aerial view taken by Gary Hayes by way of source 2.
Table Mountain was formed because a nearby volcano erupted 10 million years ago and filled an existing stream channel with a ton of lava. Then, over time the banks of that river channel eroded, exposing the much more weather resistant intact lava flow that had kept the shape of the old river. This is why Table Mountain has a sort of serpentine shape, because it took the shape of the old river bed. It is also why it is flat on top, because it was basically like pouring a liquid into a fixed container that eventually hardened over. Geologists call this an inverted river or inverted valley because the topography of the river channel is actually higher than that of the surrounding area.
As an aside, miners from the California Gold Rush recognized that Table Mountain covered what could possibly be a gold bearing stream bed, and tried to mine it. Looking through a database of old mining claims, it looks like there was one practically right next to The Grotto actually, but I didn’t see any signs of it while I was out there. I guess there wasn’t a lot of gold found underneath Table Mountain because there were never any major claims. Those silly miners were only interested in the supposed deposits underneath? I mean c’mon everyone knows the real gold is in the incredible crack climbs right above
But I digress. Getting back on track I should also mention that the lava rock that cooled to create table mountain is technically called Latite, but for the purposes of this podcast I might refer to it as basalt which is just a lava rock with less silica and more metallic composition than latite. Basalt, however, is much more readily found and studied so a lot of my sources are specific to basalts, but the same principles behind the creation of the various features apply to both rock types. So I just ask all of you out there when you head to the grotto with your friends and someone mentions how cool the basalt columns are, you don’t have to counter with a snarky “well actually rock is really Latite”, because in my opinion, close enough.
Ok, back to the grotto. To remind you we left off just when the hot molten lava filled the stream bed, so lets get into how this eventually turned into giant stone columns.
Like I mentioned at the beginning of the episode, the Grotto is vertical crack heaven because the columns are separated by cracks of varying widths giving you a ton of options for different types of crack climbing. The columns themselves are probably about 5 feet wide and have their own climbs on them as well. There is this hellish 5.10d rated climb called AC Devil Dog where you basically climb up the face of one of these outcropping columns by just hugging the damn thing since there aren’t many actual holds. My wingspan proved to just be enough to get me to the top, while hang dogging most at most of the clips.
Most of the climbs though involve jamming your hands and feet into cracks that range from hand size to finger size in between the columns. So how did these columns and cracks form?
Well when lava comes out of the ground, it is a mostly homogenous molten hot mass that flows like a liquid, hence why it took the shape of the river. When it cools, similar to freezing water, it shrinks because the once excited, heated up molecules are now packing more tightly. This rapid cooling and shrinking causes cracks in the rock. Typically these cracks break in ways that create hexagonal shapes. This is because in an ideal world when you have a completely homogeneous mixture cooling at the same rate, it will try to create the least amount of cracks it possibly has to to relieve the internal cooling stresses. So, basically what you want is the smallest perimeter to surface area ratio which is how you get the hexagonal shapes.
There’s a bunch of videos and stuff on the internet that talk about how much perfect hexagons appear in nature like in honeycombs or snowflakes apparently. They are just the most efficient shape in covering an 2D plane with the minimum amount of perimeter, or in the case of stone columns: the minimum amount of cracking.
The way these 2D hexagons turn into 3D vertical columns (and in turn create vertical cracks) is because the rate of cooling of the top of the lava section cools much faster than the bottom. Since the rate of cooling varies linearly with depth, the cracking that was initiated at the top propogates vertically downwards.
This is how we get the columns we know and love, because the bottom of the column of rock cools slower than the top, so the perturbance that was initiated at the top will just continue downwards rather than start anew. As with most things in life, these cracks take the path of least resistance, which happens to create nice vertical columns.
But wait!!! Something about all of this doesn’t add up. If you have been to the Grotto you know there are at least 3 things wrong with the idealistic hexagonal column picture I just painted. First of all, none of the columns appear to be hexagonal. I mean there is no way I would be able to grip the corners of AC Devil Dog if they were flaring at 120 degrees. This particular column is square, but I saw others that even looked triangular. Most notably none of them seemed perfectly hexagonal.
Secondly, those vertical cracks are not the only cracks in the columns. There are huge longitudinal cracks that cut across the columns sometimes causing the columns to break off. On climbs like the 5.11 rated Snake Bite, you actually start on one crack system and then you get to a point where one of the columns has broken off, and have to maneuver yourself onto another crack system while also getting around a small roof.
Finally, and most notably, the climbs don’t actually top out at the top of of the columns. There is actually at least 50 more feet of volcanic Latite that looks a lot more chaotic than the orderly vertical columns before reaching the top of the mountain. In fact, there are actually a bunch of very difficult extensions on the column climbs that go up this next section of rock.
So what’s going on here? Well lets break it down (pun intended) piece by piece. Ok so first of all, while hexagons are the ideal best shape, if the lava isn’t perfectly homogeneous in its composition, you start to get cracks that aren’t at the perfect 120 degrees to each other. It really just takes one crack that decides to protrude at a 90 degree angle instead of 120 and boom, your whole system of perfect hexagons is ruined. Take that youtube videos on hexagons in nature
Secondly, what are the other cracks in the columns that cause these things to break? Well those longitudinal cracks, in my observation, are most likely caused by spheroidal weathering. I’m not going to get into it too much in this episode since a) it opens up a whole new can of worms and b) I talked about it extensively in my Joshua Tree episode. Basically, it’s a type of chemical weathering where some of the crystals that make up the rock dissolve over time which cause cracks to expand and edges to round or break off. Some of the columns look like they have bites taken out at the bottom which is most likely from chemical weathering creating a horizontal crack that caused the bottom portion of that column to come tumbling down.
Lastly and most interestingly, lets talk about what is going on with the remaining rock above the columns. Rather than have perfect vertical cracks, this above section is a slightly overhanging with joints and fissures that propagate at random. If I didn’t know any better I would think that this was a different rock type or at least part of some different event. That’s how starkly different the upper section appears.
However, this is actually pretty common with cliffs that exhibit columnar jointing. In fact, geologists have broken down columnar jointing systems into different zones. The zones with the columns are called the colonnade and the ones with the irregular cracks and such are called the entablature, and it almost always happens where the entablature sits on top of the colonnade.
The reason the entablature exists is because in order to have those perfectly vertical cracks, you need every part of the rock horizontally to cool at the same rate and then cool slower and slower as you go further down. If for some reason the cooling process is disturbed like a particularly long and penetrative rainfall event, the isotherms (or regions with similar cooling rates) can be disturbed causing the cracking to happen haphazardly. This is more likely to be the case towards the top of a cooling lava rock section where the cooling rock is most disturbed by the elements.
So to recap: we have giant columns because of regular cooling of a homogeneous lava. However, since in reality the rock doesn’t cool regularly and isn’t perfectly homogenous, it didn’t result in perfect hexagonal columns from the top down.
But hey, if we wanted things to be perfect all the time we probably wouldn’t make it as climbers, or geologists, or enthusiasts (big shout out to the climbing and geology enthusiasts that listen to the podcast who don’t actually climb or go to these places that I talk about).
OK to finish the episode, I want to circle back to the beginning and talk about where the lava that filled the stream bed even came from, because as you will see, it is very closely related to a bunch of other rock climbing spots in California.
The lava actually wasn’t from a volcano like we normally think of them. It was from an ancient fault line pulling apart and letting magma just kind of spew outwards.
Essentially the land mass of California had been in a state of compression which created the volcanoes and volcanic fields of climbing areas such as Mt. St. Helena, Pinnacles, and Bishop Peak down in San Luis Obispo.
However, just like when you squeeze a spring and let go, after being squished in compression California went into a state of stress relaxation meaning that the land literally spread back out. This stress relaxation opened up an old fault line in the area around Table Mountain causing lava to spew out and fill the ancient stream bed, thus creating Table Mountain.
I think the interesting takeaway here though is that all these crags that I just mentioned including the grotto are related by being formed from a similar event. Granted Table Mountain was more of a reaction to the events that created those other 3 places I listed. I find it fascinating because none of these climbing locations look or climb like each other, but because volcanoes can have many different ensuing rock types, these similarly created climbing crags take wildly different forms.
You could honestly put together a pretty sweet road trip down the state of California and hit all of the coastal volcanic climbing crags, and come out a very well rounded climber. Like you could practice your crack climbing at the grotto, get ripped on the pumpy climbs at mt st helena, practice patience and slow careful climbing in pinnacles, and then finish with some sporty long link ups down at Bishop Peak. And all of these climbs would be connected by a common geologic event, which I dont know for me is freaking cool.
With that thanks so much to all of you listening! Feel free to reach out to me with any comments, questions, or critiques through any of the provided avenues in the episode description. Also, if you follow the link to my blog you can find the full episode transcript with photos. Before I close the episode I want to leave you with this quote from 1882 in a journal entry titled “The Ancient Man of Calaveras” W.O Ayers that talked about the gold mining at Table Mountain.
He writes: The question occurs to us: How came Table mountain to exist? That basalt, when it was erupted, was fluid like other lava. How could it be piled up so thick and so abrupt (for its sides are often perpendicular) on that high mountain ridge, and remain there? Why did it not spread itself out laterally and cover the plain? But one answer to these questions can be given: There was no plan.
Thanks again for listening, and I’ll catch you guys eventually on the next one. Jazzhammer out.
Sources:
https://www.tandfonline.com/doi/full/10.1080/00206810902978265
https://www.jstor.org/stable/pdf/2449590.pdf - ancient man of the calaveras
http://hayesg.faculty.mjc.edu/Gold_Rush.html - gold rush
https://www.kqed.org/science/8032/how-californias-warping-microplate-makes-its-faults-creep - Sierra Microplate and creep
https://touchstoneclimbing.com/trip-report-table-mountain/ - Table mountain climbing history
https://ohp.parks.ca.gov/?page_id=21534 - gold mining around table mountain
https://mrdata.usgs.gov/mrds/show-mrds.php?dep_id=10214295 - punch bowl gold mine that was right next to the grotto trying to mine underneath the mountain
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2007JB005018
https://pages.mtu.edu/~raman/SilverI/IRKeweenawRift/Columnar_Joints/Columnar_Joints.html - colonnade vs entablature
