Calling all rock and geology nerds: What dis? Is it quartzite?

I'm guessing quartzite. Reasons:  

Found a geological survey of the area, it claims the rock in this area is metamorphic. Rock is hard as shit and sharp as shit (I thought I had learned how to protect ropes sufficiently from granite, but I am now down two ropes, so I did not protect my ropes sufficiently for whatever this is-- was an expensive lesson). 

A ridge from this page which is known to be quartzite:

https://geology.com/rocks/quartzite.shtml

Looks extremely similar to these. It also states that quartzite often makes up large ridges. This is a long, discontinuous ridge of exposed rock which is all of apparently similar rock composition. 

There are some extremely scooped sections of the rock that look more like features found in softer rock, but from that same geology report I read that this rock is uncommonly old, and that unlike what is apparently the norm, the higher up you go the older the rock is here, and this rock I'm describing is the top of a ridge. This is an extremely geologically diverse part of California and isn't so far from Columbia where limestone and marble combine together on the same boulder, so is it instead possible that some areas have not metamorphized into quartzite? Or are there different hardness of quartzite that are possible based on differences in the metamorphosis process? Or could it just be that there are these large dishes and swooping features that are more commonly found on softer stone but exist in extremely hard quartzite simply because it's been exposed to eroding forces for millions of years? 

I don't have any photos of these features but I'm going out there tomorrow and could take some if people are curious.

I've been learning a little geology but I don't know enough to know if I can say with certainty that this is quartzite or if there are other types of rock that are likely candidates also, and I'd be extremely interested in anyone could provide some insight on the weirder features that seem like features more likely to be seen in sandstone than hard rock.

Ryan Never climbs wrote:

is that near san antonio creek falls?

yes. Do you know anything about the geology of the area? 

I tried getting ahold of the local gem and mineral society but apparently they didn't find my inquiry of interest. Have been considering just showing up to one of their meetings with a rock or two, not sure if that would be appreciated. 

The geology around Arnold is complicated. Hard to say if quartizite or meta-volcanics. I'd say don't confuse the surface appearance with the underlying rock structure, grain, etc. At that altitude a lot of rock is covered by various lichens making a clear view of the rock itself quite difficult to do. Guessing you've accommodated that in your analysis. This could be Calaveras Complex formation, which in turns has all aorts of rock types that were crammed down in a trench and got cooked to some extent. Quartzite requires both high pressure and high temps to form, and my limited understanding of the subduction zone suggests "maybe it went deep enough and got hot enough" to form?

Poking around I stumbled on this ancient manuscript :)

https://publications.mygeoenergynow.org/grc/1021057.pdf

Cherokee Nunes wrote:

The geology around Arnold is complicated. Hard to say if quartizite or meta-volcanics. I'd say don't confuse the surface appearance with the underlying rock structure, grain, etc. At that altitude a lot of rock is covered by various lichens making a clear view of the rock itself quite difficult to do. Guessing you've accommodated that in your analysis. This could be Calaveras Complex formation, which in turns has all aorts of rock types that were crammed down in a trench and got cooked to some extent. Quartzite requires both high pressure and high temps to form, and my limited understanding of the subduction zone suggests "maybe it went deep enough and got hot enough" to form?

Poking around I stumbled on this ancient manuscript :)

https://publications.mygeoenergynow.org/grc/1021057.pdf

Thanks for this, this is very interesting! Even if I don't understand much of it.

"Although quartzite occurs in most of the mountainous regions of the state, most it is feldspathic, micaceous, and in the form of thin layers associated with schist. Formations consisting largely or entirely of quartzite are most numerous in the southeastern deserts and are of Paleozoic or Pre- cambrian age. Very few of them are sufficiently pure and uniform to be potential sources of high-purity quartzite."

Earlier it states that the definition of quartzite has been broadened to the point where anything that has large amount of silica in it, so similar to granite, things can be called quartzite that are more quartzite-ish. So this could be a highly impure form of quartzite, which is apparently highly common in the state, and the amount of color variation is a sign of numerous impurities, is it not?

Isaiah aka Zay Foulks wrote:

Looks like Rhyolite to me, but my geo class was a long time ago...

Interesting. From what I can find online: 

"Rhyolite occurs in a rainbow of pale colors. It can have any texture, ranging from a smooth glass to a fine-grained rock (aphanitic) to a material containing obvious crystals (porphyritic). The hardness and toughness of the rock is also variable, depending on its composition and the rate of cooling that produced it. Typically, the rock's hardness is around 6 on the Mohs scale."

It isn't the right rock type for the area (igneous and not metamorphic) but I don't know enough about geology or that survey to know how much that matters-- does the rock in the area being metamoprhic mean that large formations of other types are unlikely, or could that still be likely? Also this rock whatever it is, is at least as hard as granite, possibly harder, which is about right for rhyolite apparently. The description of it having many pale colors is also accurate. Interestingly according to this: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/38/11-12/1204/539287/The-leona-rhyolite-Alameda-County-California

rhyolite can exist in long ridges such as is seen here, and this example is in the bay area, not too far away. 

Seems like rhyolite is potentially a serious contender?

First, I am a not a geologist and am only responding at a distance to some photographs, so not at all giving a scientific response. But I did learn how to climb on Devil’s Lake quartzite ( and that was almost a geologic age ago!!) and that rock sure looks like quartzite and the very hard and sharp-edged characteristics that you describe fit as well.

Concerning those features that you described, well, quartzite is metamorphosed sandstone, so can maintain the features of the original softer rock. One would assume that the heat and pressure forces  of metamorphism would eliminate such features, but that isn’t always the case. There are places at Devil’s Lake where you can actually see ripples from the original sand now ‘frozen’ in the quartzite.

Sure looks and sounds like quartzite. First pic shows what looks like primary structures, i.e. bedding of grains. Quartzite is known to be pretty dam hard, I was hammering away at some this weekend and I am continually impressed with how hard it is. 

Perhaps check in with the Calaveras Gem & Mineral Society. Maybe they'd know. It's a bit out of town heading north on 49 (left side.)

http://calaverasgemandmineral.net

Try a geologic map of the area. Many geologists will refrain from defining a rock without looking at a fresh surface through a magnifying glass first and at a piece that is in situ. 

Whatever it is I'm guessing it's a high feldspar content with that pink color.  Also not a geologist but I did stay at a Holiday Inn Express at some point in my life I'm pretty sure.

Christopher Smith wrote:

Whatever it is I'm guessing it's a high feldspar content with that pink color.  Also not a geologist but I did stay at a Holiday Inn Express at some point in my life I'm pretty sure.

Then it's settled.

I will say though that some of those pictures I just realized remind me of the rock on Lower Mt Scott in Wichita Mountains Wildlife Refuge in Oklahoma.  I'm pretty sure it's granite there although it's very distinct from the rest of the granite out there.

The banding (aka: foliation) is obvious in the first photo of the OP. Notice the banding on the cross-sections facing the camera. The banding, along with the hardness, and sharp, angular edges, indicate a metamorphic rock. It is not, or is no longer, a granitic rock. Quartzite is not foliated, if I recall correctly. Nor do I see the shiny, sparkly quartz crystals like grains of metamorphosed sand. It does not have the flaking characteristics of schist. Metamorphic rocks are difficult to identify, and I only had a couple courses in geology, decades ago. However, my best guess is that you found some gneiss rock.

ClimbBaja wrote:

The banding (aka: foliation) is obvious in the first photo of the OP. Notice the banding on the cross-sections facing the camera. The banding, along with the hardness, and sharp, angular edges, indicate a metamorphic rock. It is not, or is no longer, a granitic rock. Quartzite is not foliated, if I recall correctly. Nor do I see the shiny, sparkly quartz crystals like grains of metamorphosed sand. It does not have the flaking characteristics of schist. Metamorphic rocks are difficult to identify, and I only had a couple courses in geology, decades ago. However, my best guess is that you found some gneiss rock.

The banding/foliation is probably a relict texture and it is an impure quartzite. Quartzite is a metamorphic rock, typically just a package of almost pure quartz grains that have been cooked. As for the sparkly quartz grains, a lot of quartzites are wind-blown or eolian, and therefore often display "frosted grains" from the quartz grains bumping into each other in the wind etc and would probably not look so sparkly. Gneiss typically has large, discrete crystals and displays prominent banding of different colored minerals. It is difficult to identify a rock from photographs, but this doesn't like gneissic. 

Ok I think I found a document a good document to help out here: its from a geocaching site but don't hold that against it. There is an excellent primer on the complexity of the terranes and rocks of the Sierra foothills. This specific document includes a discussion about Cougar Rock located on the Arnold rim trail.

https://www.geocaching.com/geocache/GC76M4A_cougar-rock-and-the-calaveras-complex?guid=f3c0ff0f-ba48-4939-b6c0-d38c527b6b69

There is even a primer there to help you further determine the specific rocks you're evaluating.

BenJamN wrote:

Hey MP,

Geologist here.

Making speculations on a rock from a photo is doing just that - making speculations. Rock types have many gradations and everything you learned in a geology class “decades ago” was probably just the first level of identification. The Cali Geo Survey and the USGS have published geologic maps that will tell you what the rock likely is. Find one with a 1:100k or 1:24k scale so you get a more detailed description than “metamorphic rock.”

This all being said, from the pics and evidence given it’s likely a quartzite. But without cracking a sample open and looking at a fresh unweathered surface this is only a guess. Weathering can and will give all types of coloring hues including pink. Coincidentally, quartzite forms from remelted quartz rich sandstone and as Deliduka noted is extremely hard due to remelted quartz grains. Quartz is 7 on Mohs Hardness Scale.  
Rhyolite is not a metamorphic rock. It is a silica-rich volcanic rock or specifically an extrusive igneous rock. It is fine grained or glassy and can trick you unless you find larger phenocrysts. Rhyolite is the extrusive equivalent to granite. 

Cheers- Ben 

Geologist number 2 here but not local to California. I second Ben; also email someone at the geologic survey with a somewhat coherent collection of the evidence and location and they will certainly give you the answer. —> cgshq@conservation.ca.gov

Nathan Doyle wrote:

Perhaps check in with the Calaveras Gem & Mineral Society. Maybe they'd know. It's a bit out of town heading north on 49 (left side.)

http://calaverasgemandmineral.net

I've put going to one of their meetings on my calendar twice before and every time something comes up. I put it on my calendar for next month also but then just yesterday got kind of an incredible opportunity to do something cool that I definitely cannot pass up, so who knows when I'll get around to one of their meetings. Hopefully this Summer, though. 

Cherokee Nunes wrote:

Ok I think I found a document a good document to help out here: its from a geocaching site but don't hold that against it. There is an excellent primer on the complexity of the terranes and rocks of the Sierra foothills. This specific document includes a discussion about Cougar Rock located on the Arnold rim trail.

https://www.geocaching.com/geocache/GC76M4A_cougar-rock-and-the-calaveras-complex?guid=f3c0ff0f-ba48-4939-b6c0-d38c527b6b69

There is even a primer there to help you further determine the specific rocks you're evaluating.

Dude. Fucking outstanding. As someone who's done quite a lot of googling on this your Google-fu is clearly vastly superior to mine. So this is a different part of the same long ridge that Cougar Rock is a part of. It says the other common rocks besides quartzite in the area are ones quite dissimilar to the rock we're discussing here, making quartzite by far the most likely candidate. 

BenJamN wrote:

Hey MP,

Geologist here.

Making speculations on a rock from a photo is doing just that - making speculations. Rock types have many gradations and everything you learned in a geology class “decades ago” was probably just the first level of identification. The Cali Geo Survey and the USGS have published geologic maps that will tell you what the rock likely is. Find one with a 1:100k or 1:24k scale so you get a more detailed description than “metamorphic rock.”

This all being said, from the pics and evidence given it’s likely a quartzite. But without cracking a sample open and looking at a fresh unweathered surface this is only a guess. Weathering can and will give all types of coloring hues including pink. Coincidentally, quartzite forms from remelted quartz rich sandstone and as Deliduka noted is extremely hard due to remelted quartz grains. Quartz is 7 on Mohs Hardness Scale.  
Rhyolite is not a metamorphic rock. It is a silica-rich volcanic rock or specifically an extrusive igneous rock. It is fine grained or glassy and can trick you unless you find larger phenocrysts. Rhyolite is the extrusive equivalent to granite. 

Cheers- Ben 

Rad, thank you so much for taking the time to comment, Ben! Can even an ignorant knucklehead like myself break open one of these and identify it, or should I search for the assistance of my local gem and mineral society do you reckon? If I were to break one open and post pictures here might that be of use or not really? 

This has been extremely interesting and I've been learning a lot. Thank you to everyone who has contributed to this thread, this is truly fascinating to me and I think it's incredibly interesting to learn about this area where I live. 

Another question I have just in general for the climbers out there: how common are quartzite climbing areas? Is it relatively rare to climb on quartzite, or is it quite common? I haven't heard of any quartzite climbing areas in the western US and I'm curious if I just don't know about them or if it turns out that if this is quartzite that it will be a rather unique spot. 

Seth Morgan wrote:

Geologist number 2 here but not local to California. I second Ben; also email someone at the geologic survey with a somewhat coherent collection of the evidence and location and they will certainly give you the answer. —> cgshq@conservation.ca.gov

Oh wow, what an excellent recommendation. I've been wanting a resource exactly like this and imagined such a thing didn't exist; I am delighted to hear that it does. I will definitely be doing this-- I'll send them an email and let y'all know what I hear from them. Thank you!

You're welcome Ricky. I spent many years skiing at Bear Valley and just as many years learning about the complexity of northern California geology. I've always found it interesting and have spent countless hours pouring over the fine details of the geologic history of NorCal. It's a hobby of mine.

Ricky Harline wrote:

Oh wow, what an excellent recommendation. I've been wanting a resource exactly like this and imagined such a thing didn't exist; I am delighted to hear that it does. I will definitely be doing this-- I'll send them an email and let y'all know what I hear from them. Thank you!

Yup no problem! The Geos at the survey usually are extremely familiar with all the mapping and will get to the bottom of it quickly. I looked at the geologic maps on the survey’s website and I did a little google street view in the area on some of the trails and photos and it all looked very volcanic/metavolcanic. The Arnold/san antonio creek falls area has seemingly 2 main formations that outcrop: The Calaveras complex and the Mehrten formation.

Here are the relevant links.

The map:

https://www.conservation.ca.gov/cgs/Documents/Publications/Regional-Geologic-Maps/RGM_001A/RGM_001A_Sacramento_1981_Sheet1of4.pdf

Unit breakdown:

https://www.conservation.ca.gov/cgs/Documents/Publications/Regional-Geologic-Maps/RGM_001A/RGM_001A_Sacramento_1981_Sheet2of4.pdf

Edit: To reiterate, this is all armchair back of the napkin. To reinforce what Ben said: any rock identification will be outcrop and specimen specific, especially in highly metamorphosed terranes. Getting a fresh specimen, usually by cracking open a rock to ignore any surface weathering will be best.

Ricky, as to your question about quartzite climbing areas, while not as common as, say, granite, basalt,  or sandstone, they are not unusual either. Off the top of my head, besides Devil's Lake Wisconsiin, there are also several others in the Midwest (Blue Mounds, Minnesota for example). Big Cottonwood Canyon outside of Salt Lake is primarily quartzite and I believe that there are quartzite formations in some of the other canyons also the Wasatch Front in Utah.  Also in Utah, the climbs around Ruth Lake and others along Highway 50 in the Uinta Mountains are an excellent quartzite climbing area and a great way to beat the summer heat. Another, more alpine example, are the faces in the Snowy Range of the Medicine Bow Mountains in Wyoming. Up North, many of the peaks around Rogers Pass in the Selkirks are quartzite, as are sections of the Canadian Rockies --Mt. Temple and the Grand Sentinal, for example, as well as the Back of the Lake crags at Lake Louise. In the southeastern US, places such as Moores Wall and Linville Gorge in North Carolina and Talullah Gorge in Georgia are quartzite. Those are just a few examples.

A bit off topic, as it's different rock and a bit north from OP location.

Ricky (and or Geology types near Tahoe), you might find this PDF fascinating as well:

http://neotectonics.seismo.unr.edu/0_COURSES/Geo730-2021/Busby-0016-7606-120-3-274.pdf