Chickenheads and Knobs on Snow Creek Wall

[catbirdseat]

I've climbed three routes on SCW now and I've noticed that in each case the chickenheads are first seen after about three or four pitches. Am I correct in my assumption that this is because the extent of recent glacial ice was about 500 ft from the base of the wall? Any protruding features below that line were sheared off?

 

I had always thought that the ice was deep enough to completely cover the area, but this must not be so.

[miller]

CBS,

 

I'm not sure about the Snow Creek Wall features, but sometimes xenoliths and other intrusions that form dikes, knobs, etc. are more durable and weather more slowly than the surrounding granite.

 

I've been told that that is how a lot of the dikes and knobs on the Tahoe area granite came to be. This may or may not be the case at SCW - I'm far from an expert geologist.

 

-Todd

[MarkMcJizzy]

It is probably due to localized mafic enclaves, and has nothing to due with Pliestocene glaciations.

 

Snow Lake is a cirque lake, and it's former glacier seemingly projects higher than the top of the SCW. Glacial features are also found high in the Enchantments.

 

Large igneous complexes like the Mount Stuart Batholith are composed of a number of plutonic masses which vary in composition through time, and three dimensionally. I always considered the chicken heads the dispersed mass of an older, more mafic portion of the batholith, which was intruded by younger, more acid rocks. I remember seeing higher in the Snow Creek valley a site where this relationship was more clear.

[catbirdseat]

Well, presumably, these sorts of features are formed by the action of liquid water and acids produced by lichens, etc. Inclusions of more chemically resistant rock would be required, but the rock must be exposed above the ice line too. Moving ice would tend to grind, or break off any features that may have formed.

 

This is the sort of thing Dru would know about, I should think. Even if he didn't he could sound like he did.

 

So, Mark, by Mafic you are referring to rock which is darker, more basic and more rich in iron and aluminum? This sort of rock would be more chemically resistant to weathering, even though it might be a softer stone, right?

 

So your theory is that the lower part of the wall simply does not contain the necessary inclusions of mafic material.

Edited September 19, 2006 by catbirdseat

[fern]

 

have you ever taken an ice cube and tried to grind a hole in a piece of granite with it?

[catbirdseat]

Fern, that's not how glaciers grind down mountains. Stones and grit get embedded in the ice and moves with it. THAT is what grinds down solid rock.

[goatboy]

have you ever taken an ice cube and tried to grind a hole in a piece of granite with it?

 

Good question -- it never worked for me every time I tried, but then again, I only did it for a few hours instead of hundreds of years....'cuz I'm lazy.

 

I have, however, spent hours wearing away a nut tool trying to extract booty from cracks...often, on the SCW, incidentally.

[Gary_Yngve]

I'm really curious about the formation of chickenheads and knobs too, and why they only appear in certain parts of a cliff.

 

Here are some other areas that come to mind:

 

-huge chickenheads on Fote Hog in JTree (largest I've ever seen)

 

-the traversy pitch of Burgner-Stanley from the top of the trees to the start of the chimneys

 

-the final pitch on Loving Arms

[MarkMcJizzy]

Well, presumably, these sorts of features are formed by the action of liquid water and acids produced by lichens, etc. Inclusions of more chemically resistant rock would be required, but the rock must be exposed above the ice line too. Moving ice would tend to grind, or break off any features that may have formed.

 

Is there ice below the chickenheads now?

 

This is the sort of thing Dru would know about, I should think. Even if he didn't he could sound like he did.

Dru doesn't know shit about crystalline rocks

 

So, Mark, by Mafic you are referring to rock which is darker, more basic and more rich in iron and aluminum? This sort of rock would be more chemically resistant to weathering, even though it might be a softer stone, right?

Yes, mafic is more basic. They are also less resistant to chemical weathering than acidic rocks. Mineralogically, they are all similar in hardness. A huge factor in weathering is grain size.

 

So your theory is that the lower part of the wall simply does not contain the necessary inclusions of mafic material.

Yes. Does Midnight or Castle have huge masses of Chickenheads? Edited September 19, 2006 by Markmckillop

[goatboy]

Cochise Stronghold has the biggest chickenheads I've ever seen -- similar to Fote Hog, but an endless wall of them, big enough and formed in a way that allows you to belay off of them... I will upload an image of them....

 

Note the rack lying uselessly by the slung-CH anchor!

 

"What''s My Line" on Cochise Dome

 

The Rockfellow Group in the same area

[fern]

and cochise stronghold is too far south to have ever been glaciated. we have our answer. the cc.com braintrust prevails again!!

[goatboy]

and cochise stronghold is too far south to have ever been glaciated. we have our answer. the cc.com braintrust prevails again!!

 

More likely, brain RUST.

[catbirdseat]

I'm not saying glaciers cause knobs to form. I'm saying they destroy them.

[fern]

yeah exactly. cochise has big knobs because there were no glaciers to destroy them. y'all are VERY smart

[olyclimber]

all this and no mention of the knobs on cc.com? i am impressed.

[dan_forester]

also no allusions to knob-polishing yet; I'm very proud of all of you as well

[JayB]

have you ever taken an ice cube and tried to grind a hole in a piece of granite with it?

 

Come one now Fern. I haven't yet been able to bore a canyon through hundreds of feet of granite by urinating on top of a batholith, but rivers seem to do a pretty good job of it. Pommes and Oranges, eh.

 

I have no idea if CBS is correct or not, but I suspect that if you were to magically transplant a patch of chicken-head laden granite underneath the carbon glacier and then extract it 50 years later, the said chickenheads would be long gone.

[primate]

Fern, that's not how glaciers grind down mountains. Stones and grit get embedded in the ice and moves with it. THAT is what grinds down solid rock.

 

I think modern science has found that most of the weathering action of glaciers is actually done by 'quarrying' -- large blocks are removed from the glacier bed and carried along by the glacier. The action of embedded rocks in the glacial ice does account for polish, striations, etc., but this action is not sufficient to remove material quickly enough to account for much of the total erosion accomplished by the glacier.

[jmace]

Stones and grit get embedded in the ice and moves with it. THAT is what grinds down solid rock.

 

Now I have no expertise in this area, and your answer seems intuitive but would you say the same thing about a river?

 

I.E. would you say a river with silt or stones in it would have an easier time eroding a river bank or rock than a clear silt free river? Does that apply to glaciers then?

 

I.E. A silty or tiny stones embedded in a glacier would be better at eroding rock than a silt or tiny stone free glacier?

[jmace]

arge blocks are removed from the glacier bed and carried along by the glacier. The action of embedded rocks in the glacial ice does account for polish, striations, etc., but this action is not sufficient to remove material quickly enough to account for much of the total erosion accomplished by the glacier.

 

geuss I coulda read above, so the less stones in a glacier the better able it is to remove more rocks as the glacier is not already full of rocks, same goes for a river. Exact opposite of what CBS said

[Cobra_Commander]

I'd wager you don't need much sed load to grind away intrusive rock with ice. Simply apply enough pressure and time.

 

Granite can be cut and polished in "human time" with an industrial jeweled water jet.

 

But I would defer to someone with a background in glaciology. Umm, no one like that has weighed in yet have they?

[G-spotter]

Fern, that's not how glaciers grind down mountains. Stones and grit get embedded in the ice and moves with it. THAT is what grinds down solid rock.

 

That theory has been discredited since the 1950s. Wake the fuck up and read the science before you spout off numbnuts

 

It's the sediment-filled subglacial water that does all the erosion!!!!

[G-spotter]

Dru doesn't know shit about crystalline rocks

 

I'm a geomorphologist. Snow Creek Wall is a landform. Stick to mines and underground rocks where your specialty lies, dickwad Trolls like you wither in the light of day.

[MarkMcJizzy]

Thankyou Dru, that was a fine and proper responce. ScaredSilly could learn much from you, instead of bragging about molehills named after him in god forsaken ice covered lands.

 

Your fluvial fetish is clearly showing, dumbass. From Easterbrook (1999) "Because of the close association with glacially polished surfaces, cresentic fractures, and other evidences of glacial abrasion, distinguishing how much erosion occurs by sub-glacial water relative to glacial erosion can be difficult".

 

This, along with your stand on abiotic and extrophile derived petroleum deposits show what a loon you are, and no one should take you seriously. Now, fuck off.

[G-spotter]

When your mother passed the afterbirth, the doctor said "Oh my god there's two of them!"