No, I don't have endless supplies of mystery photos, but there are still a few in the files. This one is from our recent meeting of the Far Western Section of the National Association of Geoscience Teachers. We were at Road's End in the South Fork of the Kings Canyon looking at glacial moraines, when several of the group noticed the odd outcrop on the cliff above.
The rock is probably granodiorite (Lookout Peak or Paradise Pluton), and there are glacial moraines on the valley floor below. So what is going on here? Why the linear patterns on the rock face? Why the pockmarks? What do you think?
Oh, and there's a small secret in the comments...
No, I don't have endless supplies of mystery photos, but there are still a few in the files. This one is from our recent meeting of the Far Western Section of the National Association of Geoscience Teachers. We were at Road's End in the South Fork of the Kings Canyon looking at glacial moraines, when several of the group noticed the odd outcrop on the cliff above.
The rock is probably granodiorite (Lookout Peak or Paradise Pluton), and there are glacial moraines on the valley floor below. So what is going on here? Why the linear patterns on the rock face? Why the pockmarks? What do you think?
Oh, and there's a small secret in the comments...
10 comments:
And the small secret is: I don't know what all is going on there, maybe just part of it. So let's hear some good ideas!
Maybe it is where snow has over time worn into and out of cracks and started to wear away at the sides of the cliff...But im usually wrong so...
Couldn't it be some scaring from past glaciation? Just a guess.
I'm going to go with joint-controlled weathering and spalling of minerals from a glacially polished surface. Where glacially polished and unfractured the water washes off quickly and doesn't weather very rapidly; where fractures exist (joint controlled, in this case) water can linger and more weathering occurs.
That's just a first hypothesis, based on what I think I'm seeing.
You may have a light and dark generally linear pattern (subvertical) seen into the background, possibly from two kinds of intrusive rock - then I think the secondary lineation (oblique) are glacial striations that are, for some unknown reason, undergoing cavernous weathering to produce tafoni.
My only thought is basically Gaelyn's: chatter marks from glaciation.
If the smoothness and vague striations beneath the chattermarks or tafoni-weathered glacial striations are also glacial polishing and striations, then there are two directions of ice movement preserved.
That is clearly a textbook example of an "idunnoite." As in, "I dunno!"
If it's jointing, then why do we see a different geomorphic expression for each of the three sets of joints?
If it's glacial striations, why are they inclined so steeply? Do the striations match what the ice might have been doing in this valley?
The vertical dark streaks seem to be staining from water dripping down the rock face. But if so, then why hasn't the dripping water influenced the development of the tafoni?
At first I thought this might be marble rather than granodiorite, but I'll take your word for it. I would say that the colored stripes are vertical and are a surface feature derived from runoff, with the surface itself being an exfoliational joint plane exposed by glacial plucking or rockfall. I would call the pockmarks freeze-thaw weathering, and I would assign their linear patterns to compositional or textural banding in the underlying rock.
Someone's going to have to walk over there and look at it!
Post a Comment