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Brushbuffalo
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PostFri Oct 04, 2019 3:56 pm 
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Doppelganger wrote:
seem to be some interestingly flat rock surfaces

Flat [ planar] rock surfaces aren't rare. In the majority of cases the planes are joint surfaces. Joints are fractures similar in their planar nature to faults; joints are cracks without differential movement on opposite sides of the joint, while faults show displacement on opposite sides.

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Passing rocks and trees like they were standing still
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Brushwork
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PostFri Oct 04, 2019 9:24 pm 
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Already gone over my head..   
or, im getting to old to actually think.   Boy, that is scary...,

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Sculpin
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PostSat Oct 05, 2019 9:34 am 
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Found this one about 1/4 mile northeast of Windy Pass Tarn, on the west slopes of Mt. Cashmere:


The greenish rock that comprises the bottom layer was soft and frangible with no obvious crystallographic features.  The metallic green stuff at the top was in fine layers like mica, stacked like a deck of cards.

I have not perused a geological map but can attest that the geology is up there is very complicated and local.

The image was taken in my front yard.    biggrin.gif

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Between every two pines is a doorway to the new world. - John Muir
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Brushbuffalo
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PostSat Oct 05, 2019 12:32 pm 
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That's definitely a penny!
Oh, you mean the rock! biggrin.gif

( when I don't know I try lame humor).

Sculpin wrote:
geology is up there is very complicated and local.

I am guessing this is a fragment of a contact metamorphic rock associated with the margin of one of the intrusions in that area. Too much in this sample for me to be definitive without holding  it in hand and also by  looking around carefully in the field. Sorry. And thanks for the penny for scale.

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NorthBen
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PostSat Apr 11, 2020 10:35 am 
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Noticed this rock near Teneriffe Falls (outside North Bend) and recall seeing similar rocks near the summit of Mount Teneriffe along the official trail that approaches from the west.

Any ideas what it is?
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Brushbuffalo
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PostSat Apr 11, 2020 10:50 am 
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My first wild guess is it is fragmented granitic rock. If so it would have been busted up by faulting, or perhaps by intrusion.
I'll be back in this one.
EDIT: no further info hit my noggin, other than that a glance at the Geologic Map of the North Cascades (USGS Scientific Investigation Map 2940) shows the  most likely rock association is mapped as "Tcas, intrusive rocks of the Snoqualmie family" (Miocene and Oligocene), having crystallized between 22 and 28 million years ago.

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Jake Robinson
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PostMon Jun 01, 2020 8:53 am 
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Interesting layers/folds in the rock here. Taken from Diamond Peak in Idaho's Lemhi Range. Sorry the photo is kind of blurry but it was pretty impressive in person.
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Brushbuffalo
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PostMon Jun 01, 2020 9:26 am 
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That is an excellent ( extraordinary, really) example of folding, Jake. Note that the strata (layers) are horizontal on the left in your scene but as we look a relatively short distance to the right they are deformed into a vertical attitude. The best part is the intricate alternating syncline (  'downfold') and anticline ( 'upfold') array in center and right.

I used to use pictures of wild landscapes like this for teaching. Imagine the tremendous compressive stress involved to produce these folds.  There are several  rather subtle faults here as well, recognized where strata have been offset by displacement of rocks on either side of the fault plane. When folding becomes so intense that the limits of  plastic deformation, which folds rock, are exceeded, then brittle failure results in faulting.

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zimmertr
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PostMon Jun 01, 2020 1:33 pm 
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I apologize for posting a reply just so I could acquiring notifications for this thread in the future but I am unsure if it is possible to "subscribe" without leaving a comment. Very interesting stuff!
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Brushbuffalo
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PostMon Jun 01, 2020 1:41 pm 
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You did it the right way. Now you will be informed anytime there is a new post regarding this thread until you opt out. Otherwise you would have to actively follow it.

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zimmertr
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PostMon Jun 01, 2020 2:05 pm 
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In hindsight there appears be be a button on the lower left indicating you can start watching a thread without commenting. I didn't notice it until after posting though
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Brushbuffalo
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PostMon Jun 01, 2020 5:03 pm 
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You're right! I have overlooked what some call the ' lurker' box.

What's wrong with reading  without contributing ( lurking)?  I did it for a few years myself.
I believe everyone has something to contribute:
trip reports, information, even (or especially ) encouragement.

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Ski
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PostFri Jun 19, 2020 10:31 am 
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What is this please?

rock1
rock1
rock2
rock2
rock3
rock3

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pcg
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PostFri Jun 19, 2020 10:35 am 
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obsidian
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Brushbuffalo
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PostFri Jun 19, 2020 11:52 am 
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For sure, obsidian.
Note the drab outer appearance on the specimen on the left. Obsidian is a form of natural glass and it devitrifies ( alters)  relatively rapidly ( in geologic time). That's why there are few if any very old occurrences of obsidian in nature.
Obsidian is not unlike high- silica volcanic ash, granite, or rhyolite in composition, the main difference being  in eruption style. Obsidian is formed by extrusion of gas-poor very viscous magma, a principle reason that obsidian flows are thick and 'squat.' Obsidian can be black or red or both in the same sample.

Obsidian is transparent along thin edges. Note the interesting conchoidal  fracture, like broken glass....which is what it is. It can be fractured into an edge sharper than the sharpest surgical steel implement. In fact obsidian scalpels used to be the material of choice for delicate eye surgery! bugeyes.gif

Where did you find these samples of obsidian? It is virtually unknown in the North Cascades but becomes more common from Mt. Adams south into Oregon and California.

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Forum Index > Trail Talk > "What's this rock? What's that landform?"
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