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Tom
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PostTue Oct 01, 2019 10:18 pm 
Saw quite a few of these rocks at Upper Kendal Peak Lake today. Curious about the divots which seem to relate to the long and relatively straight scarring.

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Brushbuffalo
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PostWed Oct 02, 2019 8:56 am 
I checked the Geologic Map of the North Cascade Range and although the scale is small there is a unit mapped as 'Tev' near Kendall Lakes. Tev is " volcanic rocks, (early Oligocene and Eocene), described as "mostly basalt and rhyolite flows, breccia, and tuff intermixed with some sandstone and conglomerate." Although not specific for this particular specimen, it also isn't conflicting. My thought is the rock is volcanic, probably andesitic/basaltic in composition with some angular rock fragments and phenocrysts of plagioclase [ light- colored larger crystals]. The most interesting features are the lined- up cavities. I interpret those as vesicles ( gas bubbles), and the alignment is probably a flow- banding structure. If you look closely you will see how many of the vesicles are stretched and parallel, further indicating movement while still in a plastic state. It is unusual to have them in bands rather than more uniformly or randomly distributed As Doppelganger notes, these two pieces were quite likely transported at least a short distance from their bedrock source. In cases like this I try if time allows to locate a bedrock source. This is a fabulous rock! I would surely collect a piece where it is legal to do so.

Passing rocks and trees like they were standing still
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Tom
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PostWed Oct 02, 2019 1:05 pm 
Thanks for the info. You can see a smaller rock in the first image above that appears to have broken off the larger rock. Not sure if it was small enough to carry out (the larger rock was probably 6'). Next time I go up there I'll check to see if the various rocks piece together. My recollection is there were quite a few of them scattered below the talus which I believe may be an inlet early season. This area of the lake is normally under water before the level drops significantly later in season. BTW, here's another rock in the vicinity. Not sure if it's the same type of rock but it has large divots as well. Perhaps those other rocks broke off this one or the rock further out in the lake which has pieces broken off.
Note rock to lower right
Note rock to lower right

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Brushbuffalo
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PostWed Oct 02, 2019 3:20 pm 
The rock in the lower right of your final picture appears to be a more typical vesicular andesite without the flow banding.

Passing rocks and trees like they were standing still
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Brushbuffalo
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PostFri Oct 04, 2019 3:56 pm 
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.

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 
Already gone over my head.. or, i’m getting to old to actually think. Boy, that is scary...,

When I grow up I wanna play.
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Sculpin
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PostSat Oct 05, 2019 9:34 am 
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

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 
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.

Passing rocks and trees like they were standing still
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NorthBen
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PostSat Apr 11, 2020 10:35 am 
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 
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 
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 
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.

Passing rocks and trees like they were standing still
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zimmertr
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PostMon Jun 01, 2020 1:33 pm 
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 
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.

Passing rocks and trees like they were standing still
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zimmertr
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PostMon Jun 01, 2020 2:05 pm 
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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