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Alden Ryno
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PostSat Aug 08, 2020 8:39 pm 
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I encountered this rock formation during a traverse between Kimtah and Cosho Peak along Ragged Ridge in NCNP yesterday (location cited for typical rock formations/compositions in the area for context).

Slightly zoomed out
Slightly zoomed out

Initially it appears that this column fell in some way, however, it also seems like two differently composed types of rock (based solely off of color).
The large column doesn't rely match the rest of the slope yet the top portions are almost perfectly matched in angle without touch apparent erosion. I know very little of geology and my observations likely reflect that ignorance.

Thoughts?
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ThinAir
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PostSat Aug 08, 2020 8:58 pm 
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Washington is made up of 3 or 4 different landmasses that have been smushed together over the eons. You have found one of those boundaries. Congrats. Nature is beautiful, isn't it?
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Frango
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PostSun Aug 09, 2020 6:54 am 
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This is in an old avalanche field near Ferguson Lk in the Pasayten. Granite boulders as far as the eye can see....except for this chunk of what looks like pahoehoe lava. Is it? And if so, where the heck is the rest of the flow?,
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Mike Collins
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PostSun Aug 09, 2020 7:39 am 
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Assuming that gravity brought it to that locale as opposed to a glacial erratic here is a photo looking west into that area. Perhaps it is the rock composed of the old Methow Ocean (Ktf rock).
https://pubs.usgs.gov/sim/2940/photos/html/Tei-1.html
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Brushbuffalo
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PostSun Aug 09, 2020 7:58 am 
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Frango wrote:

This is in an old avalanche field near Ferguson Lk in the Pasayten. Granite boulders as far as the eye can see....except for this chunk of what looks like pahoehoe lava. Is it? And if so, where the heck is the rest of the flow?,

Excellent observation and question!

Mike Collins' explanation is a good start.

The geologic map of the North Cascades shows Ferguson Lake to be in an area mapped as Ktm, which in English translates to 'Tonalite plutons in Methow block (Late Cretaceous)".

That was English, you ask? Maybe this is a little more clear:

"These rocks crystaliized from magma 70 to 90 million years ago. They are part of the same intrusive suite [set of rocks] as the tonalitic [ granitic] plutons in the Wenatchee and Chelan blocks, but most have not been metamorphosed."

Included is the Pasayten Group, which includes some volcanic deposits formed in the now-filled ancient Methow Ocean.

That might account for the seemingly  out of place boulder that does indeed look like  well-preserved pillow structure.
I would bet there are similar rocks nearby.

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Alden Ryno
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PostSun Aug 09, 2020 12:10 pm 
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Thanks, y'all!
You've certainly help satisfy my curiosity.
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Brushbuffalo
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PostSun Aug 09, 2020 5:42 pm 
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Alden Ryno wrote:

Revised Most of Ragged Ridge is rock of the KTto formation, which is  granitic orthogneiss.However, on the geologic map there is a small fault- bounded mass of TKns, which is schist, a medium- grade metamorphic rock that is notoriously chossy.

On your epic climb of Cosho you have some excellent pictures of the disintegrated nature of the rock.  The different appearances are due to variations in composition but largely the state of alteration and both physical and chemical weathering,

Note that the prominent rib shown in your picture is partly detached such that it has developed a 'cannonhole.'  A major fracture extends upslope from the canninhole. This rib will continue  to disintegrate,  either bit by bit or as a singular sizable mass, in which event I wouldn't want to be below.

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texasbb
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PostSun Aug 16, 2020 4:04 pm 
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Okay, got a landform question.  What mechanism formed the dozens of tiny little islands here?
Marshy upper end of Swamp Lake, Wallowas
Hi-res

Here's a wider view:
H-res

And a view from about 800 feet up, directly above the inlet marsh:
Hi-res
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HitTheTrail
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PostSun Aug 16, 2020 6:05 pm 
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texasbb wrote:
H-res

OK,where is this basin?
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texasbb
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PostSun Aug 16, 2020 7:37 pm 
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HitTheTrail wrote:
OK,where is this basin?

In the western Wallowas, west of the Lostine River canyon.  Some of the headwaters of the North Minam River.
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Brushbuffalo
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PostSun Aug 16, 2020 9:49 pm 
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Based on your pictures, I think you have an example of a bio- geological runaway feedback loop.
HUH? Zzzzz😴
Let me explain the theory that I have.

Sediment, mostly gravel, accumulates in a braided stream channel during times of lessening flow velocity. If the little bars remain slightly above average water level, plants are able to become established. These plants trap more sediment thereby increasing the size of the mini islands, which facilitates more plant life on them...a runaway process by trapping more sediment and increasing the island size up to a point limited by erosion.
Key to this is in a gravel- rich stream channel we find that braiding is the dominant pattern, which causes bars to form separated by many divergent/ convergent channels (called ' anastomosing channels' by hydrologists).
I have another theory....theory the second.
And it's mine.(how about that, Anne Elk?)

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texasbb
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PostMon Aug 17, 2020 5:58 am 
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Thanks, @Brushbuffalo, that makes sense to my geologically challenged brain.  I've heard of anastomosis, but I guess I would expect to see remnants of that diverging/converging pattern in the islands.  I can't make my mind find any such patterns in this Google Earth view (the large channel at right flows toward the main lake at top):
Google Earth view of the marshy inlet to Swamp Lake

Am I missing patterns?  Am I wrong to expect patterns?

Thanks again.
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Brushbuffalo
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PostMon Aug 17, 2020 6:08 am 
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I agree, it is not clear to me now after seeing that Google Earth image that these little islands are related to a braided stream pattern. The origin is probably at least as much biological as hydrological, but right now I am stumped.

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NorthBen
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PostFri Dec 18, 2020 4:47 pm 
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A friend fished these out of the Sauk River. Any guesses on rock type and fossil species?
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Brushbuffalo
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PostFri Dec 18, 2020 6:10 pm 
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Ben, the rock is most likely a fine grained carbon-rich sedimentary rock such as mudstone or argillite, perhaps slightly metamorphosed. The fossils may be corals (I am a terrible excuse for a paleontologist).
The unusual- appearing ' ring' in the second picture is fracture filled with a mineral such as quartz or calcite ( a vein) and the rock has been eroded such we see the exposed edges that is not simply a ring but instead is a planar vein.
More veins show in two dimensions in the third photo, along with a fossil harmonica ( told you I'm weak on paleo!).

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