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Ski
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PostMon Jul 27, 2020 9:15 pm 
the rock is about the size of an orange or a baseball - 3 inches x 4 inches (approx.) absolutely no clue where it came from - possibly up near Snoqualmie Pass

"I shall wear white flannel trousers, and walk upon the beach. I have heard the mermaids singing, each to each."
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Songs2
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PostTue Jul 28, 2020 6:05 pm 
Brushbuffalo, I came across an interesting photo in a trip report from the Lake Louise - Skoki Lakes (Banff) area. It is the 7th in this TR, captioned "Painted Landscape": TR to Purple Mound The author has some close-ups of the purple talus later in the TR. I'd be interested to know what kind of rock is producing such a variety of color, especially that grand aubergine. (Apologies; I don't know how to link the photo.) Thanks!

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Brushbuffalo
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PostWed Jul 29, 2020 8:43 am 
Usually purple is due to high iron content. Not uncommon in siltsone or sometimes sandstone of deep marine origin.

Passing rocks and trees like they were standing still
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Ski
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PostWed Jul 29, 2020 8:59 am 
Brushbuffalo - thank you again very much for your help. up.gif

"I shall wear white flannel trousers, and walk upon the beach. I have heard the mermaids singing, each to each."
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Alden Ryno
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PostSat Aug 08, 2020 9:39 pm 
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 9:58 pm 
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 7:54 am 
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 8:39 am 
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 8:58 am 
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.

Passing rocks and trees like they were standing still
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Alden Ryno
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PostSun Aug 09, 2020 1:10 pm 
Thanks, y'all! You've certainly help satisfy my curiosity.

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Brushbuffalo
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PostSun Aug 09, 2020 6:42 pm 
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.

Passing rocks and trees like they were standing still
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texasbb
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PostSun Aug 16, 2020 5:04 pm 
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 7:05 pm 
texasbb wrote:
H-res
OK,where is this basin?

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texasbb
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PostSun Aug 16, 2020 8:37 pm 
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 10:49 pm 
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?)

Passing rocks and trees like they were standing still
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