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texasbb
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texasbb
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PostMon Aug 17, 2020 6:58 am 
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 7:08 am 
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.

Passing rocks and trees like they were standing still
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NorthBen
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PostFri Dec 18, 2020 5:47 pm 
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 7:10 pm 
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!).

Passing rocks and trees like they were standing still
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zephyr
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zephyr
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PostFri Dec 18, 2020 8:19 pm 
NorthBen wrote:
A friend fished these out of the Sauk River. Any guesses on rock type and fossil species?
Man, those are some beautiful rocks. ~z

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Mike Collins
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PostSat Dec 19, 2020 7:41 am 
NorthBen wrote:
My speculation is that the “harmonica” is a portion of the stem/column of a crinoid. In my own fossil collection I have cuttlebone fossils in that same rock matrix. https://www.theedkins.co.uk/jo/fossils/crinoid.htm

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NorthBen
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PostSat Dec 19, 2020 7:51 am 
Another friend-of-a-friend geologist also thought the "harmonica" was a crinoid, with the more circular fossils also being crinoids rotated 90 degrees and viewed head on. I've seen crinoid fossils in Death Valley several years ago and remember getting the impression they're pretty common as far as fossils go:

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Brushbuffalo
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Brushbuffalo
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PostSat Dec 19, 2020 8:03 am 
Mike Collins wrote:
the “harmonica” is a portion of the stem/column of a crinoid.
Mike and Ben, I initially had 'crinoid' as a possibility in my response, but deleted it. You guys are more 'paleo' than I am. wink.gif

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zephyr
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zephyr
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PostSat Dec 19, 2020 10:49 am 
The stalked forms are known as Sea Lilies, the unstalked ones are Feather Stars. They are still with us today. Here's a cool drawing by Ernst Haeckel via Wikipedia. ~z
Here's an interesting fossil form.
.

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mike
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PostSat Dec 19, 2020 3:12 pm 
One of the most studied rocks in the world.
(file name is an incorrect description)

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Malachai Constant
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PostSat Dec 19, 2020 3:24 pm 
Lots of Crinoids in Grand Canyon esp North Rim

"You do not laugh when you look at the mountains, or when you look at the sea." Lafcadio Hearn
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Brushbuffalo
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PostSat Dec 19, 2020 6:55 pm 
mike wrote:
Is that a question? I am guess it's one of the major unconformities that led to the early study of geology as a science, so it's probably in Europe, maybe in Scotland and studied by Hutton.

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mike
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PostSat Dec 19, 2020 7:32 pm 
Yes Scotland but not Hutton's Unconformity. Wish I could have visited Siccar Point. This is the Moine Thrust at Knochan Crag. First documented evidence of horizontal movement so not an unconformity as generally described. Oldest rock is thrust on top by tectonic forces as shown by Peach and Horne.

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Anne Elk
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Anne Elk
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PostSat Dec 19, 2020 11:27 pm 
This thread got bumped up; I was offline most of August and missed this:
texasbb wrote:
Okay, got a landform question. What mechanism formed the dozens of tiny little islands here?
Marshy upper end of Swamp Lake, Wallowas Hi-res
To which
Brushbuffalo wrote:
Based on your pictures, I think you have an example of a bio- geological runaway feedback loop. 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?)
But after seeing this -
Google Earth view of the marshy inlet to Swamp Lake
Brush Buffalo wrote:
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.
I like the anastamosis idea, BB (what a great word!), and your sediment trapping theory. But the photos reminded me of an arctic land formation I recently discovered wandering around Wikipedia: Pingo (Another great word!) Obviously there's no permafrost in the Wallowas, but perhaps those little round hummock formations in the anastamoses get further pushed upward by frost heaving in what looks like shallow, but permanent swamp (see the diagram in the Pingo article), providing more opportunity for plant colonization, contributing to the cycle. So there you have it: anastamosizing frost-heave pseudo-pingo feedback loops! (Maybe I should stick to brontosaurus theories). hockeygrin.gif

"There are yahoos out there. It’s why we can’t have nice things." - Tom Mahood
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puzzlr
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puzzlr
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PostSun Dec 20, 2020 12:51 am 
This thread is so great. I wish I could contribute! I spend a lot of my time on hikes asking "why" and "how" questions about the land forms around me. Sometimes I know, but mostly just wonder.

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