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Author Topic:   Origin of the Flood Layers
JonF
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(1)
Message 346 of 409 (753501)
03-20-2015 11:04 AM


High dynamic range composite:


Replies to this message:
 Message 350 by Faith, posted 03-20-2015 11:07 AM JonF has responded
 Message 355 by edge, posted 03-20-2015 11:37 AM JonF has responded

  
Faith
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Message 347 of 409 (753502)
03-20-2015 11:04 AM
Reply to: Message 344 by JonF
03-20-2015 10:47 AM


Re: Moderator Clarification
Lightening the photo in order to better visualize what's in the depression is a good idea but now it's so light I have no idea what it turns out to be.
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edge
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Message 348 of 409 (753503)
03-20-2015 11:05 AM


Okay, I haven't found an example in the Grand Canyon, but perhaps this diagram will help to explain a little bit. It shows how a gold-bearing quartz vein is weathered to produce placer deposits of different types, depending on the degree of transport. If we just ignore the gold and accept that the quartz vein will be similarly distributed by weathering we have something similar to the quartz pegmatite on the unconfomity surface.

This important thing here is the process of erosion and transport that happens at the unconformity, meaning that the rocks are exposed to surficial processes.

Does this help?


  
Admin
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Message 349 of 409 (753504)
03-20-2015 11:05 AM
Reply to: Message 328 by jar
03-20-2015 9:59 AM


Re: Formation of schist etc.
jar writes:

But Faith, you still have not presented a model. Let's try baby steps.

Faith presented a pretty detailed description of what she thinks happened, so couldn't we consider that as her model? Anyway, naturally it raises some questions, which you go on to do.


--Percy
EvC Forum Director

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Faith
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Message 350 of 409 (753505)
03-20-2015 11:07 AM
Reply to: Message 346 by JonF
03-20-2015 11:04 AM


There IS a depression there, you can see the edge of it in the lower part of the yellow circles. Not really sure what you've ended up showing here.
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Admin
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Message 351 of 409 (753507)
03-20-2015 11:16 AM
Reply to: Message 338 by edge
03-20-2015 10:31 AM


Re: Moderator Clarification
edge writes:

She wants to know how pieces of quartz broken off from the veins of quartz embedded within the Vishnu Schist came to be deposited atop the Vishnu Schist.

Probably much the same way as they would look as if they fell out of the cliff face shown and ended up in the river sediments below.

Does that help clarify things,...

It does help a bit, but not enough. It just feels like something's missing. How is it that in the distant past there was a cliff of Vishnu Schist towering above a riverbed of Vishnu Schist? I can only guess at the answer, but I assume it's one that must be very obvious to you. My guess is that the river eroded down through the Vishnu Schist, creating the cliff faces from which pieces of quartz broke off. But once I come up with that guess I question it, because if that was the case then where is that ancient cliff face now?


--Percy
EvC Forum Director

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edge
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Message 352 of 409 (753508)
03-20-2015 11:17 AM
Reply to: Message 349 by Admin
03-20-2015 11:05 AM


Re: Formation of schist etc.
Faith presented a pretty detailed description of what she thinks happened, so couldn't we consider that as her model? Anyway, naturally it raises some questions, which you go on to do.

Well, I'm not sure that I would call it a model. Perhaps we could call it an ad hoc, hypothetical scenario ... one that has been compromised on several accounts. We keep asking questions, but the only real response is that it looks like ... 'something', to one individual.
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Faith
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From: Nevada, USA
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Message 353 of 409 (753509)
03-20-2015 11:21 AM
Reply to: Message 352 by edge
03-20-2015 11:17 AM


Re: Formation of schist etc.
I suspect you aren't referring to the same post of mine that Percy is referring to.
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edge
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From: Colorado, USA
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Message 354 of 409 (753510)
03-20-2015 11:25 AM
Reply to: Message 351 by Admin
03-20-2015 11:16 AM


Re: Moderator Clarification
It does help a bit, but not enough. It just feels like something's missing. How is it that in the distant past there was a cliff of Vishnu Schist towering above a riverbed of Vishnu Schist?

Why does it have to 'tower' over itself? I've seen plenty of streams with one formation from base to the top of the bank.

I can only guess at the answer, but I assume it's one that must be very obvious to you.

Well, I've seen a lot and probably take a lot for granted, but when questions are posed in an intelligible way by someone truly interested in an answer, I don't mind spending some time.

My guess is that the river eroded down through the Vishnu Schist, creating the cliff faces from which pieces of quartz broke off.

See my previous post for a diagram that might explain. But actually, I don't think it needs to be a cliff. In fact the unconformity was the surface in question.

But once I come up with that guess I question it, because if that was the case then where is that ancient cliff face now?

Well, there could be anything a short distance into the rocks; but, as mentioned, any kind of surface, even a flat one, would suffice.

Edited by edge, : No reason given.


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edge
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Posts: 3885
From: Colorado, USA
Joined: 01-09-2002
Member Rating: 4.1


Message 355 of 409 (753511)
03-20-2015 11:37 AM
Reply to: Message 346 by JonF
03-20-2015 11:04 AM


High dynamic range composite:

This is an excellent picture. Not sure what you did to create the highlights, but it clarifies some things I suspected.
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JonF
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Message 356 of 409 (753520)
03-20-2015 12:14 PM
Reply to: Message 350 by Faith
03-20-2015 11:07 AM


There IS a depression there, you can see the edge of it in the lower part of the yellow circles.

Thaty's teh boundary between the lightened and un-lightened part. I.e the edge of the shadow of the clasts.

Not really sure what you've ended up showing here.

That the topography continues across the border between the lightened and un-lightened part with no indication of a depression.


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 Message 350 by Faith, posted 03-20-2015 11:07 AM Faith has responded

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JonF
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Posts: 3651
Joined: 06-23-2003
Member Rating: 3.6


Message 357 of 409 (753522)
03-20-2015 12:16 PM
Reply to: Message 355 by edge
03-20-2015 11:37 AM


Overlaid two copies, brightened one, selected the shadow areas in the brightened one, selected the inverse, feathered the selection edge a tad, and erased the selected area. Piece o' cake.
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Faith
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Posts: 25317
From: Nevada, USA
Joined: 10-06-2001
Member Rating: 1.1


Message 358 of 409 (753523)
03-20-2015 12:27 PM
Reply to: Message 356 by JonF
03-20-2015 12:14 PM


Thaty's teh boundary between the lightened and un-lightened part. I.e the edge of the shadow of the clasts.

No, that's a physical edge and what's on the other side is lower. However, it looks much shallower than I expected nevertheless. With the shadow completely gone I can't tell depth at all. But there is still a hint of shadow right under the clasts.


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Faith
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From: Nevada, USA
Joined: 10-06-2001
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Message 359 of 409 (753528)
03-20-2015 12:46 PM
Reply to: Message 356 by JonF
03-20-2015 12:14 PM


Look, I'd be happy enough if it was as shallow as you say because finding out the tiny scale of the image changed most of my view of it.

HOWEVER, as I analyze even your lightened version I still think that's a depression the clasts came out of. That IS the physical edge of a depression, not just the edge of the shadow that was originally there. The clasts still appear to be suspended above the depression. The lightened area where the shadow was is the schist at the bottom of the depression, it is not continuous with the surface.

The lightened area is red on your image, and the edge, which I've marked in blue on my own copy of the image, is the color of the schist. The remaining shadow that is still visible under the clasts indicates that they are suspended over the depression.

Here's yours followed by mine:

ABE: This is a corrected version. The first one had the edge marked wrong on the bottom left, so I redid it.

ABE: Here's Percy's marked image for reference to how far the surface edge extends on the left, which I marked wrong the first time.

More I look at it the more I see the depression still where I first saw it.

Edited by Faith, : No reason given.

Edited by Faith, : No reason given.

Edited by Faith, : No reason given.


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 Message 360 by JonF, posted 03-20-2015 1:52 PM Faith has responded
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JonF
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Posts: 3651
Joined: 06-23-2003
Member Rating: 3.6


Message 360 of 409 (753547)
03-20-2015 1:52 PM
Reply to: Message 359 by Faith
03-20-2015 12:46 PM


HOWEVER, as I analyze even your lightened version I still think that's a depression the clasts came out of. That IS the physical edge of a depression, not just the edge of the shadow that was originally there.

It's the edge of the shadow that was originally there. It's the boundary between the un-lightened and lightened version.

The clasts still appear to be suspended above the depression.

Oh, the clasts are definitely slightly above the Vishnu surface. That's why your idea of encroaching sandstone picking them up off the Vishnu is so silly.

The lightened area where the shadow was is the schist at the bottom of the depression, it is not continuous with the surface

Sorry. Nope. I see several continuous features crossing that line, some of them I've outlined in green

If you have any evidence other than "it sho' looks depresssionish to me", trot it out.

Edited by JonF, : No reason given.


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 Message 359 by Faith, posted 03-20-2015 12:46 PM Faith has responded

Replies to this message:
 Message 363 by edge, posted 03-20-2015 2:04 PM JonF has responded
 Message 376 by Faith, posted 03-20-2015 9:58 PM JonF has responded

  
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