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.
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?
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.
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.
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.
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.