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Author Topic:   Discussion of a specific Grand Canyon photograph
Minnemooseus
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From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
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Message 1 of 5 (787877)
07-18-2016 3:49 AM


This is a photo of the angular unconformity between the pre-Cambrian and the Paleozoic.

I don't know the actual compass directions in this photo, but for the sake of this discussion let's call away from us to be "north".

Now, this photo shows (to me) that perspectives can be deceiving. In the left ("west") 2/3s of the photo, the angular unconformity is obvious, below the thick vertically faced unit which appears to be horizontal or near horizontal.

But in the right ("east") 1/3rd of the photo, the units show no angular unconformity, and they all seem to be dipping to the lower-right ("southeast").

In reality, I think the thick vertically faced unit is still horizontal or near horizontal. The seeming dip to the lower right ("southeast") is a trick of perspective because you are looking down from above. And the lower pre-Cambrian units appear to have the same dip as the above unit because the strike of the lower units is parallel to the cliff face.

The lower pre-Cambrian units are actually dipping towards the upper-right ("northeast").

Moral of the story - Things aren't what they offhand appear to be.

Comments?

Moose

Edited by Minnemooseus, : Fill in the text of the message.


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Minnemooseus
Member
Posts: 3487
From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Joined: 11-11-2001
Member Rating: 3.1


Message 2 of 5 (787878)
07-23-2016 1:11 AM
Reply to: Message 1 by Minnemooseus
07-18-2016 3:49 AM


Bump - Text has been added to message 1
Ready for promotion consideration.

Moose


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AdminAsgara
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From: The Universe
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Message 3 of 5 (787880)
07-23-2016 1:43 AM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Discussion of a specific Grand Canyon photograph thread in the Proposed New Topics forum.
    
edge
Member
Posts: 3729
From: Colorado, USA
Joined: 01-09-2002


Message 4 of 5 (787925)
07-23-2016 5:47 PM
Reply to: Message 1 by Minnemooseus
07-18-2016 3:49 AM


Any section cut parallel to the strike direction will show apparent dips of zero. This describes the cliff section in the Precambrian rocks.

Since the Paleozoic units are not dipping significantly, they will look to be flat in any vertical section.

Hence, they appear to be parallel.

Interesting observation


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Pressie
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From: Pretoria, SA
Joined: 06-18-2010
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Message 5 of 5 (788047)
07-25-2016 6:48 AM
Reply to: Message 4 by edge
07-23-2016 5:47 PM


Yeah, to me it is quite basic.

Big difference between dip and strike. Don't think lots of laymen know that. Maybe that's a huge difference Faith doesn't realise?

Edited by Pressie, : No reason given.


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