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Author Topic:   Geologic Column
Percy
Member
Posts: 20834
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.7


Message 27 of 55 (511889)
06-12-2009 1:49 PM
Reply to: Message 25 by Peg
06-12-2009 8:26 AM


Re: mooved from
Here's an example of what JonF means by an overthrust. First you have a region of geological layers, I'll give each layer a unique character. We're looking at the layers edge on:

                ------------------------------------------
++++++++++++++++++++++++++++++++++++++++++
==========================================
||||||||||||||||||||||||| 4;||||||||||||||||

Now the region gets compressed from both ends because of the motion of the continents, and this causes it to sheer (break) along a diagonal line:

                --------------------\---------------------
+++++++++++++++++++++\++++++++++++++++++++
Thrust ====> ======================\=================== <==== Thrust
|||||||||||||||||||||||\|||� 124;||||||||||||||

Continued pressure from both ends causes one to slip along the fault and rise above the other:

                                \-------------------------
\++++++++++++++++++++++++
\=======================
\||||||||||||||||||||||
--------------------\*********************
Thrust ====> +++++++++++++++++++++\^^^^^^^^^^^^^^^^^^^^ <==== Thrust
======================\###################
|||||||||||||||||||||||\~~~~~~~~~~~~~~~~~~

The compression and pushing from each end continues and the layers that have risen are gradually pushed on top of the other layers, giving this result:

                ------------------------------------------
++++++++++++++++++++++++++++++++++++++++++
==========================================
||||||||||||||||||||||||| 4;||||||||||||||||
------------------------------------------
Thrust ====> ++++++++++++++++++++++++++++++++++++++++++ <==== Thrust
==========================================
||||||||||||||||||||||||| 4;||||||||||||||||

Now the youngest layer ("-") exists both at the top and several layers further down where it is just below the oldest layer ("|"). In reality many more layers are often involved, and subsequent erosion and deposition can make the situation very complex.

Sometimes the sheer lines still exist and can be identified, sometimes not. Geologic processes like erosion and subduction destroy a lot of evidence.

Much more complex overthrusts are common. Imagine a rug being pushed together from each end. It will gradually fold and bunch up, and bunched-up and folded portions can be folded again and can bunch up over other bunched up portions.

--Percy

Edited by Percy, : Minor correction.

Edited by Percy, : Rendering error somehow crept in - that's weird!


This message is a reply to:
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Percy
Member
Posts: 20834
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.7


Message 32 of 55 (512197)
06-15-2009 8:21 AM
Reply to: Message 30 by Peg
06-15-2009 6:47 AM


Re: consistent, pervasive, index fossils
Peg writes:

Charles Lyells book 'Principles of Geology' explains that all sedimentary rocks are deposited by extremely slow processes, such as rain washing loose sand down a mountain slope to a river. Is this theory still current today? Or has it changed?

I'm not sure why Vacate replied that Lyell's ideas were incorrect. Lyell certainly did not forget about landslides and such. While fast processes such as landslides do contribute to sedimentary layers, the vast, vast majority of sediments form from slow processes. For example, limestone layers accumulate at the rate of about a yard every 7500 years. That's pretty slow, yet the White Cliffs of Dover, hundreds of feet high, formed this way.

--Percy


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Replies to this message:
 Message 33 by Vacate, posted 06-15-2009 8:25 AM Percy has replied

  
Percy
Member
Posts: 20834
From: New Hampshire
Joined: 12-23-2000
Member Rating: 2.7


Message 35 of 55 (512209)
06-15-2009 9:01 AM
Reply to: Message 33 by Vacate
06-15-2009 8:25 AM


Re: consistent, pervasive, index fossils
I thought that might be the case but wasn't sure. I think the subtlety might be lost on Peg. I was afraid she'd conclude that you were saying that Lyell was wrong in general about how sedimentary layers form.

--Percy


This message is a reply to:
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Replies to this message:
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