Age Correlations and An Old Earth, Version 2 No 1

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In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will lie at the bottom of the sequence, while newer material stacks upon the surface to form new deposits over time. This is paramount to stratigraphic dating, which requires a set of assumptions, including that the law of superposition holds true and that an object cannot be older than the materials of which it is composed.

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• Igneous rock as is found in volcanic ash, lava flows, and igneous intrusions had been melted and so can be dated with radiometric dating methods.
• Sedimentary rock is composed of ground-up rocks so radiometric dating methods will not work on them, but rather would yield older dates from the component older rocks.

• If the sedimentary layer is above an igneous layer that had formed on the surface (important point; see below), then the sedimentary layer is younger than the underlying igneous layer. The age of the igneous layer would give us an upper bound on the sedimentary layer's age.
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• If the sedimentary layer is below an igneous layer that had formed on the surface (important point; see below), then the underlying sedimentary layer is older than the overlying igneous layer. The age of the igneous layer would give us an lower bound on the sedimentary layer's age.
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• If the sedimentary layer is between two igneous layers that had formed on the surface (important point; see below), then those two igneous layers bracket in the age of that sedimentary layer: it is younger than the older igneous layer and older than the younger igneous layer.
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  Note that that bracketing between igneous layers can be constructed from findings at two different sites as long as the sedimentary layer can be determined to be the same layer. This is determined by comparing the characteristics of the layer at the two sites (eg, composition, index fossils) as well as its relative position between other layers.
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• If a vertical igneous intrusion is found through another layer, then that means that that layer had to already exist before the intrusion. Hence, that intrusion would be younger than the layer and, of course, the layer would be older than the intrusion. In addition, an intrusion could extend through multiple layers, thus providing a lower bound on those layers' ages (ie, they'd all be older than the intrusion).
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• It is possible for an intrusion to be horizontal, such that it forces itself between two pre-existing older layers. That would make the layers above and below a horizontal intrusion older than that intrusion.
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  This is the important point I alerted you to above:
  
  This raises the question of how to tell whether an igneous layer had formed on the surface and then had more layers deposited on it, or had formed as a horizontal intrusion. Understand that the intense heat of that molten lava would have affected the pre-existing rock that came in contact with it. For example, in the Badlands of North Dakota, I found igneous intrusions into sandstone. Right along the contact between the sandstone and the intrusion the heat had changed the sandstone into a layer of sandy crust about a quarter-inch thick. It was very obvious and easy to see.

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  In the case of the lower layer, both a surface lava flow and a horizontal intrusion would have the same effect due to intense heat along that contact. In the case of the upper layer, only a horizontal intrusion would have changed the upper layer along the contact due to intense heat. A surface lava flow would have solidified and cooled down before the overlying layer would have started to form, so the overlying layer would not be affected by the heat that is no longer present.

• For an igneous layer, that would be when the molten rock (eg, lava) had solidified.
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• For a sedimentary layer, that would be when the material, usually a collection of rock particles, was deposited.
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• For metamorphic rock, I'm not sure since I haven't worked through this case. It would have an age for when it formed originally as a layer at which time it would have been either igneous or sedimentary. But then it was subjected to forces (eg, heat, pressure) which changed it, thus metamorphizing it into a different kind of rock. I do not know whether there are dating methods for determining when that metamorphic event happened.
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  Therefore, we won't discuss dating a metamorphic layer. Nor should we here since the question is about sedimentary layers.

• The material that makes up a sedimentary layer has to already exist. That means that all that material, every single particle of it, has to be older than the layer they form. Using the cake analogy, you can't bake a cake with flour that won't be milled for another month yet; the flour has to have been milled before going into the cake.
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• The layers that were the source of that material had to have formed before their material was available for forming the new sedimentary layer. Therefore, those layers had to have been older than the new sedimentary layer.
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• Parts of those older layers had to have broken up and ground up by forces such as erosion. That takes time, usually a lot of time.
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  A typical scenario would be:
  1. The older layers form. Usually they will be buried under later layers, but occasionally some of them remain as the surface (in which case, skip Step 2).
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     BTW, in order to lithify (ie, become rock -- Greek: λιθος = rock) the layer must be buried. If it is not buried, then it just remains dirt (or soil) which will get transported away with the rain or wind (unless trapped by vegetation's root systems) in the quickest form of erosion I can think of.
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     You know, actually a layer that doesn't get buried and lithifies but rather remains the surface ... that doesn't qualify as a sedimentary layer. It's just unincorporated particles waiting around to be transported or buried to form a layer. We don't have to worry about it (though I had written a lot of the following before this realization and don't have time to edit it).
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  2. Over time, those upper layers erode away, exposing those older layers. Or else (and much more commonly I would think) erosion creates a cliff (eg, the sides of a hill or of a river valley) which exposes part of those layers on the cliff face.
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  3. Whether through being exposed by earlier erosion or having remained the surface, erosion breaks off pieces of those older layers and then breaks them down into smaller and smaller particles which then get transported away by flowing water or winds (eg, the beaches of Southern California where granite mountains get broken apart and ground up into smaller particles that are transported by streams and rivers out to the ocean where they form the quartz sand on our beaches; the coral reefs around Oahu getting eroded down by the ocean waves to form the coral sand of Waikiki).
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     BTW, when geologists examine sandstone, they are able to tell whether that sand had been deposited by wind or by water.
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  4. Well, finally the particles get transported to where they form the new sedimentary layer.
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• Each of those steps take time, a lot of time. So taking the example of just a single old layer: you have the old layer forming which takes time, then you have it being buried (or not, but it won't lithify unless it is buried) which takes time. Then you have to expose it again through erosion, which takes more time. Then erosion on those exposed parts of the layer needs to break it up into particles, which takes more time. Then those particles need to get broken down into smaller particles, hence even more time. And finally those particles need to be transported to where they can form a new layer -- you guessed it: more time.
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• What we end up with is the obvious fact that the particles that comprise a new sedimentary layer must be older than that layer. Most commonly much older by an order of millions of years older.