When you look at this, which towel do you
think was folded first?  Yeah, most people
would say that the brown towel was was
folded first and then the red one on top
of that.  That just kind of makes sense
because the brown one is at the bottom;  and
we're gonna do the same idea, the same
thing, when we're looking at rocks.  We're
gonna determine what happened first, what
happened second. If you take a look at
these beautiful rocks here, we can tell,  that because they're layered,  we know
we've got some sedimentary rocks.  I've
got one Rock layer that is the labeled "A"
That's this layer that we see right
there and I've got another rock layer
labeled "B", that's this rock layer up here. 
When we take a look at those rocks,
which rock layer do you think happened first; which one was deposited first?
Well, you would probably say hey that makes sense that "A" was deposited first; "A" is
below "B".  That's the one that's gonna
happen first the same thing that
happened with our towels.  How about this
one right here?
The light-colored layer that we see down
towards the bottom, which one was
deposited first?  It was gonna be layer B.
Layer B was deposited first because we see
it at the bottom. So, we can tell by
looking at some rock layers what
happened.  The order of events or the age
of those rocks relative from one to
another and that's gonna be our first
focus of section number four.  So, here's
our geology topics that we've studied so
far: rocks and minerals, surface processes,
dynamic crust is now done and now we're
gonna move on to section four "Earth's History before we move on to mapping.
In Earth's history we're gonna focus our
attention right now on Relative Age
Dating (not relative age dating, as if
you're going out with your uncle or
something like that) relative meaning one
thing compared to another  An age dating
determining how old something is.
Not determining how old something is as in how many years old something is, but
figuring out how old something is
relative or compared to something else.
You just told me that a certain rock was
older than another.  We know that rock
layer B is older than rock layer A.  We
don't know how old rock layer B is, we
don't know how old rock layer A is we
just know their relative ages.
That's gonna be our focus, is just on
their relative age dating.  If we take a
look at our page in our notes we're
gonna focus on relative age dating.
You notice we've got a handful of diagrams
and we've got three sections that we're
gonna look at on this page, page 61 of
your notes.  The first section that I want
to focus on deals with this diagram on
the top right.  If we take a look again at
these rock layers you guys were able to
figure out that the oldest rock layer,
the rock layer that was deposited first.
Remember, if something is older it happened first.
This would be rock layer
C is the oldest rock layer.  "A" would be
the youngest rock layer and that's the
first rule or first principle that I
like to talk about. The principle of
Super Position.  So, the principle of Super
Position just simply says that, "The layer
at the bottom is the oldest layer".
If we take a look at the diagram, we've got
layers A, B, C, and D.  A is at the bottom of those layers.
And just like the towels, 
unless that whole area was flipped and
overturned (or flipped upside down, which
is not likely) the oldest rock layer is going to be at the bottom.
So, layer A is the oldest layer where layer D is the youngest layer.
We can see that here with these rock
layers.  We know that the oldest rock
layers would be down towards the bottom
and they're going to get younger and
younger as we move up towards the top where we find the younger rock layers.
We see the same thing is true with these rock layers here.
The oldest rock layers at the bottom, the youngest rock layers are at the top.
The rock rock layers at
the bottom we're deposited first; the
rock layers at the top deposited after
that.  So, relative age dating is gonna be
able to tell us the relative age.  Now we
don't know how old those rock layers are,
we don't know when they were deposited,
but we can compare one rock layer to
another rock layer to figure out how old
they are.  So, that's relative age dating;
using the principle of superposition.
Rock layers are always deposited in nice horizontal layers, but they don't always stay that way.
We see we have these beautiful sedimentary rocks.  They're in layers so we know we've got sedimentary rocks,
but something's happen to these sedimentary rocks.
Notice how we could see these layers have been bent and folded.  They were originally
deposited horizontally, but something's
happened to these layers over time.
They start in nice, horizontal layers, but
because of tectonic forces they get bent
and folded and formed these anticlines
and synclines.
This is the idea of "Original Horizontality". That sounds fancy, but it just means
originally the rocks are horizontal.
That's how they start off.  If we take a
look at the diagram there we have a
bunch of sedimentary rock layers, some
have been folded some have not. Deposition of rock layers A, B, C, D, E, and F is what happened first,
then there was some
folding and then G, H, and I.
Original Horizontality lets us know that
that folding must have happened before
G and H.  Why weren't G & H folded? 
Well, they weren't folded because
they weren't there to get folded at the time
that A through F were folded.
G &H just weren't there.  So, if they weren't
there to get folded, they must be younger.
That's this idea of Original Horizontality. That these sedimentary rock layers
are deposited horizontally
to begin with, any folding must have
happened after the deposition.
Why weren't G and H folded?  Because they
weren't there.  They must be younger.
If we
take a look at this diagram we have rock
layers G, F, E, D, C, H, and B and A.  We see
that layers A and B have not been folded.
Why???  They weren't there. 
The folding must be older than rock layers A and B.
The folding must be
younger than G, F, E, D, C, and H.
Notice that we're now talking not only about the rock layers but also about events
and features that happen to the rock layers.
That brings us to this next part.
There's an intruder.
Someone's intruding into your home.
Isn't that ridiculous to think that they can intrude into your home before your home is built?
You can't intrude into a building before the building is there.
One of the things
that intrudes our rocks is magma.
You would imagine if there's some magma
under the ground, that magma, which is
gonna be less dense, is gonna start
pushing its way through the rock and as
the magma pushes its way through the
rock, it's going to intrude into those
rock layers. And it might even burst out
at the top and give us a volcano or
maybe it doesn't at all and just
solidifies under the ground it gives us
a plutonic rock.  Or a fault, so, here we
could see if a fault (or a line a crack in the rock).
In the rock this fault that you see
right here must have happened after the
rocks were there.  Right?  That makes sense, you can see some similar layers this
layer right here
 is the same as this layer right here.
Well, why aren't they lined up?  Well,
because there's been that fault
there.  We've got one side of the fault is
moved this way this side of the fault is
moved that way and it's faulted
the rock, it's cracked through them.
Well, of course that fault can't happen until
after the rocks are there.
Here we see layers A, B, and C and layers
A, B, and C have been faulted.
Well, the fault has to come after rock layers A, B, and C.  The fault has to be younger than
rock layers A, B, and C.  It's the same
thing with the intrusion as well.
Here's an intrusion from the the darker
rock intruding into the lighter colored
rock.  That intrusion has to happen after,
so here we see this rock is intruding;
this was some magma (some melted rock) that has pushed its way into
another rock.  Well, that magma has to be younger.  We call this principle the
principle of Cross-Cutting Relations.  And
in both diagrams you could see that
layers A, B,& C were either intruded or
faulted and layer D was not intruded on
the left or faulted on the right. 
So, layer D must be younger than the
intrusion or the fault.  Layers A, B, and C
(in both cases) have to be older than the
intrusion or the fault.  There's an
igneous intrusion down towards the
bottom here.  So, here is this igneous
intrusion, that's some magma, that's solid
rock now that solidified (used to be
magma that pushed its way through some
other sedimentary rock) when that happens; when the magma touches the rock that it's intruding,
it's going to burn that rock.
I'm gonna show that by putting
some little tick marks on here to show
that that rock (when it got touched by
magma) I would imagine if you got touched
by magma too you'd get burnt a little bit.
It gets burnt there and we talked
about this concept when we talked about
different types of metamorphism.  How there's regional metamorphism and the one that's
being shown here is contact metamorphism.
Here we see another intrusion.
The dark igneous rock that
intrudes into some sedimentary rock and
when it touches that sedimentary rock
it's gonna cause some contact metamorphism.
We could use this contact
metamorphism where the magma touches the
surrounding rock to help us identify the
relative ages of some rocks.
I know you don't have any room left on your page but jump over to one of your blank pages.
Let's get this idea of
contact metamorphism and how it is going
to help us figure out the age of some
rocks.  If you take a look at these two diagrams
they look very similar to one
another but there's something that's
different about them.  Can you find the
thing that's different?
The thing that's different about them is right here. 
We see that D has some tick marks on it
While  over here D does not.
So, on the left hand diagram
limestone layer D does not have those
little tick marks on them (which for us
those little tick marks are gonna mean
contact metamorphism) those little tick
marks right there mean there's been
contact metamorphism.  Well, why wasn't
limestone layer D in the left hand
diagram, why wasn't that layer
metamorphosed?  Why didn't the magma burn rock layer D? well rock layer D is not
burned therefore it must be younger than.
Because it came after the intrusion.
Why wasn't it burned?  It wasn't there to get
burned when the magma pushed through the
shale A and the sandstone B. It burned
those layers but the limestone layer D
wasn't there at that time; it must have
come after, meaning it must be younger.
Where in the right-hand diagram D is
burned so D is burned.
It must have come before
that.  Layer D shows that it got
metamorphosed.  Well, the only reason it
could have gotten burned is because it
was there to get burnt.  In the left
hand diagram shale layer A was deposited,
then sandstone layer B, then the igneous intrusion C came through
and then after that was the deposition of D .
On the right hand layer deposition of shale A,
then deposition of B, then deposition of D
and then the intrusion C came through
and burned them.
Here's a diagram that shows an outcrop where we see some rocks.
We've got some shale and some limestone
or rock salt we see an igneous intrusion.
The igneous intrusion is going to be
this layer right here.  This is the same
igneous intrusion here and the fault is
that line there.  Can you label these in
order from oldest to youngest?
Meaning the first thing that happened label
#1, the last thing that happened
label #6.  Put them in order from oldest
to youngest
so if we look at this, the first layer that happened was the shale layer at the bottom.
Oldest rocks are at the bottom.  So, the first
thing is the deposition of
shale, the second thing that happened
after that was deposition of the sandstone,
after that was the deposition
of the limestone.  Those are pretty
straightforward and simple.  The first
thing that happened the second thing the
third thing that happened there.  You'll
notice that the salt at the top does not
get cut by the fault and it does not get
intrude either.  That's the last
thing that happened... the
salt layer the top does not get cut by the fault,
it does not get intruded,
there's no contact metamorphism on it.
That's the last thing that happens (#6).
Well, we got to figure out now
is which happened first, the igneous
intrusion or the faulting.
Notice that the intrusion gets cut,
the intrusion was split.  One part of the
intrusion moved this way, one part of the
intrusion moved that way.  How did it get
cut?  Because it had to be there before
the fault.  The fault must have happened
after in order to cut the intrusion.
So, the intrusion is gonna be #4, then
the faulting comes by in order to cut it
as well as the other layers.
Try another one.
The first is the formation of the gneiss
which is this layer right here.
I'll start you off that's the first thing
that happened. Fill out the rest.
As before, I've got a layer at the top that
does not get cut does not get faulted is
not getting treated that limestone layer
is the last thing that happened so I'm
gonna label that right now is #6
after my Gneiss was formed I have a
sandstone layer that was deposited so
deposition of sandstone is the second
thing that happened.  And then I've got
this layer here, the shale layer, and then
I have this intrusion.  Well, which one happened first?
Notice how the shale layer has contact
metamorphism.  The shale layer must
have been there in order to get burnt.
In order to get contact metamorphism, the
shale layer must have been there
previous, it's older than.  And then again
I still have this issue with the
faulting.  And the intrusion is just like
before, the intrusion gets faulted so it
must be there before the faulting.
