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On the very first day I showed you this picture of me standing in front of he
Grand Canyon asking you about these. layers
A huge part of what we know about dinosaurs depends upon these layers that
form in rock.
So how do they form and what are we going to talk about? Well this is the bottom line
about what this
little video will tell you about. Rock layers are deposited one on top
of the other,
and so the deeper we dig the, farther back in time we see.
A little bit more about how those layers form:
Well imagine you've got a river here
and it flows into the ocean or a river bottom.
There is some mud that's being carried in that river and that mud get deposited
into that bigger ocean
and slowly settles out. It
starts accumulating at the bottom of the ocean
and slowly but surely it builds up
more and more material and older layers get buried
so they stay on the bottom and new layers form at the top as stuff continues to
settle out from that ocean.
And you start seeing
patterns that look like these alternating layers;
different colors that repeat over and over and over again.
So let's take a look at how those form. You start off
with say a mountainside with trees growing on it. Here's our ocean with a boat.
This is just one of the many ways that rock layers form at the surface. This is the
simple version of it. But let's say we have
a storm come along and the raindrops fall.
Each one of those raindrops there crashes in
to this rock, breaks off a little tiny piece
and carries it along as it flows down into the ocean.
At the bottom of the ocean it starts to
settle out and you start getting a layer
of rock accumulating. Then maybe the storm goes away we have a dry season
and only a tiny little bit a sediment is going to wash down, and just from
this red area here. And again at the next year, we'll have another wet season.
Another rainstorm will crash in and wash more material
from that yellow area and you will start getting
these building up alternating layers. They might be a flood, or they might be a whole
wet season like it would be here in
in California. You have these layers on top of layers. So we are actually getting
different colors representing different material sources.
They were either looking up at the top of the mountain or lower down on it.
are and the different thicknesses represent
different amounts a sediment that were washed in. Maybe because they were different
size storms. Maybe this particular
wet season or this flood wasn't nearly as big as
the one before it. You can start accumulating
this history of what happened by looking at these rocks that have
accumulated.
Here are layers in a rock called chert. Sometimes layers can
represent a cycle that's much longer than a year.
This layer that you see right here probably took about a hundred and fifty
years to form as did most to these other layers.
Are those caused by some sort of cyclic patterns and climate?
That's what geologists love to figure out. After the layers formed lots
can happen to them.
So let's say we have this package of layers here and let's say there is a
fault in there. That fault fault causes movement and its going separate the two
layers so that they
look offset, like this. Then a rain storm will come along
and there will be some erosion at the surface. And then maybe
we'll a period of climate change where we'll start depositing more material on top.
You start seeing this very complicated history of things
such that eventually maybe they'll be exposed as cliffs along the beach
like you see here. And you can see that layers tend to start of kind of flat,
but because a tectonic forces and all sorts of great things are happening as
plates crash into each other
you get layers that are now tilted like me standing next to these
layers from millions of years ago.
You can see evidence of faults. Here is me pointing to one and you can see how
these layers go and they got offset
by the fault. That's recorded in
this history book I've layers of rock.
And here, an ancient river channel was carved out from the layers and you can see that
recorded here.
It has still stuck around millions of years later, so we can tell that there was a river
at this very spot
that was about the size of me -- not a very big one but it
happened to be record in the rock layers.
And you get very complicated histories. Here's a picture that we might show if
you take another geology class from us.
We might ask you to try and put all these different layers in order from
oldest to youngest;
knowing all these things about how the rock layers form
originally flat and what can happen to them afterwards.