that appears to be different from whatever is here, and at the
top here, see this dark band that goes across here like this?
Is another contact, above which there's very different material on top.
We're going to talk about these three
different regions as these three regions are
going to correspond to three main environmental
conditions that these rocks found themselves in.
First, let's look at just this bottom layer, in fact, the
contact between this very bottom layer and the next layer up.
Here's an image for another point of view of that contact.
And the contact is right across here, this is that middle unit.
You can see the upper unit up through here too, dark, it's always dark stuff,
with light stuff right on top of it and it goes to the top here.
And while this middle unit is a series of relatively straight layers in
through here, notice that this bottom unit has What is called cross bedding.
There are surfaces like this that intersect other
surfaces like this, that intersect other surfaces like this.
Cross bedding, in terrestrial geology, is typical of
what you find in things like dune fields.
Here is, for example, an ancient dune field
in Zion National Park, National Park Service picture.
And these cross beddings are things like this,
where you have the layers come in like this.
Cross layers like this, layers like this.
You can very clearly see the patterns going up and crossing.
You can certainly see why it is called cross bedding.
In cross bedding in general forms as I said before from things like dunes.
Imagine if you have a dune, a series of dunes, dunes crest that look like this.
The way a dune works is that sand it gets
blown eroded off the top here blown up to the top.
And falls back down on the side, and deposits here.
As it deposits here, it forms new layer, and a new layer, and
a new layer, and a new layer, and the same thing's happening over here.
New layer, new layer, new layer.
And, eventually, these new layers, will intersect, as
these dunes continue to march across here, and
you get this cross-bedding of things at this
angle, things at this angle intersecting and continuing on.
This is exactly the sorts of things that we're seeing at Mars in that bottom layer.
What forms cross-bedding like that?
Well, wind-blown dunes.
To get that cross-bedding, you need the surfaces to be easily erode And
deposit on the other side, and that is exactly how dunes are made.
That's exactly the process that makes that cross-bedding.
An interesting thing happens with that cross-bedding though.
Again, here's that contact between the, the lower and middle unit.
And notice it's not the best image to see it in but you can sort
of see that these, this cross bedding goes right up to this edge and basically stops.
What you should imagine might be going on is that there
was a surface of dunes being cross bedded like they normally are.
And for some reason the tops of the dunes all got cut off at some subsequent date.
Why are the tops of the dunes all get cut off?
And why would they get cut off at a particularly straight line like this?
These sorts of things are seen on the Earth when the
water table rises up to a certain level, the water table rises
up to right here in an, in an, dune field And cements
those grains into place, hardening them, making them harder to blow away.
The dew, the water level never rises up to here, or even higher.
It could've gone quite a bit higher.
The water level never gets any higher than this.
The wind then subsequent, subsequently comes, and removes this entire surface,
in a period of what's called erosion, erosion by the wind.
This is the first evidence we've heard of actual water at this site.
But it's a different sort of water than we've been
talking about in almost every other evidence we have for water.
Here its just everyday typical ground water
that rises and presumably falls back down again.
Well it certainly was low at the beginning because dunes like
that don't form in the presence of water table like that.
We'll see exactly what does happen in the
presence of a water table, in just a second.
Okay, so, we have this flat layer with the dunes cut off
that looks like something that would be formed because the water table rose.
Let's see what happend after that process.
Here's a small section from that middle unit where the water table had risen,
the wind had blown away the dunes
that were there, and then, something else happened.
Now, there's no way to know how much time went between, the
wind blowing away what was there and whatever else happened after that.
That's one of the problems in geology, there are big regions of
time that are lost when things get blown away or washed away.
But, whatever happened next looked something this.
This is a pretty small area, and you see these fine layers,
and you see these fine layers are a little bit lightly cross-bedded.
There are very low angle cross-beddings all throughout here.
It's not nearly as severe as we were seeing in that, in that lower layer.
And this is the sign, not of dune formation, but of, of simply sand sheet.
You can imagine, a big, flat area, that's
just getting covered with sand, and re-covered with sand.
Slightly mounded in one direction, slightly mounded in the other.
This is exactly what forms in those sorts of things.
Why do you form sand sheets, instead of dunes?
Well, one reason is, if you are slightly wetted as you're being deposited, and
so you can no longer make the dunes my marching up, and eroding and redepositing.
You just make these flat sand sheets that are kind of stuck together.
They don't erode as the wind blows nearly as much.
As that middle layer was growing up sheet by sheet by
sheet, the water table would've been close to the bottom of it.
