In this clip we're going to talk about different
types of geologic structures and what those
geologic structures look like on a geologic
map. Simplest type of geologic structure is
a fold. There are two basic types of fold.
The first is called an anticline. An anticline
is one where rock layers have been squeezed
upward. In other words, if we apply compressional
forces to these rocks, younger rocks on top
older rocks on bottom, what do we do? What
caused the rock to bend? This is called a
fold, and the particular type of fold we get
is one where older rocks are in the middle
of the fold, and younger rocks are on either
side. In other words, if this fold was to
be eroded and leveled, what would it look
like? We would see older rock in the middle
of the structure, younger rock on either side.
In other words on a geologic map, an anticline
would be an area of older rock with younger
rock on either side. This is the cross section,
a vertical slice showing what the structure
looks like. This is the map view, as if we
were looking on the structure from top down.
We have older rock in the middle, younger
rock on the edges. Syncline is the opposite
of an anticline. A syncline is a downward
fold, which allows younger rock to accumulate
in the middle with older rock 
on either side. What would a syncline look
like on a geologic map? It would have the
opposite age relationship as an anticline.
In a syncline, we're gonna have younger rock
in the middle, with older rock on either side.
Domes and basins are circular structures.
You can think of these as circular anticlines
or circular synclines. A dome is caused by
uplift across a broad region. Such that we
have older rock exposed in the middle with
younger rock surrounded by younger rock around
all the sides. In other words, we've pushed
older rock up from below, erosion has exposed
that older rock in the center of the structure.
We call that structure a dome. A basin is
the opposite of a dome. In a basin, we have
subsidence across a broad region allowing
younger rock to accumulate in the middle and
that younger rock is surrounded by older rock
on all sides. If we were to take a cross-section
through our basin, what would it look like?
It would look like a syncline, younger rock
in the middle, older rock around the edges.
If we took a cross-section through a dome,
a dome caused by uplift exposing older rock
in the middle, what would a cross-section
through a dome look like? It would look like
an anticline. This is a geologic map showing
a portion of the Midwestern United States.
The different colors on the map represent
the age of rock that is exposed to the surface.
In other words, the geologic period. A geologic
period in which that rock was formed. In the
map key or legend, older rock will be at the
bottom, younger rock will be at the top. In
other words, Precambrian rock is the oldest
rock exposed in this map area, and Cretaceous
rock is the youngest rock exposed in this
area. We can look at the relative ages of
rock units to infer the type of geologic structure
shown in any particular part of the map. For
example, what would a dome look like on the
map? Again a dome, we have older rock in the
middle, younger rock around the edges. In
other words to recognize a dome on a geologic
map, you're looking for a circular pattern
and the relative ages of those rocks should
place the older rocks in the middle, and the
younger rocks around the edges. How many domes
can you find on this map? Well this is a dome.
This dome was called the Ozark dome, it's
located in southern Missouri. We can find
that we have old Precambrian rock in the middle
surrounded by younger Cambrian, Ordovician
and even Mississippian rock. How many other
domes can you find? Well, here's another dome.
We have older Ordovician rock surrounded by
younger Mississippian rock. Are there any
other domes in the map area? There's this
one here. This is called the Cincinnati arch.
We have older Ordovician rock surrounded by
younger Silurian and Devonian rock. Again
we have identified three domes in the map
area. Places where we've had uplift across
a broad area and then as erosion wears away
at that uplift, we find progressively older
rock exposed in the middle of the structure.
Progressively older rock in the middle of
the structure surrounded by younger rock.
Basin is the opposite of a dome. Here we have
subsidence across a broad region. That subsidence
allows younger rock layers to accumulate in
the center of the structure, an older rock
layers surrounded on all sides. How many basins
can you find in the map area? There's one
basin here. This is the Illinois basin. We
have younger Pennsylvanian rock surrounded
by older early Pennsylvanian and Mississippian
rock. Can you find any other basins in the
map area? There's another one here we have
young Permian rock surrounded by compressed
progressively older Pennsylvanian and Mississippian
rock. Finally, we have a basin here on the
Lower Peninsula of Michigan. The Michigan
basin. Youngest rock and the structure is
Jurassic in age. It's surrounded by progressively
older Pennsylvanian, Mississippian, and Devonian
rock. Again, basins caused by subsidence across
a broad area, allowing progressively younger
rock layers to be formed or preserved in the
center of the structure. Older rocks surround
the edges of the structure. This figure shows
a cross-section, a vertical slice through
this part of the central US revealing the
Nashville dome, an area of uplift exposing
progressively older rock and the Illinois
basin an area of subsidence, allowing progressively
younger rock to accumulate or be formed in
the center of that structure. This vertical
slice shows that a basin is basically a circular
syncline, whereas a dome is basically a circular
anticline. Can you find any anticlines in
the map area? Anticlines would be an area
of older rock with younger rock on either
side. There's an anticline here. That anticline
has Devonian rock in the middle, surrounded
by younger Mississippian rock. There's an
anticline here. We have older Ordivician rock
in the middle, surrounded by younger Mississippian
and Pennsylvanian rock. What about synclines?
Can we find any synclines in the map area?
A syncline would have younger rock in the
middle, older rock on the edges. The opposite
of an anticline. Can you find any synclines
in the map area? Well, what about this one
here? Here's a syncline. We have younger upper
Pennsylvanian rock surrounded by older lower
Pennsylvanian rock. So this part of the map
area, the relative ages of rock units exposed
in that part of the map area reveals the presence
of a syncline.
