A map shows a bird's eye view looking down at the earth's
surface. A GEOLOGIC MAP shows the rocks at the earth's surface
and provides information about what those rocks are doing
underground. Often a geologic map will indicate both the type of
rock and the age of the rock.
A cross section shows the surface of the earth (with or without
topography or elevation) and a vertical slice of the earth. A
GEOLOGIC CROSS SECTION shows the surface and the rocks beneath
that surface. A geologic cross section can be considered similar
to looking at the side of a layer cake. The rocks are like the layers in
the slice of cake. The top of the icing
is the surface of the earth and the layers of cake and icing are
like the different rock layers within the earth.
Since a geologic map shows ONLY the surface of the earth, symbols
must be used to show what the rocks are doing beneath the
Another illustration is a BLOCK DIAGRAM. A block diagram shows the surface
of the earth and also at least one cross section. It is like a block or chunk
cut out of the earth. Again, think of it like a whole slice of cake. You can
move around and look at it from different angles and see the top and the layers of
cake and icing beneath it all at the same time. If you turn it, sometimes you can
see the top and two sides all at the same time. So, block diagrams show the surface,
and at least one side, and maybe two or more other sides, depending on how the position
of the viewer.
Example of a Geologic Map
This map shows two different rocks at the earth's surface. What
rock is at the left and what rock is on the right? This
demonstrates how the rock symbols are used. The shale on the left
has been faulted up to the sandstone on the right. This is
indicated by the letters U for up and D for down. The black line
between the two patterns is the fault. Another geologic symbol is
shown on this map. That is the symbol to show that these sedimentary
layers are horizontal beds. These beds have not been folded, only
broken by the fault.
Example of a Geologic Cross Section
On this geologic cross section, the top line is the surface of the earth. The rocks beneath
the surface are also shown. The rocks layers are horizontal, but they are broken by a FAULT.
A fault is a break in the rock layers along which movement has occurred. A JOINT is a break in the
rocks along which NO movement has taken place. A half arrow could be placed on this illustration
on each side of the fault to show the direction of movement that has taken place. Often, this is
evident from examination of the layers of rock on each side. How many rocks are found at the surface
in this area? What are the rock types? On the left is shale and on the right is sandstone.
Using the Law of Superposition, we can also determine what is the oldest rock shown in the diagram.
It is shale, the shale found at the bottom of the cross section.
Example of a Block Diagram
In this block diagram, the orientation allows us to see the surface of the earth, or the map view,
and two sides or cross sections. Examine the relationship between the part of the diagram that
represents what would be the map, and the two different cross sectional views. A block diagram gives
a better view of the rock layers of the area than either a map or one cross section alone; however, a
geologic map can provide sufficient information for construction of block diagrams through the symbols
that are provided on it. It is very important to keep in mind which you are looking at, a map, a cross
section, or a block diagram. The block diagram is fairly obvious, but you must remain cognizant of what
represents the SURFACE of the earth in each illustration and how this type of illustration communicates
the data about the rock structures.
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