How to make a stratigraphic column
Stratigraphic columns can be made by using two types of observations
- actual field observations, where a geologist can actully look at and
touch the rocks in front of them, and observations made from well logs.
Becuase well logs come from rocks that are up to 5 kilometers or more
below the surface of the earth, these rocks are not able to be seen. But
the electric well logs that are returned as the drill passes through these
rocks provides much detailed information, enough to make a very complete
To the right is a very simplified diagram of a stratigraphic column.
The most important things to notice here are the changes in rock
type, the thickness of the rock units and the order in which these
rock layers are grouped. A more complete stratigraphic column would
give us other important observations, such as color of the rocks
and the fossils contained within them. This is a simplified diagram
of the geology of Signal Hill, Long Beach, California - one of the
most important oil fields in North America in the twentieth century.
learn about the history
of Signal Hill
learn about the geology
of Signal Hill
Below is a description of how you would go about making a stratigraphic
log if you were out in the field, looking at the rocks in front of you.
1)Look at the rocks
This may seem obvious, but it is important before you begin to get a general
sense of the rocks in front of you. It will make it easier to make a description
of the rocks later if you get a good overview first.
2)Look for changes in rock type
If the rocks in one part of the outcrop appear very different, then it
is possible that they should be their own "unit". Dividing the
rock outcrop into units is based on changes either in fossils, color,
rock type, and other factors, or all of these factors combined.
3)Measure the section
Once you have determined different units, you may then measure the
thicknesses of these units. (Geologists are generally not particularly
interested in the length of the rocks, only the widths.)
Now it's time to make specific observations about the rock you are
looking at. What fossils do you see? What is the color? What kind of rock
is it (sandstone, shale, limestone, or something else)?