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 stratigraphic log.

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.)

4)Start describing!
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)?

 

 


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