Field Gear – What I Need for Measuring Section

One of the myriad of things I wind up doing in the field is “measuring section.”

Measuring section is a means of determining the actual thickness of layered sedimentary rocks in an area. To do this, I need a couple of pieces of equipment:

1) A jacob staff and level

Jacob Staff Schematic
Jacob Staff Schematic
The jacob staff and level that I use in the field.
The jacob staff and level that I use in the field. My jacob staff is 1.5 meters long.

and

2) Section measuring sheets.

Don't forget these. You need these to keep track of what you've measured.
Don’t forget these. You need these to keep track of what you’ve measured.

and also

3) A compass for measuring the strike and dip of the rocks.

Vital when measuring section for finding the strike and dip of beds as well as getting your bearings right.
Vital when measuring section for finding the strike and dip of beds as well as getting your bearings right.

This is how it works.

First you have to measure the orientation of the rock layers. Very rarely are rocks simply flat. They’re almost always at some angle. You can read about how this is done on this earlier post of mine.

What’s very important is knowing the angle at which the beds tilt away from perfectly horizontal. This is called Dip. Then you need to know the dip direction, which is the way a marble would roll if it rolled along the rock surface.

Dip (shown in red). The direction of dip is measured as an angle or bearing relative to North, and the dip amount is measured as an angle from horizontal down.
Dip (shown in red). The direction of dip is measured as an angle or bearing relative to North, and the dip amount is measured as an angle from horizontal down.

Once you know that angle, you adjust the inclinometer on the jacob staff to be that angle. Thus, when the bubble is level, the jacob staff is poised exactly perpendicular to the rock layers.

This jacob staff is at the correct angle now.
This jacob staff is at the correct angle now.

Now, you put the jacob staff on the ground and look through a sight that is perpendicular to the staff itself. You are now looking straight down a rock layer the same way that marble would roll.

Using the jacob staff.
Using the jacob staff.

The spot on the ground that you see is where you are going to move the base of the jacob staff next. Once you’ve moved, no matter how far you walked, you’ve gone up 1.5 meters in rock thickness (or however long your jacob staff is). You make some notes on your section measuring sheets describing the rocks you see and then repeat. You do this until you’ve measured all the rocks you want to measure.

You can keep measuring, whether you see rocks or not. As long as the angle is right, you’re measuring the thickness.

Measuring section in Wyoming.
Measuring section in Wyoming. This is what it looks like. Notice that the jacob staff is perpendicular to the rock layers behind my student.

Measuring section is going to be important this summer in the Hanna Basin, so you better believe my measuring gear is going with me.

Published by paleololigo

Scientist (Paleontology, Geochemistry, Geology); Writer (Speculative and Science Fiction, plus technical and non-technical Science); Mom to great boy on the Autism spectrum; possessor of too many hobbies.

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