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
and
2) Section measuring sheets.
and also
3) A compass for measuring the strike and dip of the rocks.
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.
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.
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.
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 is going to be important this summer in the Hanna Basin, so you better believe my measuring gear is going with me.
Penny Higgins - Storyteller • Artist • Scientist 