Measuring a Stratigraphic Section with a Jacob’s StaffDawn Sumner 1/2005

(Based on Compton, Robert R., 1985. Geology in the Field. Wiley Press, New York, pp. 229-234.)

The goal of measuring a stratigraphic column is to accurately characterize the thickness ofdifferent rock types. This data can be used to interpret depositional environments, variations insediment type, changes in sedimentation rate in space and time, etc. Most detailed interpretationsrequire an accurate measure of how much of each rock type is present, bed thicknesses, etc. It iseasy to measure the thickness of flat-lying beds; you can put a ruler next to them and measurefrom bottom to top. For beds that have been tilted or folded, more care is needed. Measurementsneed to be made perpendicular to the bedding. Otherwise, results will depend on the amount offolding and the direction you are looking rather than the depositional processes that formed therock.

Geologists often use a Jacob’s Staff to measure bedding thicknesses. A Jacob’s Staff is a 1.5meter-long pole that is marked off in suitable units, such as decimeters. It has an attachment for aBrunton at 1.5 m above the base of the pole. The Brunton is used as a clinometer to measure theangle of the pole from vertical and helps align the Jacob’s Staff perpendicular to bedding foraccurate measurements.

To measure bed thickness, place the Jacob’s Staff on the bedding plane at the base of the bedsyou want to measure. Next align the staff at right angles to bedding and sight downdip,perpendicular to strike, to the beds. The distance from the base of the staff to the sight point onthe Brunton is equal to the thickness of strata between the base of the staff and the point sighted.There are a number of steps for doing this measurement accurately:

1. Measure the strike and dip of bedding where you intend to measure the section; record the dataand set the clinometer on the Brunton to the angle of dip.

2. Place the Brunton securely in the attachment on the Jacob’s Staff, and open the compass lidabout 60°.

3. Place the staff at the base of the unit to be measured and tilt it downdip (exactly perpendicularto strike) until the clinometer bubble in the Brunton is centered.

4. Study the point sighted on the ground and decide if the staff can be placed on it for your nextmeasurement; if so, note the point carefully by eye or place an object at that point. You havemeasured 1.5 meters of section.

5. If the base of the Jacob’s Staff can not be placed on the point you sighted for your nextmeasurement, move the base of the staff along the lower bedding surface until a suitable pointcan be sighted.

6. Draw your stratigraphic column, describing the rocks in this unit. Measure the positions ofbeds within this 1.5 meter-thick interval using the Jacob’s Staff or a ruler.

7. Move the base of the Jacob’s Staff to the sited point, and make your next measurement.Proceed similarly to the top of the unit.

Using a Jacob’s Staff and Brunton requires sighting through a small hole, and it may be temptingto save time by estimating the alignment of the staff rather than using the clinometer. Moderateerrors in alignment, however, can cause large errors in measurements (Fig. 1). In addition, whensighting up or down a slope, one tends to tilt the staff so that it is perpendicular to the groundssurface. This gives errors that tend to accumulate through a series of measurements, giving asystematic over or under estimate of true stratigraphic thicknesses. When the staff is correctlyoriented with the clinometer, the error should be no more than a few centimeters per measure andwill tend to average out in successive measurements. Thus, it is worth learning to measureaccurately now; sighting becomes easier with practice.

Figure 1: Errors in measurement (Figure from Compton, 1985, p. 231).

Sighting with a Jacob’s Staff becomes increasingly awkward as dips become steeper. Lines ofsight typically become shorter, however, so accuracy is maintained (Fig. 2A). For dips greaterthan 70°, the geologist can kneel and look along strike, viewing the clinometer face on andmaking the projection to the ground by estimation (Fig 2B). If some beds stick up significantlyhigher than others, it may be necessary to measure the thicknesses of the low-lying beds andweathering-resistant beds separately, using modified techniques. The UCD Geology Jacob’sStaffs have a fixed mount for the Bruntons. However, you can site distances shorter than 1.5meters by removing the Brunton from the mount and carefully holding it along the edge of theJacob’s Staff in the proper orientation. You can place the Brunton anywhere along the staff tomeasure an arbitrary thickness. Be careful to accurately record the thickness represented by eachmeasurement, especially if they are different for each measurement.

Figure 2: Jacob’s Staff orientations depend on the relative dip of bedding and slope (Figure fromCompton, 1985, p. 231).