Geoscience Reference
In-Depth Information
beds when the non-resistant bed is covered by beach deposits)
or to check the cumulative thickness of a series of dipping
beds. If the thickness of a bed is critical to the objective of the
fi eldwork it is always a good idea, if possible, to measure the
thickness in more than one way, for instance by measuring
each bed individually and then by measuring the thickness of a
group of beds.
horizontal
distance(d)
d
θ
true
thickness
2
d
2.5.2 Use of the Jacob staff to measure the
thickness of inclined strata
θ
true
thickness
It is diffi cult to measure the thickness of inclined strata
exposed on a slope such as a hillside if, as is nearly always the
case, the slope of the hill does not cut the strata at 90° (Figure
2.15). A Jacob staff and Brunton-type compass-clinometer can
be used to measure such sections quickly and easily. They can
also be used together as a very accurate measure of vertical
distances.
Figure 2.14
Sketch to show the
basic trigonometry for obtaining a
true thickness by measuring the
horizontal distance between
dipping beds.
A Jacob staff can either be purchased or fashioned from a
length of wood about 2 cm × 5 cm and between about 1.25 m
and 2.0 m long (choose a length that is just below your eye
level and which is convenient for multiple measurements).
Mount the compass-clinometer with its east side (so the
clinometer is at the base) in contact with one end of the staff.
Ensure that the end of the staff is at exactly 90° to the length of
the staff so that the compass-clinometer is perpendicular to the
staff. For an exact measure, remove from the base of the staff
the distance that is equivalent to the few centimetres between
the long edge of the compass-clinometer and the centre of the
sights. Fold the peep sight at the end of the long sight
in at 90°.
Angle the mirror so that you can see the compass window
when looking through the peep sight and hole in the mirror. To
measure the thickness of the beds complete the following.
Figure 2.15
Sketch to show
how to measure thickness up a
slope with a Jacob staff of 1.5 m
length and a Brunton-type
compass-clinometer.
1. Measure the strike and dip of the strata you are
going to calculate the thickness of (Section 2.3.1) and
work out a line of traverse that is perpendicular to the
strike.
2. Set the clinometer on the Brunton-type compass-clinometer
to the angle of dip of the strata.
3. Starting at the base of the traverse, place the base of the
Jacob staff on the point in the stratigraphy you wish to
measure from and lean the Jacob staff down until the
bubble in the long spirit level is in the middle (i.e. the
Jacob staff is perpendicular to the bedding plane).
4. Sight through the peep sight and window in the mirror to a
point on the ground ensuring that you keep the long spirit
level horizontal. This point on the ground is the length of
