Field Methods Exercise 3 – Prepare a true-scale cross-section
Location: Highway 23 near Newfoundland, NJ, approx 10 miles north of I – 287. Park
in Rest Area approx. 500 feet south of work location, walk CAREFULLY to outcrop
location. PAY ATTENTION to traffic; DO NOT work too closely to the road surface.
Objectives:
 Accurately represent rock structure by creating a true-scale cross-section (X axis
equal to Y axis) using graph paper, with a legend describing map units
 Utilize Brunton compass to measure strike, and especially dip, of bedding
 Describe rock lithology where possible (i.e., use visible mineralogy, color, etc. to
define your mapping units)
Procedure:
The rock face on the northeast side of highway 23 shows an anticline paired with a
syncline. Your job is to accurately portray this cross-sectional “picture” of rock units and
rock bedding. To assist the drawing of this section, we will set flags every 10 feet along
a 150-foot length of the roadcut. You can use these flags to visually “set” your
horizontal axis. A certain number of squares of graph paper (your choice) will represent
10 feet, and the total length of your cross-section will be 150 feet. In a true-scale crosssection there is NO VERTICAL EXAGGERATION. So here at Hwy 23, you will simply
set your graph paper’s Y axis to be identical to the X axis.
You can estimate the total height of the roadcut several different ways. One of the
easiest ways is to have a person of known height (e.g., 5.0 feet) stand directly in front of
the roadcut, while you stand on the center median and use some device (a portion of
your thumb, pencil, etc.) to “scale” this 5 feet. Then you can estimate the height of the
roadcut by using this scaling device.
You can also use your Brunton to determine the angle α between the hypotenuse and
adjacent leg of a nearly-right triangle:
Outcrop
Your Brunton
opp leg
α
Adjacent leg (measure with pace – NOT TAPE!!)
Road Surface
Since tan α = opp / adjacent and you know both α and adjacent, you can solve for the
opposite leg:
Opp = adjacent x α
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A third alternative is to have someone climb around the backside of the roadcut and
drop a tape measure from the top of the roadcut to highway level. Because of the risk
of accident and/or injury from a slide or fall, this method of measurement is NOT
RECOMMENDED. For the purposes of this exercise, one of the first two methods for
estimating thickness of the mappable bedding units will be adequate.
With an accurate horizontal scale, a reasonable method of estimating vertical thickness,
and your Brunton for accurate measurement of bedding attitudes, you can develop an
accurate true-scale cross-section of the bedding units that comprise this roadcut.
You will find it necessary to define your “mapping units” as you create your crosssection. This is a bit more difficult at Newfoundland than at some other sites in NJ with
more variable lithology. Although the color changes between beds are helpful to
ascertain structural attitude, the lithology of the bedding units are somewhat difficult to
discern, because the quartz-rich beds are at least partially metamorphosed to quartzite
and the fine-grained beds, by definition, contain minerals that are too small to be
accurately defined with the naked eye or hand lens. Therefore, you are at some liberty
to define the mapping units as you see them at Newfoundland.
You will have to identify beds or bedding units with some sort of naming convention
(e.g., Unit 1, Unit 2, etc.), and a corresponding lithologic description that will help the
reader understand your critieria for separating these units one form another. For
example, there is a deep purple, fine-grained, mudstone or siltstone that can be
reasonably well-traced along the face of the outcrop. Perhaps this will be a mapping
unit. There is also a quartz-rich unit, perhaps 3-5 feet thick, that has a greenish tint that
may be another useful mapping unit.
BOTTOM LINE (here as in other exercises in this course): You are trying to accurately
communicate, through pictures and descriptions, the structural geology and lithology of
this area.
Project Deliverables:
You will produce a true-scale cross-section with a legend describing map units (color,
lithology, etc.). You will also submit a brief (one-page) description of your work
procedure. If you worked in teams, this write-up will also include a description of who
did what as part of the overall team effort. Each team member will be evaluated based
upon: 1. Quality and accuracy of the group’s work product;
2. Contribution of the individual to the overall group effort
Project Due Date:
The cross-section and write-up are due at the beginning of the second class period
following the initial day of field work.
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