GEOS 304, Lab 4: Introduction to Field Mapping
Spring 2002
Name:____________________
***Important: Be on time and bring colored pencils, a clipboard, water, and any
other field gear you need to be comfortable outdoors.***
Area of Study: Arrastre Wash in the Tucson Mountains
Rock Unit: Lower Cretaceous Amole Arkose Formation
Lithologies: Sandstone, siltstone, shale, and thin beds of limestone
Purposes:
1. To learn about measuring strike and dip in the field (i.e. what to measure and how to
do it).
2. To learn about locating yourself on an aerial photo.
3. To begin learning how to prepare a field map.
What to Do:
The whole lab will take place in the sections of the wash that appears on your photo.
Work individually, comparing notes with others only after you have decided on your
location and measured your own strike and dip. Remember that neatness counts (for 10
points, to be exact), so your map should be clear and easily legible.
1) (10 points) Basics: Mark the corners of the photo on your mylar for future reference.
Make a key, including a North arrow, scale, rock units, and any symbols used (if you
find you need more symbols, you can add them later).
2) (30 points) Take at least 10-12 strike and dip measurements of bedding and plot them
on your map with the proper symbols. These should be distributed in such a way that
you have good coverage within this small map area. Additionally, mark each strike
and dip symbol on the map with a narrow band of color to indicate which unit it was
measured in (we realize this is pretty silly since there is only one unit here, but it’s a
good habit to get into).
3) (30 points) Plot the locations of the 5 pink flags. You should be able to pin these
down on the photo to the nearest bush. They will be in the wash or on a nearby bush
and will be clearly visible.
4) (10 points) Write a brief description of the rock you’re mapping. This should include
(in the following order): color (fresh and weathered), rock name (granite,
conglomerate, etc.), mineralogy, texture (grain size, sorting, etc.), sedimentary
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structures (planar laminations, cross-bedding, etc.), and outcrop characteristics (ledgeforming, cliff-forming, etc.).
5) (10 points) Plot the locations and orientations of the four fold hinges, using the
appropriate symbols.
6) BONUS (10 points) We haven't yet talked about cross-sections, but we will spend
lots of time on them in future labs. Sketch a cross-section of one or more of the
structures you mapped. See Davis & Reynolds p 669-673 for information on crosssections.
Useful Advice
1) Location: This is often half the battle in mapping. You can take the best
measurements the world has ever seen but if you plot them in the wrong locations on your
map, then all you’ve produced is a work of fiction (and not one that will be any threat to
Tom Clancy, either). So how do you locate yourself on the photo? There’s no one way
and what you use will depend largely on what you have to work with but here are some
ideas:
A) Start at some obvious landmark that shows up clearly on the photo, like major
fork in the wash. Note the bearing (compass direction) of each leg of the fork on
your map and make sure those agree with what you observe in the field.
B) Now that you know where you are, the goal is to move up the wash without
losing track of your location. With this in mind, look around and try to correlate
some other features around you with ones on the photo. You should be able to
pick out individual bushes. This is how to confirm your location.
C) Before you move: Look up the wash and pick out a point that you’re going to
walk to. Find that point on the map. This may only mean going to the farthest
bush you can identify or it may mean going to a bend in the wash. It will also
help to pay attention to the bearing of the wash.
D) The key is keeping track of where you are. If you get lost, go back to a known
point and start again. Above all, take your time. This is not a quick process.
2) Strike and Dip: For the purposes of this exercise, you will be measuring strike and dip
of bedding planes--the interfaces between rock layers. These will be defined by a change
in the rock type or mineralogy across the plane. Beware! There are lots of planes out
here, not all of which are bedding planes.
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Map Symbols
(not all of these will be useful today)
Strike and dip of bedding (long line is oriented on map parallel to strike and
short line is oriented in dip direction with dip value written at the end)
Anticline (long line follows the crest—line of zero dip—of the anticline)
Syncline (long line follows the trough—line of zero dip—of the syncline)
Reverse fault (long line follows the trace of the fault on the map; teeth go on
the hanging wall)
Normal fault (long line follows the trace of the fault on the map; bar and ball
go on the hanging wall)
Strike-slip fault (long line follows the trace of the fault on the map; arrows indicate
sense of motion
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