| In this lab exercise we will have a
detailed look at the geology of the Trinity campus, learn how to read and use
topographic maps and get a first look at geological maps. You will
also learn how to use a geologist's compass to measure the orientation of
rock layers and how to represent your findings in a geological
cross-section. We will take a quick trip to the west edge of campus on Summit Street. Though the hike is not strenuous you should wear sturdy shoes, no sandals or open toed shoes please. Make sure you bring your lab notebook and a few pens and pencils. |
a) Topographic MapsOur first task today will consist of locating ourselves with the help of a topographic map. Even in the age of GPS units that can tell you the location of any Dunkin' Donuts within a five mile radius paper maps are still an important tool in geology. They are cheap, don't use any batteries, do not depend on any satellites and are a great way to represent the lay of the land and lots of other information in a convenient and efficient way.The United States Geological Survey (USGS) is the nation's prime maker and publisher of topographic maps. Topographic maps display the shape of the Earth's surface, its topography. All information printed on a map is a fairly straightforward and relatively intuitive way. The USGS has an excellent website about its maps, the symbols commonly used and a whole lot more. If you have never looked at a topographic map I strongly encourage you to check out these links.
Contour LinesContour lines are one way to represent topography on a map. They are the light brown lines that snake all over most topographic maps. Contour lines, or contours, represent lines of constant elevation as shown in the classic (and often copied) figure below. In order to interpret contour lines one has to know the elevation difference between two contours and which way goes up or down. The elevation difference between two contours is called the contour interval and it is usually printed somewhere on the map (most often at the bottom, near to the scale bar and other information). To figure out which way is up (or down) some (generally every fifth) contours are labeled in feet or meters above sea level. That can be the only way to figure out what is a mountain or a depression in the landscape. Often, however, it is pretty obvious what constitutes a hill or a valley. Hills have often closed contours, valleys or depressions are often filled with water or have streams running in them. Check out the example images below.The spacing of contours tells us something about the steepness of a slope. Tightly spaced contours indicate steep slopes, while gentle slopes are represented by widely spaced contours. Ihe images below illustrate this point. Contours do not necessarily have to represent topography. You might have noticed them in your newspaper, where they might represent daily temperatures on the weather page or the concentration of pesticides in groundwater. If you have taken a few math classes you might have have come across contours when studying functions. Math folks generally call them level curves. |
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| Detail from USGS topographic map, showing Dead Horse Point State Park in Utah. (view from Dead Horse Point) Note the wide contour spacing on the top of the plateau, where theroad and campsite are, and the steep canyon walls indicated by narrowly spaced contours. | Surface weather map, courtesy of the
weather channel. In this
example contours denote lines of constant barometric pressure and
separate high from low pressure areas. |
This contour map, also fro the weather
channel's website, does not display contour lines per se, but uses a color
code to separate regions bounded by contours. It also overlays the
data that was used to generate the contour
map. |
Map ScaleAnother important thing we have to know when
interpreting maps is its scale. Map scale refers to the ratio
between the distance on the map to the distance on the Earth's surface.
Map scales can be given in several ways. The map you are using
for this exercise, for example, has a scale of 1: 24,000 which is sometimes called the representative fraction. It means that one unit on the map (and any unit: in, ft, cm, mi, km will work) represents 24,000 units on the ground. In this case the scale is "one to twenty four thousand". A different way of expressing scale is by stating it explicitly as in : 1 inch equals 1 mile, which would correspond to a scale of 1:63360 (I had to look that up). Finally, scale can also be represented by a scale bar as the one shown in the figure below. Most USGS maps have both a scale bar and the scale printed on the bottom of the map sheet. The image below also gives you information regarding the contour interval and which coordinate system (map datum) was used in constructing the map.  Maps come in different scales and people talk about
large scale and small scale maps. Large and
small refer to the numerical value of the representative fraction.
The value of the ratio1:5,000 is larger than 1:2.5 Million.
Therefore a map printed at a scale of 1:5000 would have a larger
scale than a map printed at a scale of 1:2.5 Million. The images
below show examples of large and small scale maps covering the city of
Chicago. Which one shows more detail, which one covers more ground?
The three maps below were downloaded from a (unfortunately no longer available) EPA website. (http://www.epa.gov/ceisweb1/ceishome/atlas/learngeog/mapscale.html) |
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| scale = 1 : 24000 7.5 minute topographic map (large scale) |
scale = 1 : 100000 |
scale = 1 : 500000 (small scale) |
How to orient a map, or which way is North?Most of you might know that, unless its stated otherwise, North is generally on the top of the map. This is true mostly. When making maps cartographers have to project a spherical surface (the Earth's) onto a flat sheet of paper. There are several ways of doing that, with some of them being more useful than others. As a result of some unavoidable projection errors North is pretty much always somewhere near the top. In other words: a vertical line on a map points close to north, but not necessarily exactly to North.For a good overview of map projections clicke here.  A further complication results from the way we
orient a map in the field by using a compass. A compass points
towards magnetic north (the location of the Earth's magnetic Soutth pole -
yes South pole), which is often close, but not always the same as
geographic North (the location of the Earth's rotational axis, or
geographic North pole). This angular deviation is known as
magnetic declination and its magnitude and direction is shown
somewhere on the map (at the boottom for USGS topo sheets).The figure to the right shows the relationship between Geographic North, Magnetic North and vertical lines on the map. Due to the projection error vertical lines (denoted by GN, or grid-North) do not aligh perfectly with true geographic North (denoted by a small star). The different locations between the magnetic and geographic poles are responsible for the angular deviation between magnetic North and geographic North. Magnetic declination is generally rather small (less than a few degrees), but can become substantial at high latitudes. In this lab we will show you how to set your compass for the local value of declination. For a good overview of magnetic declination check out this website by the Canadian Geological Survey, from where the figure to the right was taken from. If you would like to get the geomagnetic declination for (pretty much) any location on earth, click here. |
 Besides represent topography geologic maps do show the
distribution of rock types or other geological features. One way to
distinguish geologic maps from topographical maps is that they are much
more colorful. On geological maps color represents not only the type
of rock found in a given area, but also its age. We will learn more
about this in one of the following labs.Andrew Alden, a geologist who runs a Geology site for About.com has written an excellent introduction to geological maps. The geological map of Maine was downloaded from Andrew's collection of online geological maps. |
 You've learned in part a) how contours are used to
represent topography on maps. It is sometimes useful to construct a
cross-section or topographic profile through a map in order to fully
understand the landforms and rock formations assocated with it. The
following images will guide you through the process of constructing a
topographical profile from a contour map.We start out with a 3D drawing of a hill (Fig. to right). It has contours drawn on top of it and the bottom of the figure shows an approximate (I drew this thing by hand). Also indicated on the figure is the trace of the cross-section we intend to construct. On the map view this cross-section is labeled A-B. You can click on the image to see the original. enlarged version. |
 A cross-section shows us what we could see if we sliced
through the hill as indicated in the figure to the right. In geography we
are mostly interested in the shape of the topography. In geology we
will use this simple cross-section and add geological information as it is
done in this
example, which was taken from J.D. Roger's website on
the Grand Canyon. |
 We start constructing our cross-section by taking a paper
strip (a 8.5 x 11 sheet, folded lengthwise works great) and align it with
the trace of our cross-section as shown in the figure to the right.
Then we mark the contact of each contour map with the paper strip by
a small tick mark. It is also helpful to label as many of these
tick marks as possible and add the position of prominent features to your
paper strip. This will help you later to figure out the position of
your cross-section. |
 Now that all the important information is recorded on your
paper strip we can construct our cross-section. Draw a horizontal
line. This is the baseline of your cross-section. Its scale is
determined by the scale of the map (unless you enlarge or shrink your
paper strip). The choice of a vertical scale is up to you. If
your vertical relief is small you might have to choose some vertical
exaggeration. Its up to you, but you should keep in mind that
vertical exaggeration can complicate the addition of geological features
later because angular measurements have to be adjusted to the change in
vertical scale. Instructors often tease vertical exaggerators that
they want to impress their friends with the steep mountains they climbed
in geology class... |
 We are almost done. Align your paper strip with the
baseline of your cross-section and carefully plot the contour lines at the
appropriate elevation as shown in the figure to the right. A profile
of the hill should slowly emerge. Make sure you don't skip any of
the ticks and keep your vertical elevations straight. |
 Finally, connect the dots and you are done. There is
one small, almost philosophical, aspect to be considered. In
the figure to the right the dots are connected by straight lines.
Sticklers will tell you that is all the information you recorded on
your paper strip and that's the way to do it. However, you might
have noticed that those edgy mountains are rarely observed in nature.
So it is perfectly OK to smooth out the corners a bit to make it
look more realistic. |
| Adding geological information: We will measure the orientation of rock layers in the field and add this information to our cross section. The orientation of any plane can be described by two angles, which geologists call strike and dip. The strike angle is the angle of a horizontal line that is drawn on your plane with geographic North. The dip angle is measured perpendicular to the strike measurement and measures the angle between the bedrock plane and an imaginary horizontal plane. If your bedrock plane was described by a contour map the strike would be parallel to the contour lines, while the dip angle is measured perpendicular to the contour lines. In math the dip angle corresponds to the gradient. Your book has a section on strike and dip in it the two web links below will tell you how they are measured in the field. Strike and dip measurements are best demonstrated on real rocks, so we'll cover it when we actually have something to measure. How to measure Strike and Dip (U-Calgary). The following images show you examples of some geological crossections. The image to the right combines a block diagram and a crossection. |
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| Mt. St. Helens crossection (from USGS) |
Hiiaka crater crossection (from Volcanoworld) |
from Manual of Seismic
Observation Practice |
References:
Images downloaded from other sites are either
referenced in the text or link to the original site. All other images were
drawn for this lab exercise.
