| for lab handout click here | back to GEOS 112 lab page |
| In our first lab exercise we will
have a first 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 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 labelled 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 twentyfour tousand". 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 scalebar 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 anp 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 crossection 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 crossection we intend to construct. On the map view this crossection is labelled A-B. You can click on the image to see the original. enlarged version. |
 A crossection
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 crossection
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 crossection by taking a paper strip (a 8.5 x 11 sheet,
folded lengthwise works great) and align it with the trace of our crossection
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 tickmarks 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 crossection. |
 Now that all the important information is recorded on your
paper strip we can construct our crossection. Draw a horizontal line.
This is the baseline of your crossection. Its scale is determined by
the scale of the map (unless you enlarge or shrink your paperstrip). 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
vertiacl 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 crossection 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.
