Geomorphology
11. Alpine Glacial Landforms
11. ALPINE GLACIAL LANDFORMS
40 Points
One objective of this exercise is for you be able to identify alpine glacial landforms and measure their characteristics. A
second objective is for you to improve your skills at reading and interpreting topographic maps, at correlating information on
aerial photos with information on topographic maps, and to use stereoscopes to aid in landform identification.
YOU SHOULD BE ABLE TO:
•
Correctly read and interpret topographic maps, including determining spot elevations, calculating distances, areas, and
•
Identify alpine glacial landforms on topographic maps and in stereo air photos; and
•
Create a topographic profile at a given vertical exaggeration.
gradients;
TOPOGRAPHIC MAPS
Good topographic map reading skills are important for all geomorphologists. Part of the study of geomorphology involves
determining landform characteristics, including their size and shape. Topgraphic maps provide a means to do such measurements.
Map Scale
The scale on topographic maps is presented as both a representative fraction, such as 1:24,000 − a common scale for many
topographic maps in the US, and as a graphic (Figure 11.1). The representative fraction is most useful because any unit of
measurementinches, centimeters, millimeters, or feetmay be used with this fraction. With a scale of 1:24,000, one unit of
measurement on the map equals 24,000 units of measurement on the earth’s surface. To convert measurements on the map to
actual earth measurements you can either convert the map distance straight to an earth distance (Table 11.1), or you can change
the units associated with the denominator of the representative fraction and then convert the map distance to an earth distance
(Table 11.1). If you need to calculate multiple distances from a single topographic map, the second method is probably
preferable because you only need to alter the units on the map scale once and then you can use that for all your future
calculations.
If you need to calculate areas from a topographic map, you can still use the representative fraction, but you must square
the top and the bottom of the fraction and all the units associated with the fraction (Table 11.2).
FIGURE 11.1 Graphic Map Scale
K.A. Lemke – UWSP
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11. Alpine Glacial Landforms
Geomorphology
TABLE 11.1 Using the Representative Fraction to Calculate Distances and Areas
Converting map distances to earth distances:
Assume 4 inches separate two points on a map. Calculate how many miles separate these two points on the earth’s surface.
Method 1: convert the four inches on the map to inches on the earth’s surface using the representative fraction and then convert the
inches to feet and the feet to miles:
4 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒 24,000𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒ℎ𝑖𝑖𝑖𝑖𝑖𝑖ℎ𝑒𝑒𝑒𝑒 1𝑓𝑓𝑓𝑓
1𝑚𝑚𝑚𝑚
×
×
×
= 1.5𝑚𝑚𝑚𝑚
1
1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ
12𝑖𝑖𝑖𝑖 5280𝑓𝑓𝑓𝑓
(1)
Method 2: convert the numerator of the representative fraction from inches to feet to miles and then use the modified representative
fraction to convert inches on the map directly to miles on the earth.
24,000𝑖𝑖𝑖𝑖𝑖𝑖ℎ𝑒𝑒𝑒𝑒 1𝑓𝑓𝑓𝑓
1𝑚𝑚𝑚𝑚
0.38𝑚𝑚𝑚𝑚
×
×
=
1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑐𝑐ℎ
12𝑖𝑖𝑖𝑖 5280𝑓𝑓𝑓𝑓 1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ
(2)
The map scale can now be written as 1inch:0.38 miles. Thus:
4𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒
0.38𝑚𝑚𝑚𝑚
×
= 1.5𝑚𝑚𝑚𝑚
1
1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ
(3)
TABLE 11.2 Using the Representative Fraction to Calculate Distances and Areas
Converting map areas to ares on the earth’s surface:
Assume a landform covers an area of 4 square inches on a map. Calculate how many square miles this landform covers on the earth’s
surface.
Method 1: convert the four square inches on the map to square inches on the earth’s surface using the representative fraction and
then convert the square inches to square feet and the square feet to square miles:
𝑋𝑋𝑚𝑚𝑚𝑚 2 =
=
4 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒 2 24,0002 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒ℎ𝑖𝑖𝑖𝑖𝑖𝑖ℎ𝑒𝑒𝑒𝑒 2 12 𝑓𝑓𝑓𝑓 2
12 𝑚𝑚𝑚𝑚 2
×
×
×
1
12 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑠𝑠 2
122 𝑖𝑖𝑖𝑖2 52802 𝑓𝑓𝑓𝑓 2
(4)
4 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒 2 576,000,000𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒ℎ𝑖𝑖𝑖𝑖𝑖𝑖ℎ𝑒𝑒𝑒𝑒 2
1𝑓𝑓𝑓𝑓 2
1𝑚𝑚𝑚𝑚 2
×
×
×
= 0.57𝑚𝑚𝑚𝑚 2
2
2
1
1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒
144𝑖𝑖𝑖𝑖
27,878,400𝑓𝑓𝑓𝑓 2
Method 2: convert the numerator of the representative fraction from square inches to square miles and then use the modified
representative fraction to convert the square inches on th e map directly to square miles on the earth. We can use the conversion in
equation (2) above to speed our calculations here.
0.382 𝑚𝑚𝑚𝑚 2
0.14𝑚𝑚𝑚𝑚 2
=
12 𝑚𝑚𝑎𝑎𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝ℎ2 1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ2
(5)
The map scale can now be written as 1 square inch:0.14 square miles. Thus:
4𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒 2
0.14𝑚𝑚𝑚𝑚 2
×
= 0.56𝑚𝑚𝑚𝑚 2
1
1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ2
86
(6)
K.A. Lemke – UWSP
Geomorphology
11. Alpine Glacial Landforms
Small scale maps have a smaller representative fraction than large scale maps. A of scale of 1:100,000 is a smaller scale than
1:24,000. This is obvious if the representative fractions are written as fractions rather than as ratios:
1
1
<
100,000 24,000
The same feature is smaller (i.e. it takes up less space) on a small scale map than on a large scale map. A small scale map is
like a photo taken from far away: lots of land area is included on the map but all the features on the map are really small. A
large scale map is like a close-up photo: not much land area is included on the map but all the features are really large. This
difference in scale is particularly important to keep in mind if you try to compare landforms using maps with different scales.
To calculate gradients, determine the difference in elevation of the two points of interest and divide by the distance
separating those two points:
𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺𝐺 =
∆𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸𝐸
𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑
(7)
If the two points of interest do not fall on a contour line, the best estimate of their elevation is half way between the elevations
of the neighboring contour lines. If one of the points of interest is the top of a hill, the best estimate is the elevation of the
highest contour line on the hill plus half the contour interval. If the point of interest is the bottom of a crater, the best estimate
of the elevation is the elevation of the lowest contour line in the crater minus half the contour interval. To compare the gradient
in one region to the gradient in another region examine the contour line spacing: the closer the contour lines, the steeper the
gradient. Assessing gradient in this manner, however, doesn’t give you a number; it just allows you to assess the relative gradient.
The shape of contour lines is also an important factor for interpreting topographic maps correctly. The rule of V’s states
that whenever contour lines cross rivers, they bend upstream; the contour lines form V’s and the point of the V points upstream.
Given that this is true, we should also expect contour lines crossing valleys to bend up-valley (considering a river channel
similar to a small valley). When contour lines cross ridge tops, they should bend in the opposite direction, they should bend
down-hill (Figure 11.2).
FIGURE 11.2 Ridges and Valleys on Topographic Maps
Purple lines represent ridges; note that contour lines are closed (representing high spots) or bend (point) toward lower elevations
(downhill). Blue lines represent streams in valley bottoms; note that contour lines bend (point) toward higher elevations (uphill).
K.A. Lemke – UWSP
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11. Alpine Glacial Landforms
Geomorphology
Topographic Profiles
Profiles provide scaled pictures of the shape of the landscape along a particular line or path (Figure 11.3). To draw a profile,
tic marks representing every contour line along the profile path are placed along the x-axis of a piece of graph paper. These tic
marks have the exact same spacing as the contour lines on the map; the tic marks are not evenly spaced along the axis. The easiest
way to transfer this information from the map to the profile is to lay the graph paper along the profile line and mark tics for every
contour line that intersects the graph paper. Because the tic marks have the same spacing as the contour lines on the map, the xaxis of the profile also has the same scale as the map.
The vertical axis represents elevation in either feet or meters. To draw a profile, plot a point directly above each tic mark at
the appropriate elevation. Then connect the dots with a smooth line. Just as the x-axis has a scale, the y-axis also has a scale, but
the vertical axis on most topographic profiles is exaggerated compared to the horizontal axis in order to display the topography
more clearly. Without any vertical exaggeration, many profiles (and thus many landscapes) would look flat when in reality, on a
human scale these landscapes may be quite hilly or steep. There is no rule for selecting an appropriate amount of vertical
exaggeration; selecting an appropriate amount of exaggeration depends on the purpose of the profile, the amount of detail required,
and the relief of the landscape.
Once you have determined how much vertical exaggeration you want, you need to set up the y-axis at this scale before plotting
your points. The first step involves modifying the representative fraction so that the denominator has the same units as the elevation
marks on the map, either feet or meters. The numerator of the representative fraction will remain inches or centimeters. For
example, if you have a map with a scale of 1:24,000, 1 inch on the map equals 24,000 inches on the earth. Convert the 24,000
inches to feet: 24,000/12 = 2000 ft. The scale is now 1 inch: 2000 feet. Since the x-axis of the profile has the same scale as the
map, one inch on the x-axis represents 2000 feet across the earth’s surface. For the y-axis, replace the one in the numerator with
whatever you want for a vertical exaggeration without changing the units. For example, with a 10 times vertical exaggeration, the
scale of the y-axis is 10 inches: 2000feet, in which case 1 inch on the y-axis equals 200 feet. For a 20 times vertical exaggeration,
the y-axis scale would be 20 inches: 2000 feet, in which case 1 inch on the y-axis equals 100 feet. Set the lowest elevation on the
y-axis to a value slightly less than the lowest elevation along the profile line and then add the appropriate number of feet for every
inch along the y-axis.
AERIAL PHOTOS AND STEREOSCOPES
Mirror reflecting stereoscopes allow us to see the landscape in three-dimensions provided we have two photographs of the
same location taken from two different points of view. A camera mounted on the bottom of an airplane takes a continuous series
of overlapping photos of the land beneath the plane. We can view these photos, two at a time, to see the landscape in three
dimensions. The first step for stereo viewing involves determining the area of overlap for the two photos of interest. Lay the
photos on top of one-another, lining up the area of overlap and the top and bottom edges of the two photos (Figure 11.4). Place
the photos under the mirror reflecting stereoscope and pull the two photos apart from one another. The photos will not overlap
while viewing them in stereo. Select an obvious, well-defined feature common to both photos. While looking through the
stereoscope, slowly move the photos farther away from one another until the feature you selected merges to form a single feature.
When correctly aligned, the landscape should literally pop out of the photo at you.
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K.A. Lemke – UWSP
Geomorphology
11. Alpine Glacial Landforms
FIGURE 11.3 Sample Topographic Profiles
K.A. Lemke – UWSP
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11. Alpine Glacial Landforms
Geomorphology
FIGURE 11.4 Overlapping Photos for use with Mirror Reflecting Stereoscopes
Photo 1
Photo 2
Photo 3
Area that can be seen in Area that can be seen in
stereo with photos 1 and 2. stereo with photos 2 and 3.
Photo 1
Photo 2
Photos should not overlap when looking at them under the
stereoscope. Adjust the distance separating the photos until
they come into focus and you see just one image.
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K.A. Lemke – UWSP
Geomorphology
11. Alpine Glacial Landforms
11. ALPINE GLACIAL LANDFORMS
40 Points
1.
Identify all examples of the glacial landforms listed below on the accompanying topographic map and air photos (Figures
11.5 and 11.6). Work on the photos and maps simultaneously; the photos will help you improve your skill at visualizing the
landscape shown by the contour lines, which is a critical skill for all geomorphologists. Create a legend below so that it’s
easy to tell what color or symbol goes with each type of landform.
[20]
Cirques: outline the back wall of the cirque
Arêtes: draw a line along the top of the ridge
Horns: draw a triangle on top of the peak
Glacial troughs: draw lines along both sides of the valley
Giant stair steps: draw three lines running across the valley (from one side to the other) on top of the drop-off
Tarns: place a “T” on the lakes
Paternoster lakes: place a “PL” on the lakes
2.
Create a topographic profile that follows North Boulder Creek from the cirque headwall at the upstream end of the creek
to Silver Lake (or as close to Silver Lake as can fit on the graph paper). The graph paper has bold lines every half inch.
Give your profile a 5X vertical exaggeration. Draw the lakes that occur along North Boulder Creek on your profile. Be
sure to label your axes, title your graph and include any other information necessary for readers to fully understand how
your graph relates to the map and to the real world. Neatness and accuracy count!
3.
Calculate the gradient from the top of the cirque headwall to where North Boulder Creek flows into the first of the Green
Lakes. Show your work.
4.
[2]
Calculate the gradient along the Creek from the outflow of the first lake along North Boulder Creek to the outflow of Lake
Albion.
5.
[9]
[2]
Compare the contour line spacing at the two locations where you calculated your gradients for questions 3 and 4 to the
gradients you calculated. What conclusion can you draw regarding contour lines and gradient?
K.A. Lemke – UWSP
[1]
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11. Alpine Glacial Landforms
Geomorphology
6.
How did you use information from the contour lines to help you determine the location of giant stair steps?
7.
.Calculate the approximate area, in square miles, of Arikaree Glacier located at the head of North Boulder Creek. Do this
by measuring the approximate length and width of the glacier. Show your work.
8.
[1]
[2]
At one time, Arikaree Glacier filled the entire cirque in which it sits today. Calculate the approximate former size of
Arikaree Glacier when it filled the entire cirque at the head of North Boulder Creek. Again, do this by estimating the
approximate length and width of the cirque. Show your work.
9.
Based on your calculations, what is the percent reduction in size of Arikaree glacier from when it filled the cirque to its
size today?
92
[2]
[1]
K.A. Lemke – UWSP
Geomorphology
K.A. Lemke – UWSP
11. Alpine Glacial Landforms
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11. Alpine Glacial Landforms
Geomorphology
FIGURE 11.5 Air Photo 1
N
94
K.A. Lemke – UWSP
Geomorphology
11. Alpine Glacial Landforms
FIGURE 11.6 AIR PHOTO 2
N
K.A. Lemke – UWSP
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11. Alpine Glacial Landforms
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Geomorphology
K.A. Lemke – UWSP