11. ALPINE GLACIAL LANDFORMS - University of · PDF filefrom a topographic map, you can still use the representative fraction, but you must square the top . and. ... 11. Alpine Glacial

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Geomorphology 11. Alpine Glacial Landforms

K.A. Lemke – UWSP 85

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 gradients;

• Identify alpine glacial landforms on topographic maps and in stereo air photos; and • Create a topographic profile at a given vertical exaggeration.

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 measurementinches, centimeters, millimeters, or feetmay 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

11. Alpine Glacial Landforms Geomorphology

86 K.A. Lemke – UWSP

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 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒1

×24,000𝑒𝑒𝑚𝑚𝑒𝑒𝑒𝑒ℎ𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒

1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ×

1𝑓𝑓𝑒𝑒12𝑚𝑚𝑚𝑚

×1𝑚𝑚𝑚𝑚

5280𝑓𝑓𝑒𝑒= 1.5𝑚𝑚𝑚𝑚

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.

24,000𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ

×1𝑓𝑓𝑒𝑒

12𝑚𝑚𝑚𝑚×

1𝑚𝑚𝑚𝑚5280𝑓𝑓𝑒𝑒

=0.38𝑚𝑚𝑚𝑚

1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ

The map scale can now be written as 1inch:0.38 miles. Thus:

4𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒1

×0.38𝑚𝑚𝑚𝑚

1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ= 1.5𝑚𝑚𝑚𝑚

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:

𝑋𝑋𝑚𝑚𝑚𝑚2 =4 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2

1×

24,0002𝑒𝑒𝑚𝑚𝑒𝑒𝑒𝑒ℎ𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2

12𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2×

12𝑓𝑓𝑒𝑒2

122𝑚𝑚𝑚𝑚2×

12𝑚𝑚𝑚𝑚2

52802𝑓𝑓𝑒𝑒2

=4 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2

1×

576,000,000𝑒𝑒𝑚𝑚𝑒𝑒𝑒𝑒ℎ𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2

1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2×

1𝑓𝑓𝑒𝑒2

144𝑚𝑚𝑚𝑚2×

1𝑚𝑚𝑚𝑚2

27,878,400𝑓𝑓𝑒𝑒2= 0.57𝑚𝑚𝑚𝑚2

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.

0.382𝑚𝑚𝑚𝑚2

12𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ2=


1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ2

The map scale can now be written as 1 square inch:0.14 square miles. Thus:

4𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ𝑒𝑒𝑒𝑒2

1×


1𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚ℎ2= 0.56𝑚𝑚𝑚𝑚2

(1)

(2)

(3)

(4)

(5)

(6)



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:

1100,000

<1

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:

𝐺𝐺𝑒𝑒𝑚𝑚𝐺𝐺𝑚𝑚𝑒𝑒𝑚𝑚𝑒𝑒 =∆𝐸𝐸𝐸𝐸𝑒𝑒𝐸𝐸𝑚𝑚𝑒𝑒𝑚𝑚𝐸𝐸𝑚𝑚𝐺𝐺𝑚𝑚𝑒𝑒𝑒𝑒𝑚𝑚𝑚𝑚𝑚𝑚𝑒𝑒

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).

(7)



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 x-axis 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.



FIGURE 11.3 Sample Topographic Profiles



FIGURE 11.4 Overlapping Photos for use with Mirror Reflecting Stereoscopes

Photo 1 Photo 2

Photo 3

Area that can be seen in stereo with photos 1 and 2.

Area that can be seen in 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.



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! [9]

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. [2]

4. Calculate the gradient along the Creek from the outflow of the first lake along North Boulder Creek to the outflow of Lake Albion. [2]

5. 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? [1]



6. How did you use information from the contour lines to help you determine the location of giant stair steps? [1]

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. [2]

8. 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. [2]

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? [1]





FIGURE 11.5 Air Photo 1

N



FIGURE 11.6 AIR PHOTO 2

N



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11. ALPINE GLACIAL LANDFORMS - University of · PDF filefrom a topographic map, you can still use the representative fraction, but you must square the top . and. ... 11. Alpine Glacial