Glacial_VL
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School
De Anza College *
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Course
010
Subject
Geology
Date
Apr 3, 2024
Type
Pages
8
Uploaded by CountTankPheasant43
© 2010 C. G. DiLeonardo
Objectives
By completing this exercise you will be able to: • Recognize stream valleys that have been modified by glaciation. • Contrast shape, slope gradient, and character of stream eroded and glacially eroded canyons. • Recognize common features of alpine glaciation on a topographic map. Materials: U.S. Geological Survey topographic maps of The Yosemite Valley 1:24,000; pencil; color pencils; eraser; millimeter-scale ruler/straight edge. Valley Profiles Yosemite Valley and Merced Gorge, California The Yosemite Valley may be one of the best-known examples of glaciated valleys anywhere in the world. The modification of an earlier stream canyon is recorded in the landforms here. The Merced River flows through the valley and then through the Merced Gorge, which was has not been affected by glaciation. Using the scales provided below and the index contours only construct two topographic profiles. One should cross the Merced Gorge near the southwestern edge of the map (Figure 2, A-A’) and the other across the Yosemite Valley (Figure 3, B-B’) Figure 1 Yosemite Valley Yosemite Valley is one of the most breath taking landscapes on earth. The broad valley and steep-canyon walls are testimony of an earlier period of carving by glacial ice. In many alpine areas earlier periods of glaciation have modified pre-
existing river valleys to form spectacular alpine features. These features are revealed when glaciers melt away. Photo courtesy of the United States Geological Survey. Modification of Stream Eroded Landscapes by Glaciation Christopher DiLeonardo, Ph.D. Earth & Space Sciences De Anza College
Introductory Geology Laboratory
Glacial Modification 2 Figure 2 Map of the Merced Gorge .
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Steps to constructing a topographic profile. Step 1: Identify every place a contour crosses the line of profile on the map. Step 2: Line up your vertical scale with the line of profile. Step 3: Transfer your elevation values from the line of profile to the vertical scale. Step 4 Complete the profile by connecting the points of elevation on the vertical scale with a single smooth continuous line. Make sure ends of your profile follow the slope to the of your graph. You have learned to construct topographic profiles in an earlier lab. If you need more help with them you should stop and review that lab before proceeding.
Introductory Geology Laboratory
Glacial Modification 3 Figure 3 Map of the Yosemite Valley Compare the two profiles (constructed below Figures 4a and 4b). The Merced Gorge shows the typical shape of a stream-eroded canyon in an alpine setting. The Yosemite profile is altogether quite different. 1.
How would you characterize the shape of the Merced Gorge? Hint: What letter of the alphabet does it look like?
2.
How would you characterize the shape of the Yosemite Valley? Hint: What letter of the alphabet does it look like?
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The Merced Gorge is V-shaped. Yosemite Valley is U-shaped.
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Introductory Geology Laboratory
Glacial Modification 4 Figure 4A. Profile A-A’ Vertical scale for topographic profile across the Merced Gorge. Figure 4B. Profile
B-B’ Vertical scale for topographic profile across Yosemite Valley.
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What are the biggest differences between the valley-profiles (topographic profiles A-A’ and B-B’)? Think about the shape especially but depth, etc.
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Introductory Geology Laboratory
Glacial Modification 5 Slope Gradient Yosemite Valley and Merced Gorge, California The slope gradient of alpine streams tends to be relatively steep. Slope gradient for the stream is the elevation change as you go down stream along its length. And we have seen in earlier labs on stream erosion that the slope gradient steepens upstream. So, given this you might think that the slope gradient in the Yosemite Valley would be steeper because it is upstream from the Merced Gorge. But not in this case, let’s look at the data. First, remember that the slope gradient of a stream is the change in elevation from one point on the stream to another versus the change in distance. To obtain this we measure the distance from a known elevation on a stream to another along the length of the stream. That is, you would measure along the course of the stream following it as carefully as possible. With a paper map you might use a string and carefully follow the length of the stream including any bends. Then you would straighten the string and compare it to the map scale to figure out how many miles it represented. Digital mapping allows us to use built in tools to go along the length of the stream to measure in segments eventually computing the total length between our two points. Once you have the distance between the two points of known elevation you can calculate the slope gradient of the stream using the following equation: Slope Gradient = Change in Elevation/ Change in Distance And: Change in Elevation = higher elevation (point y) – lower elevation (point x) So: Slope Gradient = (elevation of Y – elevation of X)/ Change in Distance Go to the virtual lab module page and click on the link for the Merced Gorge Map. It is a .pdf file you will want to zoom in and find point X. Note the elevation of point X. Now follow the stream from point X making note of the line segments measuring distance. No, you don’t have to add them up the cumulative distance is shown at the end. When you get to point Y along the stream on the map, note both the elevation and the cumulative distance. Now use the formula to calculate the slope gradient for this segment of the Merced River within the Merced Gorge. Your answer will be in ft/mile. Merced Gorge _______________feet/mile. Once you’ve computed the Slope Gradient for the Merced River in the Merced Gorge repeat the method to calculate the slope gradient of the Merced River flowing through the Yosemite Valley. Click on the link to the map of the Merced River course through the Yosemite Valley. Again, follow from the lower elevation at point X in the western portion of the map to the east making note of the change in distance along the way. At point X note the elevation and the cumulative change in distance. Use the equation above to calculate slope gradient in feet/mile. Yosemite Valley _______________feet/mile. 4.8
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Introductory Geology Laboratory
Glacial Modification 6 Glacial Erosion C
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Figure 5 Bridalveil Falls, Yosemite Valley Topographic map, topographic profile, and photo of Bridalveil Falls in the Yosemite Valley. Note the shape of the valley, its elevation above Yosemite, and its depth. Digital Map source Google Maps. Photo courtesy of the US National Park Service.
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Introductory Geology Laboratory
Glacial Modification 7 Where Bridalveil Falls meets the cliffs above the Yosemite Valley there is an impressive drop. It is a feature referred to as a hanging valley.
Such valleys have the distinctive characteristics of a glaciated valley. Note the “U-shaped” profile of the valley. This is similar to the Yosemite Valley profile you created, but much smaller. Understanding how the valley where Bridalveil Falls drops from is formed can help you understand the overall differences between how streams erode the landscape compared to glaciers. Review the shape of the two profiles you constructed above and your answer to question 3 about the differences in the two profiles. Merced Gorge is typical of a stream eroded canyon in an alpine environment. Above you hopefully noted its “V-shape.” If you didn’t go over the profile again. The V-shape canyon reflects the erosional mechanics of a stream. Water and sediment rushing down the stream cut down into the canyon. It is cutting down into the crust much a way a chain saw wood cut into wood. Gravity brings material down into the rushing stream completing the V or wedge-shape of the canyon. The profiles for both Yosemite Valley and the Valley above Bridalveil Falls are U-shpaed. These are typical from glaciation. The mechanism of erosion is different. It isn’t about the velocity of the glacier but rather its weight. The weight exerted on the crust digs deeply and in response the crust forms an inverted arch-shape or “U-shaped” valley. The location of Bridalveil Falls today is where a smaller tributary glacier use to flow into the larger glacier in the Yosemite Valley. The reason a hanging valley forms here is the smaller glacier being less massive exerted less weight on the underlying crust. Less weight meant the smaller glacier did not incise a deeper canyon here. And thus a “hanging valley” is formed. When the environment warmed back up the ice retreated and streams flowed through the canyons here once again. When Bridalveil Creek reaches the cliffs at the edge of the hanging valley it forms an impressive waterfall. Is there more evidence that the weight of the glacier is responsible for erosion? Yes, compare the slope gradients you calculated for the Merced River through the Yosemite Valley and and the Merced Gorge. Stream erosion being like a chain saw is dependent in part on the speed of stream. So slope gradients are worn down slowly and the stream lengthens keeping a steaper slope gradient upstream. So, the Merced River would actually have a steeper slope gradient through the Yosemite Valley if not for modification by glacial erosion. 4.
How would you compare the slope gradients you measured between the Yosemite Valley and the Merced Gorge above? It seems that the weight bearing down from the overlying glacier would tend to erode things nearly flat underneath the ice. This would tend to lower the slope gradient significantly if an alpine stream were modified by a valley glacier flowing through its course. The Merced Gorge gradient is greater than the Yosemite Valley slope.
Introductory Geology Laboratory
Glacial Modification 8 5.
Consider the width and depth of Yosemite Valley compared to the Merced Gorge. Today streams have recaptured a warmer climate and flow through this altered landscapre. But, prior to glaciation the Yosemite Valley would have looked very much like the Merced Gorge today. In general, what ways do glaciers moving through a preexisting stream valley modify its landscape? Acknowledgements The Yosemite Valley, California and Merced Gorge areas maps are courtesy of the United States Geological Survey. The digital maps used to calculate slope gradient for the Merced River flowing through the Yosemite Valley and the Merced Gorge along with the map used in the figure of Bridalveil Falls in Yosemite was constructed in part using Google Maps and available Google Map tools. About the Earth Discovery Project The Earth Discovery Project is a collaborative effort to integrate hands-on discovery-based learning with modern research tools in undergraduate geoscience education. The approach is to develop and disseminate a comprehensive set of learning resources and experiences supporting systemic educational reform. The logo of the Earth Discovery Project portrays the earth as a three-dimensional puzzle. The globe used in the logo is from NASA’s Blue Marble Project
. The Blue Marble
is a unique view of the earth, which integrates numerous data sets to construct a “true-color” three-dimensional globe. Glaciers moving through a preexisting stream valley modifies its landscape through erosion.