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EPS 50 Lab 6: Maps Topography, Geologic Structures and Relative Age Determinations

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EPS 50 Lab 6: Maps Topography, Geologic Structures and Relative Age Determinations Name:_________________________ EPS 50 Lab 6: Maps Topography, geologic structures and relative age determinations Introduction: Maps are some of the most interesting and informative printed documents available. We are familiar with simple maps that indicate places and the roads you are going to take from one place to another, but maps can reveal much more information. In this lab we will learn to interpret two very important types of maps used in geology and related disciplines: topographic maps and geologic maps. We will also learn to interpret geologic structures, and learn how they can be inferred from geologic maps. Objective: This laboratory will introduce you to topographic and geologic maps and also cover some geologic structures associated with folding and faulting. You will practice making relative age determinations of rocks and structures, and you will construct a geologic cross-section based on information you obtain from maps. Answers: All answers should be your own, but we encourage you to discuss and check your answers with other students. This lab will be graded out of 100 points. Part 1: Topographic maps (21 points) Topographic maps provide information about Earth’s surface. The main feature of topographic maps is topography: ridges, valleys, mountains, plains and other Earth surface features. Maps usually contain a lot of information (i.e. roads, highways, buildings, water bodies, railroads, etc.), but for this exercise we will focus only on topography, (i.e. elevation data). Changes in elevation are depicted on topographic maps using contour lines. Contour lines are lines representing equal elevation across the landscape. Contours are drawn at regular intervals of elevation; these intervals vary between maps, but are typically specified at the bottom of the map or in the map legend. In addition, topographic maps provide a wealth of information regarding both natural and manmade features (see the topographic map symbol key). Map scale is another critical piece of information on all maps. Scale is a number on a map that relates the distances on the map to those in the real world. For instance, a map with a 1 to 12,000 scale (1:12000), tells the user that one unit of distance on the map represents 12,000 units in the real world. The scale, therefore, indicates the level of detail for the features on the map. The key feature of topographic maps is the contour line. Contour lines connect points with equal elevation. They serve as imaginary boundaries separating areas above a given elevation from areas below it to show the general shape of the terrain. The elevation difference between two contour lines is called the contour interval. For most scientific maps, the units of the contour lines, the distances and the elevation are all in metric units: meters and kilometers. However, older American maps frequently use units of feet and miles. In order to help the user determine the elevation at certain locations on a map, the index contours (usually every fourth or fifth contour line) are often marked by thicker lines with elevations labeled. Contour maps relay extensive information. Contours that are very close to each other represent steep slopes. Conversely, widely spaced contours (or an absence of contours) represent shallow slopes. The slope direction is always perpendicular to a contour line. Alternating convergent and divergent contour lines reflect a ridge and valley topography. A ridge line is an ideal line parallel to divergent contours passing through the point of maximum divergence (where the curvature of the contour is greatest). Similarly, a valley axis passes along contours of maximum convergence. Streams tend to follow the valley axes and contour lines intersecting them have a V (or U) shape with the bottom of the V (or U) pointing uphill. Closed contours without other contour lines inside are either high points (e.g., a peak) or low points (e.g., a depression). A saddle is an area bound by higher contours in two opposite directions and lower contours in the other two opposite directions. A watershed (or catchment) is the extent of land which can drain water to a common pour point. It can be identified on a topographic map by tracing two uphill lines in two directions running perpendicular to the contours until a ridge is found. The ridge line is then followed (possibly through peaks and saddles) until the other line perpendicular to the contours is met. On the previous page is a topographic map of the East Bay. The contour intervals (black) are 10 m, and the streams (light blue) are automatically generated. The Wildcat Creek (dark blue) runs from the Berkeley Hills northwestward into Richmond. 1) What kind of structural feature determines the linearity and flow path of Wildcat Creek? (1 pt) 2) Outline on the map or sketch below the alluvial fan that Wildcat Creek has built. (1 pt) We will now make some measurements from this map. In particular, we are interested in determining the slope of Wildcat Creek near its source, in an intermediate reach and near the mouth. The slope between two points is defined by the difference in elevation divided by the distance (rise over run). Elevation difference can be obtained by counting the contours between two points (multiplied by the contour interval), while distance can be measured using a ruler. As the units on your ruler are not the same as on the map, determine the appropriate conversion by measuring the scale bar at the bottom of each map. For example, if 1000 m on the map scale is 5 cm on your ruler, you would know that 1 cm on your ruler is 200 m on the map. Note that since we will work on some maps with detail views of the East Bay area in different scales, the scale bars will not be the same on different maps. There is no path of Wildcat Creek in the maps, so the first step of each exercise will be recognizing Wildcat Creek from the contour lines. When tracing the path of the creek between the indicated points, remember that contours intersect streams forming a V or a U with the apex of the V pointing upstream. Below is a detail showing the source area of Wildcat Creek. 3) Trace the path of the creek between point A and point A’. What is the distance (in map units) between A and A’? The elevation change? The slope? Show your work. (3 pts) Below is a detail showing an intermediate reach of Wildcat Creek. 4) Trace the path of the creek between point B and point B’. What is the distance (in map units) between B and B’? The elevation change? The slope? Show your work. (3 pts) Below is a detail showing the mouth of Wildcat Creek. As no information is available between the contour lines between C and C’, refer to the large map to estimate the path of the creek. 5) Trace the path of the creek between point C and point C’. What is the distance (in map units) between C and C’? The elevation change? The slope? Show your work. (3 pts) 6) Using the measurements you just made, estimate the type of particles (e.g. boulders, cobbles, pebbles, sand, silt) you may find on the bed of Wildcat Creek in reaches A, B and C. In what depositional environment would you expect to find conglomerate deposits? (3 pts) The burst of light detection and ranging (LiDAR) technology gives the ability of generating high-resolution data to topographic maps. LiDAR is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The map shown below is a detail of the Lone Tree watershed in Marin Co., California. The LiDAR data are in sub-meter spacing. Notice how details of the terrain are captured. Below is another detail of the same area. Notice the streams and ridges. 7) Outline the watershed above point B. In other words determine the region from which water could flow to point B. You will need to trace from B outward, and then follow the ridge all the way around. This region is the contributing area of point B. For extra credit, estimate the contributing area in square meters. (4 pts) The figure below shows a detail of the landslide scar. Notice the kinks in the contours above and below the landslide. At these kinks the contours become closer to each other, indicating an increase in slope. Use this information to visualize the outer perimeter of the landslide and think about how deep it might be. The 5 contours closest to each other at the top of the scar (on the right in the map) could give you an indication of its maximum depth. In contrast, the opposite end is much shallower. 8) Estimate the volume of this landslide in cubic meters. Use an approximate perimeter and an average depth. Do not worry about being exact, but show your work. You can quickly obtain a ball-park figure by assuming a rectangular shape and vertical walls. (3 pts) Part 2: Geologic maps (13 points) The geologic map of Marble Canyon, Arizona is on your table. The purpose of a geologic map is to show surface geologic features (e.g., rock types, ages) in terms of different colors and symbols. All geologic maps have several common features: [1] colored areas and letter symbols to represent the rock units on the surface in any given area, [2] lines to show the types and locations of contacts and faults and [3] strike and dip symbols to show the orientations of the formations.
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