PHYSICAL GEOLOGY LABTopographic and Geologic MapsDr. Gregg Wilkerson and Michael Oldershaw

To study the Earth we need to be able to convey information about its surface. We need to know about the shape of the surface to tell what kind of geologic features are present, and a topographic map can help us identify the type of river, locate evidence of sea level change, or show us evidence of glaciers. Geologic maps show us the rocks present at the surface and locate features such as faults and folds to help us interpret the history of an area.

Map ConventionsGenerally (and with all the maps we will use in this class), maps are laid out with north at the top. That means the left or the map is the west side, the right is to the east, and the bottom is to the south.

ScaleEach maps is drawn to a specific scale, that is, a certain distance on the paper represents a certain distance in the real world. It would be impractical to carry around a piece of paper as big as California, so the California map is shrunk down to practical size.

The topographic maps we use depict scale in two ways, graphic and with a ratio. Most of us are familiar with the graphic scale where a bar on the map legend represents a certain distance. On the topographic maps, this bar is located at the bottom center of the map. You can see that miles and kilometers are depicted on the bar. Look closely to note where 0 is on the bar scale, it is not on the left.

The topographic maps also use a ratio to depict scale, and if you look at the maps we’ll use in this lab, you will see the numbers 1:24000 or 1:62500. Note that there are no units (miles, or kilometers). These numbers just mean the map is shrunk down either 24,000 or 62,500 times from real size. If you measure an inch on either map, that inch is equal to 24,000” or 62,500” in the real world. Of course we would not measure such distance in inches, but by dividing by 12 (12” = 1’), we can come up with something more familiar. 1” on the 1:24000 maps equals 2000’ feet in the real world. On the 1:62500 maps, 1” = 5208’, which is pretty close to 1” = 1 mile. Knowing this will help you get a feel for distance when you use the maps.

TOPOGRAPHIC (TOPO) MAPSTopographic maps show us the topography or shape of the surface by means of an intricate series of brown-colored lines that depict elevation. These contour lines are drawn along specific elevations or height above some known point, usually mean (average) sea level. If you could walk along a contour line you would walk a perfectly level path. When you look at a topo map, these lines will show you mountains and valleys and how they all fit together.

Each contour line represents one specific elevation. Crossing the lines means you are either going uphill or downhill. The difference in elevation from one line to the next is the contour interval. In most of the maps we look at this is either 40’ or 80’, but there are others. You will gain or lose this amount of elevation each time you cross a line. The maps include labels for the contour lines, but generally only every fifth line has a label. The ones with the labels are a little thicker and these are called index contour lines, but they have the same value as the others.

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If the contour interval is 40’, and you are walking uphill, you will gain 40’ each time you cross a line. If you have to walk a long ways to gain 40’ the ground is pretty flat and the lines are spread apart. If you cross several lines in a short distance the incline is steep (look at the Panorama Bluffs on the map below). Because our campus is not too steep, a map that covers BC uses a 20’ spacing. If your map is up in the mountains an 80’ spacing is appropriate so you can depict bigger elevation changes with fewer lines.

Take a close look at the figure below and you can see the BC campus. The 700’ contour line runs right through the middle of the campus. Look how it turns around the stadium. You can see that there are two other lines showing the steep slope that bounds the football field and track. Follow University Avenue to the west and you can see that it cross the 600’ contour line just off campus. So that means you are going uphill from southwest to northeast as you cross campus. You can also figure the contour spacing by counting the lines. Starting at 600’ you cross 5 lines to get to the 700’ and so each line is worth 20’ (200’/5 = 20’). Now that you know that, you can figure the value of each line.

The southwest corner of the campus (right next to the red “16” – more about this later) is between the 640’ and 660’ contour lines so the elevation is somewhere between these values. It is closer to the 640’ foot line, so a good estimate is ~645’. If you were to answer that the elevation is 640’ that would be incorrect; I expect you to estimate elevations by reading between the lines. What is a good estimate of the elevation at the northeast corner? You can see the 740’ contour is near the corner, and the corner is about midway between the 740’ and 760’, so an estimate of ~750’ is good.

The geology lab is just below the “e” in “College” on the map (you can see the entrance drive from Mt. Vernon). What is your estimate of the elevation of the lab?

Look to the east of campus along Panorama Drive and you will see a couple of black circles that represent water tanks. Surrounding these tanks you can see the 800’ index contour line, and if you look carefully you will see how it sticks out to the northwest a little ways forming a ridge. On either side of this promontory the hillside drops very steeply. Look carefully on the south side and you can see a steep gully (canyon, arroyo) that comes up and crosses Panorama. You can see the gully by looking for the points where each contour turns back on itself forming sharp “V”. You can follow the Vs all the way down gully to the highway below. And, you can always tell which way is up in a canyon by remembering that the “contour Vs point upstream”.

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Relief, Gradient, Depression ContoursIf we want to know the difference in elevation in an area or for a whole map, we would subtract the lowest elevation from the highest to get the relief. For example on the map above, I estimate the lowest elevation as ~430’ (at the base of the bluffs), and the highest as ~865’ (by the water tanks), for a relief of 435’.

Gradient is the average slope. You might want to know the gradient of a river for example or a highway. Gradient is computed by calculating the relief for, let’s say a river, and dividing by the distance between the high and the low points. Another way to say this is “Rise over Run”. You will come up with an average slope expressed as feet/mile or meters/kilometer. If you are calculating the elevation of river, you have to find a contour line crossing the river and use that to estimate.

Where a big hole is depicted on a topo map, such as the middle of a volcanic crater or a large open pit mine, the elevation is decreasing inside the hole/crater. If we just drew the contour lines normally, it would look like a peak rather than a crater. Where the contours decrease in value, they have “tick” marks and these denote depression contours. If you look closely at the map above, you can see depression contours just above the “ld” in Bakersfield College. This is the Outdoor Theater.

Other Topo Map ConventionsMagnetic Declination

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Think of a compass. The needle points north right? It turns out that the needle points to “magnetic north” which is not in quite the same location as the Earth’s North Pole (“true north”). This difference between true and magnetic north is called the magnetic declination. Because topo maps are laid out on true north, if you want to use a compass to navigate, you have to make a correction. On the maps in our area, this difference is about 15° E (at the center of the map). Each map depicts this correction with a graph.

Adjacent MapsYou may need to know which topo maps surround yours. On many of our maps, the name of the adjacent map is written on the margins and at the corner. On the more modern map, this is depicted graphically on the bottom of the map. Your map is in the center of a rectangle, and the surrounding maps are labeled next to it.

Publication and Revision InformationAt the southwest corner of the map is information at when the map was published, and when it was revised. Topo maps are checked against aerial photos and this data is displayed as well.

Bench Marks and Elevation MarksYou will note little triangle symbols scattered around topo maps. Some of these have names, but they all have elevations. These are precisely surveyed locations called bench marks. If you go to one you will find a small concrete monument with a brass plaque cemented in the middle. There is actually a tiny little dent in the middle of the plaque for a surveyor to measure from. Less precisely know elevations are also depicted on the maps as numbers with an “x” or sometimes next to a hilltop.

Topo Map SymbolsThere is a legend that goes with all the topo maps in the lab. This separate sheet has all of the symbols used on the maps.

LOCATING THINGS ON MAPSWe have to be able to specify locations on maps and we use Topo maps to do this in two ways; with Latitude and Longitude and with the Public Land Survey System.

Let’s take these in order and to do so we’ll need to agree on some terms. Latitude and Longitude (Lat/Lon) use angular measurement to specify locations on Earth. This makes sense, angles divide up circles, and the Earth is spherical; we can look at a sphere as a whole bunch of circles flying in formation. By convention we divide up a full circle into 360 equal parts. Each of these is a degree (°). Because the Earth is pretty big (circumference is ~24,900 miles or 40,090 km) each degree is a lot of real estate (about 70 miles or 111 km). If we want to locate anything precisely we need finer units. Each degree (°) is divided up into 60 minutes (‘), and if we really want to get precise each minute (‘) is divided into 60 seconds (“). Using the numbers from above, a second of Longitude at the equator is about 100’ or 30 meters, and this is close enough.

Now that we have our measurements ready to go, let’s start at the equator. We can measure north or south from the equator to the poles, 90° (1/4 of the full circle) each way. This is how we measure Latitude (think rungs of a ladder). For example, this classroom is about 35° 24’ north of the equator (you would write that as 35° 24’ N Lat). Note that you have to specify the direction or you will leave people guessing which half of the globe you are talking about.

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Is Latitude going to be good enough to locate this classroom? No, the 35° 24’ N Lat only describes a circle that goes all the way around the Earth. We need something else that is measured east or west, and that is Longitude. Instead of the equator, we’ll arbitrarily start in Greenwich England (the English pioneered the use of Latitude and Longitude for navigation so of course they started at home). We can measure east or west from Greenwich (it is just east of London). We only go half way around (180°) each way to avoid overlapping. As an example, this classroom is about 118° 58’ west Longitude (remember to include the west). Now we can locate anything very precisely on Earth. GPS is essentially using a digital version of this system.

Lines of Latitude are always parallel to the equator, and the distance around gets shorter as you move from the equator (90° north or south is essentially just a dot). Lines of Longitude are always the same length because they run from pole to pole. Lines of Longitude do not run parallel to each other.

Topographic maps are laid out with the Lat/Lon system. For example, the north and south lines at the top and bottom of your maps are Latitude lines just as the east and west map edges are lines of Longitude.

The Lat/Lon coordinates are specified at the corners of your Topo maps, and you can use these to estimate the location of any point on the map. To make things a little easier, each map also includes the Lat or Lon at 1/3 of the way NS or EW on you maps. You just have to ignore all of the other numbers on the sides to see them.

If you look at the corners of your map and do a bit of math you will see that the maps are named for how many minutes of Latitude or Longitude they cover (compare the Lat/Lon at the corners and do the subtraction). Your maps are either 7.5’ or 15’ of both Lat and Lon.

In the exercises you will have several opportunities to locate things with this system.

Public Land Survey SystemWe have our third president, Thomas Jefferson, to thank for this system. It was intended to provide a way of locating property to provide for a “Nation of yeoman farmers.” This means small farmers and Jefferson was very much an advocate for agriculture. It is a simple system that uses an imaginary grid starting from an arbitrary point. The spaces in the grid can then be designated as, for example, the square in the 2nd row north and the 4th column east. In this system the horizontal rows (that are numbered north and south from the origin) are called Townships. The vertical columns (numbered east or west from the origin) are called Ranges. Each Township and Range is 6 miles across and 6 miles from top to bottom.

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The arbitrary starting point for the grid is called a Base and Meridian. In California we have three of these, one up north (the Humboldt Base and Meridian), one down south (the San Bernardino Base and Meridian), and the one that we use most, starting at its namesake, the Mount Diablo Base and Meridian. You can see this system is used to divide up the entire country, including Alaska.

Now we can locate things using this system. For example BC is in Township 29 South and Range 29 East. However, as each of these is 6 miles wide, that is not a very precise location. Each Township and Range is dived up into 36 Sections, each a square mile. BC is in Section 16 (the red number on the Topo map above). Now we have things broken down a little better. You can see this on the figure below, and note that the sections are numbered differently than you may expect. We can further divide sections into quarters (such as the NE or SW quarter), and this is precise enough for our purposes. By convection we always go from smallest to largest, and Township comes first. So BC is in the northeast quarter of Section 16 in Township 29 South and Range 29 East, Mount Diablo Base and Meridian. Abbreviated this is: NE/4 Sec 16 – T 29 S/R 29 E MDB&M.

On your maps the PLSS is shown with red lines and numbers. The heavier lines denote Township or Range boundaries. Follow the lines to figure out where they wrote the Township or Range.

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GEOLOGIC MAPSWe will also use geologic maps in the course of our work. These maps may also include topography, but the main point is to depict the rocks exposed on the Earth’s surface. From these maps we can infer what happened in the geologic past. For example, if we look at the geologic map in the area east of Bakersfield (Hart Park area), we can see that the area has been under the ocean two times in the last few million years.

To create such a map, you have to study the rocks in the area and divide them into groups of similar rock types called formations. The map is created by locating the formations and the boundaries with other formations or contacts. Formations are depicted with different patterns and colors and with letters or numbers. The formations on the map are listed in an Explanation or Legend. The formations are grouped by similar rock type (such as igneous or sedimentary), and are organized by age with the oldest on the bottom. Geologic time is depicted to the left of the rocks, and on some maps is also included in the description along with rock type, and formation names.

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Contact types vary and are depicted on the maps. We will learn more about this when we talk about Structural Geology and Geologic Time. Other geologic features are shown on the maps such as faults and folds and the way the formations slope into the ground.

The point of this exercise is a simple introduction to the maps and the use of the map legend.

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TOPOGRAPHIC MAP Exercises: Names: _______________________

Kernville Quadrangle, 15’

1 How many minutes of Latitude does this map cover? ____________________

2 How many minutes of Longitude? ____________________

3 Why is the map not a square? __________________________________

4 Where on Earth would the map be closest to square shaped? ____________________

5 Where on Earth would the map be a triangle? ____________________

6 What are the Lat/Lon boundaries (edges) of the map ___________________________

7 What is the Lat/Lon of Manter Meadow (E) _____________________

8 What is the Lat/Lon of Packsaddle Cave (NW) _____________________

9 About how many miles south of Mt. Diablo is the Gold Lodge Campground (W)?______________

10 What is the Section Township and Range of Cannell Peak (C)? _____________________

11 What is the S-T/R of Packsaddle Cave? _____________________

12 What is the elevation of Sirretta Peak? _____________________

13 What is the highest elevation on the map? _____________________

14 Where is the lowest elevation on the map? _____________________

15 What is the lowest elevation on the map (you’ll have to estimate)?___________________

16 What is the relief of the map? _____________________

17 What is the map to the southwest? _____________________

18 What is the straight line distance from Sirretta Peak to Split Mountain (SW) in miles and km?

_______________ Miles

_______________ Km

19 What is the area of this map in square miles and square km? _______________ Miles2

_______________ Km2

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Miracle Hot Springs, 7.5’ Quadrangle1 What is the Lat/Lon of Rankin Peak (SE)? _____________________

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3 What is the S-T/R of O’Brian Hill (to the ¼ section) (SE)? _____________________

4 What is the S-T/R of Greenhorn Cave? ____________________

5 What is the elevation of the Lone Star Mine (NW)? ____________________

6 What is the straight line distance from Rankin Peak to Rough and Ready Mountain (NW) in miles and km?

_______________ Miles

_______________ Km

7 What is the area of this map in square miles and square km? _______________ Miles2

______________ Km2

8 What is the gradient in feet/mile of the Kern River from the east of the map (use Mile 69) to the west edge of the map at Mile 58? ___________________

9 What is the gradient in m/km of highway 178 across the entire map? ___________________

10 Draw the symbols used for mining on this map. ___________________

11 When was this map published? ___________________

12 What is the magnetic declination on this map? ___________________

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Bakersfield Sheet of the California Geologic Map

1 Draw a normal contact, include a section where the location is unsure. ___________________

2 ___________________

3 What is the symbol and age of the formation at Emerald Mountain (SE)? ___________________

4 Specifically name the heavy black line ~15 km east of Tehachapi (SE)? ___________________

5 Specifically name the heavy black line ~15 km west of Tehachapi? ___________________

6 What is the symbol and age of the formation under BC? ___________________

7 What marks the east edge of the Carrizo Plain (SW)? ___________________

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Extra CreditRio Bravo Ranch, 7.5’ Quadrangle

1 Calculate the gradient of the Kern River over three distances, use feet/mile.

a. From the east edge of the map to the mouth of the canyon: ___________________

b. From the mouth of the canyon to the west of the map: ___________________

c. Across the entire map: ___________________

2 Which direction does Cottonwood Creek flow (S)? ___________________

3 How do you know this? ___________________

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