Geology 2, Historical Geology Your Name___________________Lesson 9 - Geologic Maps

Geologic maps are a representation of the landscape, rocks, folds, faults and other geologic features present at a location. This lab is intended to provide an understanding of what is shown on a geologic map and how to use that information to interpret the geologic history.

Part I. Contour Lines.Many geologic maps are shown with topographic contour lines. A contour line connects all points on the map that have the same elevation above sea level. The pattern of these lines show elevation changes and consequently topography (landscape).Rules for Contour Lines

Every point on a contour line is at the same elevation. The line connects points of equal elevation Contour lines separate areas of different elevation – either uphill or downhill Contour lines close to form irregular circles, but sometimes the line extends beyond the edge of the map

without creating a closure. Contour lines never cross each other, but they can merge at a vertical cliff. The closer the contour lines are to one another, the steeper the slope. Concentric contour lines represent a hill If the concentric lines define a depression, hachured marks are used. Contour lines form a V pattern when crossing a stream.

Question for Part I. Draw on the map above to show the following1. Circle an area with a steep slope, Write the letter S within your circle2. Write the letter P on a hill top or mountain peak3. Circle an area with a shallow slope. Write the letter L within your circle.4. Circle an area that represents a canyon or drainage. Draw a line down the center of the

drainage and put an arrow on your line to show the downhill direction of water flow.

Part II Map Symbols – formation and age.Rock units (formations, groups, igneous or metamorphic units) are shown on a map using color and a symbol. These symbols and colors are then described in a map key. The following provides a description of how these map symbols are defined. Remember that the term “System” is used (instead of Period) when talking about rocks.

Question for Part II - Complete the exercise below. Write the map symbol and write the color that would be used to show the age of the rock formation.

Geologic Map Symbols: The first capitalized letter stands for the geologic System (see above). The second portion designates a specific mapped unit (such as a formation) and is shown in lowercase letters. For example the Mississippian age Stark Formation is designated as Ms. Since the Tertiary is subdivided into Epochs, the symbols for the Tertiary begin with T, then the epoch is put in lower case letters, followed by lower case for location. For example Tpy.

Color

Part III – Folds and Faults.The following figures show how a “strike and dip” symbols are used to show how rock layers dip into the earth. The two lines of this “T” symbol are always drawn perpendicular to show the dip direction and a line 90 degrees to the dip direction (called the strike). The strike is parallel to the contact line between layers.

Strata may be folded into either an arched up fold called an Anticline or an arched-down fold called a Syncline. The crest of an Anticline or the trough of a syncline define a line called the Axis. The strata of an Anticline dip away from the axis. The oldest strata are at the center of an Anticline, near the axis, and get progressively younger away

from the axis. The strata of a Syncline dip into the axis. The youngest strata are at the center of a Syncline, near the axis, and get progressively older away from

the axis.

Folds (Anticlines and Synclines) may be “plunging”. This means the axis is not horizontal. A plunging Anticline has the oldest strata in the center and plunges toward the closed end of the map pattern (toward the nose).A plunging Syncline has the youngest strata in the center and plunges toward the opening of the map pattern (away from the nose).

Three main types of Faults are shown in the figure to theright. Each fault type is associated with one of the principletypes of stress.The symbols for each fault type are shown on the previouspage. The symbol for a Normal fault is a line with U and D

(sometimes it has little round circles to show the D side)

The symbol for a Reverse fault is a line with teeth.The teeth are on the overriding side (side moving up)

The symbol for a strike-slip fault is a line with arrows showing the horizontal motion.

Questions for Part III1. Use the map symbols shown to complete the blank front side of the block. Show how each layer on the surface continues below the surface. This is a cross section.

2. Use the rules for interpreting geologic maps on the previous page to complete the cross section on each of the models below. The numbers are from oldest (1) to youngest (2) strata.

Complete the front side of each block to show how the strata dip or are folded. Write the strike and dip symbol on the surface for the blocks that only have numbers. For the folds, write the appropriate geologic map symbol on the top (map) surface). Use the line below each block to write the name (type of fold) of the structure shown.

Part IV. Cross SectionsThe diagram below shows how a “Cross Section” is created from a geologic map. A strip of paper is laid across the map and the contact and strike and dip information is transferred to the strip. The strip of paper is then laid on a separate sheet where the information is used to draw the contacts and dip angles. This information is then extended lower on the cross section to make an image of what the geology looks like beneath the surface.

Question for Part IV Use the geologic map below to construct a cross section from A to B to C to D. Identify the folds present at B and C, and the type of fault shown near location D.

Part V – Geologic Map QuestionsGeologic Map of the Bright Angel Quadrangle, Arizona

1. What is the contour interval for this map? Given this interval, how precisely (in feet) may map locations not located on a contour line be determined?

2. How does the map pattern indicate that the Paleozoic formations are approximately horizontal?

3. Find the strike and dip symbols in the Kaibab Plateau area (northern edge of the map). What degree of bedding dip is shown?

4. Find the Zoroaster Granite and the Brahma Schist in the Inner Gorge. According to the map legend, which one is younger?

5. The Tapeats Sandstone is shown adjacent to the Zoroaster Granite and Brahma Schist. Does the contact between the Tapeats and Zoroaster represent an unconformity? Cite your reasoning.

6. Find a monocline on the map. List the name of the monocline and the approximate position. What is a monocline? Draw a picture and show how the arrows shown on the map relate the monocline in your drawing.

7. What clue on the map suggests which formations are the most resistant to weathering?

8. Look at the cross section for line A-A’ on the map .What type of fault is the Phantom Fault and other nearby faults?What type of stress produces these types of faults?

Geologic Map of the Williamsville Quadrangle, VA1. What type of fold is Chestnut Ridge (southeastern corner). Use the age relationships and strike

and dip symbols to answer this question. Cite your evidence

2. What type of fold is Shenandoah Mountain (middle-east section). Use the age relationships and strike and dip symbols to answer this question. Cite your evidence.

What type of fault is the major SW-NE fault shown cutting across the central portion of this map?

Based on the folding and faulting, describe the direction of stress that these rock formations experienced to produce this folding and faulting.

Based on the age of the rock, what can you say about when this deformation (folding and faulting) occurred?

Geologic Map of the Southern Half of Somerset County , PAConstruct a cross section for a portion of the section that extends from C to D.Your cross section should include the portion from Allegheny Mt. to the Caselman River. The instructor will help you with the method used to construct the cross section.

Geologic Map of the Eel River Valley Area, CA1. When was this map made?

2. What is the oldest rock unit shown on this map – provide the name and age

3. What formation are included in the Wildcat group? What is the approximate age range of this Group?

4. List the major unconformities that exist between the rock units shown on this map

5. The Eel River occupies the axis of a fold. What type of fold is this?

6. What is the dip direction of strata south of Ferndale?

7. What is the name of the major fault that cuts NW-SE through the map in the vicinity of Fortuna to Field’s Landing?

What type of fault is this? (Look at the map cross-section to determine the type of fault).

Is the fault mapped as a high- or low-angle fault?

Based on the cross section, what can you say about the most recent movement on this fault and the age of the folding (use the age of the formations involved)?