Sandstone strata and a mass of granite

2/26/2016 Homework 5 Geologic Time

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Homework 5 Geologic Time Due: 11:59pm on Sunday, February 28, 2016

You will receive no credit for items you complete after the assignment is due. Grading Policy

Interactive Animation: Relative Geologic Dating

When you have finished, answer the questions.

Part A

Which of the following statements about relative and absolute age dating is most accurate?

ANSWER:

Correct

Part B

What is the principle of original horizontality?

ANSWER:

Relative age dating places rocks and events in chronological order and can provide information about absolute age.

Relative age dating provides information about absolute ages but does not place rocks and events in chronological order.

Relative age dating places rocks and events in chronological order but does not provide information about absolute age.

Relative age dating does not provide information about absolute ages, nor does it place rocks and events in chronological order.

2/26/2016 Homework 5 Geologic Time

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Correct

Part C

What is the principle of superposition?

ANSWER:

Correct

Part D

What is the principle of cross­cutting relationships?

ANSWER:

Correct

Part E

Five layers of rock are cut by two faults. Both faults cut through all five layers of rock. Fault A breaks through to the surface, whereas fault B does not. Which of the following statements about faults A and B is most accurate?

Metamorphic rocks are close to horizontal when deposited.

Sedimentary rocks are close to horizontal when deposited.

Sedimentary rocks are close to horizontal when eroded.

Metamorphic rocks are close to horizontal when eroded.

Within a sequence of rock layers formed at Earth's surface, rock layers in the middle of a sequence are older.

Within a sequence of rock layers formed at Earth's surface, rock layers higher in the sequence are older.

Within a sequence of rock layers formed at Earth's surface, rock layers lower in the sequence are older.

Geologic features that cut through rocks must form at roughly the same time as the rocks that they cut through.

Geologic features that cut through rocks must form before the rocks that they cut through.

Geologic features that cut through rocks must form after the rocks that they cut through.

2/26/2016 Homework 5 Geologic Time

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ANSWER:

Correct

Part F

Which principle of relative age dating is important for determining the relative age of igneous rock that has intruded into overlying rock?

ANSWER:

Correct

Part G

A fault (F) breaks three layers of sedimentary rock (S). An igneous intrusion (I1) has broken through the bottommost layer of rock. A second igneous intrusion (I2) has moved up the fault and pooled on top of the uppermost layer of rock. Which event would be considered the youngest?

ANSWER:

Correct

Faults A and B are about the same age, and both are older than the five layers of rock.

Fault A is younger than fault B, and both are older than the five layers of rock.

Faults A and B are about the same age, and both are younger than the five layers of rock.

Fault A is younger than fault B, and both are younger than the five layers of rock.

the principle of original horizontality

the principle of cross­cutting relationships

the principle of intrusive relationships

the principle of superposition

Faulting of rock along F is the youngest event. We know this because all three layers of sedimentary rock have been broken.

The intrusion of I2 is the youngest event. We can know this because I2 sits on top of all other rocks.

Deposition of the three sedimentary layers, S, is the youngest event. We know this because the fault underlies the igneous rocks.

The intrusion of I1 or I2 is the youngest event. Without more information, we cannot know which igneous rock is youngest.

2/26/2016 Homework 5 Geologic Time

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SmartFigure: Relative Dating

Launch the SmartFigure Video

When you have finished, answer the questions.

Part A

A sandstone contains inclusions of metamorphic rock. An igneous dike cuts both the sandstone and inclusions. List the rocks from youngest to oldest.

Hint 1.

Use your knowledge regarding the principles of cross­cutting relationships and dating by inclusions to answer this question.

ANSWER:

Correct

Part B

metamorphic rock, igneous dike, sandstone

igneous dike, sandstone, metamorphic rock

metamorphic rock, sandstone, igneous dike

sandstone, metamorphic rock, igneous dike

igneous dike, metamorphic rock, sandstone

2/26/2016 Homework 5 Geologic Time

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If a sequence of sedimentary units is cut by a fault, what does the principle of cross­cutting relationships tell a geologist?

Hint 1.

Recall what the principle cross­cutting relationships states and how it is used for relative age dating.

ANSWER:

Correct

Part C

Which of the following describes the principle of original horizontality?

Hint 1.

The video showed a sequence of folded sedimentary rocks. What had to occur to form this feature?

ANSWER:

Correct

Part D

The sedimentary units on the left side of the fault are the same as those on the right side.

All of the sedimentary units must have been deposited and lithified before being cut by the fault.

The fault is older than the sedimentary sequence.

Sedimentary layers are laid down horizontally.

The oldest sedimentary unit is located at the base of the sequence, while the youngest is at the top.

Inclusions within a sedimentary rock are older than the sedimentary rock itself.

Folded sedimentary layers were originally laid down flat and later deformed.

A fault or dike that cut a sedimentary sequence is younger than the sedimentary rocks it breaks through.

Undeformed sedimentary layers present on one side of a river­cut canyon are the same as those on the opposite side.

The oldest sedimentary unit is located at the base of the sequence, while the youngest is at the top.

2/26/2016 Homework 5 Geologic Time

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An undeformed sequence of sedimentary rocks is exposed in a large river canyon. Which two principles would be demonstrated by the rocks?

Hint 1.

Think back to the five principles you learned about in the video. Which two would be the most applicable to an undeformed rock sequence that has been eroded by a large stream?

ANSWER:

Correct

Part E

An igneous dike cuts through limestone, but not through the overlying sandstone. Which of the following statements is most accurate?

Hint 1.

Think about how the principles of superposition and cross­cutting relationships are used for this question.

ANSWER:

Correct

principles of lateral continuity and inclusions

principles of superposition and lateral continuity

principles of cross­cutting relationships and superposition

principles of superposition and dating by inclusions

principles of lateral continuity and cross­cutting relationships

First, the sandstone was laid down, next the limestone was deposited, and finally was cut by the igneous dike.

The limestone and sandstone were deposited and then cut by the igneous dike.

First, the limestone was laid down, then intruded by the igneous dike, and lastly the sandstone was deposited.

The igneous dike represents the oldest rock, while the sedimentary rocks are relatively younger.

First, the limestone was laid down, folded and cut by an igneous dike, and finally the sandstone was deposited.

2/26/2016 Homework 5 Geologic Time

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GeoTutor: Constructing an Order of Sequence of Geologic Events ­ Geologic Time Scale

Geologists have divided the whole of history into units of increasing magnitude. This is called the geologic time scale. The entire time scale was originally based on relative dating, since radiometric dating was not available at the time. Absolute dating techniques determine a numerical age of strata given in number of years. Relative dating techniques, on the other hand, determine the age of a stratum relative to other strata (i.e., if it is younger or older), without providing any numerical data. Geologists have been able to determine the relative ages of rocks and any fossils they contain to reconstruct a history that reveals the evolution of Earth's continents and living organisms using four laws of stratigraphy:

1. Law of Superposition: Younger strata are deposited on top of older strata. 2. Law of Original Horizontality: Strata are deposited horizontally. Tilted strata had been tilted by some geologic event after the time of deposition. 3. Law of Lateral Continuity: Layers of sediment initially extend laterally in all directions. As a result, rocks that are otherwise similar, but are now separated by a valley or other erosional feature, can be assumed to be originally continuous.

4. Law of Cross­Cutting Relationships: Magma intrudes and crystallizes (forming features such as faults and dikes). These features are younger than the strata they cut through.

The geologic time scale subdivides the 4.6­billion­year history of Earth into several units, outlining the time frames of several events of the geologic past. See below for the geologic time scale chart.

2/26/2016 Homework 5 Geologic Time

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Part A ­ Laws of stratigraphy

In the figure below, a series of geologic events, A­J, shows the configuration of rocks as seen from a road. Some strata have been tilted, and a volcanic dike has intruded some

2/26/2016 Homework 5 Geologic Time

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of the rocks. Use the laws of stratigraphy to rank these strata.

Rank the strata from oldest to youngest.

Hint 1. The Law of Cross­Cutting Relationships

The volcanic dike (H) must be older than any strata it does not cut through and younger than any strata it does cut through, because the strata it cuts through must have been there before the intrusion of magma.

Hint 2. The Law of Original Horizontality

Pretend the tilted strata are horizontal. That is, "D" is above "A," "C" is above "A," and so on. The Law of Original Horizontality states that strata are deposited horizontally in their original states. Tilted strata had been tilted by some geologic event after the time of deposition, but still retain their relative order.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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All attempts used; correct answer displayed

Notice that the tilted strata are immediately overlain by horizontal strata. This can only occur if erosion has partially removed the tilted strata so they all terminate at the same depth.

Part B ­ The geologic time scale and unconformities

Gaps in the rock record are called unconformities. Unconformities are caused by periods of erosion that have occurred between periods of deposition, which have erased a portion of the rock record. There are three types of unconformities: (1) angular unconformities occur when tilted strata are overlain by horizontal strata—Click here to see an angular unconformity; (2) disconformities occur when strata are separated by an erosional surface—Click here to see a disconformity); (3) nonconformities occur when strata overlay igneous or metamorphic rocks that are resistant to erosion—Click here to see a nonconformity.

Now use the figure below, which has labeled each of the rock strata/layers from Part A with their respective geologic time periods, to fill in the gaps in the following sentences.

2/26/2016 Homework 5 Geologic Time

https://session.masteringgeology.com/myct/assignmentPrintView?assignmentID=1211587 11/41

Match the words in the left column to the appropriate blanks in the sentences on the right. Make certain each sentence is complete before submitting your answer.

Hint 1. How to determine the missing time period

Identify the youngest and oldest strata in the diagram, and use the geologic time scale provided above to find all of the geologic periods between these ages.

Hint 2. The types of unconformities

The volcanic dike terminating abruptly at a stratigraphic boundary would indicate that erosion has occurred.

Hint 3. The age of unconformities

An unconformity must be at least the age of the strata overlying it and can be as old as the strata below it.

ANSWER:

2/26/2016 Homework 5 Geologic Time

https://session.masteringgeology.com/myct/assignmentPrintView?assignmentID=1211587 12/41

Correct

The tilting of the Triassic rocks could have occurred in the Triassic, Jurassic, or Cretaceous periods. This amounts to an uncertainty of at least 55 million years.

Interactive Animation: Angular Uncomformities, Noncomformities, and Discomformities

When you have finished, answer the questions.

Help

1. The Quaternary and Tertiary rocks are separated by this type of unconformity:

a disconformity .

2. Due to an unconformity, the Jurassic period is missing from the rock record.

3. The Triassic rocks must have been most likely tilted during or after the Triassic period

4. The dike dates at least to the Quarternary period.

5. The Triassic and Cretaceous rocks are separated by this type of unconformity:

an angular unconformity .

Reset

2/26/2016 Homework 5 Geologic Time

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Part A

Which image is an example of an angular unconformity?

SEE IMAGES BELOW FOR ANSWER SELECTIONS.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Correct

Part B

In the images below, which contains a disconformity?

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Correct

Part C

What does the term unconformity mean?

Hint 1.

un = NOT; conform = go along with

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Correct

Part D

In the following rock sequence, how much erosion might have occurred between rock layer A and rock layer B?

ANSWER:

Correct

Part E

What characteristic most directly DISTINGUISHES an angular unconformity from a nonconformity?

Hint 1.

The word angular is the key hint.

ANSWER:

a missing rock layer in a sequence that represents a period of deposition

an extra rock layer that represents a period of deposition

a missing rock layer in a sequence that represents a period of erosion or nondeposition

an extra rock layer that represents a period of erosion

at least 10,000 years

none or only a very small amount (Time does not equate to erosion.)

more time than it took to deposit rock layer B

at least 1 million years

more time than it took to deposit rock layer A

2/26/2016 Homework 5 Geologic Time

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Correct

Part F

Which list best describes the events that would lead to the layering of sedimentary rocks in this diagram?

ANSWER:

Correct

GeoTutor: Constructing an Order of Sequence of Geologic Events – Relative Dating

The ordering of events in geological history has long been a difficult task, but once simple principles were determined observation and logic could be used to determine the order of events. With these principles, one cannot calculate the exact number of years ago an event occurred, but instead the sequence of events can be determined. This is referred to as relative dating. The principles are as follows:

1. The law of superposition: In sedimentary rocks, the rock bed on the bottom must be older than the rock bed on the top. 2. The principle of original horizontality: Sedimentary rocks were originally deposited as flat­lying, horizontal layers.

Angular unconformities represent missing time, whereas nonconformities do not.

Conformities represent missing rock layers.

Nonconformities separate parallel rock layers of the same rock type.

Nonconformities separate two different rock types, whereas angular unconformities form only between strata of the same rock type.

Angular unconformities separate rock layers along nonparallel surfaces.

deposition, erosion, deposition, erosion, deposition

erosion, deformation, erosion, deformation, erosion

deposition, deformation, deposition, deformation, deposition

erosion, deposition, erosion, deposition, erosion, deposition, erosion

2/26/2016 Homework 5 Geologic Time

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3. The principle of cross­cutting relationships: Any rock or feature, cutting through another rock or feature, must be younger than the material through which it cuts. (For example, with faults, igneous intrusions such as dikes, or fractures, the first rock must be there for these secondary features to cut through.)

4. Inclusions: Any rock fragments included within another rock must be older than the rock in which they are included. (For example, if eroded fragments of one rock layer become part of another sedimentary rock layer, the rock with the included fragments must be younger than the fragments themselves.)

Part A ­ Basic Principles for Relative Geologic Dating

Below is a geologic structure that illustrates the various principles of relative dating. You will identify the basic principles used in relative geologic dating by dragging labels to their corresponding targets in the image below.

Drag the appropriate labels to their respective targets.

Hint 1. Inclusions in sedimentary rock layers

According to the principle of inclusions, the layer of rock that has inclusions from another rock layer must be younger.

Hint 2. A dike cutting through sedimentary rock layers

The rock layers that the dike cut through must have been there first. This is the principle of cross­cutting relationships.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Correct

As you can see from above, using the logic of these principles when observing sedimentary rock, we can determine a sequence of events.

Now that we have investigated the principles of relative dating, we can use these principles to determine how to read the sequence of geologic events in a location.

Part B ­ Ordering of Geologic Events

The principles of relative dating can be used to understand the order of geologic events. A geologic event can be anything: the deposition of horizontal layers of sedimentary rock, the faulting or folding of rock layers, the tilting of rock layers, the erosion (or wearing away) of rock, the intrusion of volcanic rock within existing rock layers, and so on. Refer to these relative dating principles:

1. Inclusions: Any rock fragments included within another rock must be older than the rock in which they are included. (For example, if eroded fragments of one rock

2/26/2016 Homework 5 Geologic Time

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layer become part of another sedimentary rock layer, the rock with the included fragments must be younger than the fragments themselves.) 2. The principle of cross­cutting relationships: Any rock or feature, cutting through another rock or feature, must be younger than the material through which it cuts. (For example, with faults, igneous intrusions such as dikes, or fractures, the first rock must be there for these secondary features to cut through.)

3. Angular Unconformity: It consists of tilted or folded sedimentary rocks that are overlain by younger, more flat­lying strata. An angular unconformity indicates that during the pause in deposition, a period of deformation (folding or tilting) and erosion occurred.

4. Tilting or deformation could occur to an otherwise horizontally layered sedimentary rocks. Most layers of sediment are deposited in a nearly horizontal position. Thus, when we see rock layers that are folded or tilted, we can assume that they must have been moved into that position by crustal disturbances after their deposition. In such an instance, the tilted structure will be younger than the orginal horizontal layers.

Order the five images below along the timeline based on the sequence of geologic events. To find the oldest, look for the image that shows the least geologic changes. To find the youngest, look for the picture that has the most geologic changes.

Rank from oldest to youngest.

Hint 1. Inclusions from rock layers above and below

In the picture where the gray layer first appears, the layer must be younger than the layers above and below because it has inclusions of both layers of rock within it according to the principle of inclusions. Therefore, this event must have happened after the picture without the gray layer. This can occur when igneous rock intrudes between layers of sedimentary rock and incorporates pieces of the rock layers above and below into the cooling magma.

Hint 2. The oldest and the youngest geologic features/events

The oldest geologic feature should have the least geologic changes and the youngest should have all features from the previous events.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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All attempts used; correct answer displayed

As you can see, you can apply the logic of the principles of relative dating to successfully sequence the order of geologic events in a scene. The principles allow you to tell the geologic story of a landscape.

Lab Activity 8.2.1 ­ Relative Dating

Now that you have practiced ordering geologic events that occurred within a scene or outcrop, you will relate the five geologic laws to this process. First, apply geologic laws to an outcrop in the order that they are invoked by events within said outcrop. Then examine a second scene, where you will identify the geologic laws that explain the relative orders of pairs of events.

Part A ­ Applying Geologic Laws in Order

Please rank from first to last the geologic laws that are used to determine the relative order of the four events that are labeled (but not ordered) in the drawing of the outcrop below.

2/26/2016 Homework 5 Geologic Time

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Please rank the geologic laws used for the history of this outcrop from first to last.

You did not open hints for this part.

ANSWER:

Part B ­ Supporting an Outcrop’s History with Geologic Laws

For each rectangle associated with a pair of geologic structures or events, please identify the name of the geologic law that determines which of the two events within the pair

2/26/2016 Homework 5 Geologic Time

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occurred first.

Drag the appropriate labels to their respective targets.

You did not open hints for this part.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Gigapan: Virtual Fieldwork—Relative Dating and Unconformities

Geologists can determine the geologic history of an area by describing rock outcrops and analyzing the layers of rock. Today you will be a geologist visiting a rock outcrop virtually. You will be able to zoom in and out of the Gigapan image to explore the outcrop and determine the relative ages of rock layers and the geologic history of the area by applying your knowledge of the principles of geology and unconformities.

The principles of geology that you will use in this example are:

The law of superposition: A sedimentary rock bed on the bottom must be older than the rock bed on the top. The principle of original horizontality: Sedimentary rocks were originally deposited as flat­lying, horizontal layers. The principle of lateral continuity: Sedimentary layers, when formed, extended horizontally in all directions.

You will also use your knowledge of unconformities, features created when deposition stopped, uplifting and erosion occurred, and, after a period of time, sedimentation began anew above the eroded layer. There are three main types of unconformities:

A nonconformity is found where igneous or metamorphic rocks have eroded and then sedimentary rock layers are deposited above. A disconformity is a break between parallel sedimentary rock layers above and below. Disconformities represent times when sediments were not deposited or were eroded. An angular unconformity is found where sedimentary layers were tilted and eroded and younger and more flat­laying sedimentary layers were deposited above.

In this exercise, you will use Gigapan technology to:

become familiar with interpreting rock outcrops, understand the sequence of events that occurred as these rocks formed and changed over time, and identify the location of an unconformity in this outcrop and provide evidence for its type.

Gigapan technology mosaics thousands of photos together into a single image, allowing you to zoom in and see the tiniest of details. Imagine zooming in on a grain of sand on a photo of a beach!

Instructions for all Parts:

1. Launch the Gigapan image http://www.gigapan.com/galleries/10030/gigapans/129421 2. You can zoom into the image to take a close look at the angular unconformity.

Instructions for Part A:

1. Scroll down and click on the Google Earth link on the Gigapan site to launch the Gigapan image in Google Earth. 2. Close the photo by clicking on Exit Photo to see your field site location in Google Earth. 3. Zoom in or out to determine your location. Also, on the upper right side, your will find the north arrow. If "N" is not aligned with "North" move it to North. This will ensure that the alignment of your field site is directly facing you in an east­west direction.

4. You can reopen the Gigapan image by clicking on Angular Unconformity, west of El Paso, Texas on the left pane of Google Earth. 5. Do not close Google Earth.

Part A ­ Locating your field site

As a geologist, you always want to first locate your field site on a map. It helps other geologists to locate the field site for future studies and helps you look for relationships with data from other nearby field sites. Now, determine where you are (your field site) in the world. Choose the map that best locates your field site.

http://www.gigapan.com/galleries/10030/gigapans/129421
2/26/2016 Homework 5 Geologic Time

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You did not open hints for this part.

ANSWER:

Instructions for Part B:

1. Go back to the Gigapan image for the Angular Unconformity, west of El Paso, Texas. 2. Examine the outcrop carefully. Make note of any features that would show up on a map (e.g., roads, trees, etc.). 3. Now switch back to Google Earth and zoom in or out to determine how the outcrop is oriented (runs north to south, runs northeast to southwest, etc.) compared to where you are standing and viewing the outcrop. If "N" is not aligned with "North" move it to North.

Part B ­ The orientation of the outcrop

2/26/2016 Homework 5 Geologic Time

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Rock units tell us about Earth’s past, so if we find evidence of geologic processes that have directional components (direction of plate movement, folds and faults, mineral foliation, wind and water currents, etc.), we need to be able to accurately reconstruct those directions. Also, in terms of the scientific method, it helps other geologists to be able to recreate the field investigation step­by­step to confirm or refute any previous findings.

Imagine visiting this outcrop, standing at the location where the Gigapan image was taken, and observing the natural and built features around you. Choose the most accurate representation of the outcrop’s orientation and your vantage point (where you are standing in relation to the outcrop). The representations below depict you and the outcrop as viewed from above. Similar to how you identified the location of this outcrop in the previous part, use Google Earth at a multiple zoom levels. The yellow dot is the point where the Gigapan image was taken.

You did not open hints for this part.

ANSWER:

2/26/2016 Homework 5 Geologic Time

https://session.masteringgeology.com/myct/assignmentPrintView?assignmentID=1211587 27/41

Instructions for Parts C and D:

1. Exit Google Earth and go back to the Gigapan image for Angular Unconformity, west of El Paso, Texas. 2. Examine the outcrop carefully, and make note of the orientation of the layers of sedimentation in this image. Are all of the rock layers running in the same direction? Does the formation contain layers running at different angles?

3. Recall that angular unconformities refer to the junction between sedimentary rocks at an angle and rocks that are more horizontal and represent a time when the rocks were uplifted and eroded. Can you see the evidence of uplift and erosion in the image?

Part C ­ Analysis of an outcrop sketch

Where you see layers of sedimentary rock at an angle in contact with rocks that are horizontal, they are separated by a surface called an angular unconformity. This erosion surface represents a time when rocks were eroded before new layers of rock were formed. This can also occur during a pause in deposition, when a period of deformation (such as folding or tilting) has occurred.

Choose the sketch that best represents the rock outcrop.

You did not open hints for this part.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Part D ­ Making observations I

Simple yet thoughtful observation exposes the history of an outcrop. The sedimentary rocks in the Gigapan image were formed as sediment accumulated as layers that stacked atop older layers. As layers became lower in the stack sequence and covered by newer layers, they became rocks.

If this area had been under water, the shells of organisms would have become limestone, a rock that can't be identified visually but can be identified using field­based tests. Underwater movement of sediment may also create mixes of fine and coarse grains. This sediment becomes conglomerate, a rock clearly identifiable given its combined

2/26/2016 Homework 5 Geologic Time

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coarse and fine grains. Over time, some layers would have become exposed as the water retreated and the rock layers above them were eroded. Additionally, some layers would have been tilted by tectonic forces.

Classify the observations according to the rock that they describe, or choose “Not enough information to tell.”

Drag the appropriate items to their respective bins. Each item may be used only once.

You did not open hints for this part.

ANSWER:

Part E ­ Making observations II

Choose the location of the unconformity.

You did not open hints for this part.

ANSWER:

2/26/2016 Homework 5 Geologic Time

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Part F ­ Making observations III

Now that you have identified the unconformity in this outcrop, can you explain why it is an angular unconformity? Review the statements below, and indicate which are correct.

Select all that apply.

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