44 Why the Grand Canyon is Grand Stage D (Synthesis Essay)

GPH 112 Lab: Analyzing the topography of the Grand Canyon 1

Lab Title What makes the Grand Canyon so grand?

What is
this lab all
about?

The Grand Canyon of the Colorado River is a “must see” for people who
live all over Earth. Some save up for years to make the journey. Certainly,
for anybody living in Arizona, it’s a point of pride as Arizona is “The
Grand Canyon State”. So what’s the big deal? Here, you will explore the
Grand Canyon from the perspective of physical geography.

Lab Worth The points you accumulate for correct answers count towards your grade.
Incorrect answers do not hurt your grade.

Computer
program
used in this
lab

You will be given instructions in a canvas module page on how to
download virtual world of the Grand Canyon that shows the geology
and the topography. In this program, you are a virtual character able to
wander around the Grand Canyon’s landscape and rock types.
WARNING: There are two different Grand Canyon geovisualizations –
this one, and the other focusing on microclimate and vegetation.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 2

Interesting
maps to
download –
not
necessary
to do the
labs, but
helpful to
some
students

National Park Service map of Grand Canyon National Park:
https://www.nps.gov/carto/hfc/carto/media/GRCAmap1.jpg

NPS 3D map of the Grand Canyon
https://www.nps.gov/carto/hfc/carto/media/GRCB3DMap.jpg

Interactive geologic map of the Grand Canyon:
https://azgs.arizona.edu/interactive-geologic-map-grand-canyon

Topographic map of the Grand Canyon:
https://www.loc.gov/resource/g4332g.np000100/

Shaded relief map of the Grand Canyon area:
https://legacy.lib.utexas.edu/maps/national_parks/grand_canyon_map.jpg

Bright Angel Topographic Map:
http://legacy.lib.utexas.edu/maps/topo/arizona/pclmaps-topo-az-bright_angel-1903.jpg

SQ general
studies
criteria

Students analyze geographical data using the scientific method, keeping in
mind scientific uncertainty. Students also use mathematics in analyzing
rates to change in the landscape.

TABLE OF CONTENTS OF THIS DOCUMENT

1. Preface: What makes the Grand Canyon so grand? Page 3
2. Overview of lab activities Page 5

Stage A Basic Background: Written material for the basic background quiz
that corresponds with the matching audiovisual presentation

Page 8

Stage B Exploration: Making some basic observations related to the
landforms of the Grand Canyon

Page 22

Stage C more detailed analysis: Exploring connections between topography
and rock types in the heart of the Grand Canyon

Page 31

Stage D synthesis: Short essay analysis of why the Grand Canyon is so grand
(or perhaps not grand at all)?

Page 59

GPH 112 Lab: Analyzing the topography of the Grand Canyon 3

Photo: Courtesy of NASA

1. Preface: What makes the Grand Canyon so grand?

The Grand Canyon is deep, with an average depth of about a mile (1600 m) and a
maximum depth of about 7800 feet (2377 m) if you are standing on the North Rim. Hells
Canyon along the border of eastern Oregon and western Idaho, however, is deeper with a
maximum depth of 7993 feet (2436 m). The Yarlung Tsangpo in the Himalayas is much
deeper at 17,567 feet (5382 m). Some would argue that the Kali Gandaki Gorge
(between the peaks of Dhaulgari and Annapurna) in the Himalayas is even deeper.
The Grand Canyon is long, about 277 miles (446 km) by most starting and ending
points. Again, the Yarlung Tsangpo’s “Grand Canyon” is longer at 308 miles (396 m).
The Grand Canyon is not wide by international standards or even national
standards. On average, its just 10 miles (16 km) wide as the helicopter flies, and its very
narrow at Marble Canyon spanning only 1798 feet (548) meters. Hell’s Canyon, for
comparison, is 10 miles wide (16 km).
Perspective certainly matters in analyzing this lab’s question, and different sorts
of people would probably answer the question differently. The historian Dr. Stephen
Pyne wrote an entire book on “How the Canyon Became Grand.” The Hopi consider the
Grand Canyon as the place of their creation and hence sacred. A geologist might argue
that it’s the rocks on the canyon sides that make it so grand, exposing rocks as old as 1.75
billion years near the bottom and 230 million years near the top with abundant fossils and
the ability to see clearly such things as unconformities. Other types of scientists would
make claims to grandness as well, for example, biogeographers study a variety of plant
and animal life that ranges from harsh desert at the bottom to spruce forests on the North
Rim.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 4

The perspective of the writers of this laboratory admit to seeing the Grand
Canyon as physical geographers. Also, our perspective was influenced heavily by taking
geography students on field trips to the Grand Canyon and listening to their views. In the
end, for a physical geographer, the grandness comes down to a combination of factors
that could be summarized in this diagram showing the components of physical
geography. All different aspects of physical geography would influence our answer.

Image: Courtesy of Ron Dorn

This lab, thus, starts to answer the question of “Why is the Grand Canyon so
grand” from the perspective of physical geography in general, and especially from the
perspective of the geomorphology (landforms) part of physical geography. Because the
landforms of the Grand Canyon depend heavily on the geology and the water/hydrology,
these subjects are going to be part of this lab.

Caveat about the lab: There is no doubt that an online lab about the grandness of
the Grand Canyon is missing out on our five traditional sense of sight (and the changes in
lighting), hearing of the wind whistling through the canyon, the taste of trail and camping
food, the smell of plants, and touching of different rock textures. In the end, you will just
have to experience these at the Grand Canyon for yourself.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 5

2. Overview of lab activities

The purpose of this section is to provide you an overview of the activities you will
complete. Before you dig into the lab, you are also welcome to learn extra background
information about the geology and hydrology of the Grand Canyon in the third section.
You certainly do not have to read the third section in detail to do this lab, but you will
probably find that this enrichment material will help you get more out of “playing the
video game” and the other lab activities.

2.1 Parts of this lab:

You should have already completed Stage 0 (begin). If you did not, please stop
now. Go back to canvas. Find the Stage 0 for this lab, and do that first.
Stage A gives you some basic background about the topography of the Grand
Canyon and its link to the different geological formations. Stage A is followed up with a
short quiz. It does not matter whether you read the background material or watch the
audio-visual presentation. Its all the same material.
In the exploration of this lab (Stage B), you will get a chance to enhance your
grade by learning a bit about the Grand Canyon and the sorts of activities you will
engage in if you decide to move onto Stage BC.
In the detailed analysis part of the lab (Stage C), you will use the video game
geovisualization to explore in greater detail the connection between the geomorphology
and the rock types of the Grand Canyon.
Then, Stage D of the lab encourages you to synthesize what you have learned in
writing a short four-paragraph essay on why you think the Grand Canyon is grand. Most
of this essay tasks you with covering what you learned in lab activities, but you are also
encouraged to explain your own personal perspective on the lab question.

2.2. The study area

The entirely of the Grand Canyon is far too big to analyze in this introductory
laboratory. Thus, all of the laboratory activities will focus on what many consider to be
the heart of the Grand Canyon, centered between the two National Park visitor centers on
the North Rim and South Rim. The frame of the study area can be seen inside this map
of northern Arizona, courtesy of the Arizona Geographic Alliance:

GPH 112 Lab: Analyzing the topography of the Grand Canyon 6

A NASA space shuttle image from the winter of 2019 captures this study area as well,
where up is to the East:

GPH 112 Lab: Analyzing the topography of the Grand Canyon 7

This National Park Service map shows the study area and a bit of the surrounding area.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 8

STAGE A: BASIC BACKGROUND MATERIAL FOR THIS LAB

THIS CLASS HAS NO PREREQUISITES. The assumption is that you’ve never
heard of anything like this material before. Thus, this stage provides the basic
background that you will find helpful in completing this lab.
You can learn this basic material in different ways. You can read
this section, or you can watch an audiovisual presentation that you can
access through the Starting Page for the lab and also Canvas Stage A
module page.
When you access that audiovisual presentation you will be asked for a logon
and a password. It is not your ASURite ID. It’s a general logon (gph111) and password
(gaia).

Background on Grand Canyon Geology

This is not a class about geology. However, to understand the landscape of the
Grand Canyon, it is very helpful to understand more about the rocks. Also, you will
come to understand that geologists think differently about time than anybody else. They
think in terms of eras (hundreds to tens of millions of years long) and periods (tens to
millions of years long) distinguished by the nature of fossils found in the rocks. Not only
that, the oldest rocks seem to excite geologists more than younger rocks.
This excitement over old rocks comes out in the in the Grand Canyon with the
two oldest eras exposed. Deep in the inner canyon are Proterozoic Era rocks that are most
metamorphic and igneous with very steep slopes because these rocks are hard:

[Note: most of the figures in this section are courtesy of National Park Service materials.]

GPH 112 Lab: Analyzing the topography of the Grand Canyon 9

The second oldest geological era exposed in the Grand Canyon is the Paleozoic
with lots of great fossils. Within the Paleozoic, Grand Canyon rocks include the
Cambrian with fossils of crab-like animals and shells. Above (younger) than the
Cambrian are Devonian and Mississippian periods (also known as early Carboniferous)
with shells and coral fossils. Then comes the Permian era at the top with a mixture of sea
fossils (shells, corals) and terrestrial fossils (e.g. insect wings, animal tracks).

Quick check: What are the two geological eras exposed on the walls of the Grand
Canyon?

This is the sequence of Paleozoic layers (called formations by geologists) you will
see in the game, but ‘painted’ on a photograph. You don’t have to memorize these
formations, but I guarantee you will come to know the Kaibab Formation (a limestone
rock) as the top and youngest layer of the sequence! The Kaibab Limestone is what
makes the plateaus on the South and North rims of the Grand Canyon.

You will also get to know the Kaibab Limestone, the Coconino sandstone, and the
Redwall Limestone because they make up the big cliff faces. In contrast, the Bright
Angel Shale makes up the lower-angled slopes (ramps). So right away, you can see in

GPH 112 Lab: Analyzing the topography of the Grand Canyon 10

the picture above that rock type can influence the landscape, where rocks that are harder
to erode make cliff faces and rocks easier to erode make the lower slopes.

Quick Check: What is the top (youngest) formation in the Paleozoic sequence of the
Grand Canyon that also makes up the plateaus that surround the Grand Canyon?

A founding “father” of geomorphology, G.K. Gilbert interpreted slope steepness
as a function of a rock’s resistance to rock decay (weathering) and erosion. Gilbert went
to the Grand Canyon and felt that the rocks of the inner gorge have very steep slopes
because they are so resistant. Rocks that are very weak do not need steep slopes to erode
them. However, very strong rocks (resistant to decay and erosion) will just get steeper
and steeper slopes until they do start to erode. Gilbert, thus, discovered that slopes adjust
their angle to be able to transport rock material (or soil if there is a vegetation cover).
Thus, the inner gorge slopes are so steep because they are highly resistant to both decay
and erosion.
Grand Canyon Unconformities: These are not really a part of this lab, but they
should be mentioned here because geologists get very excited when they see an
unconformity, especially when it is so well exposed and so dramatic as in the Grand
Canyon; in the Grand Canyon, it is called the “Great Unconformity”.
What’s the big deal? A very long amount of time and a lot of geological events
are between what’s underneath and what’s above this unconformity. Geologists also
stress that because the top of the Great Unconformity is pretty flat, it represents a long
time for erosion to erode the land surface to low relief.

Underneath the Great Unconformity are rocks of the “Grand Canyon Supergroup” that
represent a lot of geological events including deposition of Proterozoic Era rocks,
faulting, and then erosion of these rocks.
All of the rocks above the Great Unconformity are from the Paleozoic Era, a time
when organisms like marine shelled life, fish, amphibians, reptiles, and land plants first

GPH 112 Lab: Analyzing the topography of the Grand Canyon 11

appeared. [Sorry if you like dinosaurs, birds and flowering plans; you will have to travel
north of the Grand Canyon to see Mesozoic rocks with those types of fossils.] All of the
rocks underneath the Great Unconformity are much older from the Proterozoic Era.

Quick Check: Geologists make such a big deal of unconformities, especially the Great
Unconformity in the Grand Canyon. What is an unconformity?

You may be amused by this cute way to learn the Paleozoic layers in the Grand
Canyon in order from YOUNGEST (top of the sequence) to OLDEST (bottom of the
sequence). No reason to memorize these. You will learn them in playing the game and
doing the lab.

•Know – Kaibab Limestone
•The – Toroweap Formation
•Canyon’s -Coconino Sandstone
•History: -Hermit Shale
•Study – Supai Formation
•Rocks – Redwall Formation
•Made -Muav Limestone
•By -Bright Angel Shale
•Time -Tapeats Sandstone

In addition to the rock layers, ancient faults (no longer active) will influence the
topography seen in the lab. In particular, there is a very straight feature called Bright
Angel Canyon. The rocks were faulted (broken) in the Proterozoic Era and then again in
the time frame of 50-80 million years ago (long after the Paleozoic rocks were deposited)
in a time geologists call the “Laramide Revolution” when the Grand Canyon area was
uplifted (along with the Rocky Mountains).
The fault itself certainly did not make the Grand Canyon, but crushing the rocks
made them easier to erode – making them the location of Bright Angel Creek.
The reason why Bright Angel Creek is a canyon is that the main river – the
Colorado River, eroded down to a lower elevation. Each time the Colorado River eroded
down just a bit, Bright Angel Creek could also erode down some more. Also, as Bright
Angel Creek eroded down to lower-and-lower elevations, it also extended the canyon
further by eroding headward (in an uphill direction). Geomorphologists call this
“headward erosion”.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 12

Quick Check: Why are some of the side canyons (e.g. Bright Angel) of the Grand
Canyon so straight?

If you want to learn some more and really nerd out about the rock types of the
Grand Canyon, for enrichment, you are welcome to watch a presentation made by
Professor Larson of Mankato State University when he was a Ph.D. student at ASU. It’s
a tour of what you would see walking down to the bottom of the Grand Cayon from the
South Rim:

You will need a special logon (landforms) and password (rock)
https://www.asu.edu/courses/gph211/ArizonaLandscapes/PublishGrandCanyonHike/

Background on some aspects of Grand Canyon geomorphology

This section provides some insight into the following questions:
• how did the Colorado River start (come into existence in the Grand Canyon area and
start to erode downward?
• when did the Colorado River come into existence in the Grand Canyon area and start to
erode downward?
• are canyon deepening and canyon widening processes different?
• how did the Colorado River deepen the canyon?
• how do the slopes on the side of the Grand Canyon erode back from the river?
• why do you see so much bare rock in the Grand Canyon?
• how did all of the small drainages (watersheds) develop on the side of the Grand
Canyon?

GPH 112 Lab: Analyzing the topography of the Grand Canyon 13

How did the Colorado River start (come into existence in the Grand Canyon area
and start to erode downward?

The answer to this question has to explain how the Colorado River was able to cut
through a mountain. The image below from the International Space Station shows a
forested region of higher elevation topography that the Grand Canyon splits into two
parts. The darker green forest identifies the location of a geological feature that formed
about 50 to 80 million years ago in the “Laramide Revolution” when the entire Colorado
Plateau was uplifted. This “Kaibab Upwarp” is a very large mountain, and the Colorado
River cuts right through this mountain in the core of the Grand Canyon between the
North Rim and the South Rim.

Image: Courtesy of NASA, International Space Station Photo

Quick check: what is the name of geological upwarp structure (mountain) that the
Colorado River crosses (and is the location of the heart of the Grand Canyon)?

There are only four ways that rivers can cross geological uplifts (mountains) like
the Kaibab Upwarp. Dr. John Douglass is the world’s foremost expert on the formation of
the Grand Canyon, and he created a diagram to show the four ways:

GPH 112 Lab: Analyzing the topography of the Grand Canyon 14

John Wesley Powell thought that Colorado River was antecedent, in that it predated the
uplift that took place between 50 and 80 million years ago (Laramide Revolution). As
you will learn in the next section, the age of the river itself is much younger – even
though some geologists think there could have been a super ancient canyon in this
location (a view that the developers of this lab do not hold).
All of the other three processes have their advocates. Arthur Strahler suggested
that some aspects of superimposition could apply in the Grand Canyon, although not
getting across the Kaibab Upwarp. Newberry, Blackwelder, and then John Douglass and
Norman Meek all think lake overflow caused the Colorado River to be born. Many
different investigators favor piracy by different processes such as headward incision and
groundwater sapping. You will not learn here what the developers of this lab think. If you
want to dig deeper into this topic, take GPH 211 (the landform processes class) that has
the formation of the Grand Canyon as an assignment. What’s important is that the origin
of the Grand Canyon does not matter to you finishing this lab. Its just a question that
everybody asks. The next question of when is important to you doing this lab.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 15

When did the Colorado River come into existence in the Grand Canyon area and
start to erode downward?
About 4.8 million years ago is the answer. Recent research conducted by the
Arizona Geological Survey and colleagues1 have done award-winning research in
establishing when the Colorado River came into existence. I encourage you to read the
article in the footnote. In brief, before the Colorado River came into existence, there were
a series of closed depressions between present-day Arizona and Nevada-California
sometimes forming lakes with deposits at the bottom. The first influx of river water into
these lakes started about 4.8 million years ago. As Dr. Jon Spencer concludes in this
article: “Rapid incision of the Grand Canyon began at this time.”
The Colorado River then eroded downward through the Paleozoic rocks down to
close to its current level by 1.2 million years ago, as explained below.

Quick question: how long ago did the Colorado River start to incise into the rocks of the
Grand Canyon? And when did this incision (down cutting) of the Colorado River each its
present-day elevation (level)?

1 https://azgeology.azgs.arizona.edu/article/feature-article/2011/12/48-ma-age-inception-modern-colorado-river

GPH 112 Lab: Analyzing the topography of the Grand Canyon 16

Are canyon deepening and canyon widening processes different? Yes! Although
many people think that the Grand Canyon was created by the Colorado River, this
thinking is incorrect.

How did the Colorado River deepen the canyon?
Rivers incise (cut down into) bedrock when the gradient (steepness) of the river
increases. Sometimes, the river’s longitudinal profile (elevation profile along its length)
steepens because mountains are uplifted, but this is not the case here. The Rocky
Mountains (and the Kaibab Upwarp) lifted 50 to 80 million years ago and have been
slowly eroding ever since.
In this case, the river’s gradient became steep when it started flowing over the
western edge of the Colorado Plateau down into the lowlands in the modern-day Lake
Mead area. The giant drop in elevation from the starting elevation of the Colorado River
in the Grand Canyon area down to the Basin and Range was about 6000 feet. This big
drop gave the river enough steepness to have its flow be very turbulent. Professor Mark
Schmeeckle at ASU is a world leader in understanding the importance of turbulence2 and
how this allows rivers to transport sediment and hence incise downward. Thus, the steep
river profile allowed a high enough velocity of flow with a lot of turbulence to cut
downward (incise).

If there is only river incision going on, the canyon would be only as wide as the
river. The form would be a slot canyon much more massive than Antelope Canyon3. The
Grand Canyon widened from two categories of processes: slope retreat and development
of drainages.

How do the slopes on the side of the Grand Canyon retreat (erode back) from the
Colorado River?
Clarence Dutton’s study of the region in the 19th century included tremendous
artwork that included this view exemplifying a series of steep cliffs, gentle slopes, and
steep slopes. The processes operating on these slopes lead to their retreat from the river.
The Tonto Platform (fairly low relief surface INSIDE the canyon) was made by retreat of
the cliffs above.

2 http://www.public.asu.edu/~mschmeec/
3 https://antelopecanyon.az/

GPH 112 Lab: Analyzing the topography of the Grand Canyon 17

The cliff faces erode by mostly mass wasting (landsliding). The rocks decay and
weaken to the point where they detach and produce rock falls and larger events called
rock slides. This is a screenshot video showing a rock slide in the Grand Canyon and the
aftermath of another:

A completely different way that the Grand Canyon sides retreat (erode back)
involves water moving through drainages. No matter whether the drainages are
ephemeral small creeks or larger creeks, flash flooding occurs and the turbulence of the
flood waters erodes both hard rock and soft rock. When the drainages encounter a hard
layer of rock, the flow becomes a waterfall. This all results in the erosion of rock material
and the deepening of these drainages as seen in a NPS Video, a video screenshot and a
still photo. Clicking on this link brings you to an enrichment video (Deer Creek falls)

GPH 112 Lab: Analyzing the topography of the Grand Canyon 18

In general, when layered sedimentary rock erodes back (retreats), in this case back
from the Colorado River, you will see the weakness or hardness of the rock through the
slope steepness. Often, there are big differences in weakness that produce cliff faces
(caprock) made up of strong rock and the footslope made up of weak rock. Erosion of the
weak rock undermines the harder rock above, leading to collapse and rock falls (and rock
slides).
There is another way that steep cliff faces can be produced through collapse and
rock fall, and this is spring sapping. Ground water can leak out of cliff faces if the cliff
stores water and the rock underneath (e.g. shale) cannot, creating springs. The slow water
leak decays the rock, which erodes away and produces cavernous alcoves. Eventually, the
roof of the caverns collapse and a new cliff face is born.

Image courtesy of TM Oberlander.

Image courtesy of TM Oberlander.

GPH 112 Lab: Analyzing the topography of the Grand Canyon 19

Quick check: How do the cliff faces on the side of the Grand Canyon retreat (erode back)
from the Colorado River?

Sedimentary layers in the Grand Canyon are tilted. This occurred when the
Colorado Plateau was uplifted during the Laramide Revolution (50 to 80 million years
ago). A cross-section of the rocks in the Grand Canyon you see below is oriented North
Rim (Kaibab Plateau) on the left and South (Coconino Plateau) on the right. You can
probably perceive that the tilt (what geologists call dip) of the rocks are mostly north-to-
south.
The dip of the sedimentary rock is important in explaining the different widths of
the canyon on the north and south sides between the North Rim and the South Rim.
Higher elevations are on the north side, giving more opportunity to develop canyon
systems (drainages). Also, the tilt towards the south on the north side tends to favor more
springs.
Rim

Why do you see so much bare rock in the Grand Canyon?
Part of the reason why physical geographers consider the Grand Canyon so grand
is the exposure of bare rock. While trees and soil can cover slopes near the very top on
the North Rim, the view everywhere is of exposed rock. This allows dramatic views.
An obvious answer is that you see bare rock because the climate is so dry. Once
you drop below the rims, the amount of precipitation decreases to the point where trees
cannot grow and shrubs are scattered. Since plants tend to hold soil in place, a lack of
plants means that loose soil can be eroded.
A not so obvious answer to this question has to do with the balance between rock
decay (weathering) and erosion (transport of loosened rock material). All of widening of
the Grand Canyon starts with the decay of rock (called weathering). The rock must be
decayed to the point where it detaches from the hard rock, creating loose material. G.K.
Gilbert discovered that slopes that expose lots of bare rock have rates of weathering much
slower than rates of transport of rock. He called these slopes “weathering limited”.
You have to think about this slowly and in this sequence … If rock decay was
faster than transport, then lots of loose rock would accumulate and you would see lots of
soil. This is not the case in the Grand Canyon. You see lots of bare rock, because as fast
as the rock decays – it is then transported (by water flowing over the surface, by rills, by
gullies, by rock fall, by rock slides). This is called a “weathering-limited landscape”.

Quick check: Why do you see so much bare rock in the Grand Canyon?

GPH 112 Lab: Analyzing the topography of the Grand Canyon 20

How did all of the small drainages (watersheds) develop on the side of the Grand
Canyon?
A view of the Grand Canyon from the Space Station below shows a lot of side
canyons on both sides. These tributaries of the Colorado are all cutting down (incising)
for the same reason as the main Colorado River: there is a steep gradient (slope) from the
Colorado River up to the rims on both sides. This steepness gives the water flowing in the
creeks (typically only after an intense rain) more velocity and turbulence to transport rock
and erode their beds.
You will probably notice that these canyons are much bigger on the north side
than the south side. The Kaibab Plateau on the north side is a much higher elevation. The
higher elevation means that there’s more precipitation falling and there’s more
topography to erode into as a canyon system (drainage basin) develops. Also, notice the
plateau (flat surface) that surrounds the Grand Canyon. It is the plateau topography that
contrasts so greatly …