The Geology of the
Grand Canyon…
…from the Bottom Up
PowerPoint presentation by Thomas McGuire, September 2006
Contact: cavecreekdigital@msn.com
Notes/Narration by slide number.
Title
The Grand Canyon is arguably the deepest cleft in Earth’s surface. A few rivers around
the world have dug deeper than the Grand Canyon, but those canyons are not as long nor
do they cut though a nearly flat plateau landscape.
2
The canyon is a mile deep and 200 miles long. Upstream is Glen Canyon, which is now
submerged by the Glen Canyon Dam and Lake Powell, at the top right corner of this map
Below the Grand Canyon is another major water supply reservoir for the desert
Southwest. Lake Meade. The South Rim is the primary tourist location at the canyon.
3
Most visitors view the canyon from the South Rim. The distant North Rim, about 20
miles away, is another part of the Colorado Plateau that is 2000 feet higher than the South
Rim. Note the benches where soft rocks, often shale, have been worn away interrupted by
steep cliffs of more resistant strata. In the eastern United States, plants hold the soil to
obscure a stepped bedrock landscape.
4
Many people look at the river a mile below and cannot appreciate the powerful river and
dangerous rapids. As United State rivers go, the Colorado drains a very large area, but the
annual discharge is actually only about the same as New York’s Hudson River. Most
geologists believe that the Colorado River, visible here, carved the Grand Canyon over a
period of 5-10 million years.
5
The canyon was first run by a small group of volunteers led by John Wesley Powell.
They used wooden boats not very different from this one. Major Powell was a survivor of
the Civil War whose right arm had been amputated after a severe wound at the Battle of
Shiloh. Nine men in four boats started down the river in May of 1869. He later became
the director of the United States Geological Survey.
6
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River-level access to the Grand Canyon is at Lee’s Ferry, named after the Mormon
pioneer who settled here with this 17th wife to guide travelers across the Colorado River.
Several companies offer raft trips through the Grand Canyon. They start here at Lee’s
Ferry where at least a week of supplies must be loaded. Here is also the highest level of
Grand Canyon Stratigraphy with the early Mesozoic Moenkopi Sandstone overlying the
Kaibab Limestone formation. (When combined with exposures at nearby Zion and Bryce
canyons, this provides one of the best exposures of geologic time anywhere.)
7
Navajo Bridge carries the last highway that crosses the river for more than 300 miles. The
first section of the river is known as Marble Canyon, named for the steep cliffs of
limestone and sandstone that the river cuts through for the next 65 miles.
8
Stretches of calm water are interrupted by rapids every mile or so where side canyons
have washed down large rocks that block the river. Note the trees and other vegetation
that exist only near river level in this arid desert environment. These “green zones” are
called riparian habitat.
9
The boulder filled rapids provide a challenge for the boatmen who must navigate them
with skill. It’s a wet ride but welcome relief from the summer desert heat. Unfortunately,
dozens of people have drowned in the river from overturned boats and swimmers who
succumb to the icy water. Even in the summer, water that drains from the lower layers of
Lake Powell is near freezing.
10
The erosive power of the river and the sediment it carries is evident where hard,
crystalline rocks have been worn almost as smooth as glass by floodwaters.
11
One of the more unusual geologic features is the concave exfoliation in North Canyon.
Similar weathering features are found on granite domes of the Sierra Mountains. But
those surfaces are convex like a dome. In those places expansion and cracking due to
mineral changes are evident.
12
Explorations up the side canyons is a highlight of Grand Canyon raft trips. Most of them
are box canyons. That is, you can follow them for a distance, but access is usually
blocked by a high ledge or waterfall. The green plants growing here indicate spring water
seeping from springs in the canyon wall.
13
Jointing (cracking) of the rock strata allow water left by summer monsoon and winter
rains to circulate deep into the bedrock. Some of that water from the plateau above exits
at Vasey’s Paradise high on the canyon wall. This is one of the most dramatic water
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springs anywhere. Calcite rich water flow as much as 50 cubic feet per second erupts
from the canyon wall making a lush riparian habitat and deposits of calcite travertine long
the river.
14
Even fresh water for the resorts along the South Rim must be pumped up thousands of
feet from the Colorado River. Springs occur along the canyon where the water table
intercepts the canyon wall. However, the highly fractured sedimentary strata in this area
make the level of a traditional water table hard to locate.
15
Erosion along the outside of a meander undercutting the hard limestone has created
Redwall Cavern. The limestone is actually gray on fresh surfaces, but colored by iron rich
water seeping down from the layers above.
16
100 feet deep, this cavern is almost as large as a grid ion football field. Powell estimated
that it could hold as many as 50,000 people. But he only came with 9. Sandy beaches like
this are common all along the bottom of the canyon.
17
The color of the water is a function of its source. Water draining out of Lake Powell is
clear as the river has dropped it’s sediment at the upper end of the lake. Here, the baby
blue water of the Little Colorado enters the canyon. This color comes from limestone
springs several miles upstream.
18
No only blue in color, but much warmer than the main river, the Little Colorado drains a
vast area of the Colorado Plateau in Arizona. During summer monsoon storms or spring
snow melt, this raging river turns a muddy brown like most streams that drain into the
canyon.
19
Native Americans made the long trek down to the river to obtain salt to supplement their
diet of game and native plants. This salt is deposited here by groundwater as it exits an
impermeable stratum at river level.
20
At about mile 65 the narrow walls of Marble Canyon open into the official beginning of
the Grand Canyon. For about 12 miles of the next part of the Grand Canyon, the river
meanders broadly before it descends into the very old and hard Precambrian igneous and
metamorphic rocks of the Inner gorge. Very faintly visible on the left at the horizon is
Mary Coulter’s six-story watchtower, a favorite stop for tourists on the eastern end of the
South Rim.
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21
Soon the views of the canyon rim are obscured by the steep walls of the crystalline Inner
Gorge. This is a crossing maintained by the United States Geological Survey to monitor
the flow of the river. Precise measurements are needed to regulate water released from
Lake Powell and to estimate the river discharge. The entire discharge is used by the six
states of the Colorado Basin and Mexico. The river water hasn’t reached its mouth in the
Gulf of California in many decades,
22
Hikers cross this suspension bridge to Phantom Ranch, a nearby guest resort and
campground. The trails to the South and North rims are steep and narrow. The only
access is by foot, mule or, for emergencies only, by helicopter.
23
Experienced river runners who know the river well generally guide small rafts without
motors through the rapids. Dangerous rocks, haystacks (standing waves) and holes can
swallow a small raft and its riders. Most of the erosion by the river occurs in floods.
Since the construction of the Glen Canyon Dam about 1960, the largest annual floods
have been prevented, as has the low water conditions of summer. Floodwaters from
tributaries now cause the greatest floods.
24
Changes in the course of the river are rare, but one such re-routing of the river by a
massive landslide caused the Colorado to excavate a new channel. This left Deer Creek a
plunge of 150 feet to the edge of the river. Such abrupt drops at the mouth of a stream are
called hanging valleys and hanging falls.
25
Many side canyons are worn smooth by monsoon floods. Undulations in the top of the
Precambrian rocks, an unconformity, are responsible for three sections of steep inner
gorges between other places where the sedimentary rocks like these come to river level.
26
Sometimes the side canyons are blocked by large bounders that fall from the cliffs above.
If the boulders are washed into the river, they may cause new rapids and hazards to
navigation.
27
This boulder of Redwall Limestone was released during a violent rainstorm in 2003. The
boulder left several small craters as it bounced across the canyon floor and broke apart.
This is the largest piece.
28
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Fossils are abundant in many of the Paleozoic layers of the canyon walls. This is the stem
of an ancient animal called a crinoid that looked like a flower growing on the bottom of
the ocean.
29
At the top of the crinoid stem is the calyx, which is the filter feeding part of the animal.
Crinoids are relatives of starfish and sea urchins. Both the mouth and anus are located
near the top of this structure.
30
Recent volcanic features are very evident in the western parts of the canyon. Vulcan’s
Anvil, an intrusive body, is only 1-2 million years old. Through vents like this one,
molten magma rose to the surface to make many small volcanoes and spill lava into the
canyon.
31
One of the steepest parts of the canyon is at the remote Toroweap viewpoint on the North
Rim. This overlook features one of the most unobstructed views of the river below. Late
Paleozoic Toroweap limestone caps a cliff that plunges more than half a mile to the
Colorado River below.
32
From here you can look straight down on Vulcan’s Anvil and the rafts that stop to view
it. It’s a dizzying view with no railings or artificial supports to hold on to.
33
Across the canyon from Toroweap is Vulcan’s Throne, a small basaltic crater with a lava
flow spilling into the canyon. Many similar volcanic cones can be seen in this area. Look
for this feature in slide 36 to see where the river is.
34
Lava flows actually dammed the river creating a natural reservoir that extended hundreds
of miles upstream. The basalt is more resistant than the surrounding sedimentary rocks.
Therefore the river cut a new channel to our right around the lava dam. Note the large raft
in the river.
35
Columnar jointing and even flower-like patterns can be seen on the canyon walls. Note
the trees for scale. When fluid basaltic lava cools and hardens, it shrinks and the rock
breaks into these interlocking columns. Similar structures can be seen at Devil’s Postpile
in California and even along the west bank of the Hudson River as viewed from Yonkers,
NY.
36
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From Toroweap, one of the most exciting rapids can be seen where Prospect Creek has
built a delta into the river. Again, note the stair step nature of this desert landscape.
Notice the volcanic feature you saw in slide 33.
37
From above this 13-foot drop over Lava Falls, one of the most treacherous rapids, looks
quite tame. Note the boats pulled ashore on the right to allow the boatmen to survey the
rapids for unknown obstructions.
38
A dangerous hole in the middle of Lava Falls has toppled even the largest river running
rafts. Several people have drowned here. Wary boatmen usually take a line to the right.
39.
Most groups leave the river just below Lava Falls. But with no road access, they
generally ride a helicopter up onto the rim at the Bar-10 Ranch. This is followed by a half
hour ride in a small fixed wing aircraft over the Coconino Plateau and back to Lee’s
Ferry. The rafts must continue another hundred miles to a takeout along Lake Meade.
40
Many questions still confront geologists and hydrologists:
1. Why does the Colorado River cut through a high area of the Colorado Plateau rather
than taking a lower route a few miles to the south?
2. Was the canyon dug by the Colorado River, or did other streams flowing in opposite
directions create the valley before the river took that route?
3. What caused the Colorado Plateau to uplift to over a mile above sea level?
4. Should more reservoirs be created along the river to store water for times when it’s
most needed (even a dam site deep in Marble Canyon was proposed and explored) or
should dams such as the Glen Canyon Dam be torn down to allow the river to run free?
5. How can Colorado River water be equitably apportioned among the states and Mexico
that border the river?
6. Will river flow continue to supply the water needed for farming and household uses in
southern California, Arizona and other southwestern states?
7. How will the canyon change in the near and distant future?
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
These images and PowerPoint presentation by Thomas McGuire, Cave Creek, AZ
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Textbook, Review Book & Lab Book Author Amsco School Publications of New
York
www.amscopub.com
Arizona State University geology trips, 2004 & 2005: Dr. Paul Knauth, trip leader
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