Geology of the Grand Canyon area

From Academic Kids

The Grand Canyon from Navajo Point. The Colorado River is to the right and the North Rim can be seen to the left in the distance. Also visible is nearly every sedimentary layer described in this article.
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The Grand Canyon from Navajo Point. The Colorado River is to the right and the North Rim can be seen to the left in the distance. Also visible is nearly every sedimentary layer described in this article.

The geology of the Grand Canyon area exposes one of the most complete sequences of rock anywhere, representing a period of 1.4 billion years of the Earth's history in that part of North America. The major sedimentary rock layers exposed in the Grand Canyon and in the Grand Canyon National Park area range in age from 2 billion to about 200 million years old. Most were deposited in warm shallow seas and near ancient, long-gone sea shores. Both marine and terrestrial sediments are represented, including fossilized sand dunes from an extinct desert.

Uplift of the region started about 75 million years ago in the Laramide orogeny, a mountain-building event that is largely responsible for creating the Rocky Mountains to the east. Accelerated uplift started 17 million years ago when the Colorado Plateaus (on which the area is located) were being formed. In total these layers were uplifted an estimated 10,000 feet (3000 m) and as a result the ability of the ancestral Colorado River to cut its channel into the four plateaus that constitute this area was increased. But the canyon did not start to form until 5.3 million years ago when the Gulf of California opened up and thus lowered the river's base level (its lowest point) from that of large inland lakes to sea level.

Wetter climates brought upon by ice ages starting 2 million years ago greatly increased excavation of the Grand Canyon, which was nearly as deep as it is now by 1.2 million years ago. Also about 2 million years ago volcanic activity started to deposit ash and lava over the area. At least 13 large lava flows dammed the Colorado River, forming huge lakes that were up to 2000 feet (600 m) deep and 100 miles (160 km) long. The nearly 40 identified rock layers and 14 major unconformities (gaps in the geologic record) of the Grand Canyon form one of the most studied sequences of rock in the world.

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Grand_Canyon_geologic_column.jpeg
Figure 1. A geologic cross section of the Grand Canyon. Black numbers correspond to subsection numbers in section 1 and white numbers are referred to in the text
Contents

Deposition of sediments

Some important terms: A formation is a rock unit that has one or more sediment beds and a member is a minor unit in a formation. Groups are sets of formations that are related in significant ways and a supergroup is a sequence of vertically related groups and lone formations. The various kinds of unconformities are gaps in the geologic record. Such gaps can be due to an absence of deposition or due to subsequent erosion removing the rock units.

Vishnu Group

About 2000 million years ago in Precambrian time, thousands of feet of ash, mud, sand, and silt were laid down in a shallow forearc basin similar to the modern Sea of Japan. This basin was between Laurentia (proto-North America/Europe) and an orogenic belt of mountains and volcanoes in an island arc not unlike today's Japan. 1800 or 1700 to 1600 million years ago the process of plate tectonics compressed and accreted these marine sediments onto Laurentia. Essentially the island arc slammed into the growing continent and the marine sediments in-between were squeezed together and uplifted out of the sea.

This is the metamorphic rock now exposed at the bottom of the canyon in the Inner Gorge. Geologists call this dark-colored, garnet-studded layer the Vishnu Schist. Combined with the other schists of this period, the Brahma and the Rama, this makes up the Vishnu Group (see 1a in figure 1). No identifiable fossils have been found in these strata but lenses of marble now seen in these units were likely derived from colonies of primitive algae.Template:Inote

The Vishnu Group was intruded by blobs of magma rising from a subduction zone offshore as recently as 1660 million years ago. These plutons slowly cooled to form the Zoroaster Granite (seen as light-colored bands in the darker Vishnu Schist; see 1b in Figure 1). Some of this rock eventually was metamorphosed into gneiss.

At about the same time a major orogeny (mountain-building event) uplifted the region and created the 5 to 6 mile (8 to 9.5 km) high Mazatzal Mountains.Template:Inote For at least 200 million years and possibly as long as 500 million to even 1000 million years in some places erosion stripped much of the exposed sediments and the mountains away, reducing the once very high range (possibly as high as or higher than the modern Himalaya) to small hills a few tens to hundreds of feet (tens of meters) high and leaving a major angular unconformity. The once deeply buried mountain roots were all that remained of the Mazatzal Mountains as the sea reinvaded.

Grand Canyon Supergroup

In late Precambrian time, extension from a large tectonic plate or smaller plates moving away from Laurentia thinned its continental crust, forming large rift basins (this rifting ultimately failed to split the continent). Eventually a region of Laurentia from at least present day Lake Superior to Glacier National Park in Montana to the Grand Canyon and the Uinta Mountains was invaded by a shallow seaway.Template:Inote The Grand Canyon Supergroup of sedimentary units is composed of nine varied formations that were laid down from 1250 million to 825 million years ago in this sea. The total thickness of the sediment and lava deposited was well over 2 miles (3.7 km). Rock outcroppings of the Grand Canyon Supergroup appear in parts of the Inner Gorge and in some of the deeper tributary canyons.

The oldest section of the supergroup is the Unkar Group (a group is a set of two or more formations that are related in notable ways). It was laid down in an offshore environment

  • Bass Limestone (averages 1250 million years old) - Wave action eroded the land, creating a gravel that later lithified into a basal conglomerate. This formation is known as the Hotauta Member of the Bass Limestone. The Bass Limestone formation was deposited in a shallow sea near the coast as a mix of limestone, sandstone, and shale. It is 120 to 340 feet (37 to 104 meters) thick and grayish in color. This is the oldest layer exposed in the Grand Canyon that contains fossils - stromatolites.
  • Hakatai Shale (averages 1200 million years old) - The Hakatai Shale is made of thin beds of non-marine derived mudstones, sandstones, and shales. This formation indicates a short-lived regression (retreat) of the seashore in the area that left mud flats. Today it is very bright orange-red and gives the Red Canyon its name.
  • Shinumo Quartzite - This formation was a resistant marine sandstone that later formed islands in Cambrian time. Those islands withstood wave action long enough to become re-buried by other sediments in the Cambrian Period. It was later metamorphosed into quartzite.
  • Dox Sandstone (averages 1190 million years old) - A shallow formation made of ocean-derived sandstone with some interbedded shale beds and mudstone. Ripple marks and other features indicate it was close to the shore. Outcrops of this red to orange formation can be seen in the eastern parts of the canyon. Fossils of stromatolites and algae are found in this layer.
  • Cardenas Lava (1250 to 1100 million years old) - This is the youngest formation of the Unkar Group and is made of layers of dark brown basaltic rocks that flowed as lava up to 1000 feet (300 m) thick.

The Nankoweap Formation averages 1050 million years old and is not part of a group. This rock unit is made of coarse-grained sandstone and was deposited in a shallow sea on top of the eroded surface of the Cardenas Lava. The Nankoweap is only exposed in the eastern part of the canyon. A gap in the geologic record, an unconformity, follows the Nankoweap.

All formations in the Chuar Group (about 1000 to 825 million years old) were deposited in coastal and shallow sea environments.Template:Inote

  • Galeros Formation - A mainly greenish formation composed of interbedded sandstone, limestone, and shale with some shales ranging in color from red to purple. Fossilized stromatolites are found in the Galeros.
  • Kwagunt Formation - The Kwagunt consists of black shale and red to purple mudstone with some limestone. Isolated pockets of reddish sandstone are also found around Carbon Butte. Stromatolites are found in this layer.
  • Sixtymile Formation - Sixtymile is made of tan-colored sandstone with some small sections of shale.

About 800 million years ago the supergroup was tilted 15° and block faulted in the Grand Canyon Orogeny.Template:Inote Some of the block units moved down and others moved up while fault movement created north-south trending fault-block mountain ranges. Some 100 million years of erosion took place that washed most of the Chuar Group away along with part of the Unkar Group (exposing the Shinumo Quartzite as previously explained). The mountain ranges were reduced to hills and in some places the whole 12,000 feet (3700 m) of the supergroup were removed entirely, exposing the Vishnu Group below. This created what geologist John Wesley Powell called the Great Unconformity, itself one of the best examples of an exposed nonconformity (an unconformity with bedded rock units above igneous or metamorphic rocks) in the world. In all some 250 million years of the area's geologic history was lost in the Great Unconformity.Template:Inote Good outcrops of the Grand Canyon Supergroup and the Great Unconformity can be seen in the up-stream portion of the Inner Gorge.

Tonto Group

When the ocean started to return to the area 550 million years ago in the Cambrian, it began to concurrently deposit the three formations of the Tonto Group as the shoreline moved eastward:

  • Tapeats Sandstone (averages 545 million years old) - This formation is made of cliff-derived medium to coarse-grained sand and conglomerate that was deposited on an ancient shore (see 3a in figure 1). Ripple marks are common in the upper members of this dark brown thin-bedded layer. Fossils and imprint trials of trilobites and brachiopods have also been found in the Tapeats. Today it is 250 to 300 feet (75 to 90 m) thick cliff-former.
  • Bright Angel Shale (averages 530 million years old) - Bright Angel is made of mudstone shale interbeded with small sections of sandstone and shaly limestone with a few thin beds of dolomite. It was deposited mostly as mud just off-shore and contains brachiopod, trilobite, and worm fossils (see 3b in figure 1). The color of this formation is mostly various shades of green with some brownish tan to gray parts. It is a 325 to 400 feet (100 to 120 m) thick slope-former.
  • Muav Limestone (averages 515 million years old) - The Muav is made of gray thin-bedded limestone that was deposited further off-shore as calcium carbonate precipitates (see 3c in figure 1). It is fossil poor yet trilobites and brachiopods have been found in it. The western part of the canyon has a much thicker sequence of Muav than the eastern part.Template:Inote The Muav is a 250 to 375 feet (75 to 115 m) thick slope-former.

These three formations were laid down over a period of 30 million years from early to middle Cambrian time. Fossils of trilobites and burrowing worms are common in these formations. We know that the shoreline was transgressing (advancing onto land) due to the fact that finer grade material was deposited on top of coarser-grained sediment. Today the Tonto Group makes up the Tonto Platform seen above and following the Colorado River with the Tapeats Sandstone and Muav Limestone forming cliffs, and the Bright Angel Shale forming slopes. Unlike the Paleozoic units below it, the Tonto Group's beds lie in pretty much their original horizontal position. The Bright Angel Shale in the group forms a aquiclude (barrier to groundwater seeping down) and thus collects and directs water through the overlying Muav Limestone to feed springs in the Inner Gorge.

Temple Butte, Redwall, and Surprise Canyon

The next two periods of geologic history, the Ordovician and the Silurian, are missing from the Grand Canyon geologic sequence. Geologists do not know if sediments were deposited in these periods and were later removed by erosion or if they were never deposited in the first place. Either way, this break in the geologic history of the area marks an unconformity of about 165 million years.

Geologists do know that deep channels were carved on the top of the Muav Limestone during this time. Streams were the likely cause but marine scour may be to blame. Either way, these depressions were filled with freshwater limestone about 350 million years ago in the Middle Devonian in a formation that geologists call the Temple Butte Limestone (see 4a in figure 1). Marble Canyon in the eastern part of the park displays these filled purplish colored channels well. The Temple Butte Limestone is a cliff former in the western part of the park where it is gray to cream-colored dolomite. Fossils of animals with backbones are found in this formation; bony plates from freshwater fish in the eastern part and numerous marine fish fossils in the western part. An unconformity marks the end of this formation. The Temple Butte is 250 to 375 feet (75 to 115 m) thick.

The next formation in the Grand Canyon geologic column is the cliff-forming Redwall Limestone, which is 450 to 525 feet (137 to 160 meters) thick (see 4b in figure 1). The Redwall is composed of thick-bedded dark brown to bluish gray limestone and dolomite with white chert nodules mixed in and was laid down in a retreating shallow tropical sea near the equator in early to middle Mississippian time (about 335 million years ago). Many fossilized crinoids, brachiopods, bryozoans, horn corals, nautiloids, and sponges, along with other marine organisms such as large and complex trilobites have been found in the Redwall. Caves and natural arches are also found. After this formation was deposited the Grand Canyon region was slowly uplifted and part of the upper Redwall was eroded away in late Mississippian. The exposed surface of the Redwall gets its characteristic color from rainwater dripping from the redbeds of the Sunpai and Hermit shale that lie above.

The Surprise Canyon Formation is a sedimentary layer of purplish red shale that was laid down in discontinuous beds above the Redwall (see 4c in figure 1). It was created by evolving tidal estuaries in very late Mississippian and possibly in very earliest Pennsylvanian time. This formation, which only exists in isolated lenses that are 0 to 40 feet (0 to 12 m) thick, can only be reached by helicopter and was unknown to science until the 1980s.Template:Inote An unconformity marks the end of the Surprise Canyon Formation and in most places this unconformity has entirely removed the Surprise Canyon and exposed the underlying Redwall.

Supai Group

The Supai Group was deposited in Pennsylvanian and early Permian time in swamppy and riparian environments from clastic sediment mostly derived from the Ancestral Rocky Mountains (the average age of this group is 285 million years). The Supai in the western park of the canyon contains limestone, indicative of a warm shallow sea, while the eastern part was likely a muddy river delta. This formation consists of red siltstones and shales capped by tan-colored sandstone beds that together reach a thickness of 600 to 700 feet (180 to 210 meters). Shale in the early Permian formations in this group were oxidized to a bright red color. Fossils include amphibian footprints, reptiles, and plentiful plant material in the eastern part and increasing numbers of marine fossils in the western part. The formations of the Supai Group are (from oldest to youngest; each ends in an unconformity):

  • Watahomigi (see 5a in figure 1): Slope-forming gray limestone with some red chert bands, sandstone, and purple siltstone that is 90 to 175 feet (27 to 53 m) thick.
  • Manakacha (see 5b in figure 1): Cliff and slope-forming pale red sandstone and red shale that is 200 to 275 feet (60 to 85 m) thick.
  • Wescogame (see 5c in figure 1): Ledge and slope-forming pale red sandstone and siltstone that is 100 to 225 feet (30 to 70 m) thick.
  • Esplanade (see 5d in figure 1): Ledge and cliff-forming pale red sandstone and siltstone that is 225 to 300 feet (70 to 90 m) thick.Template:Inote

An unconformity marks the end of the Supai Group.

Hermit, Coconino, Toroweap, and Kaibab

Like the Supai Group below it, the Hermit Shale was deposited in a swampy environment (see 6a in figure 1). The alternating thin-bedded iron oxide, mud and silt were deposited via freshwater streams in a semi-arid environment an average of 265 million years ago. Fossils of winged insects, cone-bearing plants, and ferns are found in this formation as well as tracks of amphibians and reptiles. It is a soft deep red shale and mudstone slope-former in the canyon that is 160 to 175 feet (49 to 53 m) thick.. Slope development will periodically undermine the formations above and car to house-sized blocks of that rock will cascade down onto the Tonto Platform. An unconformity makes the end of this formation.

The Coconino Sandstone formed as the area dried out and sand dunes made of pure quartz sand invaded a growing desert some 260 million years ago (see 6b in figure 1). Today it is a golden white to cream colored cliff-former near the canyon's rim that is 375 to 650 feet (115 to 200 m) thick. Eolian (wind-created) cross bedding patterns of the frosted, well-sorted and rounded sand can be seen in its fossilized sand dunes. Also fossilized are arthropod and early reptile tracks along with some burrows. An unconformity makes the end of this formation.

Next in the geologic column is the 200 to 250 foot (60 to 75 m) thick Toroweap Formation (see 6c in figure 1). It consists of red and yellow sandstone and shaly gray limestone interbeded with gypsum that were deposited in a warm shallow sea as its shoreline transgressed (invaded) and regressed (retreated) over the land (average age of the rock is about 250 million years). In modern times it is a ledge and cliff former that contains fossils of brachiopods, corals, and mollusks along with other animals and various terrestrial plants. The Toroweap is divided into the following three members:

  • Seligman: Slope-forming yellowish to reddish sandstone and siltstone.
  • Brady Canyon: Cliff-forming gray limestone with some chert.
  • Wood Ranch: Slope-forming pale red and gray siltstone and dolomitic sandstone.

An unconformity makes the end of this formation.

One of the highest, and therefore youngest, formations seen in the Grand Canyon area is the massive 250 to 350 foot (75 to 105 m) thick Kaibab Limestone (see 6d in figure 1). A prominent ledgy cliff-former, the Kaibab Limestone was laid down in middle Permian time an average of about 225 million years ago in the deeper parts of the same advancing warm, shallow sea that deposited the underlying Toroweap Formation. The Kaibab is typically made of sandy limestone sitting on top of a layer of sandstone but in some places sandstone and shale are near or at the top.Template:Inote This is the cream to grayish white rock that park visitors stand on while enjoying the spectacular vistas of the canyon from both rims (some calling it 'Grand Canyon's bathtub ring' due to its appearance). It is also the surface rock covering much of the Kaibab Plateau just north of the canyon and the Coconino Plateau immediately south. Shark teeth have been found in this formation as well abundant fossils of marine invertebrates such as brachiopods, corals, mollusks, sea lilies, and worms. An unconformity makes the end of this formation.

Mesozoic deposition

Cedar Mountain is a mesa made of the Mesozoic-aged Moenkopi Formation.
Enlarge
Cedar Mountain is a mesa made of the Mesozoic-aged Moenkopi Formation.

Uplift marked the start of the Mesozoic and streams started to incise the newly dry land. Broad, low valleys deposited sediment eroded from nearby uplands in Triassic time creating the once 1000 foot (300 m) thick Moenkopi Formation. The formation is made from sandstone and shale with gypsum layers in between. This easily eroded formation may have been deposited above the rim of the Grand Canyon. Moenkopi outcrops are found along the Colorado River in Marble Canyon, on Cedar Mountain (a mesa near the southeastern park border), and in Red Butte (located south of Grand Canyon Village). Remnants of the Shinarump Conglomerate, itself a member of the Chinle Formation, are above the Moenkopi Formation near the top of Red Butte but below a much younger lava flow.Template:Inote

Formations totaling over 5000 feet (1500 m) in thickness were deposited in the region in the Mesozoic and Cenozoic but were almost entirely removed from the Grand Canyon sequence by subsequent erosion (see below). For details on these layers see geology of the Zion and Kolob canyons area, and geology of the Bryce Canyon area. All these rock units together form a super sequence of rock known as the Grand Staircase.

Creation of the Grand Canyon

Uplift and nearby extension

Uplift of the Colorado Plateaus forced rivers to cut down faster.
Uplift of the Colorado Plateaus forced rivers to cut down faster.

The Laramide orogeny affected all of western North America by helping to build the Cordellian Mountain Range (of which the Rocky Mountains are a major part). This major mountain-building event started near the end of the Mesozoic (around 75 million years ago) and lasted well into the early Cenozoic. A second period of uplift started 17 million years ago, creating the Colorado Plateaus (the Kaibab, Kanab, and Shivwits plateaus bound the northern part of the canyon and the Cococino bounds the southern part). However for reasons poorly understood, the beds of the Colorado Plateaus remained mostly horizontal through both events even as they were uplifted an estimated 9000 feet (2700 m). Before the uplift the plateau region was about 1000 feet (300 m) above sea level and bounded by high mountains to the south and west.

In middle Tertiary time (about 20 million years ago) tensional forces (crustal stretching) created and expanded faults in the area and caused some moderate volcanic activity. To the west, these forces created the Basin and Range province by forming long north-south trending faults along which basins (grabens) dropped down and mountain ranges (horsts) were uplifted. The extreme western part of the park is intersected by one of these faults, the Grand Wash.

The Colorado River is born and cuts down

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Colorado_River_from_Desert_View-geologic_column.jpeg
The Colorado River had cut down to nearly the current depth of the Grand Canyon by 1.2 million years ago.

Continued uplift of the Colorado Plateaus created monoclines and also increased the elevation of its plateaus. This steepened the gradient of streams flowing in the Colorado Plateaus province. The ancestral-Colorado River was a land-locked river until 5.3 million years ago (see below). Before that it had a series of temporary base levels (lowest points) in large lakes in the Colorado Plateaus in the early Tertiary and possibly the Basin and Range by the middle Tertiary.Template:Inote

The opening of an arm of the Gulf of California 5.3 million years ago changed the direction of nearby streams toward the sagging and rifting region. The up-steam uplift and down-stream sagging caused streams flowing into the gulf to run and downcut much faster. Soon (geologically speaking) headwater capture consolidated these streams into one major river and associated tributary channels - the modern Colorado drainage system. The most important consolidation occurred when a separate pre-existing river that was carving a channel into the San Andreas Fault and out into the gulf likely captured the land-locked Colorado.Template:Inote Excavation of the eastern part of the Grand Canyon began previous to this but was greatly accelerated and expanded west afterward.

Ice ages during the Pleistocene brought a cooler and wetter pluvial climate to the region starting 2 to 3 million years ago. The added precipitation increased runoff and the erosive ability of streams (especially from spring melt water and flash floods in summer). With a greatly increased flow volume, steepened gradient, and lower base level, the Colorado cut faster than ever before and started to quickly excavate the Grand Canyon two million years before present, almost reaching the modern depth by 1.2 million years ago.Template:Inote

Volcanic activity dams the new canyon

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Vulcans_Throne_and_Lava_Falls_in_Grand_Canyon_NP.jpg
Vulcan's Throne volcano above Lava Falls. Lava flows like this heavily-eroded remnant once dammed the Colorado River.

In later Pleistocene time, around one to two million years ago, basaltic lava covered part of the area and in places cascaded down side canyons, even damming the western part of the canyon between miles 178 and 188 (286 and 302 km) in the Mount Trumbull area. The river was dammed in this way at least 13 times from 1.8 million to 400,000 years ago.Template:Inote

Three of these lava dams were over 1000 feet (300 m) high, forming lakes similar to reservoirs such as Lake Mead or Lake Powell. Some of the lakes were over 100 miles (160 km) long and 200 to 2000 feet (60 to 600 m) deep for many years before finally over-topping the dam and eroding much of it away in massive cascading waterfalls (it took about 20,000 years from start of each dam's formation to its destruction). Cinder cones and the remnants of lava flows are visible in the Toroweap area and the remains of some of the dams exist today as rapids such as Lava Falls.

Recent geology, human impact, and the future

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Glen_Canyon_Dam_from_Colorado_River.jpg
Glen Canyon Dam has greatly reduced the amount of sediment transported by the Colorado River through the Grand Canyon.

The end of the Pleistocene ice ages and the start of the Holocene began to change the area's climate from a cool, wet pluvial one, to dryer semi-arid conditions similar to that of today (although much of the rim then as now receives enough precipitation to support large forests). With less water to cut with, the erosive ability of the Colorado was greatly reduced (the rocks of the Inner Gorge are also relatively resistant to erosion). Mass wasting processes thus began to become relatively more important than they were before, creating steeper cliffs and further widening the Grand Canyon and its tributary canyon system.

In modern times, the building of the Glen Canyon Dam and other dams further upstream, have regulated the flow of the Colorado River and have substantially reduced the amount of water and sediment it carries. This has lessoned the river's ability to scour rocks and the demand for water is so great that in most years the Colorado does not reach its delta in the Gulf of California.

About 45 earthquakes occurred in or near the Grand Canyon in the 1990s. Of these, five registered between 5.0 and 6.0 on the Richter Scale. Dozens of faults cross the canyon, with at least several active in the last 100 years.

The stream gradient of the Colorado River is still steep enough to suggest that the river could cut another 1200 to 2000 feet (370 to 600 m) assuming no additional uplift in the geologic future. This does not account for human impact, which would tend to slow the rate of erosion.

References

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Grand_Canyon_from_Grandview_Point-1000px.jpeg
Grand Canyon from a trail below Grandview Point

In order of greatest use. Template:Explain-inote

  • Geology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D. Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7
  • Geology of U.S. Parklands: Fifth Edition, Eugene P. Kiver, David V. Harris (New York; John Wiley & Sons; 1999; pages 397-409) ISBN 0-471-33218-6
  • Kaibab.org, Grand Canyon Rock Layers (http://www.kaibab.org/geology/gc_layer.htm), The Geology of the Grand Canyon (http://www.kaibab.org/geology/gc_geol.htm) (viewed 19-20 March 2005)
  • Secrets in The Grand Canyon, Zion and Bryce Canyon National Parks: Third Edition, Lorraine Salem Tufts (North Palm Beach, Florida; National Photographic Collections; 1998) ISBN 0-9620255-3-4
  • Grand Canyon: The Continuing Story, Connie Rudd (KC Publishing, Inc.; 1990) ISBN 0-88714-046-7
  • The Colorado River Super Guide Map of the Grand Canyon, Bronze Black (Flagstaff, Arizona; Dragon Creek Publishing; 2003)
  • Geology of Grand Canyon National Park, North Rim by Annabelle Foos (http://www2.nature.nps.gov/geology/education/foos/grand.pdf)

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