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Geology of the Grand Canyon area

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   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.
   Enlarge
   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 nearly 2 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 200 million to nearly 2
   billion 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) which enabled the ancestral Colorado
   River to cut its channel into the four plateaus that constitute this
   area. 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.
   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
   Enlarge
   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

Deposition of sediments

   Some important terms: A geologic 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

   The Vishnu Group had its beginnings about 2 billion years ago in
   Precambrian time when thousands of feet of ash, mud, sand, and silt
   were laid down in a shallow forearc basin similar to the modern Sea of
   Japan. During this time period the 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. From 1.8 to 1.6
   billion years ago the Yavapai and then the Mojave island arcs collided
   and accreted with the Wyoming craton of the proto-North American
   continent. This process of plate tectonics compressed and accreted
   marine sediments onto Laurentia. Essentially the island arcs 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.

   The Vishnu Group was intruded by blobs of magma rising from a
   subduction zone offshore as recently as 1.66 billion 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. The
   intrusion of the granite occurred in three phases: two during the
   initial Vishnu metamorphism period, and a third around 1.5 billion
   years ago. This third phase was accompanied by large-scale geologic
   faulting, particularly along north-south faults that caused some
   rifting, and a possible partial breakup of the continent..

   Studies of the sequence of rocks show that the Vishnu Group underwent
   at least two periods of orogeny mountain-building. These orogenies
   created the 5 to 6 mile (8 to 10 km) high Mazatzal Mountains (
   Yavapai-Mazatzal orogeny). This was a very high mountain range,
   possibly as high as or higher than the modern Himalaya. Then, for over
   500 million years, erosion stripped much of the exposed sediments and
   the mountains away. This reduced this very high range to small hills a
   few tens to hundreds of feet (tens of meters) high, leaving a major
   angular unconformity. The once deeply buried mountain roots were all
   that remained of the Mazatzal Mountains as the sea reinvaded.

   During the late Cretaceous or early Tertiary time the Farallon tectonic
   plate subducted under the west coast of the North American plate
   causing a compressional force across the region that resulted in an
   uplift and the formation of the Colorado Plateau.

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.
   The resulting 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 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 (40 to 100 m) thick and grayish in
       colour. 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 shale. 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.
     * Galeros Formation – A mainly greenish formation composed of
       interbedded sandstone, limestone, and shale with some shale ranging
       in colour 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. 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. Good outcrops of the Grand Canyon Supergroup and the
   Great Unconformity can be seen in the upstream 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 a cliff-former, 250 to 300 feet (75 to 90 m) thick.
     * 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 mostly deposited as mud just offshore, and contains brachiopod,
       trilobite, and worm fossils (see 3b in figure 1). The colour of
       this formation is mostly various shades of green with some
       brownish-tan to gray parts. It is a slope-former, 325 to 400 feet
       (100 to 120 m) thick.
     * Muav Limestone (averages 515 million years old) – The Muav is made
       of gray thin-bedded limestone that was deposited further offshore
       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. The Muav is a cliff-former, 250 to 375 feet
       (80 to 120 m) thick.

   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) because 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 basically lie in their original horizontal
   position. The Bright Angel Shale in the group forms an 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 top of this formation. The Temple Butte is 250
   to 375 feet (80 to 120 m) thick.

   The next formation in the Grand Canyon geologic column is the
   cliff-forming Redwall Limestone, which is 450 to 525 feet (140 to 160
   m) 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 colour from rainwater
   dripping from the redbeds of the Supai 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 up to 40 feet (12
   m) thick, can only be reached by helicopter. It was unknown to science
   until the 1980s. An unconformity marks the top 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 swampy 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 shale capped by tan-colored sandstone beds that
   together reach a thickness of 600 to 700 feet (180 to 210 m). Shale in
   the early Permian formations in this group were oxidized to a bright
   red colour. 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; an unconformity is present at the top of
   each):
     * 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 (30 to 50 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.

   An unconformity marks the top 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
   semiarid 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 marks
   the top 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 375 to 650 ft (115 to 200 m) thick
   golden white to cream-colored cliff-former near the canyon's rim.
   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 marks the top of this formation.

   Next in the geologic column is the Toroweap Formation, 200 to 250 feet
   (60 to 75 m) thick (see 6c in figure 1). It consists of red and yellow
   sandstone and shaly gray limestone interbedded 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 marks the top of this formation.

   One of the highest, and therefore youngest, formations seen in the
   Grand Canyon area is the massive Kaibab Limestone, 250 to 350 feet (80
   to 110 m) thick (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. 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 call 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 marks the top of this formation.

Mesozoic deposition

   Reddish Moenkopi outcrop below volcanic rubble on Red Butte
   Enlarge
   Reddish Moenkopi outcrop below volcanic rubble on Red Butte

   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.

   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 Cordilleran 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

   The Colorado River had cut down to nearly the current depth of the
   Grand Canyon by 1.2 million years ago.
   Enlarge
   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 landlocked 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.

   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 upstream uplift and downstream 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 preexisting river that was carving a channel into the
   San Andreas Fault and out into the gulf likely captured the landlocked
   Colorado. 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.

Volcanic activity dams the new canyon

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

   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

   Glen Canyon Dam has greatly reduced the amount of sediment transported
   by the Colorado River through the Grand Canyon.
   Enlarge
   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, received enough precipitation to support large forests).
   With less water to cut, 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 diminished 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.

   The dam has also changed the character of the river water. Once both
   muddy and warm, with only bottom feeding fish, the river is now clear
   and cold and now supports planted trout. This in turn has changed the
   migration patterns of the bald eagle, which previously would transit
   the canyon to favorable fishing sites downstream, but now use the river
   as their seasonal feeding site.

   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 (400 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.
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