One of the most well
known features of the ancient lands of the Southwest are the unusually shaped
and strikingly colorful rock formations that attest to millions and billions
of years of Geologic Time.
The Grand Canyon in Arizona, The Arches in Utah, and the Painted Desert,
Canyon DeChelly, and Monument Valley in Arizona are well known, often photographed
The mysterious caves formed beneath the earth's surface are another source
of ancient wonder.
The Grand Canyon
Grand Canyon is itself a geologic museum of epic proportions, where nearly
half of the earth's 4.6-billion-year history is displayed. Erosion has
exposed rock strata ranging from the 1.7-billion-year-old Vishnu Schist
of the Inner Gorge, to the pale caprock Kaibab Limestone deposited 250
million years ago, to the 1-million-year-old black lava flows in the western
Canyon. The debris-filled floodwaters of the Colorado River cut through
all the rock layers in a mere 4 to 6 million years, as it rushed from
high in the Wyoming mountains to the sea.
Forming the Rock
Although the region today is a semiarid,
high-desert plateau, the rocks of Grand Canyon tell of past mountains
as high as the Himalayas, now eroded to their roots, and climatic changes
that allowed numerous seas, sluggish rivers, and dune-filled deserts to
dominate the region. The passage of each geologic era is recorded in successive
rock strata. The youngest rocks in the Canyon were formed by volcanoes,
but the bulk of the rocks are sedimentary, the result of marine and river
deposits and tall sand dunes, which over millennia were compacted and
hardened into the easily eroded sandstones, shales, and limestones found
throughout the Southwest.
Plate Tectonics and the Carving
of the Canyon
Most of the western portion of the
North American continent was at or below sea level until sometime after
the Mesozoic Era, 240,000 - 65,000 years ago. The ancestral Colorado River
meandered over a large plain. Then, a 130,000-square-mile area of the
southwestern United States called the Colorado Plateau was gradually squeezed
up a mile high as the Pacific continental plate crashed against and went
under the North American plate (close to the modern-day California shoreline),
sending powerful geologic reverberations eastward that created the Rockies.
In time, this tilting caused the Colorado River to rush downhill, carving
a pathway to its new outlet in the Gulf of California. The Grand Canyon
began to appear. For another 2 to 3 million years, the sediment-laden
Colorado and its tributaries worked to deepen and, with the aid of wind,
rain, ice, and gravity, widen the Canyon to a present 18 miles in places.
The floor of the Grand Canyon is the lowest
point of the "Grand Staircase," a series of enormous geological "steps"
that climb up over the Kaibab Plateau into southern Utah, through Zion National
Park, up again through Bryce Canyon, before finally reaching the 11,311-foot
summit of Brian Head Mountain.
The Grand Staircase
As it rises up through successive layers of rock strata, the Grand Staircase
witnesses several billion years of geologic history.
All through out this Grand Staircase there have formed amazing works of
wind and water erosion over billions of years, giving us today a magical
landscape unlike anything else on earth.
Rio Grande Rift
Another major geologic formation
in the Southwest is the Rio Grande Rift. Beginning about 30 million years
ago, tension caused by movement in the earth's mantle created a huge valley,
and immense tear that runs across New Mexico from Colorado to northern Mexico.
Now known as the Rio Grande Rift, this pulling apart of the earth's crust
resulted from separation along two parallel fault zones. Here again plate
tectonics and a river have combined forces to shape a major geographic feature
in the Southwest.
Utah Arches and Canyons
One of the best kept geologic secrets in the world are the fantastic canyons
in Utah - fantastic in that they come in shapes, sizes, and colors that
are nothing less than amazing.
There are arches of sandstone creating bridges across the sky, windows worn
through sand stone buttes, towering spires, huge rocks balanced on tiny
pedestals, multi-colored canyons, and thousands of ancient petroglyphs carved
in these canyon walls.
Combinations of salt left from when the area was under arctic seawater in
the Mesozoic Era (240 - 150 million years ago), water, ice, and minerals,
all combine to make this region one of the most amazing in topography and
From the soft pink and brown hues of Navajo Sandstone at Zion, to the rainbow
of mineral colors at Bryce, to the fiery red Entrada Sandstone at Arches,
the natural canyon walls and nearly unbelievable rock formations have left
human visitors awe struck for thousands of years.
One step above the Grand Canyon in the Grand Staircase formation is Zion
This canyon is a surround of warm pink and buff Navajo Sandstone and ranges
in elevation from 3,666 to 8,726 feet. Zion speaks of eons of time, some
when lakes covered the region and left layers of debris on their beds that
became thousands of feet of deep rock, and Zion also tells us of other times
when their were hot dry winds blowing sand dunes across the Southwest.
Then 13 million years ago when the Colorado Plateau starting lifting up,
a river began to run through to form the canyon. Today that river still
runs, and is called the North Fork of the Virgin River.
The top step in the Grand Staircase is Bryce Canyon at elevations of 6,600
to 9,100 feet.Bryce Canyon National Park is not really a canyon at all,
but is a series of fourteen amphitheaters that have been eroded out of the
eastern rock face of the Paunsaugunt Plateau. The colored soils we see there
are from the silt, sand, and limy skeletons of freshwater lake creatures
deposited in 2,000 foot thick lake beds nearly 60 million years ago, soon
after the dinosaurs ceased to walk the land. Around 13 million years ago,
the lands were lifted, slowly, and split along fault lines. Water, snow
and ice began to work away at the lake beds once they were exposed to the
air. This erosion carved the soft rock into spires, pinnacles, windows,
arches and natural bridges. As much as the forms are astounding, the colors
are also unexpected. Hematite gives the rocks their red and brown colors,
limonite lends a surprising soft yellow, and manganese oxides provide a
lavender blue. Straddling the Colorado River and several large tributaries,
Canyonland National Park is another sandstone marvel and contains a wonderful
combination of ancient cliff dwellings, rock art, an the by far more ancient
sandstone arches, teetering boulders on thin pedestals, and horizontally
banded canyon walls. The area contains Navajo Sandstone 'slickrock' that
are petrified sand dunes from the Jurassic Era, a geologic process that
began when dinosaurs were dominating the land.
Rainbow Bridges and Natural Bridges National Monuments are two more remarkable
geologic wonders in Utah.
Science expeditions in the 1900s sough to determine which were the largest
natural bridges in the world.
Ultimately it was proven that Rainbow Bridge, named after a Navajo legend,
was the largest natural bridge in the world spanning 275 feet. This bridge
was formed as plate tectonics lifted the Colorado Plateau up a mile in the
sky about 65 million years ago, increasing the pressure of small creeks
into larger streaming forces, carving away at the sandstone as it traveled
to a lower elevation to the southwest. Here, at the Rainbow Bridge, the
softer Navajo sandstone was more easily carved away than the harder Kayenta
sandstone that lay above it, and the result is a standing Kayenta sandstone
bridge no longer supported by the soft Navajo sandstone it had once lain
At Natural Bridges three impressive bridges span the canyons and were named
in 1908 using Hopi words - Sipapu (place of emergence) being the largest,
Kachina (spiritual guardians), next in size, and Owachomo (rock mound) the
third in size. These bridges are made of cedar Mesa Sandstone and have ancient
rock art carvings on them. Also carved by stream flows, these bridges were
formed when debris from growing rivers actually punched through canyon walls
that they had previously would around.
Further north in Utah, sits the masterpiece of sandstone bridge collections,
Arches National Park. At Arches National Park there are more than 1,700
natural rock spans in that park alone. These gravity-defying wonders of
natural rock took millions of years to create and area fragile protected
wonder today. Their stone is primarily the fiery red of the Entrada sandstone,
but there is the softer pink hues of the Navajo sandstone as well. No river
runs through these arches as at Rainbow Bridge or Natural Bridges. Wind
and ice instead carved these majestic arches.
Caves - click here to learn about Ancient Caves in the Southwest
the Geology & Cave photo album