Tag Archive for: Glad you Asked

SURVEY NOTES

What Gives Utah’s “Red Rock Country” its Color?

by Lance Weaver


Coloration of the Navajo Sandstone caused by post-depositional movement of the iron mineral hematite. (Photo credit Peter Fitzgerald, GNU Free Documentation License)

Utah’s Colorado Plateau is famous for its striking vistas and dazzling colors. Hues of red, pink, maroon, yellow, brown, and white create an array of stunning rock colors that attract visitors from all over the globe. From the red rocks of the Navajo Sandstone to the Vermilion Cliffs of the Moenave and Kayenta Formations to the pink, crimson, and chocolate cliffs of the upper Grand Staircase, many who visit the Colorado Plateau wonder what gives the rocks their brilliant colors. This question has spurred much research by geologists, involving chemical and physical analysis. The answers can be complicated, as many different minerals can cause coloration in rocks; however, for the most part, the red, pink, yellow, and brown colors of Utah’s “Red Rock Country” simply comes down to one element—iron.

Since minerals form the basis for many pigments and dyes, it should be no surprise that they are also responsible for the coloration of rocks. Of all the common colorful minerals found in Earth’s crust, few are as abundant, dynamic, and multi-colored as iron. Depending on how it combines with other elements, iron can form a veritable rainbow of colors. When iron combines with oxygen it becomes iron oxide, and its degree of oxidation largely determines its color. Ochre, a mixture of clay, sand, and iron oxide, has been one of the most commonly mined mineral pigments for tens of thousands of years and is composed of the same minerals that often color rocks. Obtained from iron-bearing clays, ochre can produce several colors and hues that are used as natural coloring agents. Red ochre comes from hematite (Fe2O3), a mineral named for the same Greek root word for blood, and has long been used as a red pigment. Some iron oxides, when hydrated (combined with hydrogen and oxygen), can form bright yellows such as yellow ochre which comes from the mineral limonite (FeO(OH)+H2O). Brown ochre comes from the mineral goethite (FeO(OH)) and is a partially hydrated iron oxide. Iron can also form black pigments from minerals such as magnetite (Fe3O4), or even blue and green hues from minerals such as glauconite and illite. For the most part, these iron minerals, and particularly hematite, are responsible for coloring the Colorado Plateau’s sedimentary rock layers.

The Amazon River’s “meeting of waters” is a fantastic example of the different water chemistries likely responsible for the coloration of ancient sediments. The Rio Negro, a tributary of the Amazon, is a “blackwater” river which is clear, slightly acidic, and contains high concentrations of reduced iron. The Amazon, however, has lower concentrations of iron and dissolved solids, but a higher sediment load and oxidized iron giving it its reddish-brown color. (Photo credit Gabriel Heusi, Wikimedia, Creative Commons license.)

Researchers have questioned how the pigment-bearing iron minerals get into rocks like sandstone and shale as well as how the minerals are dispersed within the rock. One might suspect that the brightly colored minerals might be sprinkled throughout the sand and clays or cements that composed the sandstone and shale units—something like chili powder, evenly mixed within the salt. However, by looking at thinly cut sections of rock under a microscope, it becomes clear that this is typically not the case in Utah’s Color Country rock. Instead, the very sand grains that form the matrix of the rock units are actually “frosted” or coated with a layer of iron-rich mineralization. These grains are then cemented together with a pale to white calcite or silicate glue. In the case of sandstone units like the prominent Navajo or Wingate Sandstone, the sand is composed almost entirely of translucent or white quartz grains that are coated with a thin veneer of red hematite mineralization. Although the exact timing is debated among geologists, this “coating” of iron-bearing minerals likely began forming as the grains were transported from their place of erosion to their respective areas of deposition. The same process can be seen today as mineral-rich waters of semi-arid to tropical rivers mineralize large amounts of sediment as it is transported and deposited into adjoining basins.

Multicolored sections of the Navajo Sandstone in the Zion National Park area.

After the sediment is buried, moving groundwater can further mineralize and alter the red rock to change it to varying shades of pink, vermilion, maroon, or even white. In southern Utah, the upper parts of the Navajo and Entrada Sandstones often exhibit areas referred to as “bleached zones.” This term refers to areas where reducing groundwaters have partially removed the iron oxide coating from the sand grains. A reducing agent is a solvent that can remove oxygen from a compound. So in the case of the iron pigments that colored the Navajo Sandstone, groundwater that was slightly acidic or contained other reducing agents seems to have dissolved large amounts of iron mineralization from the upper sections, often redepositing the iron in cracks, joints, or different sections of the sandstone that possess irregularities in grain size. Areas that have lost iron oxide become lighter shades of pink and white, whereas areas that gained additional iron oxide from the groundwater movement become darker shades of maroon and even black. In most cases, these color alterations likely happened while the units were deeply buried beneath the surface. However, because these units are so permeable, allowing water to flow easily through them, water has continued migrating, dissolving bits of iron and other minerals even after they have been exposed by erosion. The dissolved minerals often get left behind on canyon walls and surfaces as the water evaporates, contributing to the creation of the well-known “desert varnish” on the rock face.

Iron nodules, often called “Moqui marbles,” weathering out of the Navajo Sandstone. The nodules here range from about 1 to 4 inches in diameter.

Another interesting feature of post-depositional iron-oxide movement within southern Utah’s sandstones are Moqui marbles (see “Glad You Asked” article in the September 2017 issue of Survey Notes). Moqui marbles are spherical concretions or nodules of hematite and sandstone that are formed as large amounts of reducing water dissolve hematite and illite minerals from one part of the sandstone and redeposit them around a point of nucleation. It is unclear what creates the nucleation spot for these iron concretions, but once the hematite begins to bind to some type of ionized nucleus, a chemical reaction begins causing more dissolved hematite to precipitate out of solution around existing nodules.

The amount of iron-oxide mineralization that gives Utah’s sandstones their color is typically very small. One in-depth analysis of rock coloration in the Navajo Sandstone found that minuscule differences in iron-oxide mineralization can mean the difference between red, pink, and white sandstone. For instance, red sandstone contained an average of 0.7 percent of iron oxide within the samples, whereas a sample of “bleached” white sandstone contained 0.2 percent. Pink samples seem to have nearly the same amount of iron minerals as the deep red samples; however, the iron in the pink sections of rock is largely stripped from the original grain coatings and redeposited in voids between the sand grains.

Although geologists are confident about the minerals involved in coloring Utah’s red rocks, many questions remain. Some of these involve the extent to which ancient folding, petroleum migration, or even deep geothermal waters might have played a role in the mineralization and coloring of the rocks. Regardless of the answers, all can agree that the colors of the rocks in Utah’s Colorado Plateau region make for some of the most spectacular scenery on Earth.



For more information see:

Nielson, G. B., Chan, M. A., and Petersen, E.U., 2009, Diagenetic coloration facies and alteration history of the Jurassic Navajo Sandstone, Zion National Park and vicinity, southwestern Utah, in Tripp, B.T., Krahulec, K., and Jordan, J.L., editors, Geology and geologic resources and issues of western Utah: Utah Geological Association Publication 38, p. 67–96.

A Google Earth© oblique view of Split Mountain and surrounding regions. Browns Park Gates of Lodore Canyon of Lodore Diamond Mountain Plateau Yampa Plateau Yampa River Echo Park Island Park Green River Green River Split Mountain Whirlpool Canyon A Google Earth oblique view

SURVEY NOTES

Glad You Asked: The Curious Case of the Green River in the Uinta Mountains

by Douglas A. Sprinkel


People often ask, Why does the Green River flow toward and through the Uinta Mountains? Or, Why does the Green River cut through the middle of Split Mountain when logically it should have flowed around it? Those questions also puzzled John Wesley Powell during the first-ever scientific exploration of the Green and Colorado Rivers in 1869 (see main article in this issue of Survey Notes).

During their month-long stay in the eastern Uinta Mountains, Powell and his companions explored the Green and Yampa Rivers as well as the side canyons and surrounding peaks. Some of our earliest state-of-the-art scientific measurements and descriptions of the geology were the result of that first expedition. Powell made a second trip down the Green and Colorado Rivers in 1871, as well as several pack trips into the territories of Colorado, Wyoming, and Utah. These overland traverses in 1868, 1869, 1874, and 1875 helped Powell formulate his understanding of the geology of the eastern Uinta Mountains.

During Powell’s first expedition, he noted that the Green River flows from its headwaters in the Wind River Mountains of Wyoming southward across a basin, toward the Uinta Mountains. However, at the foot of this mighty mountain range, instead of being deflected by the obstruction, the river flows directly into the range! This and other examples throughout the Uinta Mountains of where the Green River flows through obvious obstructions instead of around them sparked Powell’s imagination to explain such a peculiarity.

So how did the Green River achieve what seems to be an impossible route? At first Powell thought that the river simply followed established fissures within the range. However, Powell rejected that concept because “…very little examinations show that this explanation is unsatisfactory. The proof is abundant that the river cut its own channel; that the cañons are gorges of corrasion.” Powell concluded that the Green River had to have been running its course before the mountain range formed. Powell recognized that the Uinta Mountains are a large anticlinal fold that formed by uplift. He hypothesized that the uplift had to be slow enough for the Green River to cut into the rising fold without altering the river course. Thus, he surmised that the Green River cut its canyon as the range rose around it, and tributaries that fed into the Green River were agents of erosion that stripped the rock off the axis of the great arch. Had the uplift been rapid, the river would have been deflected to a new course that would take the Green River around the flanks of the Uinta Mountains. Powell coined the term “antecedent valley” for this hypothesis. He also coined the terms “consequent valley” for channels that form as a result of the existing topography and “superimposed valley” for channels that form on low-gradient surfaces over which the channel maintains its course even as the stream cuts down into pre-existing structures. The term “superimposed” has since been shortened to “superposed.” Powell also considered but rejected the notion that the Green River is a superposed stream.



Was Powell right? Work by geologists like Julian Sears in 1924 and Wilmot Bradley in 1936 provided evidence that a network of streams flowed away from the Uinta Mountains into the adjoining basins at the end of Uinta uplift during the Laramide orogeny (mountain-building event) 70 to 34 million years ago including where the Green River now flows toward the range. Sears also provided evidence that later faulting formed Browns Park and argued that the Green River was a superposed stream. However, work by Wallace Hansen from 1965 to 1986 provided the clearest picture of drainage reorganization and capture of the Green River. Stream or river capture (also called stream or river piracy) is the diversion of a stream from its current channel to the channel of the capturing stream, often causing the captured stream to reserve its flow. The lowering of the eastern Uinta Mountains by Neogene (20 million years ago to present) extensional faulting was the agent that caused the change in the eastern Uinta Mountain drainage system and the
ultimate capture of the Green River.

Powell and his companions reached Island Park on June 22, 1869, where they camped on one of the islands on the Green River. While there they explored the area noting topography and geology. They climbed the flank of Split Mountain to view the core of the mountain and the rapids within. Powell noted that the Green River entered the range through its north flank, then turned west down the axis of the fold before it turned south again to exit the mountain through its south flank.



Split Mountain is an anticline formed by uplift along the Island Park reverse fault zone, which is on the north limb of the fold. This faulting and folding are part of the general uplift of the Uinta Mountains. At the time of this deformation, the Mesozoic formations that typically overlie the Paleozoic formations were still present and involved in the folding. During a period of relative tectonic quiescence 34 to 25 million years ago, the flanks of the Uinta Mountains, including Split Mountain and the anticline, were buried by a thick blanket of sediments that became the Oligocene-age Bishop Conglomerate. The Bishop Conglomerate formed a low-gradient slope on which streams meandered. Before the Green River was captured and redirected toward the Uintas, it is thought that the Yampa River meandered across the landscape at nearly the present elevation of the Diamond Mountain and Yampa Plateaus (A). The relatively unobstructed Yampa River incised a canyon into the Bishop Conglomerate generally southwest to the Uinta Basin crossing what would become Echo Park, Whirlpool Canyon, Island Park, and Split Mountain. As downcutting continued, the canyon deepened and eventually cut into the emerging Split Mountain (B). The ancestral Yampa River probably took advantage of Laramide fractures on the flanks and crest of the anticline. The Yampa could have been captured by a stream that flowed down the anticlinal crest. Intriguing, beheaded valleys on nearby Blue Mountain, which could be either an ancestral channel of the Yampa, or of a major tributary, support this theory. Regardless, it seems clear to geologists that Split Mountain is a classic example of a superposed stream (C).

The final chapter in this story is the integration of the Green River into the Colorado River basin. Researchers now think that at Echo Park a tributary of the ancestral Yampa River occupied what is now Canyon of Lodore. In 2005, geologists Joel Pederson and Kevin Hadder argued that the tributary formed as Browns Park filled with sediment around the Gates of Lodore. This accumulation of sediment allowed the tributary to spill over and connect the drainage system in Browns Park with the Yampa River drainage and flow into the Uinta Basin, eventually joining the Colorado River below Green River, Utah. The Green River was now captured and redirected to flow toward the Uinta Mountains, through Flaming Gorge and Red Canyon, and down Browns Park to the spillpoint near the Gates of Lodore, completing the river system we see today (perhaps the river in the Uinta Basin should have been called the Yampa since the Yampa was there first!). So, Powell’s keen observations were amazingly astute, though evidence now shows a much more complicated story than he surmised.

Survey Notes v.48 no.1, January 2016

Survey Notes v.48 no.1, January 2016

Our latest issue of Survey Notes is here! Find articles on the new Ogden 30′ x 60′ geological map, the Markagunt Gravity Slide, and more among our regular feature columns.

VIEW THE LATEST ISSUE

Check out past issues of Survey Notes too!

This soil might just look like some crusty dirt. However it’s the crust on the dirt that makes this soil so very important. Tread lightly through this “Glad you Asked” article on soil crusts to find out more!

READ MORE

Have you ever been on an outdoor adventure when you found yourself faced with some kind of geological feature, only you weren’t sure which one? ..It looks like Paul Bunyan’s Woodpile, but is this it?..

Check out our “Glad You Asked” article where you can learn more about how Geographic Names came to be officially recognized, and explore the online database of where these places are located!

READ MORE

Trilobites are always a fun find when you’re exploring the outdoors, but how much do you know about Trilobites as living organisms? Maybe you’re just interested in finding your own Trilobite fossils. Read more about the little critters in our “Glad You Asked” article HERE!

Cooler weather is on its way, so we’ve got a cool “Glad You Asked” article to compliment the changing seasons! It’s a beautiful time of the year to get out into Utah’s geology. Maybe some of you have noticed these groovy rocks out on your outdoor adventures. What are those grooves in the rocks, and how did they get that way?

Read more about Glacial Striations and Slickensides HERE!

We’ve got some Great Salt Lake trivia for you to end the day on—how many think you can answer correctly?? Check out our “Glad You Asked” article below for the answers.

1. What do Great Salt Lake, the Bahamas, the old Hansen Planetarium in downtown Salt Lake City, the Manti LDS Temple, and Hearst Castle in San Simeon, California, have in common?

2. What does the original Saltair resort on the south shore of Great Salt Lake have in common with the coasts of Indonesia, Thailand, and northwestern Malaysia?

3. What two things do Great Salt Lake, Apollo 16, and northern shovelers and common goldeneyes (ducks) have in common?

Find the answers HERE

photo by Stevie Emerson

When I was a child, my family would often go camping in the summers. I would pick up various rocks and ask my dad what they were. “They’re called Leavarite, so you leave em’ right there.” While this is no “Leavarite,” it is something a lightning strike left behind. Most people have never seen it, and those who have may have never realized what it was at the time. This remnant is called a Fulgurite. Fulgurites are natural tubes or crusts of glass formed by the fusion of silica (quartz) sand or rock from a lightning strike. Their shape mimics the path of the lightning bolt as it disperses into the ground.

Read more about fulgurites in our Glad You Asked article HERE!

Current Issue Contents:

• The Uinta Mountains: A Tale of Two Geographies
• In Memoriam: Lehi F. Hintze
• Students Fill the GIS Gap
• The 2014 Crawford Award
• GeoSights: Roosevelt Hot Springs Geothermal Area, Beaver County
• New Publications
• Teacher’s Corner
• Core Center News
• Glad You Asked: What are keeper potholes & how are they formed?

GET IT HERE

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Tag Archive for: Glad you Asked

SURVEY NOTES

What Gives Utah’s “Red Rock Country” its Color?

by Lance Weaver


Coloration of the Navajo Sandstone caused by post-depositional movement of the iron mineral hematite. (Photo credit Peter Fitzgerald, GNU Free Documentation License)

Utah’s Colorado Plateau is famous for its striking vistas and dazzling colors. Hues of red, pink, maroon, yellow, brown, and white create an array of stunning rock colors that attract visitors from all over the globe. From the red rocks of the Navajo Sandstone to the Vermilion Cliffs of the Moenave and Kayenta Formations to the pink, crimson, and chocolate cliffs of the upper Grand Staircase, many who visit the Colorado Plateau wonder what gives the rocks their brilliant colors. This question has spurred much research by geologists, involving chemical and physical analysis. The answers can be complicated, as many different minerals can cause coloration in rocks; however, for the most part, the red, pink, yellow, and brown colors of Utah’s “Red Rock Country” simply comes down to one element—iron.

Since minerals form the basis for many pigments and dyes, it should be no surprise that they are also responsible for the coloration of rocks. Of all the common colorful minerals found in Earth’s crust, few are as abundant, dynamic, and multi-colored as iron. Depending on how it combines with other elements, iron can form a veritable rainbow of colors. When iron combines with oxygen it becomes iron oxide, and its degree of oxidation largely determines its color. Ochre, a mixture of clay, sand, and iron oxide, has been one of the most commonly mined mineral pigments for tens of thousands of years and is composed of the same minerals that often color rocks. Obtained from iron-bearing clays, ochre can produce several colors and hues that are used as natural coloring agents. Red ochre comes from hematite (Fe2O3), a mineral named for the same Greek root word for blood, and has long been used as a red pigment. Some iron oxides, when hydrated (combined with hydrogen and oxygen), can form bright yellows such as yellow ochre which comes from the mineral limonite (FeO(OH)+H2O). Brown ochre comes from the mineral goethite (FeO(OH)) and is a partially hydrated iron oxide. Iron can also form black pigments from minerals such as magnetite (Fe3O4), or even blue and green hues from minerals such as glauconite and illite. For the most part, these iron minerals, and particularly hematite, are responsible for coloring the Colorado Plateau’s sedimentary rock layers.

The Amazon River’s “meeting of waters” is a fantastic example of the different water chemistries likely responsible for the coloration of ancient sediments. The Rio Negro, a tributary of the Amazon, is a “blackwater” river which is clear, slightly acidic, and contains high concentrations of reduced iron. The Amazon, however, has lower concentrations of iron and dissolved solids, but a higher sediment load and oxidized iron giving it its reddish-brown color. (Photo credit Gabriel Heusi, Wikimedia, Creative Commons license.)

Researchers have questioned how the pigment-bearing iron minerals get into rocks like sandstone and shale as well as how the minerals are dispersed within the rock. One might suspect that the brightly colored minerals might be sprinkled throughout the sand and clays or cements that composed the sandstone and shale units—something like chili powder, evenly mixed within the salt. However, by looking at thinly cut sections of rock under a microscope, it becomes clear that this is typically not the case in Utah’s Color Country rock. Instead, the very sand grains that form the matrix of the rock units are actually “frosted” or coated with a layer of iron-rich mineralization. These grains are then cemented together with a pale to white calcite or silicate glue. In the case of sandstone units like the prominent Navajo or Wingate Sandstone, the sand is composed almost entirely of translucent or white quartz grains that are coated with a thin veneer of red hematite mineralization. Although the exact timing is debated among geologists, this “coating” of iron-bearing minerals likely began forming as the grains were transported from their place of erosion to their respective areas of deposition. The same process can be seen today as mineral-rich waters of semi-arid to tropical rivers mineralize large amounts of sediment as it is transported and deposited into adjoining basins.

Multicolored sections of the Navajo Sandstone in the Zion National Park area.

After the sediment is buried, moving groundwater can further mineralize and alter the red rock to change it to varying shades of pink, vermilion, maroon, or even white. In southern Utah, the upper parts of the Navajo and Entrada Sandstones often exhibit areas referred to as “bleached zones.” This term refers to areas where reducing groundwaters have partially removed the iron oxide coating from the sand grains. A reducing agent is a solvent that can remove oxygen from a compound. So in the case of the iron pigments that colored the Navajo Sandstone, groundwater that was slightly acidic or contained other reducing agents seems to have dissolved large amounts of iron mineralization from the upper sections, often redepositing the iron in cracks, joints, or different sections of the sandstone that possess irregularities in grain size. Areas that have lost iron oxide become lighter shades of pink and white, whereas areas that gained additional iron oxide from the groundwater movement become darker shades of maroon and even black. In most cases, these color alterations likely happened while the units were deeply buried beneath the surface. However, because these units are so permeable, allowing water to flow easily through them, water has continued migrating, dissolving bits of iron and other minerals even after they have been exposed by erosion. The dissolved minerals often get left behind on canyon walls and surfaces as the water evaporates, contributing to the creation of the well-known “desert varnish” on the rock face.

Iron nodules, often called “Moqui marbles,” weathering out of the Navajo Sandstone. The nodules here range from about 1 to 4 inches in diameter.

Another interesting feature of post-depositional iron-oxide movement within southern Utah’s sandstones are Moqui marbles (see “Glad You Asked” article in the September 2017 issue of Survey Notes). Moqui marbles are spherical concretions or nodules of hematite and sandstone that are formed as large amounts of reducing water dissolve hematite and illite minerals from one part of the sandstone and redeposit them around a point of nucleation. It is unclear what creates the nucleation spot for these iron concretions, but once the hematite begins to bind to some type of ionized nucleus, a chemical reaction begins causing more dissolved hematite to precipitate out of solution around existing nodules.

The amount of iron-oxide mineralization that gives Utah’s sandstones their color is typically very small. One in-depth analysis of rock coloration in the Navajo Sandstone found that minuscule differences in iron-oxide mineralization can mean the difference between red, pink, and white sandstone. For instance, red sandstone contained an average of 0.7 percent of iron oxide within the samples, whereas a sample of “bleached” white sandstone contained 0.2 percent. Pink samples seem to have nearly the same amount of iron minerals as the deep red samples; however, the iron in the pink sections of rock is largely stripped from the original grain coatings and redeposited in voids between the sand grains.

Although geologists are confident about the minerals involved in coloring Utah’s red rocks, many questions remain. Some of these involve the extent to which ancient folding, petroleum migration, or even deep geothermal waters might have played a role in the mineralization and coloring of the rocks. Regardless of the answers, all can agree that the colors of the rocks in Utah’s Colorado Plateau region make for some of the most spectacular scenery on Earth.



For more information see:

Nielson, G. B., Chan, M. A., and Petersen, E.U., 2009, Diagenetic coloration facies and alteration history of the Jurassic Navajo Sandstone, Zion National Park and vicinity, southwestern Utah, in Tripp, B.T., Krahulec, K., and Jordan, J.L., editors, Geology and geologic resources and issues of western Utah: Utah Geological Association Publication 38, p. 67–96.

A Google Earth© oblique view of Split Mountain and surrounding regions. Browns Park Gates of Lodore Canyon of Lodore Diamond Mountain Plateau Yampa Plateau Yampa River Echo Park Island Park Green River Green River Split Mountain Whirlpool Canyon A Google Earth oblique view

SURVEY NOTES

Glad You Asked: The Curious Case of the Green River in the Uinta Mountains

by Douglas A. Sprinkel


People often ask, Why does the Green River flow toward and through the Uinta Mountains? Or, Why does the Green River cut through the middle of Split Mountain when logically it should have flowed around it? Those questions also puzzled John Wesley Powell during the first-ever scientific exploration of the Green and Colorado Rivers in 1869 (see main article in this issue of Survey Notes).

During their month-long stay in the eastern Uinta Mountains, Powell and his companions explored the Green and Yampa Rivers as well as the side canyons and surrounding peaks. Some of our earliest state-of-the-art scientific measurements and descriptions of the geology were the result of that first expedition. Powell made a second trip down the Green and Colorado Rivers in 1871, as well as several pack trips into the territories of Colorado, Wyoming, and Utah. These overland traverses in 1868, 1869, 1874, and 1875 helped Powell formulate his understanding of the geology of the eastern Uinta Mountains.

During Powell’s first expedition, he noted that the Green River flows from its headwaters in the Wind River Mountains of Wyoming southward across a basin, toward the Uinta Mountains. However, at the foot of this mighty mountain range, instead of being deflected by the obstruction, the river flows directly into the range! This and other examples throughout the Uinta Mountains of where the Green River flows through obvious obstructions instead of around them sparked Powell’s imagination to explain such a peculiarity.

So how did the Green River achieve what seems to be an impossible route? At first Powell thought that the river simply followed established fissures within the range. However, Powell rejected that concept because “…very little examinations show that this explanation is unsatisfactory. The proof is abundant that the river cut its own channel; that the cañons are gorges of corrasion.” Powell concluded that the Green River had to have been running its course before the mountain range formed. Powell recognized that the Uinta Mountains are a large anticlinal fold that formed by uplift. He hypothesized that the uplift had to be slow enough for the Green River to cut into the rising fold without altering the river course. Thus, he surmised that the Green River cut its canyon as the range rose around it, and tributaries that fed into the Green River were agents of erosion that stripped the rock off the axis of the great arch. Had the uplift been rapid, the river would have been deflected to a new course that would take the Green River around the flanks of the Uinta Mountains. Powell coined the term “antecedent valley” for this hypothesis. He also coined the terms “consequent valley” for channels that form as a result of the existing topography and “superimposed valley” for channels that form on low-gradient surfaces over which the channel maintains its course even as the stream cuts down into pre-existing structures. The term “superimposed” has since been shortened to “superposed.” Powell also considered but rejected the notion that the Green River is a superposed stream.



Was Powell right? Work by geologists like Julian Sears in 1924 and Wilmot Bradley in 1936 provided evidence that a network of streams flowed away from the Uinta Mountains into the adjoining basins at the end of Uinta uplift during the Laramide orogeny (mountain-building event) 70 to 34 million years ago including where the Green River now flows toward the range. Sears also provided evidence that later faulting formed Browns Park and argued that the Green River was a superposed stream. However, work by Wallace Hansen from 1965 to 1986 provided the clearest picture of drainage reorganization and capture of the Green River. Stream or river capture (also called stream or river piracy) is the diversion of a stream from its current channel to the channel of the capturing stream, often causing the captured stream to reserve its flow. The lowering of the eastern Uinta Mountains by Neogene (20 million years ago to present) extensional faulting was the agent that caused the change in the eastern Uinta Mountain drainage system and the
ultimate capture of the Green River.

Powell and his companions reached Island Park on June 22, 1869, where they camped on one of the islands on the Green River. While there they explored the area noting topography and geology. They climbed the flank of Split Mountain to view the core of the mountain and the rapids within. Powell noted that the Green River entered the range through its north flank, then turned west down the axis of the fold before it turned south again to exit the mountain through its south flank.



Split Mountain is an anticline formed by uplift along the Island Park reverse fault zone, which is on the north limb of the fold. This faulting and folding are part of the general uplift of the Uinta Mountains. At the time of this deformation, the Mesozoic formations that typically overlie the Paleozoic formations were still present and involved in the folding. During a period of relative tectonic quiescence 34 to 25 million years ago, the flanks of the Uinta Mountains, including Split Mountain and the anticline, were buried by a thick blanket of sediments that became the Oligocene-age Bishop Conglomerate. The Bishop Conglomerate formed a low-gradient slope on which streams meandered. Before the Green River was captured and redirected toward the Uintas, it is thought that the Yampa River meandered across the landscape at nearly the present elevation of the Diamond Mountain and Yampa Plateaus (A). The relatively unobstructed Yampa River incised a canyon into the Bishop Conglomerate generally southwest to the Uinta Basin crossing what would become Echo Park, Whirlpool Canyon, Island Park, and Split Mountain. As downcutting continued, the canyon deepened and eventually cut into the emerging Split Mountain (B). The ancestral Yampa River probably took advantage of Laramide fractures on the flanks and crest of the anticline. The Yampa could have been captured by a stream that flowed down the anticlinal crest. Intriguing, beheaded valleys on nearby Blue Mountain, which could be either an ancestral channel of the Yampa, or of a major tributary, support this theory. Regardless, it seems clear to geologists that Split Mountain is a classic example of a superposed stream (C).

The final chapter in this story is the integration of the Green River into the Colorado River basin. Researchers now think that at Echo Park a tributary of the ancestral Yampa River occupied what is now Canyon of Lodore. In 2005, geologists Joel Pederson and Kevin Hadder argued that the tributary formed as Browns Park filled with sediment around the Gates of Lodore. This accumulation of sediment allowed the tributary to spill over and connect the drainage system in Browns Park with the Yampa River drainage and flow into the Uinta Basin, eventually joining the Colorado River below Green River, Utah. The Green River was now captured and redirected to flow toward the Uinta Mountains, through Flaming Gorge and Red Canyon, and down Browns Park to the spillpoint near the Gates of Lodore, completing the river system we see today (perhaps the river in the Uinta Basin should have been called the Yampa since the Yampa was there first!). So, Powell’s keen observations were amazingly astute, though evidence now shows a much more complicated story than he surmised.

Survey Notes v.48 no.1, January 2016

Survey Notes v.48 no.1, January 2016

Our latest issue of Survey Notes is here! Find articles on the new Ogden 30′ x 60′ geological map, the Markagunt Gravity Slide, and more among our regular feature columns.

VIEW THE LATEST ISSUE

Check out past issues of Survey Notes too!

This soil might just look like some crusty dirt. However it’s the crust on the dirt that makes this soil so very important. Tread lightly through this “Glad you Asked” article on soil crusts to find out more!

READ MORE

Have you ever been on an outdoor adventure when you found yourself faced with some kind of geological feature, only you weren’t sure which one? ..It looks like Paul Bunyan’s Woodpile, but is this it?..

Check out our “Glad You Asked” article where you can learn more about how Geographic Names came to be officially recognized, and explore the online database of where these places are located!

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Trilobites are always a fun find when you’re exploring the outdoors, but how much do you know about Trilobites as living organisms? Maybe you’re just interested in finding your own Trilobite fossils. Read more about the little critters in our “Glad You Asked” article HERE!

Cooler weather is on its way, so we’ve got a cool “Glad You Asked” article to compliment the changing seasons! It’s a beautiful time of the year to get out into Utah’s geology. Maybe some of you have noticed these groovy rocks out on your outdoor adventures. What are those grooves in the rocks, and how did they get that way?

Read more about Glacial Striations and Slickensides HERE!

We’ve got some Great Salt Lake trivia for you to end the day on—how many think you can answer correctly?? Check out our “Glad You Asked” article below for the answers.

1. What do Great Salt Lake, the Bahamas, the old Hansen Planetarium in downtown Salt Lake City, the Manti LDS Temple, and Hearst Castle in San Simeon, California, have in common?

2. What does the original Saltair resort on the south shore of Great Salt Lake have in common with the coasts of Indonesia, Thailand, and northwestern Malaysia?

3. What two things do Great Salt Lake, Apollo 16, and northern shovelers and common goldeneyes (ducks) have in common?

Find the answers HERE

photo by Stevie Emerson

When I was a child, my family would often go camping in the summers. I would pick up various rocks and ask my dad what they were. “They’re called Leavarite, so you leave em’ right there.” While this is no “Leavarite,” it is something a lightning strike left behind. Most people have never seen it, and those who have may have never realized what it was at the time. This remnant is called a Fulgurite. Fulgurites are natural tubes or crusts of glass formed by the fusion of silica (quartz) sand or rock from a lightning strike. Their shape mimics the path of the lightning bolt as it disperses into the ground.

Read more about fulgurites in our Glad You Asked article HERE!

Current Issue Contents:

• The Uinta Mountains: A Tale of Two Geographies
• In Memoriam: Lehi F. Hintze
• Students Fill the GIS Gap
• The 2014 Crawford Award
• GeoSights: Roosevelt Hot Springs Geothermal Area, Beaver County
• New Publications
• Teacher’s Corner
• Core Center News
• Glad You Asked: What are keeper potholes & how are they formed?

GET IT HERE

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