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By Author Messages in Stone: Colorado's Colorful Geology
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First published August 31, 2003
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November 7, 2018
I have learned more about Utah and Colorado Plateau geology than Rocky Mountain geology since it has always intimidated me with all the giant heights of rock and folds and faults but my curiosity is finally stronger than the complexity. As the amazing author John McPhee has written, there is big picture geology that interests me, and I am on the hunt for understanding. Colorado’s Rocky Mountains are on the edge of the Colorado Plateau and have intensely affected the landscape I love in Utah. I also did some research as I attempted to piece together a succinct story and am indebted to an amazing site of the geology of Rocky Mountain National Park that helped place the barebones of the book into a richer context. https://www.nps.gov/parkhistory/onlin...
For some perspective, Pre-Cambrian time was very long, being estimated at 1500 million years, or three quarters of all known geologic time. It is not certain how many eras of time are represented; however, two have been generally recognized: the Archeozoic and the Proterozoic…However, as is very often the case, these rocks have suffered intense deformation during the great length of time since their formation, and have been intruded by igneous masses. They are like a manuscript that has been scattered and torn, leaving us only "internal evidence" as a clue to the order in which the recovered portions should be fitted together. Moreover, the almost general absence of fossils leaves us with no secure means of correlating portions of the record now widely separated by erosion or by intervening areas of younger rocks. We might easily compare the early records of geologic history, so beset with difficulties, to the bare inklings of early human history.”
There have been more than a few mountain building events here in Colorado, and there are mellifluously named orogenies. 1.8 to 1.7 billion years ago, mountains were lifted in the Colorado Orogeny as part of an island arc and continent collision; the Ancestral Rocky Mountains rose 320-300 million years ago when Africa collided with that crustal plate; the Laramide Orogeny 80-40 million years ago may have been caused by plates diving under off the coast of the west coast of North America; and the current Rockies have been actively building for the last 20 million years affected by Basin and Range extension to the west and the San Andreas Fault. The wondrous part is that we still aren’t sure of each mechanism, so I think geology is one of the most vibrant sciences where people can decipher grand and sublime mysteries. I am impatient as well, so if someone can achieve that quickly, that would be great.
Going back to the beginning of what we do know, there was a tectonic plate that carried an oceanic part and an Archean continent of Wyoming and northwestern Colorado with it. Then arrived the plate with the rest of Colorado, in an island arc, and when they collided, our present Colorado was created. The proof if this in geologic terms is the Cheyenne Belt, a suture of the two with 2.7 billion year old rocks on one side, and the younger Precambrian rocks of Colorado that were only 1.7 billion years old. Of course, now I immediately want to find it and see it and experience that feeling, but of course, I already have crossed it to go the Snowy Mountains (a subrange of the Medicine Bow Mountains that stretches from Colorado for 100 miles to Wyoming) northwest of Laramie and just didn’t know. A little digression: the name Medicine Bow is quintessentially western and native American; legend has it that the tribes of the area created the most wonderful bows and in the linguistic acrobatics to be able to communicate, “making-medicine” and “making-bows” were combined to name the area.
“Unlike the richly vegetated slops of today’s mountains, Colorado’s Precambrian mountains did not have a single plant growing on them. For that matter, neither did the hills and plains because plants had not yet colonized the land. By the end of the Proterozoic, all trace of mountainous topography was gone, flattened into a featureless plain by erosion. The Precambrian story ends at the Cambrian Period when abundant fossils of shell-bearing animals appeared.” The erosion of these bare mountains must have been accelerated without plants to hold the soil, if there was soil and dirt and sand accumulating, and the idea that plants hold our landscapes together is never more present in my mind. Imagine the entire planet, not green or even brown, but perhaps grays and blacks of volcanic rock. What a sight to see.
Some of the most ancient rocks on the planet are found in the Canadian Shield, exposed, and elsewhere in a giant North American Craton that has stood relatively unwarped and solid through millions of years. Of course, it traveled around the globe, but was exempt from mountain building forests. There are parts of these exact rocks in Greenland and Scotland. It stretches from the Artic Circle to Texas, although not as far as the Gulf of Mexico, and from the edges of the Appalachians to the Rocky Mountains, where the island arc collided and sutured on, or more poetically, where the continent accumulated, amassed, accreted more land to coalesce to make the West as it is now.
As mentioned above, the continent and Colorado wandered around the globe, or more aptly, grinded, travailed, and crushed it way around. For example, in the early part of the Paleozoic, it was in the Southern Hemisphere, but then at the equator in Pennsylvanian time, and in the northern Horse Latitudes along the latitude of Florida in the mid-Permian time. I learned that there is currently no way to fix the east-west position (longitude) of ancient continents, only their latitude. So it could have been exactly where Florida is now, or on the other side of the globe, its antipode, in the middle of the Indian Ocean (courtesy of www.antipodesmap.com).
Moving forward in time, about 500 million years ago, Cambrian time, while Colorado was close to the equator, the sea advanced, and created shoreline and near-shoreline sandstones in Glenwood Canyon and other places; life was arising in the ocean, but still no plants or animals on land. Some are found in the San Juan and Front Ranges in Colorado.
Per the National Park Service, this time is described as: “The waters of the sea found the surface of decayed rock much as one finds the surface of the lower granitic areas of today, strewn with broken rock, sand, and clay, grading downward into the unweathered bedrock. The rivers which formerly carried the sediments beyond the region into the oceans bordering the continent, now dropped them in the shore waters of the invading sea. The waves worked over the residual surface material, and the sediments brought in by the streams, sorting them and spreading them over the sea floor in layers or strata of conglomerate, sandstone and shale. In some parts of the sea the shore waters were clear and made a favorable habitat for animals and plants. The animals took lime from the waters, made it into their shells and other parts, and as the animals died, these limy parts accumulated on the bottom. Century after century these accumulations grow, and the waves broke and ground many of them into a mud-like mass and spread a mixture of limestone mud, shells, and shell fragments over the sea floor in layers which afterward consolidated into limestone strata of the upper part of the Cambrian formation.”
The Ordovician period (450 million years ago) saw the sea drain and then inundate the land again, depositing dolomite and limestone and indicating wildly fluctuating shoreline with rivers draining into it (estuaries), in stormy settings, and they know that by rock features that show the rock being deposited had periods of exposure to air, and then coverage again. The ancient estuary habitat near Florence Colorado has preserved agnathids, one of the earliest known vertebrates or fish as well as conodonts, a type of eel.
This is where I get most awestruck; that this land I walk on was once and many times covered by a shallow sea, and that sense of wonder is hard to share. You either feel it or you don’t, but it demonstrates the power of science to expose the world in a way that enriches your appreciation and experience of it, so that beauty is in every grain of sand, and every rock.
The Silurian period, where Colorado was below the equator, possibly where the Andes are, or off the coast of Thailand and Vietnam where the Gulf of Thailand is now, was the one of the highest points of sea level, until it dropped precipitously and allowed erosion of much of the depositions of the Silurian. During that time, the Devonian Period, about 375 million years ago, sea level rose again slowly, and Colorado was under a large bay without a lot of drainage from rivers, eroding the former layers and depositing fossil rich shale beds that include corals and mollusks. There are no Devonian era rocks ever found in Colorado, but it is known as the Age of the Fishes and lush, richly evolving vegetation started to cover the land like ferns and mosses, and by the end of the Devonian, the first forests and the first air-breathing vertebrate appeared.
From the NPS: “Colorado and much of the surrounding territory was land area during late Ordovician, all of Silurian and the greater part of Devonian times. It is known from a study of Silurian rocks elsewhere that the climate was mild and equable over North America even far north in the Arctic circle, for coral reefs built up by organisms very sensitive to cold flourished there. Invertebrates continued to dominate the marine life. It is in Silurian rocks that the earliest remains of supposed land plants have been discovered. These consist of a few fragmentary stems, some of which bear small bract-like leaves. It is possible that the land surface existing in Colorado at this time was clothed by these primitive land plants occupying perhaps the lowlands along the rivers and shores but probably far from being profusely distributed. Fishes undoubtedly lived in the streams; and scorpions and thousand legged worms (millipeds) are known from the upper Silurian rocks and may have been the first animals to inhabit the lands.”
Shallow seas covered Colorado again in Mississippian time (340 million years ago) as North American and Greenland collided with Europe (super-continent Laurasia) and gray limestone was deposited that is best exposed in the famous Leadville area and contains mineral riches. When that limestone eroded in late Mississippian, it created karsts and terra rosa, a red colored soil preserved in southwestern Colorado. Near the end of the period, the slightly mysterious rise of the Colorado Plateau region happened, lifted into mountain ridges, due to disturbances of the earth’s crust, which caused the seas to withdraw. These mountains are called the Colorado Mountains.
The humid and rainy Pennsylvanian Period, slightly above the equator, about 300 million years ago, had shallow seas again, dark colored marine shales until the Ancestral Rocky Mountains and the Ancestral Uncompahgre Mountains rose. The Ancestral Rockies were about 2,000 feet high and extended from Boulder to Steamboat, Colorado. “The mountains rose faster than the rising seas and were attacked by erosion. They shed sediment into basins between the ranges and formed aprons of coarse sediment on both flanks of the two ranges. since it is rare to find faults and folds during this time, the distribution is the primary evidence…examples of the reddish sediment shed off the east side of the Ancestral Rockies are displayed in the Flatirons near Boulder, Red Rocks, and Roxborough State Park. Erosion of these rocks created the ski runs of Vail…and the gorgeous Maroon Bells of Aspen.”
Interestingly, if you ever drive across the Rockies via I-70, there are puzzling badlands of Eagle and Gypsum in the central Rockies that were caused by evaporation basins and deformation of the softer gypsum, other salts, and algal mounds. The rising mountains blocked the seas. This areas seem incongruous with the surrounding mountains to the east (Vail and the Gore Range and Glenwood Canyon to the west.) They have found 20 marine cycles near Vail and the subsurface of Denver caused by the supercontinent Gondwana’s glaciers that formed and melted several times, and caused an unusual fluctuation of sea level. Colorado was not part of Gondwana, but affected by it vai worldwide sea levels; Gondwana lay on the South Pole and consisted of Africa, South America, and Australia.
Per NPS: “Early in the Pennsylvanian there began another period of subsidence allowing the seas to slowly creep over the land, the waves assorting the materials they found on the surface, carrying the finer sands and clays into deep water to make sandstone and shale, and leaving the coarse material as a conglomerate. The transgression of the sea was so extensive, as is shown by the present distribution of Pennsylvanian rocks, that the recently uplifted Colorado Mountains stood as island masses surrounded at times by the sea. These land areas were undergoing vigorous erosion and the streams were carrying great quantities of waste material into the surrounding lowlands and out into the sea. The shoreline in the Colorado area was probably very uneven; in some regions there were extensive bays which were being rapidly filled with coarse deposits laid down as deltas at the mouths of streams. Some of these delta deposits are very extensive and were evidently laid down in continental basins wholly cut off from the sea; other deposits seem more likely to have been formed along river courses and flood plains, and in fresh water basins. There were abundant insects, one with a wing span of 29 inches, and the first reptiles formed.
The Permian period was marked by arid conditions, about 275 million years ago, and large dune fields spread across eastern Colorado, but the western part still had shallow seas. The Ancestral Rockies were leveled flat, and all the continents joined to form Pangaea supercontinent, with Colorado on the edge of a shallow sea.
From NPS: These conditions were generally adverse to life and the struggle for existence was intense. All forms of life were greatly reduced in numbers and many forms disappeared entirely. Mountain-making movements of the earth's crust were very marked; for it was during this time that the principal movements occurred, producing the Appalachian mountains of eastern North America. The Urals of Europe, and the Variscan chains were completed across southern England, Germany, and Northern France. In the Orient there were also folding and thrusts along the arc of the Japanese Islands. The widespread aridity and other great changes wrought in climate by these major movements in the earth's crust had marked effect upon the vertebrate life. The amphibians which had dominated the land during the warm and humid Mississippian and Pennsylvanian, and which were dependent upon bodies of water for at least a portion of their life history, found that environment becoming very scarce. However, the relatively insignificant reptiles of the late Paleozoic were increasing greatly in number during the Permian. Before the close of the period they had undoubtedly mastered all the land, and heralded in a time when they were to reign supreme.
To wrap up, all of that detail was in the Paleozoic Era, the age of Invertebrates, and it was essentially a time of multiple incursions of the sea and the beginning of life in and out of the sea. The Precambrian that preceded it is shrouded in more mystery since there was more time to alter the rocks so their story is harder to decipher. Next is the Mesozoic Era which includes the Triassic, Jurassic and Cretaceous periods and was known as the Age of Reptiles aka Dinosaurs.
The Triassic Period was a time of eroding mountains with a warm dry climate, and much of the fabulous scenic “redbeds” were created from mudflats, alluvial plains, and dune fields as sandstones, siltstones, and shales. There is little preservation of flora during this time, but the dinosaurs began their meteoric rise and the very first, small mammals appeared. The Jurassic was next, 175 million years ago, and had a warm, desert climate as North America separated from Africa, and drifted into the northern Horse Latitudes. There were vast plains of sand dunes, four short periods of the ocean again, and then a freshwater lowland of lakes, swamps, and braided and meandering streams similar to the present day Amazon interspersed with sand dunes, covering 10 states and part of Canada, called the Morrison Formation and it was rich with hundreds of fossils including the dinosaurs, including all of the American Jurassic dinosaurs, meaning they haven’t been found anywhere else.
The Cretaceous, about 100 million years ago, saw the Gulf of Mexico approach from the South while the Arctic Ocean flooded in from the north, while other oceanic waters came in from all directions, again and again. To view a timelapse of that is my idea of heaven. The Western Interior Seaway covered most of Colorado while volcanoes were active in Utah and possibly every western state, and is similar to the coast of Lousiana. The book asks us to imagine the hurricanes that could have been formed with such a long coastline (1300 miles!) There is evidence in the Mesa Verde formation that preserves washover channels and fans similar to Gulf Coast landforms. A sight to see.
The beginning of the Laramide orogeny ended the ocean’s incursions, never to return. Colorado was tropical at this time, and bees and butterflies appeared for the first time, as well as flowering plants and palm trees. “ Cretaceous rocks represent four basic depositional environments. Sediment deposited in deep to moderate depth marine waters yielded limestone and chalk. Sediment deposited into shallow to intermediate depths of marine waters produced dark shale with interbedded thin limestone, siltstone, and sandstone. Massive sandstone formed near shorelines or in intertidal areas. Sediments in coastal plain, lagoon, swamp, and alluvial plain environments became claystone, shale, lenticular sandstone, coal, and conglomerate.”
NPS: ”Soon after the maximum inundation of the sea (Benton Time) the northern end of the geosyncline emerged and the great eperic sea began a southward retreat that was hastened by the rapid filling of its basin with sediments pouring in from the rising highlands to the west. The sea lingered longest in an elongate embayment extending northward from the Gulf of Mexico across the western Great Plains states, known as the Lance sea. The final retreat of the sea transformed its old floor into a vast swampy lowland over which the streams spread thick non-marine sediments during the closing stages of the period.
In the swamps of this lowland accumulated the vegetation that was to make the vast coal beds of the latest Cretaceous formations of the Rocky Mountain region from Alberta to Mexico. The closing stages of the Mesozoic are marked by one of the most extensive mountain making movements North America had experienced since the Pre-Cambrian. This orogeny involved a region fully 3000 miles wide, extending from eastern Colorado to eastern Nevada and central Idaho; and from western Alaska to Mexico. At this time the Cordilleran geosyncline which had been the site of so many marine transgressions during the Paleozoic and Mesozoic and which had only recently been covered by the Cretaceous sea, was folded and faulted on a grand scale. Great forces within the earth's crust brought about extensive compression from the west.
1 of 2, to be continued in comments
For some perspective, Pre-Cambrian time was very long, being estimated at 1500 million years, or three quarters of all known geologic time. It is not certain how many eras of time are represented; however, two have been generally recognized: the Archeozoic and the Proterozoic…However, as is very often the case, these rocks have suffered intense deformation during the great length of time since their formation, and have been intruded by igneous masses. They are like a manuscript that has been scattered and torn, leaving us only "internal evidence" as a clue to the order in which the recovered portions should be fitted together. Moreover, the almost general absence of fossils leaves us with no secure means of correlating portions of the record now widely separated by erosion or by intervening areas of younger rocks. We might easily compare the early records of geologic history, so beset with difficulties, to the bare inklings of early human history.”
There have been more than a few mountain building events here in Colorado, and there are mellifluously named orogenies. 1.8 to 1.7 billion years ago, mountains were lifted in the Colorado Orogeny as part of an island arc and continent collision; the Ancestral Rocky Mountains rose 320-300 million years ago when Africa collided with that crustal plate; the Laramide Orogeny 80-40 million years ago may have been caused by plates diving under off the coast of the west coast of North America; and the current Rockies have been actively building for the last 20 million years affected by Basin and Range extension to the west and the San Andreas Fault. The wondrous part is that we still aren’t sure of each mechanism, so I think geology is one of the most vibrant sciences where people can decipher grand and sublime mysteries. I am impatient as well, so if someone can achieve that quickly, that would be great.
Going back to the beginning of what we do know, there was a tectonic plate that carried an oceanic part and an Archean continent of Wyoming and northwestern Colorado with it. Then arrived the plate with the rest of Colorado, in an island arc, and when they collided, our present Colorado was created. The proof if this in geologic terms is the Cheyenne Belt, a suture of the two with 2.7 billion year old rocks on one side, and the younger Precambrian rocks of Colorado that were only 1.7 billion years old. Of course, now I immediately want to find it and see it and experience that feeling, but of course, I already have crossed it to go the Snowy Mountains (a subrange of the Medicine Bow Mountains that stretches from Colorado for 100 miles to Wyoming) northwest of Laramie and just didn’t know. A little digression: the name Medicine Bow is quintessentially western and native American; legend has it that the tribes of the area created the most wonderful bows and in the linguistic acrobatics to be able to communicate, “making-medicine” and “making-bows” were combined to name the area.
“Unlike the richly vegetated slops of today’s mountains, Colorado’s Precambrian mountains did not have a single plant growing on them. For that matter, neither did the hills and plains because plants had not yet colonized the land. By the end of the Proterozoic, all trace of mountainous topography was gone, flattened into a featureless plain by erosion. The Precambrian story ends at the Cambrian Period when abundant fossils of shell-bearing animals appeared.” The erosion of these bare mountains must have been accelerated without plants to hold the soil, if there was soil and dirt and sand accumulating, and the idea that plants hold our landscapes together is never more present in my mind. Imagine the entire planet, not green or even brown, but perhaps grays and blacks of volcanic rock. What a sight to see.
Some of the most ancient rocks on the planet are found in the Canadian Shield, exposed, and elsewhere in a giant North American Craton that has stood relatively unwarped and solid through millions of years. Of course, it traveled around the globe, but was exempt from mountain building forests. There are parts of these exact rocks in Greenland and Scotland. It stretches from the Artic Circle to Texas, although not as far as the Gulf of Mexico, and from the edges of the Appalachians to the Rocky Mountains, where the island arc collided and sutured on, or more poetically, where the continent accumulated, amassed, accreted more land to coalesce to make the West as it is now.
As mentioned above, the continent and Colorado wandered around the globe, or more aptly, grinded, travailed, and crushed it way around. For example, in the early part of the Paleozoic, it was in the Southern Hemisphere, but then at the equator in Pennsylvanian time, and in the northern Horse Latitudes along the latitude of Florida in the mid-Permian time. I learned that there is currently no way to fix the east-west position (longitude) of ancient continents, only their latitude. So it could have been exactly where Florida is now, or on the other side of the globe, its antipode, in the middle of the Indian Ocean (courtesy of www.antipodesmap.com).
Moving forward in time, about 500 million years ago, Cambrian time, while Colorado was close to the equator, the sea advanced, and created shoreline and near-shoreline sandstones in Glenwood Canyon and other places; life was arising in the ocean, but still no plants or animals on land. Some are found in the San Juan and Front Ranges in Colorado.
Per the National Park Service, this time is described as: “The waters of the sea found the surface of decayed rock much as one finds the surface of the lower granitic areas of today, strewn with broken rock, sand, and clay, grading downward into the unweathered bedrock. The rivers which formerly carried the sediments beyond the region into the oceans bordering the continent, now dropped them in the shore waters of the invading sea. The waves worked over the residual surface material, and the sediments brought in by the streams, sorting them and spreading them over the sea floor in layers or strata of conglomerate, sandstone and shale. In some parts of the sea the shore waters were clear and made a favorable habitat for animals and plants. The animals took lime from the waters, made it into their shells and other parts, and as the animals died, these limy parts accumulated on the bottom. Century after century these accumulations grow, and the waves broke and ground many of them into a mud-like mass and spread a mixture of limestone mud, shells, and shell fragments over the sea floor in layers which afterward consolidated into limestone strata of the upper part of the Cambrian formation.”
The Ordovician period (450 million years ago) saw the sea drain and then inundate the land again, depositing dolomite and limestone and indicating wildly fluctuating shoreline with rivers draining into it (estuaries), in stormy settings, and they know that by rock features that show the rock being deposited had periods of exposure to air, and then coverage again. The ancient estuary habitat near Florence Colorado has preserved agnathids, one of the earliest known vertebrates or fish as well as conodonts, a type of eel.
This is where I get most awestruck; that this land I walk on was once and many times covered by a shallow sea, and that sense of wonder is hard to share. You either feel it or you don’t, but it demonstrates the power of science to expose the world in a way that enriches your appreciation and experience of it, so that beauty is in every grain of sand, and every rock.
The Silurian period, where Colorado was below the equator, possibly where the Andes are, or off the coast of Thailand and Vietnam where the Gulf of Thailand is now, was the one of the highest points of sea level, until it dropped precipitously and allowed erosion of much of the depositions of the Silurian. During that time, the Devonian Period, about 375 million years ago, sea level rose again slowly, and Colorado was under a large bay without a lot of drainage from rivers, eroding the former layers and depositing fossil rich shale beds that include corals and mollusks. There are no Devonian era rocks ever found in Colorado, but it is known as the Age of the Fishes and lush, richly evolving vegetation started to cover the land like ferns and mosses, and by the end of the Devonian, the first forests and the first air-breathing vertebrate appeared.
From the NPS: “Colorado and much of the surrounding territory was land area during late Ordovician, all of Silurian and the greater part of Devonian times. It is known from a study of Silurian rocks elsewhere that the climate was mild and equable over North America even far north in the Arctic circle, for coral reefs built up by organisms very sensitive to cold flourished there. Invertebrates continued to dominate the marine life. It is in Silurian rocks that the earliest remains of supposed land plants have been discovered. These consist of a few fragmentary stems, some of which bear small bract-like leaves. It is possible that the land surface existing in Colorado at this time was clothed by these primitive land plants occupying perhaps the lowlands along the rivers and shores but probably far from being profusely distributed. Fishes undoubtedly lived in the streams; and scorpions and thousand legged worms (millipeds) are known from the upper Silurian rocks and may have been the first animals to inhabit the lands.”
Shallow seas covered Colorado again in Mississippian time (340 million years ago) as North American and Greenland collided with Europe (super-continent Laurasia) and gray limestone was deposited that is best exposed in the famous Leadville area and contains mineral riches. When that limestone eroded in late Mississippian, it created karsts and terra rosa, a red colored soil preserved in southwestern Colorado. Near the end of the period, the slightly mysterious rise of the Colorado Plateau region happened, lifted into mountain ridges, due to disturbances of the earth’s crust, which caused the seas to withdraw. These mountains are called the Colorado Mountains.
The humid and rainy Pennsylvanian Period, slightly above the equator, about 300 million years ago, had shallow seas again, dark colored marine shales until the Ancestral Rocky Mountains and the Ancestral Uncompahgre Mountains rose. The Ancestral Rockies were about 2,000 feet high and extended from Boulder to Steamboat, Colorado. “The mountains rose faster than the rising seas and were attacked by erosion. They shed sediment into basins between the ranges and formed aprons of coarse sediment on both flanks of the two ranges. since it is rare to find faults and folds during this time, the distribution is the primary evidence…examples of the reddish sediment shed off the east side of the Ancestral Rockies are displayed in the Flatirons near Boulder, Red Rocks, and Roxborough State Park. Erosion of these rocks created the ski runs of Vail…and the gorgeous Maroon Bells of Aspen.”
Interestingly, if you ever drive across the Rockies via I-70, there are puzzling badlands of Eagle and Gypsum in the central Rockies that were caused by evaporation basins and deformation of the softer gypsum, other salts, and algal mounds. The rising mountains blocked the seas. This areas seem incongruous with the surrounding mountains to the east (Vail and the Gore Range and Glenwood Canyon to the west.) They have found 20 marine cycles near Vail and the subsurface of Denver caused by the supercontinent Gondwana’s glaciers that formed and melted several times, and caused an unusual fluctuation of sea level. Colorado was not part of Gondwana, but affected by it vai worldwide sea levels; Gondwana lay on the South Pole and consisted of Africa, South America, and Australia.
Per NPS: “Early in the Pennsylvanian there began another period of subsidence allowing the seas to slowly creep over the land, the waves assorting the materials they found on the surface, carrying the finer sands and clays into deep water to make sandstone and shale, and leaving the coarse material as a conglomerate. The transgression of the sea was so extensive, as is shown by the present distribution of Pennsylvanian rocks, that the recently uplifted Colorado Mountains stood as island masses surrounded at times by the sea. These land areas were undergoing vigorous erosion and the streams were carrying great quantities of waste material into the surrounding lowlands and out into the sea. The shoreline in the Colorado area was probably very uneven; in some regions there were extensive bays which were being rapidly filled with coarse deposits laid down as deltas at the mouths of streams. Some of these delta deposits are very extensive and were evidently laid down in continental basins wholly cut off from the sea; other deposits seem more likely to have been formed along river courses and flood plains, and in fresh water basins. There were abundant insects, one with a wing span of 29 inches, and the first reptiles formed.
The Permian period was marked by arid conditions, about 275 million years ago, and large dune fields spread across eastern Colorado, but the western part still had shallow seas. The Ancestral Rockies were leveled flat, and all the continents joined to form Pangaea supercontinent, with Colorado on the edge of a shallow sea.
From NPS: These conditions were generally adverse to life and the struggle for existence was intense. All forms of life were greatly reduced in numbers and many forms disappeared entirely. Mountain-making movements of the earth's crust were very marked; for it was during this time that the principal movements occurred, producing the Appalachian mountains of eastern North America. The Urals of Europe, and the Variscan chains were completed across southern England, Germany, and Northern France. In the Orient there were also folding and thrusts along the arc of the Japanese Islands. The widespread aridity and other great changes wrought in climate by these major movements in the earth's crust had marked effect upon the vertebrate life. The amphibians which had dominated the land during the warm and humid Mississippian and Pennsylvanian, and which were dependent upon bodies of water for at least a portion of their life history, found that environment becoming very scarce. However, the relatively insignificant reptiles of the late Paleozoic were increasing greatly in number during the Permian. Before the close of the period they had undoubtedly mastered all the land, and heralded in a time when they were to reign supreme.
To wrap up, all of that detail was in the Paleozoic Era, the age of Invertebrates, and it was essentially a time of multiple incursions of the sea and the beginning of life in and out of the sea. The Precambrian that preceded it is shrouded in more mystery since there was more time to alter the rocks so their story is harder to decipher. Next is the Mesozoic Era which includes the Triassic, Jurassic and Cretaceous periods and was known as the Age of Reptiles aka Dinosaurs.
The Triassic Period was a time of eroding mountains with a warm dry climate, and much of the fabulous scenic “redbeds” were created from mudflats, alluvial plains, and dune fields as sandstones, siltstones, and shales. There is little preservation of flora during this time, but the dinosaurs began their meteoric rise and the very first, small mammals appeared. The Jurassic was next, 175 million years ago, and had a warm, desert climate as North America separated from Africa, and drifted into the northern Horse Latitudes. There were vast plains of sand dunes, four short periods of the ocean again, and then a freshwater lowland of lakes, swamps, and braided and meandering streams similar to the present day Amazon interspersed with sand dunes, covering 10 states and part of Canada, called the Morrison Formation and it was rich with hundreds of fossils including the dinosaurs, including all of the American Jurassic dinosaurs, meaning they haven’t been found anywhere else.
The Cretaceous, about 100 million years ago, saw the Gulf of Mexico approach from the South while the Arctic Ocean flooded in from the north, while other oceanic waters came in from all directions, again and again. To view a timelapse of that is my idea of heaven. The Western Interior Seaway covered most of Colorado while volcanoes were active in Utah and possibly every western state, and is similar to the coast of Lousiana. The book asks us to imagine the hurricanes that could have been formed with such a long coastline (1300 miles!) There is evidence in the Mesa Verde formation that preserves washover channels and fans similar to Gulf Coast landforms. A sight to see.
The beginning of the Laramide orogeny ended the ocean’s incursions, never to return. Colorado was tropical at this time, and bees and butterflies appeared for the first time, as well as flowering plants and palm trees. “ Cretaceous rocks represent four basic depositional environments. Sediment deposited in deep to moderate depth marine waters yielded limestone and chalk. Sediment deposited into shallow to intermediate depths of marine waters produced dark shale with interbedded thin limestone, siltstone, and sandstone. Massive sandstone formed near shorelines or in intertidal areas. Sediments in coastal plain, lagoon, swamp, and alluvial plain environments became claystone, shale, lenticular sandstone, coal, and conglomerate.”
NPS: ”Soon after the maximum inundation of the sea (Benton Time) the northern end of the geosyncline emerged and the great eperic sea began a southward retreat that was hastened by the rapid filling of its basin with sediments pouring in from the rising highlands to the west. The sea lingered longest in an elongate embayment extending northward from the Gulf of Mexico across the western Great Plains states, known as the Lance sea. The final retreat of the sea transformed its old floor into a vast swampy lowland over which the streams spread thick non-marine sediments during the closing stages of the period.
In the swamps of this lowland accumulated the vegetation that was to make the vast coal beds of the latest Cretaceous formations of the Rocky Mountain region from Alberta to Mexico. The closing stages of the Mesozoic are marked by one of the most extensive mountain making movements North America had experienced since the Pre-Cambrian. This orogeny involved a region fully 3000 miles wide, extending from eastern Colorado to eastern Nevada and central Idaho; and from western Alaska to Mexico. At this time the Cordilleran geosyncline which had been the site of so many marine transgressions during the Paleozoic and Mesozoic and which had only recently been covered by the Cretaceous sea, was folded and faulted on a grand scale. Great forces within the earth's crust brought about extensive compression from the west.
1 of 2, to be continued in comments
March 17, 2021
I really enjoyed this book! This was my first taste with geology and I appreciated that basic terms were defined. The photography was phenomenal. It was so interesting to learn more about Colorado, giving me words to describe the beauty I saw! Now I can’t wait to visit again!
September 17, 2019
This is a good book with a lot information. It just really wasn't for me. Too much science talk.
June 10, 2020
I've always been sorry that I didn't take a Geology class in college, so when I saw this title I knew it was a book that I needed to read. And it was well worth the time. The diagrams and photos make the info very accessible to the Geologically illiterate. Here are a few tidbits that I flagged:
p 10: There are at least 19 calderas in Colorado making the state one of the world's best outdoor laboratories in which to study their formation.
p 49: The oldest dated rocks on Earth are about 4 billion years old. The oldest dated rocks in Colorado are about 2.7 billion years old and lie in a small area in the very NW corner of the state.
p 78: Before their disappearance 12,000 years ago, large glaciers thousands of feet thick filled valleys and left their marks on the mountainous landscapes of Colorado. ... Today's small glaciers are not remnants of the large glaciers, but were formed in sheltered mountainous landscapes in Colorado during the Little Ice Age between 1200 and 1880 A.D.
p 95: Speleologists continue to catalogue Colorado's more than 265 caves and have compiled an impressive list of types ...
I particularly appreciated the geologic descriptions of places I have visited. I'll put this on my shelf of books that I need to read again in a couple of years.
p 10: There are at least 19 calderas in Colorado making the state one of the world's best outdoor laboratories in which to study their formation.
p 49: The oldest dated rocks on Earth are about 4 billion years old. The oldest dated rocks in Colorado are about 2.7 billion years old and lie in a small area in the very NW corner of the state.
p 78: Before their disappearance 12,000 years ago, large glaciers thousands of feet thick filled valleys and left their marks on the mountainous landscapes of Colorado. ... Today's small glaciers are not remnants of the large glaciers, but were formed in sheltered mountainous landscapes in Colorado during the Little Ice Age between 1200 and 1880 A.D.
p 95: Speleologists continue to catalogue Colorado's more than 265 caves and have compiled an impressive list of types ...
I particularly appreciated the geologic descriptions of places I have visited. I'll put this on my shelf of books that I need to read again in a couple of years.
June 20, 2012
Awesome book for those, such as I, who are into this subject!
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