Rocky Mountains

The various mountain ranges and basins that make up the Rocky Mountains represent a region of tremendous mineral wealth that has played an essential part in the economic development and prosperity of the United States and Canada. In addition, the ranges constitute a recreational resource of inestimable value. Finally, the Rockies have proved to be an invaluable key to understanding the earth's past as well as the processes that continue to transform it.

Mountain Formation

Numerous mountain ranges are sometimes grouped under the general definition of the Rocky Mountains by various geologists and geographers. Canadian scientists often define the Rockies differently from the way their counterparts in the United States define the Rockies. During the past few centuries, the defined extent of the Rockies has expanded or contracted to include first one range or feature, then yet another. To define the Rockies in a geologically meaningful way, it is necessary to include those features that are surface expressions of particular systems of formative processes. The Rockies and their geologic history are products of long-term and very large-scale processes that shaped not only the North American continent but also the configuration of the earth's continents and oceans as a whole. Thus, the term "Rockies" will mean the more easterly of those mountain ranges and closely associated features that constitute the North American cordilleran region.

The cordillera is a long, largely continuous highland structure extending from eastern Alaska southward to northern New Mexico. For most of their length, the Rockies front the Great Plains to the east and, in the United States, border a geophysical feature called the Basin and Range Province to the west. The Rockies owe much of their present conformation to the Cordilleran orogeny, of which the Laramide orogeny was one of the last great pulses. Thus, a geologic history of the Rocky Mountains is, for the most part, a few localized acts in the greater drama of the Cordilleran orogeny and the formation of North America.

Orogenies are mountain-building episodes or events occurring over periods of time, typically measured in tens of millions of years. In modern geologic theory, they are considered consequences of large-scale forces operating in the realm of plate tectonics. Plate tectonics theory, aspects of which were once called continental drift, holds that the crust of the earth is composed of about twelve major rigid units called lithospheric plates. Some of these plates are oceanic, while others are continental in nature. All are involved in some type of motion relative to one another.

Plate motion can take various forms: Plates can move away from a common center, slide alongside each other, or collide with each other with various effects. One collision type involves a process called subduction, in which a heavier plate is forced to dive beneath the edge of a lighter plate. This effect is believed to be taking place along the western coast of South America as the edge of a heavier oceanic plate is subducted beneath the lighter continental plate where South America rides. The Andes mountain chain is considered a direct result of this subduction. Geologists point to the Andes and the associated subduction as a model for what probably occurred in relation to the formation of the Rockies. In both cases, a long, high belt of mountains possessing similar rocks, minerals, and geologic structures was formed as a product of plate collision.

Much of the evidence of the way the Rockies formed exists in the findings of two subbranches of geology: stratigraphy and sedimentology. Both sciences deal with the layers of rock termed strata and the story they tell about past environments in which deposition and erosion took place. Findings indicate that the ranges known as the Rockies did not exist in their present incarnation before the close of the Mesozoic era, 65 to 70 million years ago. Across great durations of time previous to that period, however, evidence exists for other incarnations of the Rockies occupying somewhat similar geographic orientations. The incarnation just prior to the present range has been termed the Ancestral Rockies. Before the appearance and elimination through erosion of this range, it appears that other Rocky Mountain-like ranges existed at least two other times in earth's history.

These very ancient pre-Ancestral Rockies are evidenced by recrystallized, tightly folded rocks, which represent orogenies estimated to have occurred during the Late Precambrian eon, also termed the Proterozoic eon, which ended about 600 million years ago. Very general ages for two orogenies of these Precambrian Rockies have been estimated at about 1.5 billion and 2.5 billion years before the present. After about 1 billion years before the present, orogenic activity seems to have terminated, and a very lengthy period of crustal stability and erosion appears to have dominated. During this period, the last of the Precambrian Rockies seems to have been totally beveled down to the roots by erosion during a time referred to as the Lipalian interval, which lasted for hundreds of millions of years.

Paleozoic and Mesozoic Eras

With the advent of the Paleozoic era about 600 million years before the present, stratigraphic evidence indicates marine conditions dominated in the region formerly occupied by mountains. Sandstones, limestones, and shales indicative of quiet, marine depositional environments prevailed. This situation continued during much of the Early Paleozoic over much of what later would become the North American cordilleran region. Beginning with the Late Devonian period in the Late Paleozoic, about 380 million years before the present and extending into the following Mississippian period, tectonic disturbances began to have effects again. An orogeny called the Antler took place, and at least 1,000 meters of sandstone and conglomerate sediments were deposited in places such as present-day Nevada. These sediments accumulated as the mountains began to erode. Other evidence of this orogeny exists in the form of the clastic (broken rock) layers lying unconformably above older marine strata.

Unconformable stratigraphic rock sections indicate that a break in sedimentary deposition occurred, often as a result of erosional processes outpacing depositional ones, which is exactly what one would expect if a mountain range were rising in an area. Although the Antler orogeny is not directly related to the formation of the present Rockies, it is an event representative of the collage of many smaller orogenic events that contributed, over time, to the Cordilleran orogeny and, thus, to the formation of the Rockies. Like most, if not all, orogenesis, plate tectonic activity is believed to be the driving force behind the Antler event. It is theorized that either a relatively small lithospheric plate called a microcontinent or another tectonic feature called a volcanic arc was being subducted somewhere to the west.

Following the Antler orogeny by tens of millions of years, another orogenic event occurred in the area now part of the western United States termed the Colorado orogeny. This mountain-building event was responsible for uplifting the Ancestral Rockies. During the Pennsylvanian period, about 300 million years ago, two large, mountainous islands or island chains arched up in the Colorado area. They rose above a shallow sea that covered, with the exception of the Appalachia cordilleran area to the east, most of the North American continental landmass. A sea of this epeiric, or epicontinental, type represents a transgression, or advance, of marine waters over a core area of a continental tectonic plate, known as a craton.

The Colorado orogeny produced what are believed to have been two distinct subranges: Front Range and Uncompahgre. A narrow, linear, shallow seaway is thought to have separated them. As the two island ranges eroded, they deposited extensive aprons of sediment into the surrounding seas. These sediments accumulated particularly in the narrow intervening region and are represented by the Fountain and Maroon geologic formations of Colorado. Evaporite basins also formed in the intermontane region as some arms of the surrounding seas became trapped or isolated. Sedimentary environments typical of such basins are represented by gypsum (calcium sulfate) and salt deposits in the west-central Colorado area. Erosion of the Ancestral Rockies continued for tens of millions of years, resulting eventually in their complete leveling. Remnants of these ranges are thought to have persisted in the form of minor hilly topographic relief until the middle of the following geologic era—the Mesozoic, famous for its dominant reptilian fauna such as the dinosaurs.

Tectonic forces had configured the arrangement of the continents into one great landmass by the Late Paleozoic era. Epeiric seas virtually disappeared for a time from North America by the close of the Paleozoic—the Permian period, 280 to 240 million years ago. Arid conditions prevailed for millions of years in western North America and in other areas that made up the supercontinent, which geologists call Pangaea. By the Triassic period, at the onset of the mid-Mesozoic, Pangaea slowly began to break up into smaller units. The following Jurassic period witnessed not only the formation of a large seaway between North America and Eurasia and the southern continents, termed Gondwanaland, but also an intensification of tectonic activity along the western margin of North America. This intensification culminated in the long process of mountain building in the west termed the Cordilleran orogeny.

As the North American continental plate progressively overrode various microcontinents, also known as suspect terrains, as well as volcanic arcs and sea floor to the west, an Andean-type plate margin was initiated. This convergent-type tectonic margin generated the various cordilleran ranges. Widespread crustal deformation and orogenic activity began during the Jurassic near the Pacific Coast with such events as the Nevadan orogeny. Orogenic activity spread eastward, initiating more uplifting activity such as the Sevier event in the Late Cretaceous period, near the close of the Mesozoic era. Finally, the Laramide event was generated as one of the last, most eastward pulses of the Cordilleran orogeny, starting in the Late Cretaceous period. This orogeny lasted for about 20 million years, from roughly 65 million to 45 million years ago, up into the Eocene epoch of the early Cenozoic era. Its end product was the easternmost ranges of the North American Cordillera—the ranges known as the Rocky Mountains.

Cenozoic and Pliocene Eras

By the onset of the Laramide orogeny, an epicontinental sea had formed by the transgression of marine waters from the Arctic to the north and the Gulf of Mexico to the south. These seas joined in midcontinent to form a long, wide but shallow, epeiric sea, which left marine sediments behind known generally as the Sundance formation. To the west of these marine strata, a famous, nonmarine sequence of strata formed, termed the Morrison formation, famous for numerous dinosaur fossils. As the land still farther to the west began to uplift, erosion accelerated, depositing continental sediments to the east, prograding, or extending out, into the epeiric, marine sediments. These sediments are termed the Morrison clastic wedge and are the first evidence, in Late Jurassic times (about 180 million years ago), of the eastward spreading effects of the Cordilleran orogeny. Lower Cretaceous period deposits in the central Rocky Mountain area exhibit a much thicker clastic wedge of up to 10,000 meters (32,808 feet). This wedge's accumulation coincides with the draining of the epeiric sea from the continental interior and immediately presages the Laramide event.

By the Paleocene and Eocene epochs of the Early Cenozoic, the Laramide orogeny had created the long, high ranges of mountains of the Rockies. Between the ranges, intermontane basins had formed as lowlands between the north-south trending, linear uplifts. These basins began to fill with sediments eroded off the adjacent mountains as well as with river and lake sediments. Numerous basins became large lakes, which, over millions of years, gradually filled completely. During their existence as active lake environments, the larger lakes were hosts to thriving ecosystems of plants and animals. The economically important deposits of oil shale found widely in Colorado, Wyoming, and Utah formed from the petrochemical mineral kerogen (which yields oil when heated) trapped within the shale. The kerogen is believed to have formed primarily from the decay of lake-dwelling planktonic organisms during the lifetime of the larger lake bodies. Indications suggest the lakes were in existence for at least 5 to 8 million years.

The filling of the intermontane basins and lakes is believed to have been completed by the end of the Eocene or perhaps the Early Oligocene epoch, about 35 million years ago. At roughly the same time, regional uplift had come to a halt and the effects of the Laramide orogeny dwindled to a standstill. Erosion by Oligocene times had made great inroads toward leveling the landscape of a generally stable tectonic region. For something like 5 to 10 million years, the eastern Rocky Mountain region saw no new uplift, and erosion proceeded to wear the mountains away. A huge, sloping apron of eroded sediments began to develop out onto the Great Plains to the east, forming a deep clastic wedge during the Oligocene and most of the following Miocene epoch of the Cenozoic. The remnants of the once-lofty Laramide summits stood only as minor prominences, sticking out of huge, surrounding aprons composed of their own eroded debris. As erosion proceeded still further, these aprons began to coalesce into a solid sheet of sediment termed the Tertiary pediment, which was once erroneously called the Rocky Mountain peneplain.

This trend was thwarted, however, during the Late Miocene by a great regional upwarping of the crust over most of the Rocky Mountain region as well as adjacent areas. To the west, the huge area of the present Colorado Plateau began to lurch upward toward its present altitude during this period. As the regional uplift proceeded into the Pliocene, rivers and streams were rejuvenated by the increased gradients they now flowed down, and stream-cut erosion accelerated. A lengthy period of canyon cutting ensued, which has continued. This phenomenon of stream rejuvenation by regional uplift explains much of the unusual entrenched meander and superimposed drainage found in the Rocky Mountain region and in the western states at large. The Miocene-Pliocene regional uplift added 1,000 to 2,000 meters (3,280 to 6,562 feet) of new height to the peaks of the Rockies and also lifted surrounding terrain, such as the Tertiary pediment, so that these former lowlands were considerably elevated and now seem like anomalous terrain.

Finally, as the cooling of the Pliocene gave way to the intermittently frigid period of continental glaciation dominating the Pleistocene epoch of the past 2 million years, large alpine glaciers formed at higher altitudes in the ranges of the Rockies. At least three episodes of alpine glaciation are believed to be represented in places such as the Colorado Rockies of the United States. Glaciation helped to scour sedimentary layers from mountaintops, exposing the harder and older crystalline rocks lying beneath. Glaciation also widened and deepened downslope valleys, giving some a U-shaped cross section, characteristic of glacially influenced topography. Glaciers also left their mark on the Rocky Mountain landscape in the form of myriad examples of sculpted terrain: glacial landforms such as cirques (steep-walled basins), moraines (accumulations of rock debris), and razor-sharp ridges called arêtes. The end of the Pleistocene epoch's cycle of glacial advance and retreat is not certain, however, and the few thousand years since the waning of widespread glacial effects has not been enough time for significant geologic processes to leave a new page in the history of the Rocky Mountains.

Human Activity

Humans entered the Rocky Mountains region as Paleo-Indians migrated into North America following the last ice age. Archaeological evidence shows that for thousands of years, humans used the natural resources of the mountain landscape, and their hunting and burning activity likely had a significant effect on the ecosystem. The earliest inhabitants were followed by the ancestors of many modern Indigenous tribes, many of whom adopted a migratory way of life that saw them live in the mountains in the warmer seasons and the plains in the winter. Some of the peoples who lived in or around the Rockies by the time of European contact included the Kutenai, Sioux, Ute, Crow, Shoshone, Cheyenne, Arapahoe, Apache, and Blackfoot.

The first people of European descent to visit the Rockies were those in the party of Spanish explorer Francisco Vázquez de Coronado in the 1540s. French and later British fur traders followed in the 1700s. Notably, in 1793 Sir Alexander MacKenzie was the first European to cross the mountain range, and from 1804 to 1806 the Lewis and Clark expedition scientifically documented the Rockies. As European exploration and then settlement spread throughout the region, bolstered by the expansion of the United States, Indigenous populations were devastated by disease, fighting, loss of natural resources, and cultural suppression. Starting in the 1840s, many settlers passed through the mountains on the Oregon Trail, with outposts springing up along the way. In subsequent decades, gold rushes and the completion of the transcontinental railroad brought fresh waves of settlers.

As negative impacts of mining, logging, oil exploration, and other problematic developments began to be realized, the US government established national parks to protect swaths of land in the Rockies. Some of the major early parks include Yellowstone National Park, Glacier National Park, and Rocky Mountain National Park. A similar movement set aside significant amounts of land in the Canadian Rockies. These and other protected areas continue to be vital to the economy of the Rocky Mountains region as a major draw for tourists looking for outdoor recreation or simply to view natural splendor. Despite these efforts, the damage to the soil cover on the mountain caused erosion, which unearthed minerals and metals. These proved harmful for the wildlife in the mountains and for the groundwater.

Human activity that has caused climate change has contributed directly to changes in the Rocky Mountain range. For example, as average temperatures rise, there is less precipitation and less snowfall in the area, coupled with snows that melt more quickly. Because of this, the growing season is longer, dryer, and less productive. The average amount of frost-free days has risen over time, going from 65 frost free days in 1940 to 100 frost free days in 2011. This forty day change is very significant because frost provides a natural, slow watering system for the mountain range and its valleys. Though drastic change is slow, the previously listed effects continue to become more noticable and more severe.

Principal Terms

Ancestral Rockies: the mountain ranges that preceded the Rockies by many millions of years and no longer have surface topographic expressions of their former existence

cordillera: a major mountain chain or system of such chains, especially one that is a dominant feature of a continent-sized landmass

craton: a large, tectonically stable core area of a continental-type tectonic plate, usually of great age; also termed a continental shield

epeiric sea: a shallow sea that temporarily (in geologic terms) covers a portion of a craton; also termed an epicontinental sea

Laramide revolution: an orogeny that occurred between the Late Cretaceous and the Early Tertiary periods of geologic time, instrumental in producing the ranges termed the Rocky Mountains in the United States

orogeny: a mountain-building episode or event that extends over a period usually measured in tens of millions of years; also termed a revolution

plate tectonics: a widely accepted theory in geology that contends that the earth's crust is composed of about twelve moving, rigid plates

subduction: a process, according to the theory of plate tectonics, whereby one crustal plate is thrust beneath another at a convergent boundary

transgression: the flooding of a large land area by the sea either by a regional downwarping of continental surface or by a general global rise in sea level

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