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In the next flyby, we leave the Great Plains abruptly as the Front Range of the Rocky Mountains rises dramatically from the gently sloping high plains. In the Denver area, much of this zone is taken up by steep-dipping sedimentary rocks that make a feature known as "hogbacks". Both a summer and a winter Landsat MSS full scene show the plains, the Front, and the high mountains of Colorado in the Denver area. Other areas in the Southern Rockies are also examined.


Denver, Colorado and the Southern Rocky Mountains


In the eastern U.S. we passed over an old mountain system - the Appalachian Belt, now rejuvenated by uplift starting some few million years ago. We flew across the vast lowland of the continental interior. From Central Colorado and along the trend from Mexico to Canada, we encounter an array of geological and physiographic expressions of orogenic (mountain-building) events that began in the late Paleozoic and continue today. This map sets a stage overview of the geology of the western 1/3rd of the U.S.

Physiographic Provinces and their subdivisions in the western U.S.

The Rocky Mountains is a vast, generally north-south collection of joined or separated ranges that extend from northern New Mexico well into Canada. The Rockies (as they are often called) within the U.S. are shown here, first as a shaded relief map and then as a physiographic subdivisions map:

Shaded relief map of the Rocky Mountains and adjacent physiographic provinces.

Subdivisions of the Rocky Mountains

Parts of the Rocky Mountains were being built geologically since the Paleozoic. The main orogeny occurred through subduction to the east at the end of the Mesozoic, with periodic rejuvenation since then. This sketch illustrates the general conditions during the Cretaceous Period as uplift was maximized:

Diagram showing how subduction created the Rocky Mountains.

The boundary zone between the Great Plains and the eastern Front of the Rocky Mountains is abrupt, with the transition zone generally less than 1-2 km (0.6-1.2 miles) wide. The 700 km wide scene taken by the Day-Vis channel on the Heat Capacity Mapping Mission (HCMM) sensor shows much of the southern end of the Rockies from northern New Mexico into the southern half of Wyoming. The map below it indicates that the entire state of Colorado is included in the image.

HCMM view of the Colorado Rocky Mountains and areas in neighboring states.

Sketch map locating main geographic features in the above HCMM image.

In this part of the United States the uplifted and folded rocks making up the true Rocky Mountains occupy a relatively narrow strip passing through the central part of Colorado and New Mexico, and swinging northwest through most of Wyoming (the Wind River Mtns mark the shift that carries the Rockies into Idaho and Montana). The principal units in Colorado are the Front Range and the Sawatch Range. This limited width of the orogenic belt comprising the Rockies may seem a surprise because once in these mountains, their lofty grandeur gives an impression that they extend well to the west. In fact, from western Colorado into Utah, mountains of a different type, found in the Colorado Plateau (next page), are predominant. The San Juan Mountains look to the traveler much like the Rockies but they are actually a great pile of highly dissected volcanic flow units intermixed with sedimentary rocks and are considered a separate unit. Note the Uinta Mountains in Utah - the only dominantly east-west mountain system in the U.S.

The Front is well displayed by this perspective natural color image using Landsat 7 data merged with DEM topographic data.The area shown are the high plains around Greeley, Colorado, with a reservoir in the foothills near Fort Collins.

Landsat 7 natural color image, redisplayed in perspective, showing the High Plains and Colorado Rocky Mountains near Fort Collins.

The Rocky Mountain National Park is in the background, near the snow-capped high mountains near Estes Park. This popular destination for tourists is shown in the image below, made from combining a Landsat TM scene with DEM data:

Perspective view of Rocky Mountain National Park, made from a Landsat TM image and DEM data.

The Park contains a considerable number of high peaks above 11000 feet, which offer challenges to both experienced and novice mountain climbers. In 1955 the writer (NMS) climbed first Hallett's Peak (12743 ft) and then the second tallest in Colorado, Longs Peak, at 14255 ft (the last 1000 of which proved an ordeal). Longs Peak is in the cluster of snow-covered mountains off to the east near the bottom right. The main line of snow-capped mountains form the Continental Divide; those at the top of the image, offset east of the Divide, are the Mummy Range group. This next photo was taken at 10000 ft looking at the East Face of Longs Peak; our ascent was not that vertical facing but along the ridge line running up from right to left.

The approach to Longs Peak from the east.

The Park is included in this false color composite of central Colorado acquired in early October, 1973 soon after the launch of Landsat 1.

Landsat MSS color composite image, taken on October 10, 1973, of the Colorado Rocky Mountains, Denver, and the Great Plains to the East.

Red colors within Denver and around Boulder and Fort Collins signify trees and crops still bright in the near-IR in this October scene. Their distribution is affected by proximity to the Platte and other rivers. These rivers provide irrigation for the alfalfa, hay, oats, corn, beets, and barley grown in this part of the Plains. Uncultivated areas, such as the Pawnee Grasslands (brownish patch at the right center), contain buffalo and grama grasses.

A January 1973 winter view of Denver places the region in a rather different perspective. One consequence is the "heat island" effect of towns and cities, in which melting and snow-plot removal cause the streets and roofs to appear very dark from the loss of the snow cover.

Landsat MSS color composite image, taken on January 11, 1974, of the Denver scene; the snow cover and urban melting create distinctive new patterns.

6-9: This winter scene brings out the infrastructure of much of Denver. Four large towns are now visible to the north; name them. Are they visible in the summer scene. Explain the brownish color associated with the Rockies. Is there snow above the treeline? What factor(s) help(s) to indicate drainage patterns. What happened to the lakes so visible in the summer image? ANSWER

The Rocky Mountains form the eastern edge of the North American Cordillera, which is made of groups of diverse, usually complexly-folded and faulted blocks of crust. The blocks are uplifted or thrust against other blocks separated by structural basins. They were deformed and emplaced during periods of major orogenies that were often disconnected in time and place. Rock units in the Rockies were finally compressed and shoved upwards about 65-70 million years ago. The present topography of these mountains is the result of strong erosion thereafter that has lowered their original heights to under 4,420 meters (14,500 ft). Alpine glaciation has steepened and widened already deep valleys, creating the rugged vistas that make this region especially popular for tourists and skiers. The barren (whitish) area along the crest of the Front Range in the October image coincides with the Continental Divide where it sits above the treeline. The slopes below are forested with Douglas fir, spruce, pine, and aspen trees. Rocky Mountain National Park is in the left center of the image. The southern end of the Medicine Bow Range in Wyoming extends into the image at the top left. The photo below shows a typical landscape within this part of the Rocky Mountains.

Aerial oblique photo of the High Rockies west of Denver.

Denver, CO, appears at the edge of the plains in the lower center in both the Fall and Winter scenes. This burgeoning city is shown first from space and then in two aerial oblique views, with the Rockies in the background.

Denver, as seen by Landsat-7

Panorama of Denver, Colorado.
Courtesy: Carolina Map Distributors

The City of Denver, looking west to the Rocky Mountains; the golden dome of the State Capital Building is in the left foreground.

Just west of downtown Denver in Invesco Stadium, home of the Denver Broncos. This IKONOS image shows the details in this part of the city.

IKONOS view of the section of Denver that includes Invesco Stadium.

Northeast of downtown Denver is a large square which is the Rocky Mountain Arsenal, once used for storage of military weapons. This can be easily seen in this 5 meter resolution Indian IRS-2 satellite subscene; note also the new International Airport east of the city.

Part of the Denver area as imaged by IRS-2

To the west of central Denver, the land slowly rises until it abuts against the foothills of the actual Rocky Mountains. This is shown here in a Landsat-7 ETM+ image.

Landsat-7 view of the high plains as this physiographic unit abruuptly meets the Rocky Mountain Front.

To Denver’s west, against the mountains, are narrow outcrops of red sandstone (Lyons Formation) that bend upwards at about 45° to heights up to about 50 meters (160 ft) or more. These landforms are "hogbacks," which were steeply dipping sedimentary rocks, inclined down to the east as the Front Range rocks were pushed upwards. Those hogbacks are quite evident in this aerial oblique view taken along the Front south of Denver.

The Rocky Mountain Front, south of Denver, with hogbacks.

The structural configuration of the rocks at the Front are shown in this geologic cross-section, beneath which is an exposed section of the geologic units at a roadcut along I-70 (there is a sign at the cut which explains the local geology).

Geological cross-section that illustrates the sudden changes as the Rocky Mountain Front pushed up and warped the sedimentary units at the edge of the Great Plains.

I-70 roadcut.

Within the hogbacks just north of Interstate 70 is the famed Red Rocks Amphitheatre, first opened in 1941. There each summer are classical concerts given by the Colorado Symphony and concerts by rock bands and other popular music groups. With hogbacks on either side, the acoustics are surprisingly good in these outdoor conditions. Here is a ground photo:

The Red Rocks Ampitheatre; South Table Mountain - against which lies the town of Golden, home of Coors beer - is a lava-capped Mesa in the background.

The second largest city in Colorado is Colorado Springs, about 170 km (100 miles) south of Denver, which also is situated at the Rocky Mountain Front. It is the home of the Air Force Academy. Look first at a Landsat-DEM perspective view that shows Pikes Peak in the high Rockies. Then at a satellite photo looking down at C.S. itself. Just north of the city is the beautiful Garden of the Gods, another series of red sandstone hogbacks (also called "flatirons" from their shape) such as we saw west of Denver.

Perspective view of Colorado Springs and the Rocky Mountains beyond.

Vertical view from a satellite of Colorado Springs

Red sandstone hogbacks near Colorado Springs.

Moving southward from Colorado Springs, note this perspective view (Landsat + DEM) of the Rocky Mountain Front near Walsenburg, CO :

Another perspective of the Rocky Mountains.

Before leaving Colorado, we point out that on the west side of the Sangre de Cristos above the New Mexico border lies America's largest sand dunes field - The Great Sand Dunes National Monument. Although small in area, windblown sand from western winds piles up against the mountains to produce dunes as high as 220 meters (700 ft), as seen in this Landsat-7 subscene, below which is an IKONOS image that covers a part of the dune field.

The Great Sand Dunes National Monument in the San Luis Valley.

Great Sand Dunes N.M.; IKONOS image

Our last look near the south end of the Southern Rocky Mountains is in New Mexico in this Landsat image that includes Albuquerque, the Sandia Mountains to its east, and the Rio Grande to the west:

Landsat image of the city of Albuquerque, NM and the countryside around it.

East of the main surface expression of the Rocky Mountains in New Mexico are several mountain masses that can be considered as uplifted basement rocks and folded sedimentary flank rocks. The Black Hills have already been described. Below is a scene that has a true western flavor but lies east of both the Rocky Mountains and block mountain in the Basin and Range southward extension (see page 6-8)

Southcentral New Mexico, most notable is the White Sands National Monument.

The most conspicuous feature in the scene iw the White Sands National Monument. The sand is composed not of quartz, as is usual, but of Gypsum (CaSO4.2H2O). It was there that the U.S. for nearly a decade launched captured German V-2 rockets and some early U.S. rockets. The program was under the direction of Dr. Wernher von Braun, the brilliant scientist-engineer who led the German program during WWII. The writer (NMS) had the opportunity to interview him and watch a V-2 launch in 1947 when I was stationed at Fort Bliss near El Paso, about 85 km (50 miles) south and was also a "reporter" for the Army newpaper. I also spent several weekends in the Sacramento Mountains (dark red), covered mostly with evergreens (Ponderosa Pine); these mountains are the southernmost extention of the Rockies. Mountains of the Basin and Range eastern extension (the San Andres in the center) occur in the center and to the west of the scene (two small segments of the Rio Grande River appear at the left of the image). The towns of Alamagordo and Tucumcari lies at the western base of the Sacramento Mts; not far to the northwest, in the Tularosa Basin is the site of Trinity, where the first atomic bomb was detonated in July 1945. The dark patch is the Malpais basaltic lava field, a young extrusion of late Pleistocene age.

Reversing direction, on this page we will next move north of the Colorado Front into the Middle Rocky Mountains. These lie mostly in the state of Wyoming. The Rockies there are exposed as a group of separated mountain ranges with deep intermontane basins (filled with erosional sediments from the mountains) in between. This map (from E. Raisz; Landforms of the United States) shows the distribution of these uplifts and basins; read the caption to identify individual ranges:

Landforms map of Wyoming and parts of adjacent states. Within Wyoming, the lower right shows the Laramie and Medicine Bow Mountains (the Black Hills, page 6-5, are in the upper right; the Bighorn Mountains are in top center - these transition into the Owl Creek Mountains (east-west); the prominent slanted range in left center is the Wind River Mountains; to the west are the Hoback and Wyoming Ranges which extend north to meet the Grand Tetons; the Absaroka Mountains are above the Wind River Mtns and lie just east of Yellowstone Park.

These self-same ranges appear in the mosaics of Wyoming on page 7-1.

As an example of the Middle Rocky Mountains, this Landsat-1 scene (which was the principal study area for the writer's (NMS) Wyoming Landsat study) shows the Wind River Mountains (left) and the Owl Creek Mountains. The low central area is the Wind River Basin; note Ocean Lake and the Boysen Reservoir:

Landsat-1 false color composite image of the central part of Wyoming as described above.

Nestled within the broad stretch of the Middle Rocky Mountains as they pass through northwestern Wyoming is America's first National Park: Yellowstone. This next image shows Yellowstone Park in context with the surrounding mountains"

Regional view of Yellowstone and surrounding natural features.

Here is a Landsat view that zeros in on the Park itself; for reference note Yellowstone Lake.:

Landsat-2 image of the Yellowstone Park and surroundings.

In the mid-1990s, Yellowstone Park and some surrounding areas were plagued with wildfires that extended over huge areas. The next two images - one Landsat, the other SIR-C radar - show some of the burn scars:

Landsat image of Yellowstone, with burned areas in dark purple.

SIR-C radar (left) encompassing Yellowstone National Park (dark grey areas are burned forest; colorized version of the radar image to single out the scars in orange-brown.

Yellowstone is the site of widespread volcanism erupting from now buried calderas. Activity has been recent - the last eruptive flows were in the Pleistocene less than 1,000,000 years ago. Groundwater that seeps down to the still hot subsurface lavas is responsible for the many geysers that are the trademark of this Park. Yellowstone Lake occupies a darker area (evergreens) near the image center. The Absaroka Mountains line the east side of the Park; these continue into the Beartooth Mountains to their north. Look in this image about due west (left) of Yellowstone Lake - this is how it appears in a Landsat-7 ETM+ subscene: grazing and clearcut forests are to the left (west); fully preserved forestlands are on the right; the sharp boundary is related to a fenceline (this effect is similar to the U.S.-Canadian boundary shown on the previous page):

Landsat-7 subscene showing the straight boundary between forested public land to the right and private lands to the left.

The next scene to the south of Yellowstone contains some of the most spectacular scenery in America. See caption for identification of natural features:

Landsat-1 scene showing the Grand Tetons (center top), the Gros Ventre Range (to its right), the Wyoming and Hoback Ranges (vertical pair), and the east end of the Snake River Plains.

The Tetons are the up-down range at top center. To their east are the E-W Gros Ventre Mountains (those who know French should try to translate these names, given by French trappers in the 18th century). The two curving ranges below are the Wyoming and Hoback ranges. The beginning of the Snake River Plains (page 6-8) appears in the upper left.

The Tetons are dramatically photogenic (backdrop in the movie "Shane"). Here they are in a panoramic view from the alluvial plains through which the Snake River flows":

Panoramic view of the Grand Tetons.

For sentimental reasons, the writer (who has done geological field work in all these ranges) includes this last photo - the Tetons from Jenny Lake, where he and his wife spent their honeymoon in 1961 (which did not include any mountain-climbing).

View across Jenny Lake of Mt. Moran and the Grand Teton.

Fit these images, which show the beauty of the Tetons close-up, with this perspective view of the entire range, the Snake River Plains, and the Hoback Range to the south as constructed from a Landsat-7 natural color composite and STRM elevation data:

The Tetons and surroundings shown as an aerial perspective using combined Landsat and STRM data.

This perspective view discloses the Tetons to be a block fault mountain range in which the east side is thrust upwards as a swinging motion. Thus, the west slope is gently inclined, so much so that bicyclists have reached the crest whereas those climbing from the east side must use ropes and other such advanced scaling equipment.

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Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net