Key Words and Themes

Objectives

• Describe the faults created by compression.
• Contrast faulting in the Rockies with the Basin and Range.
• Compare and contrast the style of faulting and folding int Rocky Mountain National Park and Glacier-Waterton National Park.
• Explain why in the Northern Rockies older Precambrian and Paleozoic rocks are found on top of younger Mesozoic and Cenozoic Rocks.
• List and discuss the evidence leading geologists to believe that the Rocky Mountains were uplifted at least three times.
• Discuss when the parks were glaciated and how glaciers modified their landscape.
• Describe and explain the formation of the following alpine features: cirque, U-shaped valley or glacial trough, arete, horn, tarn, patre-nostra lakes, lateral moraine, and end moraine.
• Explain why rocks exposed in the Rockies are so much older than those exposed on the Colorado Plateau.
• Describe the Sevier and Laramide orogenies and their influence on the Rocky Mountains.
• Compare and contrast the rock types found in Rocky Mountain National Park with those in Glacier-Waterton National Park.
• Describe the general lithology, age, and location of the Precambrian Belt Supergroup.
• Identify and describe the age and formation of the oldest fossils preserved in Glacier National Park.

Introduction to the Rocky Mountains

The Rocky Mountains, which rise sharply from the Great Plains, compose the eastern margin of the Western Cordillera. Extending from central New Mexico through Canada, they form the spine of the continent, separating drainages flowing west into the Pacific from those flowing east into the Atlantic.   The U.S. and Canadian Rockies are subdivided into three regions (fig. 1) characterized by differences in structural style and lithology.  As shown in figure 1,  Rocky Mountain National Park is in the Southern Rockies, Grand Teton National Park is in the Middle Rockies, and Waterton-Glacier National Park is in the Northern Rockies. Styles of Deformation Although the Rockies have a long and varied history of uplift and erosion, their distinctive structural styles evolved during the Late Cretaceous - Early Tertiary Sevier-Laramide Orogeny driven by collisions along the west coast. The less severe upwarps and monoclines of the Colorado Plateau were also formed during this time.  The Southern and Middle Rockies rose as blocks of ancient crystalline basement and sedimentary cover were thrust upward along steep reverse faults.   This style of deformation is known as thick-skinned tectonics (fig. 2B). Deformation in the Northern Rockies was largely restricted to faulting and minor folding within the sedimentary cover, which slid over the crystalline basement.   Layers were stacked one on top of another along low-angle thrust faults.  This type of deformation is referred to as thin-skinned tectonics (fig. 2A).   Thin-skinned deformation is analogous to the pealing and stacking layers from an onion by running your fingers across its surface, whereas thick-skinned deformation would involve cutting deep into the onion with a knife and pulling up wedges.

Figure 1. Subdivisions of the Rocky Mountains.  Red dots locate from south to north Rocky Mountain National Park, Grand Teton National Park and Glacier National Park.

Figure 2. Styles of deformation caused by orogenic compression.  A. Thin-skinned deformation: Rocks are stacked by thrust faulting within the sedimentary cover.  Note that the underlying basement composed of ancient metamorphic an igneous rocks is not involved. This is somewhat analogous to deformation experienced by snow forced to slide over a sidewalk in front of a shovel.  This style of deformation characterized the Sevier Orogeny in Nevada, California, and the Northern Rockies (Glacier National Park).  B. Thick-skinned deformation: Compression pushes both the basement and cover rocks up along high angle faults. This style of deformation characterized uplift of the Southern Rockies (Rocky Mountain National Park) during the Laramide Orogeny.

Image modified from Tozer, Butler and Corrado (2001) Comparing thin-and thick-skinned thrust tectonic models of the Central Apennines, Italy. EGU Series 1, pp. 181-194.

Rocks in Rocky Mountain and Glacier National Parks

Sedimentary rocks underlie all the parks we’ve studied so far, although some contain a smattering of volcanic rocks. In contrast, Rocky Mountain National Park is composed of igneous and complexly deformed metamorphic rocks (fig. 3). Sedimentary rocks are found along the flanks of the range, but outside the park. Glacier National Park contains sedimentary rocks that have been metamorphosed at such a low grade that they retain all the details of their sedimentary origin and exhibit very little deformation. For the most part, they are still considered sedimentary, but are much more indurated and resistant to weathering. What were once shale (c.f. fig. 4) are now harder, less fissile, cliff-forming argillites (c.f. fig. 5).

 

Figure 3. Rocks exposed in Rocky Mountain National Park.  The  outcrop on the right is an intrusive igneous rock called granite. The rock is composed of interlocking mineral grains of quartz and feldspar, and is quite homogeneous. The outcrop on the right is a metamorphic gneiss composed of contorted layers of light and dark minerals.  These rocks were once marine sediments and volcanic rocks recrystallized and deformed by plate collisions around 1.6 billion years ago.  Image source: LSH

 

Figure 4. Shale in the Temple Butte Formation, Grand Canyon. Shales split easily along bedding (a property known as fissility) allowing water to penetrate, react with the clays, and crumble the rock.  For this reason shales typically form broad slopes or alcoves on the Colorado Plateau. Image Source: LSH	Figure 5. Red Argillite of the Grinnell Formation in Glacier National Park. Argillite was shale subjected to very low grade metamorphism.  The rock was recrystallized enough to destroy fissility, but not enough to erase other sedimentary structures. Image Source:  http://picasaweb.google.com photo#5225300242042814914

Rocky Mountain National Park and Glacier National Park are distinctly different in their lithologies. Mountains in Rocky National Park (figs. 7 and 8) are composed of uplifted Precambrian basement composed of granite and gneiss. The Mountains in Glacier National Park are composed of stacked layers of ancient sedimentary rock (fig. 10)--the Precambrian Belt Supergroup. At a glance the Rockies easily reveal the rocks from which they're built. Granite mountains (fig. 7) lack layering and appear quite homogeneous, barring any scattered dikes or other younger intrusions. Mountains built of metamorphic rock exhibit swirls of folded and contorted rock (fig. 8) resembling praline ice cream. In contrast, mountains of sedimentary rock (fig. 10) exhibit uniform layers that lack evidence of complex folding.

Figure 7. Granite mountain in Rocky Mountain National Park. Long's Peak (14,259 ft.), the tallest mountain in Rocky Mountain National Park is composed of the Silver Plume Granite. Image Source: Wikipedia

Figure 8. Mountain of Metamorphic rock in Rocky Mountain National Park. Mt Ypsilon in Rocky Mountain National Park is composed of metamorphic rock. At first glance the horizontal foliation looks like sedimentary strata.  However, upon close inspection you can see complex folding of the layers.  (Click to enlarge.)   The mountain gets its name for the avalanche scars that form the Greek letter Y.  Image Source:LSH

Figure 9. Sedimentary rocks in Glacier National Park.  Note horizontal layers and ripple marks formed by ancient waves on the surface of the beds.  Image Source: Geology National Parks CD - Robert Smith	Figure 10. Horizontal layers of sedimentary rocks exposed in the Garden Wall, Glacier National Park.  Glacier National Park contains rocks of the Belt Supergroup  deposited in an ancient basin between 1.4 and 1.47 billion years ago.

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Rocky Mountain National Park, Colorado

Facts: Area: 265,727 acres; 415 square miles.  Established January 26, 1915. Contains 42 mountains above 12,000 feet along the Continental Divide. The tallest mountains Long's Peak (14,259 ft).  Headwaters of the Colorado River.  One third of the park if fragile alpine tundra.

Description: The Colorado Rockies are the highest mountains in the Front Range of the Rockies with several peaks above 12,000 feet.  The mountains are composed of crystalline igneous and metamorphic rock of Precambrian age.  The rugged topography was sculpted  by mountain streams, frost wedging, and Pleistocene alpine glaciers, which carved deep valleys and cirque basins into flat raised uplands.  Visitors are drawn to the park to view and experience its spectacular mountain scenery and to park hike the over 350 miles of trails.  The park contains some of the best climbing peaks in the U.S. (visit explorerocky.com/Gorp/ Gordan Novak's High Peaks). 

Hiking the Rockies - a personal note.

I've hiked the White Mountains and Mount Katahdin in Maine. So for those who want to know how the Rockies compare, here is a brief synopsis. First, the vertical climb in elevation is actually less in RMNP. The mountains are higher, but so is the base elevation for the trailheads. For example, the starting elevation for Long's Peak (fig. 7) is ~9,409 feet, and you climb 4,850 ft. to get to the summit (14,259 ft).  Long's Peak is clearly the most difficult hike in the park. The elevation gain for most summit hikes in RMNP is under 4,000 feet. However, the elevation gain for Mount Washington is 4250" and ~4,200 for Mount Katahdin, and it's pretty much straight up over ledges and boulders. The principle disadvantage of hiking RMNP is the low oxygen at high elevations. Some people have a real problem with this, I didn't after the first few hours of hiking. It's also very easy to get caught in lighting and hail storm if you're not off the peak by noon. So you need to start early, and think twice about carrying aluminum-hiking poles. Well-traveled trails at RMNP are nicely groomed, and the trail builders in RMNP actually believed in switchbacks. The trails are longer, but their gradients are less, as is the tendency to degrade through erosion. The Civilian Conservation Corps (CCC) built most of the hiking trails, as well as the Trail Ridge Road that cuts through the park along the Continental Divide, during the Great Depression (1930s). Trails to peaks that are less travel are not groomed and vague so make sure to carry a good map. The Trail Ridge Road (fig. 11) is incredibly scenic but resist the desire to look at the landscape while driving. The road is narrow, full of bikers and other cars, guard rails are lacking and drop-offs frighteningly large.   For a list and description of the summit hikes go to summitpost.com.

Figure 11.  Trail Ridge Road crossing the Continental Divide.  This is without a doubt one of the most scenic roads in North America Image Source: LSH

Formation of the Southern Rocky Mountains & Rocky Mountain National Park

Although the present mountains in Rocky Mountain National Park are very young (<5 ma and still rising), the region has experienced many cycles of uplift and erosion.  Its early Precambrian history is not unlike that seen in the lower gorge of the Grand Canyon, where we saw 1.8 billion-year-old Precambrian igneous and metamorphic rocks exposed beneath a nearly mile-thick-assemblage of sedimentary rocks. The metamorphic rocks were once sediments deformed and metamorphosed during the assembly of Rodinia, the ancient Proterozoic supercontinent. Mountains in Rocky Mountain National Park are composed of similar Precambrian rocks.  Around 1.75 billion years ago ancient sedimentary and volcanic rocks were intensely deformed and metamorphosed into swirling schists and gneisses, and later intruded by large bodies of granite, such as the 1.4 billion-year-old Silver Plume pluton that forms Long's Peak (fig. 7).  These rocks compose the crystalline core of the Southern Rockies. Unlike the crystalline (metamorphic and igneous) rocks in the Grand Canyon that lie deep beneath a sedimentary cover, those in the Southern Rockies have been ratcheted up along reverse faults so many times that their sedimentary cover has been entirely stripped off by erosion. Rocks that once lay miles beneath the surface now stand at elevations up to and over 12,000 feet (3,660 m).

The Southern Rockies felt the first pulse of uplift during the Pennsylvanian Period (~300 ma). Gondwana collided from the south as Pangea assembled pushing up the Ancestral Rockies to elevations of around 2,000 feet (610 m).  By the Jurassic Period these mountains were eroded to a level plain and buried by a thick blanket of sediment. During the Mid-Cretaceous Period the Western Interior Seaway covered the region.  The Ancestral Rockies are long gone, but the basement faults formed during their uplift would be reactivated during subsequent periods of compression.  Sediments shed from the Ancestral Rockies into the nearby eastern basin form the Permo-Pennsylavanian Fountain Formation.  Its beds of red sandstone and muddy conglomerate were subsequently tilted and eroded  forming a hogback ridge and the dramatic flatirons seen in Redrock State Park and the Garden of the Gods flatirons (Roxborough State Park), which great travelers approaching the Front Range from the east. 

Faulting responsible for the modern Rockies started during the Late Cretaceous to Tertiary (75 -35 Ma) Laramide Orogeny.   The orogeny was a mountain-building event characterized by basement-involved uplifts along high-angle reverse faults (thick-skinned tectonics), located 800 miles (1,287 km) inland from the subduction zone purported to cause it.  Uplift occurring hundreds of miles from a plate boundary is considered unusual and is attributed to a phase of nearly flat subduction (fig. 13) of the Farallon Plate.  See also The history of subduction beneath western North America by Steven Earle.

Figure 12. Laramide Orogeny.  Basement uplifts driven by shallow subduction of the Farallon Plate beneath western North America during the Late Cretaceous to Tertiary.  After Harris, Tuttle, and Tuttle (2004).  See also this animation of Farallon-Plate-related-uplift from the NASA/Goddard Space Flight Center Scientific Visualization Studio.

Successive mountain-building events reactivated Laramide faults and raised deep basement rock to progressively higher levels.  The original Laramide uplifts were eroded by the mid-Tertiary, and have since experience periods of repeated uplift, and erosion, as well as sporadic volcanism from mid-Tertiary to the Pliocene. Multiple cycles of uplift and erosion are inferred from flat upland surfaces believed to be uplifted erosional surfaces and terraces carved by ancient streams. (See readings 3 and 4.) However, many of the flat surfaces my in fact be lithologically controlled.  The latest uplift, to which the mountains owe their present elevation (~12,000ft), began within the last 7-5 million years with the rifting of the Basin and Range. A a broad fault-bound anticlinal arch (fig. 14) defines the present structure of the of the Rockies in Rocky Mountain National Park.

Figure 13. Structure Rocky Mountain National Park.   The mountains are a fault-bound anticlinal arch with Precambrian rocks exposed in the core.  Foothills expose Mesozoic sedimentary rocks in eroded cuesta and hogback ridges. From Harris, Tuttle, and Tuttle (2004).

The multiple episodes of uplift and erosion experienced by the Southern Rockies are interpreted from the sediments deposited in adjoining basins, presence of flat upland surfaces, and by the unroofed Precambrian core the modern mountains. Without protracted periods of uplift and erosion these rocks would not be exposed.  The flat summits of Flattop Mountain (fig. 14) and Deer Mountain have been interpreted as remnants of an old erosional surface called a peneplain.  (However, my personal observation indicates that there may be substantial lithologic and structural control to these surfaces.)  Following the Laramide Orogeny erosion reduced the mountains to base level creating a broad peneplain surface on the Precambrian rock. Renewed uplift on the order of 9,000 feet (2743 m) rejuvenated the mountains and initiated incision of the surface.  Uplift of an erosional surface is referred to as rejuvenation because the resultant increase in stream gradient causes renewed incision and steepening of the topography.  The Southern Rocky Mountains have been rejuvenated twice in the last 20 million years.

Figure 14.  View looking northwest across Flattop Mountain. The flat surface of this mountain and other mountains along the Continental Divide lead geologist to hypothesize that the surface was erosional peneplain that was uplifted 9,000 feet above base level.

Glaciation

Read Glacier Basics
Alpine and continental glaciers both modify landscapes, but in a different ways.  Glaciers by definition are flowing bodies of ice.  Ice over 100 feet (60m) thick will plastically deforms (flows) under its own weight.  Once it moves it becomes a glacier.  A continental glacier ( ice sheet) such as the Laurentide Ice Sheet that covered New England can be over a mile thick and covered the entire landscape.  It's an unconfined glacier that reduced a landscape's relief by scouring protuberances and filling in small basins with sediment. Alpine glaciers (fig. 6) are smaller glaciers confined in upland basins and mountain valleys where they concentrate their scouring.  Glacial erosion occurs locally beneath the glacier resulting in a sharpening of the landscape. Areas not covered by ice remain high, while ice-covered areas are lowered by scouring.  Amplitheater-shaped basins called cirques are excavated where ice accumulates in the heads of streams and on rocky shelves high up on the mountains.  As ice continues grow it descends down-valley, widening the floor and steepening the valley walls. Eventually an alpine glacier will carve a broad, deep, steep-sided valley called a alpine trough or U-shaped valley (figs. 6-8).   Interfluve ridges between adjacent alpine troughs are sharpened and narrowed forming serrated ridges called aretes.   During the Pleistocene (1.8 ma - 10 ka) the Rockies were dissected by alpine glaciers that greatly modified and sharpened the landscape.  Two major periods of glaciation occurred during this time.  The oldest, the Bull Lake  glaciation, occurred 200,000-120,000 year ago. Most of the prominent glacial features (figs. 7 and 8) we see today are the result of the more recent Pinedale glaciation that occurred 30,000-10,000 year ago. Since the the retreat of the Pinedale glaciers, small glaciers have formed, expanded and retreated.  Most of the alpine glaciers that saw a resurgence during the Little Ice Age (1620-1850) are nearly gone today. Mountain glaciers exist only where low temperatures at high elevations allow them to persist.  In this modern time of global warming most glaciers in the U.S. Rockies will be gone within 50 years.  The U-shaped valleys, cirque basins, sharp knife-edged aretes, (e.g. Half Mountain) and glacial horns (e.g. Little Matterhorn) that distinguish the Rockies are all the work of alpine glaciers.  Important to early explorers and Indian moving through the mountains were mountain passes called cols. These saddles or ridge depressions were created where the headward expansion of adjacent cirque glaciers breached the ridge between them.  

Figure 15.  Alpine glaciers and features.  Alpine glaciers sharpen landscape by locally scouring deep cirque basins and U-shaped valleys (alpine troughs).  Interfluve ridges are are sharpened into steep-sided aretes. Horns are isolated peaks steepened on all sides by glacial erosion.  Both Rocky Mountain National Park and Glacier-Waterton National Peace Park are noted for their dramatic glacial landscape. Modified from Wikimedia Common file by Luis María Benítez.

Go to Summitpost.com and view the Thatchtop Summit Panorama the following are examples of features carved by glacial erosion: a) aretes: the serrated ridge left and right of Arrowhead  and Keyboard of the winds, b) horns: Chiefs Head Peak and Long's Peak, c) cols: Stoneman Pass and McHenrys Notch

Broad valleys, bowl-shaped cirques  (figs.  16 and 21), and jagged serrated ridges are common alpine glacial features in Rocky Mountain National Park.  However, other features commonly overlooked in the park are glacial pavements (fig. 17), roche mountonnees (figs. 18 and 19) and moraines (fig. 20 and 21).  Although less dramatic than the alpine valleys and basins they reveal important information about the movement of a glacier and and how far it advanced.

Glacial Scouring

Glacial ice is quit soft relative to the rock beneath it.  By itself, ice is incapable of eroding the deep valleys and basins so typical of alpine landscapes.  Glacial scouring is actually accomplish by the hard rocky debris embedded in glacier's basal ice.  Silt, sand, pebbles and boulders frozen to the bed grind away at the bedrock as the ice flows over it.  The pebbles and boulders carve striations and grooves while the sand and silt polish the rock's surface. The faster and thicker the glacier the greater the scouring.  Grooves and striations are important because they give the general orientation of ice flow. Striated and grooved glacial pavements (fig. 17) are evidence of glacial scouring and can only be seen on the lower walls and floors of glacial valleys.  Weathering since deglaciation has removed much of the glacial polish. However, the deeper striations and grooves are often still visible (fig. 17).

Figure 16. Cirque basin at the head of glacier gorge.  Cirques are carved by small glaciers created when ice accumulation in local summit depressions. Freeze-thaw activity and rotational sliding of the cirque glacier enlarges the depression forming an amplitheater-shaped basin. The headwall between adjacent cirque basins is thinned and steepened into a serrated ridge or arete. Image source: LSH

Figure 17. Glacial pavement exposed in Glacier Gorge.  This pavement is composed of jointed Precambrian granite that has been smoothed and grooved by debris carried at the base of a glacier. The granite weathers rapidly, so the fine glacial polish is absent. However, the and grooves are still visible. Can you distinguish glacial grooves from joints? Roll over the image to find out. Image source: LSH

Figure 18. Formation of asymmetrical stream-lined bedrock knobs (roche mountonnees) beneath a glacier.  Debris embedded in the ice abrades the stoss side of a bedrock obstacle. The higher pressure on the stoss side causes local melting of the ice along the bed.  The meltwater produced flows to the lee (low pressure) side where it refreezes in rock fractures and to the glacier, thus enabling the glacier to extract (pluck) blocks of rock from the knob. Image source: LSH

Figure 19. Small roche montonnee exposed on the Glacier Gorge Trail.  The feature is about a meter in length.  Identify the stoss and lee sides and determine the direction of ice flow.  Roll over the image to check your interpretation.  Image source: LSH

Moraines

Because alpine moraines are found in the lowlands where they are often hidden in forests they are commonly overlooked when viewed from the road.  They are more readily recognized from the air or looking down from a mountain peak.  Moraines are ridges of glacial debris deposited along the margins (sides or end) of a glacier. Lateral moraines form along the sides.  End moraines along the snout.  A terminal moraine delineates the maximum advance of a glacier, while smaller recessional moraines trace the glacier's retreat upvalley.   Reconstructing the past positions of glaciers is accomplished through the mapping of moraines.  Surrounding Moraine Park in RMNP is a terminal moraine formed during the Pinedale glacial stage.  It consists of two lateral moraines that loop and join across a smaller end moraine (fig. 20 and 21).  The surface of the moraine is scattered with large rounded boulders call glacial erratics that were transported and dumped by the ice in the moraines.  The presence of these boulders is a clear indicator of glacial deposition. 

Figure   20. Google Earth image of Moraine Park.  This satellite image nicely displays the looping lateral and end moraines of a Pinedale-age glacier that flowed from Forest Canyon, Spruce Canyon, Odessa Gorge (map) and numerous tributary valleys all flowed together forming a tongue of ice that terminated at Moraine Park.

Figure 21.  View of Moraine Park from Moraine Park Visitor's Center. (Roll over image to  see features.) Moraine Park is the flat meadow marking the location of a glacier originating from tributary valleys in the mountains.  The ridges around the park are moraines of rocky debris that accumulated along the glacier's margin.  The boulders in the foreground are glacial erratics, boulders transported and dumped by the glacier. They are an essential component of the moraine. By identifying the lithologies composing erratics geologist can determine where the glacier traveled from.  Scattered on the floor of the park are a couple of isolated roche mountonnees.

Distribution of Modern Glaciers

The remaining glaciers in Rocky Mountain National Park are small and slowing disappearing.  They are not in equilibrium with the present climate and owe their survival to the prevailing westerly winds that transport snow from the flat uplands and deposit it in lee-side basins along the eastern flank of the Continental Divide.  In addition to glaciers of ice there are also over 150 rock glaciers in the park. A large rock glacier on Long's Peak may be 100 feet thick and extend over a mile in length.  Rock glaciers occur where rocky talus created by frost shattering is filled with interstitial ice. As the ice deforms the rocky mass slowly flows like a glacier.  The Tyndall Glacier rest upon a rock glacier.

Figure 21.  Glaciated upland surface of the Southern Rocky Mountains.  The white line is the Continental Divide.  Route 36 (thin yellow line) is the Trail Ridge Road that traverse the Continental Divide and Rocky Mountain National Park. There are  a few cirque basins and alpine valleys on the west side.  However, most are on the east. Glaciers on the east side are sheltered from the warm afternoon sun. But more importantly, the eastern basins trap snow carried across the mountains by persistent westerly winds. For this reason glaciers along the Front Range are known as "wind-drift-glaciers" (KellerLynn, 2004). Without this wind effect they would not exist.

Figure 22.  Partial Map of Rocky Mountain National Park. (Click here to view entire park map.)   Bighorn Flats and Flattop Mountain are the remains of an eroded peneplain formed when an older mountain range was reduced to base level.  The surface was uplifted to its present elevation (12,000 ft) by renewed faulting over the last 5 million. The serrated eastern margin of the plateau was shaped by alpine glaciers. (Roll over image to see glacial features.)  Explore features of the park--view these image by Gordon Novak: Bighorn Flats , Andrews Glacier, McHenry's Peak, Hallet Peak, Glacier Gorge, Sky Pond and Taylor Glacier, Long's Peak, Little Matterhorn, and Half Mountain.

Figure 23. Ice accumulated along the east side of the Continental Divide Near the Alpine Visitor's Center.  Prevailing westerly winds blow from left to right.	Figure 25. Tyndall Glacier viewed from Flattop Mountain.  Hallett Peak in the background lies to the south.  This dying glacier overlies a rock glacier responsible for some of its movement.

Figure 26. Google Terrane Map of Rocky Mountain National Park. Click to view in a larger map and explore.

Rocky Mountains NP - References and Sites

On to Glacier National Park