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Rocky Mountain National Park

Geologic Features & Processes

Rocky Mountain National Park abounds in geologic features that may be of concern for park planning, public safety, or resource protection. Geologic features (or landforms) and processes have scientific and aesthetic significance, as well as continually affecting human beings and other living things. These features and processes may not be readily apparent on the park’s geologic map.

Colorado River
The Colorado River originates in Kawuneeche Valley in Rocky Mountain National Park. Here it is very narrow, contrasting greatly with the segment of the river that flows through Grand Canyon National Park. The river flows 1,400 miles (2,253 km) from its birthplace in the park to the Gulf of California.

Columnar Jointing
High above the floor on the east side of Kawuneeche Valley in the vicinity of the Bowen- Baker Trailhead, a lava flow of rhyolite displays a peculiar type of jointing. This ornate rock feature, called columnar jointing, is not caused by weathering or erosion but by contraction of molten rock as it cools. This process produces parallel, prismatic columns that are polygonal in cross section. The lava flowed from one of the volcanoes in the Never Summer Mountains about 26 million years ago (Raup, 1996). The position of the lava flow high above the valley floor indicates that this valley was filled to that level with volcanic ash and lava at the time of the eruption. Erosion by water and ice in the ensuing years has removed most of the volcanic rocks and cut the valley to its present level (Raup, 1996).

Exfoliation Domes and Ornate Erosion Features
The rounded knobs on McGregor Mountain and Lumpy Ridge are homogeneous masses of granite with very uniform structure. As erosion of overlying rocks— presumably thousands of feet of sedimentary rock— slowly released pressure from the granite, exfoliation domes were formed. The granite responded by expanding and cracking into concentric slabs that resemble the rings of an onion.

Fractures that form in response to the removal of overlying rock may be quite prominent, in which case they often result in “sheeting” and sometimes form bizarre landforms. Helped along by frost action, sheeting produced the Keyhole on the popular route to the summit of Longs Peak.

Over millions of years, wind erosion and freeze- thaw action have sculpted the rocks in the park into many ornate forms. On Lumpy Ridge, for example, fantastically shaped rocks include balanced rock and clusters of oddly shaped boulders, such as “hen and chicks.”

Glacial Features
Cirques and Tarns
Few landforms have caught the imagination of geomorphologists more than the glacial cirque: a bowlshaped, amphitheater- like hollow or basin eroded into a mountain mass. The small glaciers that exist in the park occupy cirques once excavated by their large predecessors. After the glacier that created the cirque melts, a small lake, called a tarn, may occupy the basin. One of the most spectacular cirques is the area below the East Face of Longs Peak. Chasm Lake (a tarn) fills the basin. Other distinctive cirques in the park are located on Terra Tomah and Ypsilon Mountains.

Glacial Erratics
In many glaciated areas, large boulders end up stranded when glaciers recede. These out- of- place rocks are called erratics. Erratics testify to the effectiveness of glacial erosion and transport, and may have glacial striations. They also testify to glacial deposition and lie scattered on bedrock surfaces different from their own compositions. Visitors can see nice examples of erratics along the Cub Lake trail.

Glacial Polish, Striations, and Grooves
Rocks and sediment frozen to the base and sides of glaciers act like sandpaper and grind, scratch, and polish the rocks over which they pass. Transported rocks also become smoothed and rounded themselves. Glacial polish, striations, and grooves line Old Fall River Road, which leads to the head of the glacial valley at Fall River Pass.

Glacial Till and Moraines
A glacier carries all sizes of debris at its base, sides, and surface, and deposits this material along the sides and floor of the valley down which it flows. Till is the general term for the poorly sorted mixture of fine to coarse rock debris deposited directly from glacier ice. The most obvious landforms composed of till are moraines. Moraines can be undulating mounds or sharp ridges depending on how long a glacier remained stable in a particular position or how much erosion and weathering have taken place in the intervening time between deposition and the present.

Lateral moraines form on the sides of glaciers and merge with an end or terminal moraine, an arc- like ridge that forms at the terminus of glaciers. Excellent examples of lateral moraines are on the north and south sides of Moraine Park. The south lateral moraine is nearly 1,000 feet (305 m) high. An end (terminal) moraine forms when ice stabilizes for a time prior to retreat. An end moraine may dam meltwater to create a lake on its upvalley side. Moraine Park Museum sits on an end moraine, which also happens to be the terminal moraine, and represents the furthest point of glacial advance. A good viewing point of moraines is Many Parks Curve.

Hanging Valleys
Few landscape features are aesthetically more beautiful than a hanging valley with a high cascade or waterfall. A hanging valley that is formed by glacial erosion (there are other kinds of hanging valleys) is a side valley with its mouth at a relatively high level above the main glacial valley; it is smaller than the main valley. A “trunk” glacier eroded the larger, main valley, and a tributary glacier eroded the smaller hanging valley. The discordance of the levels of the valley floors, as well as their difference in size, is due to the greater erosive power of the main, trunk glacier. Some lakes, such as Nanita and Nokoni, lie in hanging valleys. Streams that occupy hanging valleys are Roaring River, Chiquita Creek, and Fern Creek.

Horns, Arêtes, and Cols
Mountains that are, or have been, surrounded by glaciers tend to have characteristic forms caused by the fracturing action of ice, leading to steep rockwalls at the heads of cirques that flank peaks. Where isolated, such mountains may form upstanding horns with three or four distinct faces. The most famous horn is the Matterhorn in the Swiss Alps. The Little Matterhorn, tucked away in Glacier Gorge, is less spectacular than its Swiss namesake but it still possesses distinctly carved faces. Sometimes steep, straight ridges, called arêtes, may link horns and cols—open, U- shaped passes. A good example of a col lies above Andrews Glacier in Loch Vale. This particular col directs wind- blown snow, carried over the Continental Divide, onto the surface of this “wind drift” glacier.

Outwash Deposits, Kettles, and Kettle Ponds
During a warming trend as a glacier recedes, a stream laden with sediment is “washed out” from the glacier (hence the term “outwash”) and deposited in a flat area below. In mountainous areas, these deposits are called valley trains. Depressions, known as kettles, often pockmark outwash and moraines. Kettles form when a block of stagnant ice becomes wholly or partially buried in sediment and ultimately melts, leaving a pit behind. Kettles can be feet or miles long but are usually shallow. In many cases, water eventually fills the depression and forms a pond or lake, called a kettle pond or kettle lake. Sheep Lakes are kettle ponds that lie in the outwash on the floor of Horseshoe Park. These small ponds are intermittent and usually dry up late in the summer. Bighorn sheep visit these kettle ponds, primarily during their lambing season in May and June, to drink the water and eat the mud, which is rich in nutrients for lactating ewes. We have the glaciers to thank for this prime wildlife- viewing area.

Roches Moutonnées
Roches moutonnées are characteristic of glacial erosion on massive rocks. Roches moutonnées are asymmetrical, elongate knobs or hillocks of resilient bedrock that have been smoothed and scoured by moving ice on the upglacier (stoss) side. On the down (lee) side, the rock is steep and hackly from glacial quarrying. Moutonnées are the wigs—with smooth bangs and curly backs—that barristers and judges wore in early European and British courts and resemble this glacial form. People have also interpreted roches moutonnées as “rock sheep,” as they are thought to resemble grazing sheep. The knob in the middle of Moraine Park is a fair example of a roche moutonnée.

U-shaped Valleys
Prior to the appearance of glaciers, alpine valleys are characteristically V- shaped, as is typical for valleys cut by streams. As a glacier moves down a valley, it makes the valley wider, steeper, and straighter, so that the previous V- shaped valley is transformed into a U- shaped one. Good examples of U- shaped valleys are abundant in the park, for instance, the valley of Fall River and Kawuneeche Valley. When glaciers retreat, the bottoms of U- shaped valleys may become flat, as sediment deposited in lakes impounded by the terminal moraines fills them.

Today, the park’s small glaciers are restricted to high elevations above 11,000 feet (3,350 m) and north- and east- facing cirques, where they are sheltered from the Sun’s direct rays. Local topography helps to shelter the glaciers and directs wind- blown snow onto their surfaces. In general, during the winter blowing snow occurs over 50% of the time, with 95% of the days in January having blowing snow. On average, over 30 blowing snow events occur each winter, with each event averaging 36- hours long (Berg, 1986). Hence, glaciers in the Front Range are referred to as “wind- drift glaciers” because they receive most of their snow as wind- blown snow, which falls predominantly on the western slope and is transported to the eastern slope. Notably, these glaciers have been able to form well below regional snowline because of wind drift. A fine example of a wind- drift glacier in the park is Andrews Glacier.

Braddock and Cole (1990) identified 34 snow banks and ice masses in the vicinity of Rocky Mountain National Park. These include the snow and ice bodies shown on 1:24,000- scale topographic maps published between 1957 and 1962. Fourteen of the ice masses have been named: Rowe Glacier (between Rowe Peak and Hagues Peak), Sprague Glacier (at Irene Lake in Spruce Canyon), Tyndall Glacier (at the head of Tyndall Creek), Andrews Glacier (east of Andrews Pass), Taylor Glacier (at the head of Icy Brook), Chiefs Head Peak Glacier (above Frozen Lake), Mills Glacier (on the east side of Longs Peak), Moomaw Glacier (south of The Cleaver), and the six St. Vrain Glaciers (outside of the park at the head of Middle St. Vrain Creek).

All but one of these snow banks or ice masses are on the east side of the Continental Divide; the exception is the mass above Murphy Lake (near Snowdrift Peak). All but one occur at the heads of cirques: north- , northeast- , or east- facing; the exception is the large snowbank northeast of Rowe Mountain, which is in a northeastfacing gully. Some of the ice masses, such as Andrews Glacier, are actively moving and can be considered actual glaciers; others are stagnant. Over long periods of time, it is not known whether these masses are growing or shrinking (Braddock and Cole, 1990).

Rock Glaciers
Rock glaciers are distinctive from ice glaciers in that their movement is characterized by a large amount of embedded and overlying rock material. A rock glacier may be composed of (1) ice- cemented rock formed in talus that is subject to permafrost, (2) ice- cemented rock debris formed from avalanching snow and rock, or (3) rock debris that has a core of ice, either a debris- covered glacier or a remnant moraine. Two types of rock glaciers occur in the Front Range. The first type forms on the floors of modern cirques and closely resembles the tongues of small valley glaciers. They are referred to as cirque- floor rock glaciers (Outcalt and Benedict, 1965) or tongue- shaped rock glaciers (Madole, 1972). Because they contain cores of banded glacial ice and grade upvalley into lateral moraines, investigators determine rock glaciers of this type to represent the debris- covered tongues of former glaciers in the Front Range. This may not be true everywhere, however (Madole, 1972). Most cirque- floor rock glaciers consist of two or more superimposed lobes, bounded by longitudinal furrows, resulting from independent ice advances. Despite their compound nature, the complexes now appear to be moving downslope as single units (Outcalt and Benedict, 1965).

Rock glaciers of an entirely different character occur beneath steep valley walls, where they are supplied with debris from avalanche couloirs. Interstitial ice, responsible for the movement of the “valley- wall” rock glaciers, probably results from the metamorphism of snow buried beneath rockfall debris or supplied by winter avalanching (Outcalt and Benedict, 1965). Geologists also refer to this type of rock glacier as a lobate- shaped rock glacier (Madole, 1972).

In Rocky Mountain National Park, rock glaciers either accumulate at heads of cirques and flow down the length of the main valley or accumulate along the sides of valleys and flow outward toward the center. Rock glaciers are abundant on both sides of the Continental Divide and both sides of the Never Summer Mountains, and have flowed down slopes that face all compass directions (Braddock and Cole, 1990). Rock glaciers occur on Ships Prow, Pagoda Mountain, and Storm Peak on the east side of the park, and near Azure, Julian, and Hayden lakes on the west side of the park (Braddock and Cole, 1990).

Numerous lakes—nearly 150—punctuate the landscape of Rocky Mountain National Park. They come in many shapes and sizes, and their origins vary. As the large valley glaciers retreated, they left basins in their paths. Today, chains of glacial lakes are found in these deprressions and appear as treads of a rising staircase. The term paternoster, i.e., “Our Father,” is an apt description for these chains of lakes, as they resemble the beads on a rosary. An example of paternoster lakes is the linked Fifth–Fourth–Spirit–Verna–Lone Pine lakes on the west side of the park. Some, such as Gorge Lakes (seen from Trail Ridge looking across Forest Canyon), fill glacially carved but do not form chains. Others, such as Forest Lake and Bierstadt Lake—additional examples of kettle ponds—formed behind dams of glacial moraine (Emerick, 1995).

A number of small pothole ponds, formed by weathering of exposed bedrock, lie among the granite outcrops along Lumpy Ridge. The largest of these is Gem Lake, a rain- fed pond with no inlet or outlet (Emerick, 1995).

Ponds and lakes are among the most temporary features of the landscape. Though they may seem long- lived in terms of human generations, lakes gradually fill with sediment and become shallower and smaller (Emerick, 1995).

Meandering Streams
Streams constantly erode their banks and deposit new sediment; hence, riparian zones change more than any other type of ecosystem in the park. In flat valleys, streams tend to meander, widening their bends and occasionally short- circuiting them, leaving the abandoned meanders to form oxbow lakes, which over time fill in with sediment. Floods that result from snowy winters and wet springs may scour channels or form new stream courses.

Meandering streams are typical of streams in the park because gradients are low behind the Pleistocene moraine dams. In many cases, meandering streams represent lake floors of past glacial lakes or outwash deposits. Good examples of meandering streams are in the area surrounding the Bowen- Baker Trailhead, where the Colorado River meanders across the valley, and Horseshoe Park. Horseshoe Park earned its name through this geomorphic process: loops in the meandering stream that have been cut off and left behind as isolated sections, resemble horseshoes.

Patterned Ground
Today the term periglacial is used to describe processes and landforms associated with very cold climates in areas not permanently covered with snow and ice. Periglacial features are distinctive from glacial features and in many cases are located far from glaciers. One particular periglacial features in Rocky Mountain National Park is patterned ground, which forms at high elevations, above about 11,500 feet (3,505 m) in the Front Range (Ives and Fahey, 1971). An interesting effect of ice- crystal growth in soils (ground ice) is the moving of soil and rock fragments upward toward the surface to form mounds and rows of soil or rock. Rock fragments lying close to the surface conduct heat causing a cycle of freezing and thawing and growth of ice under rocks. Continued
thickening of ice layers heaves rocks upward, causing them to rise to the surface. Frost action moves rocks both sideways and upward. Heaved rocks form patterns of bands, circles, nets, and polygons called patterned ground.

Large angular blocks of rock in an accumulation known as felsenmeer (German for “rock sea”) are a conspicuous display of frost action above treeline. People have also described them as “tombstone rocks” (Kiver and Harris, 1999). A popular stopping point along Trail Ridge Road is Rock Cut. Tundra Communities Trail leading from the road provides easy access through felsenmeer and patterned ground.

Many geologic features do not fit nicely into just one category; sackung features are a case in point being both glacial and structural. As thick, mountain glaciers carved their way down preexisting valleys, they caused two changes: they steepened the valley walls, and to a lesser degree, they deepened the valleys. When the ice filled these valleys, the glaciers provided lateral support to the valley walls. When the ice melted, the steep valley walls lost their support. Gravity caused the mountains between the over- steepened valleys to actually spread laterally into the valleys. Small faults along the tops and sides of ridge crests were created that commonly have uphillfacing scarps called sackung features. In the past, geologists have thought these faults were caused by mountain- building processes; they now know they were caused by mountain “falling” processes.

The term solifluction was proposed by Andersson (1906) as “the slow flowing from higher to lower ground of masses of waste saturated with water.” Because Andersson did not state explicitly that it referred to flow over frozen ground, some geomorphologists have extended the term to include similar movement in temperate and tropical regions. It is preferable to restrict the term to slow soil movement in periglacial areas, however (Bates and Jackson, 1987). (See also Geologic Issues section of this report.)

The term solifluction implies the presence of permafrost. Permafrost (i.e., permanently frozen ground) has no doubt existed over most of the alpine zone in the past, but it is discontinuous today (Rich Madole, written communication, 2003).

During the summer season in the high country, water is unable to percolate into an impervious layer of frozen ground below the surface. As a result an “active layer” of soil becomes supersaturated and flows. Flowage can occur on slopes as gentle as two or three degrees. Where there is a well- developed mat of vegetation, a sheet may move downward in a series of well- defined lobes and form terrace- like features. Features formed by soil flowage appear as wavy slopes and are quite prominent along Trail Ridge Road, for example, between Forest Canyon Overlook and Rock Cut.

Tors are isolated rock towers rising prominently above otherwise level terrain. They are typically composed of granite, which is very jointed and made more so by sheer jointing that develops because of dilation as rock is unloaded. Tors may assume peculiar or fantastic shapes. Visitors can see excellent examples along the nature walk above Rock Cut on Trail Ridge Road. Investigators think that periglacial processes may be important in the formation of tors (Bates and Jackson, 1987). Investigators have identified tors as indicators of non- glaciation (Street, 1973). Tors remain in areas that were beyond the limit of glaciation, otherwise glaciers would have modified or destroyed them.

Uplifted Erosion Surface
The so- called “Roof of the Rockies” is a remnant of an ancient rolling plain that has survived despite being broken by faults, uplifted several thousand feet,
intersected by great canyons, and subjected to the vicissitudes of the Ice Age (Richmond, 1974). Big Horn Flats and the flat, plain- like surface that spans across the landscape between 11,000 and 12,000 feet (3,350 to 3,660 m) on Trail Ridge are parts of this uplifted erosion surface.

Since the erosion surface was first recognized and reported (Marvine, 1874), it has caught the attention of geologists and visitors alike, but not without controversy. Questions still remain regarding: (1) the number of surfaces—investigators have recognized as few as one to as many as 11 surfaces; (2) the age—early or late Tertiary, that is 5 million or 50 million years old; and (3) the genesis—peneplain (forming at low elevations and with low river gradients) vs. pediment (forming under arid conditions along mountain fronts or plateaus).

Geologists have paid so much attention to erosion surfaces because of the structural implications. For decades, geologists used erosion surfaces as a clue to the post- Laramide deformational history of the middle and southern Rocky Mountains. Because peneplains were believed to form at low elevations and with low river gradients, substantial uplift was required to bring them to their present elevations. Using this kind of evidence, investigators estimated late Cenozoic uplift to between 5,000 and 9,000 feet (1,524 and 2,743 m) (Davis, 1911; Chamberlain, 1919). Reclassifying peneplains as pediments greatly reduced the amount of uplift required (Johnson, 1931, 1932; Mackin, 1947). Uplift was then estimated from displaced flora and fauna, for example
using fossils from Florissant, Colorado (Epis and Chapin, 1975). The curious irony is that the magnitude of uplift based on paleontology is approximately the same as it was when based on peneplains (Bradley, 1987).

The most recent theory regarding erosion surfaces identifies one major subsummit (lower) surface and a second (higher) summit surface. The lower surface formed in late Tertiary time when conditions were arid to semi- arid, classifying it as a pediment. The higher surface is much less extensive than the lower subsummit surface. Studies have concentrated on the lower erosion surface that has been called “Rocky Mountain,” “Sherman,” “Late Eocene,” and “Subsummit,” which leaves the full significance of the higher surface, which has been called “Flattop” and “Summit,” inconclusive (Bradley, 1987). Terra Tomah Mountain, Flattop Mountain, and Longs Peak are all part of the higher summit surface.


Bates, R.L., and Jackson, J.A., eds., 1987, Glossary of geology (3rd ed.): Alexandria, Virginia, American Geological Institute, 788 p.

Berg, N.H., 1986, Blowing snow at a Colorado alpine site—measurement and implications: Arctic and Alpine Research, v. 18, p. 147–161.

Braddock, W.A., and Cole, J.C., 1990, Geologic map of Rocky Mountain National Park and vicinity, Colorado: U.S. Geological Survey Map I- 1973, scale 1:50,000.

Bradley, W.C., 1987, Erosion surfaces of the Colorado Front Range—a review, in Graf, W.L., ed., Geomorphic systems of North America: Boulder, Colorado, Geological Society of America, Centennial Special Volume 2, p. 215–220.

Chamberlain, R.T., 1919, The building of the Colorado Rockies: Journal of Geology, v. 27, p. 145–164, 225–251.

Davis, W.M., 1911, The Colorado Front Range—as study in physiographic presentation: Association of American Geographers Annals, v. 11, p. 21–83.

Emerick, J.C., 1995, Rocky Mountain National Park natural history handbook: Niwot, Colorado, Roberts Rinehart Publishers, 158 p.

Epis, R.C., and Chapin, C.E., 1975, Geomorphic and tectonic implications of the post- Laramide, late Eocene erosion surface in the Southern Rocky Mountains, in Curtis, B.F., ed., Cenozoic history of the Southern Rocky Mountains: Geological Society of America Memoir 144, p. 45–74.

Ives, J.D., and Fahey, B.D., 1971, Permafrost occurrence in the Front Range Colorado Rocky Mountains, U.S.A.: Journal of Glaciology, v. 10, p. 105–111.

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Kiver, E.P., and Harris, D.V., 1999, Rocky Mountain National Park (Colorado), in Geology of U.S. Parklands (5th ed.): New York, John Wiley & Sons, chapt. 11, p. 630–644.

Mackin, J.H., 1947, Altitude and local relief of the Bighorn area during the Cenozoic: Wyoming Geological Association, 2nd annual field conference, Guidebook, p. 103–120.

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Raup, O.B., 1996, Geology along Trail Ridge Road, Rocky Mountain National Park, Colorado: Helena and Billings, Montana, Falcon Press Publishing Company, 73 p.

Richmond, G.M., 1974, Raising the roof of the Rockies: Estes Park, Colorado, Rocky Mountain Nature Association, 81 p., 1 pl. in pocket.

Street, F.A., 1973, A study of tors in the Front Range of the Rocky Mountains in Colorado, with special reference to their value as an indicator of nonglaciation [M.S. thesis]: Boulder, University of Colorado, 241 p.

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