and Vicinity

Northern Wyoming 

Backpacking

Backpacking Wyoming

Geology
The Laramide Rocky Mountain Ranges

Copyright © 2013 Travis N. Wood


Ancestral and Laramide Rockies.

The Rocky Mountains may be considered in two parts:

  1.  The older Ancestral Rockies extend from northern Montana northward through the Canadian provinces of Alberta and British Columbia and almost to the Yukon provincial border.
  2. The younger Laramide Rockies extend from southern Montana southward to northern New Mexico.

 So the Rocky Mountains as a whole consist of the Ancestral Rockies in the north and the Laramide Rockies in the south. The Laramide Rockies are those that were uplifted during the Laramide Orogeny, that is, the Laramide period of  mountain-building. The period is named after the Laramie Mountains in southeast Wyoming.

 In geologic context, all mountain ranges of  Laramide origin in the area are considered part of the Rocky Mountains. But the Rocky Mountains are not limited to only those of Laramide origin.

All ranges in Wyoming and virtually all those along Wyoming state lines are Rocky Mountain ranges of Laramide origin. That includes the Black Hills on the east, the mountains around Yellowstone to the northwest, the Beartooths and others to the north, and the Uintas to the southwest.

 Virtually all ranges in Colorado are also of Laramide origin. That includes the Front Range, Sawatch, Sangre de Christo, and San Juan mountain ranges. Each is a Laramide range and each is part of the Rocky Mountains.

(See the map to the right for other Laramide Ranges in the Rockies. Click on the map for a larger image.)

"Island Ranges" of the Laramide Rockies.

 Some folks have thought of the Rocky Mountains as only those ranges lying along the Continental Divide. And that probably accounts for why early explorers, seeing the Black Hills from a distance, may have referred to them as "east of the Rocky Mountains."

Regardless of origin, the substantial Black Hills tourist industry has latched onto that phrase, "east of the Rockies," as somehow the Black Hills' claim to fame. But scientists today, in a variety of disciplines, consider the Black Hills to be part of the Rockies rather than east of the Rockies. In the earth sciences and associated life sciences, the Black Hills are considered the "easternmost extension of the Rocky Mountains."

About 150 miles to the west of the Black Hills is another "island range" called the Bighorn Mountains. And about 110 miles southeast are the Laramie Mountains. Further southwest from the Bighorns about 120 miles is the Wind River Range, perhaps not quite an "island" but nearly so. Each mountain range is oriented somewhat northwest to southeast. And those who have backpacked the interior wilderness areas of the Wind Rivers and the Bighorns can easily find similarities between those two ranges. Yet similarities to the Black Hills, another Laramide Range, are not so obvious, so a closer look in in order.

The Black Hills Mountain Range.

In 1990, Dr. Sven G. Froiland, in Natural History of the Black Hills, wrote that the mountain range called the Black Hills was "the easternmost extension of the Rocky Mountains." He continues, "There are many formational similarities between the Black Hills and the Bighorn Mountains, especially."*  Those formational similarities involve plate tectonics as discussed below.

It suffices for now to say that the Wind Rivers are similar to the Bighorns, which are similar to the Black Hills. If, as part of the Rockies, the Black Hills seem somewhat different, that may be because the mountain range is considered the oldest in the Laramide Rockies. There were times in geologic youth when the Black Hills reached elevations of up to 14,000 feet — similar to the Bighorns, Wind Rivers, or even Front Range of Colorado. Now those lofty elevations have worn away to leave a high elevation of  about 7,240 feet.

Mountain versus Hill.

Historically and currently, the Lakota people have called the Black Hills "He Sapa," that is, "Black Mountains." Locally, the more popular notion is that they were called "Paha Sapa," or "Black Hills." The context in which the Lakota people may have used either term is not clear. But it is important to note that the Natives considered them mountains. So official naming by non-Natives does not necessarily represent the historical conception of those who first lived in the vicinity of the mountain range.

By most any objective measure, the Black Hills are mountains. According the the US Geological Survey:

"There are no official definitions for generic terms as applied to geographic features. Most descriptions are ambiguous at best. . . . ¶ As for mountains and hills...the British Ordnance Survey once defined a mountain as having 1,000 feet of elevation, anything less was a hill, but the distinction was abandoned sometime in the 1920's. . . .  The U.S. Board on Geographic Names once stated that the difference between a hill and a mountain in the U.S. was 1,000 feet of local relief, but even this was abandoned in the early 1970's."

According to the Peaklist Organization, Harney Peak in the Black Hills at 7242 feet elevation has a prominence of 2932 feet. And there are several other peaks in the Black Hills above 7000 feet. So even according to older definitions, the Black Hills are mountains. But as the USGS says, there is no standardized definition for the difference between mountain and hill.

Plate Tectonics and the Laramide Ranges.

Current theory for formation of the Laramide Rockies is based upon plate tectonics. In simplest terms, the interior of the planet is molten and fluid and overlain by a solid crust and upper mantle, the lithosphere. While that "lithosphere" may be 25 to 125 miles thick, it is thin in comparison to the earth's diameter of nearly 8000 miles. The relatively thin lithospheric "shell" is divided into plates that are pressed against each other by forces from within the earth's more fluid interior.

So over long distances and millions of years, the stressed and fragmented shell of the earth buckles and wrinkles. And one plate may be forced beneath another. One such plate was the Farallon plate off what is now the West Coast of North America. That Farallon plate, pushing against the North American plate, was forced beneath the North American plate. That is to say, the Farallon Plate "subducted" the North American plate. In this process, as the theory goes, the Farallon plate caused the North American to buckle up into mountains as much as a thousand miles from the boundary of this "subduction."

This buckling up and wrinkling of the North American plate is called an "orogeny," or process of mountain building. And in this occurrence of the Farallon plate subducting the North American plate, the mountain building is called the Laramide Orogeny. The resulting mountains so formed are the Laramide Rockies.

The area of the Laramide Mountains was, millions of years ago, under ancient seas. At that time, the lower or basement igneous rock beneath those seas, was overlain by thousands of feet of sediment that became sedimentary or metamorphic rock. Subsequent to the buckling and wrinkling uplift of these mountain ranges, the sedimentary and metamorphic layers were worn away by millions of years of erosion. And the basement rock was uplifted to elevations often above the outlying outcrops of layers once deposited by ancient seas.

 The result is that the Laramide Rockies today tend to have a common geologic structure of high elevation, basement igneous rock surrounded by outcrops of slanting sedimentary and metamorphic layers forming the perimeter of each mountain range. This is apparent today in the Bighorns, the Wind Rivers and other Laramide Mountains as interior wilderness areas of high granite peaks surrounded by slanted ridges of worn-away sedimentary outcrops.

But if all this is seems overly simplistic, it undoubtedly is. The Laramide Orogeny is dated at from about 72 to 40 million years ago. In the 40 million years since, other forces have been at work. Cracks or fault lines have developed in the layers and magma "intrusions" filled some of those channels with what became cooled igneous rock. The Black Hills, being one of the earlier "wrinkles" from the Laramide forces, through a long period of erosion, have exposed those intrusions in the rock shapes of Black Elk Wilderness and other areas of the central Black Hills. Mount Rushmore, the enormous sculpture near Black Elk wilderness was "carved" from one of those granitic intrusions.

And finally, those intrusions were responsible for structures like Devils Tower and for the gold mined from the Northern Black Hills. Summed up, to a greater extent than in the Bighorn Mountains or Wind River Range, the long wearing away of the Black Hills mountain range made gold more accessible and roads easier to construct. None the less, some areas of the Black Hills remained nearly impenetrable Thus, to the backpackers' chagrin, the Black Hills in the past has been overrun with roads, mining claims, and small ranches. But many roads are currently being closed to protect resources and wildlife. And there remain many primitive areas where a hiker can wander for days or hours with very little contact with other humans or their machines.


Links and References:

The Laramide Orogeny:

Colorado Geology Overview, Dr. Jeremy McCreary, 2003 — Probably the most thorough discussion of the Laramide Orogeny, but predating other research.

Structural Geology of the Rocky Mountains, Dr. James S. Aber, Emporia State University, 2003 — This site begins with a discussion of the Ancestral versus Laramide Rockies.

Laramide Mountain Ranges and Yellowstone Park, Dr. D.C. DeMets, University of Wisconsin — Shows basic outline of Laramide boundaries.

NASA/Goddard Space Flight Center, Scientific Visualization Studio — Video illustrations of Farallon Plate subduction.

National Park Service, Devils Tower, Geologic Setting — Brief summary of geologic events at Devil's Tower, including the Laramide uplift and later magma intrusions.

"Easternmost Extension of the Rockies":

* Sven G. Froiland, Natural History of the Black Hills and Badlands, 2nd ed. (Sioux Falls, South Dakota: The Center for Western Studies, 1990), 11.

Diversity and Distributions, A Journal of Conservation Biogeography: ". . . the Black Hills of Wyoming and South Dakota, an isolated, easternmost extension of the Rocky Mountains."

Common Ground: The Struggle to Define Ownership of the Black Hills: "The Black Hills are 'the easternmost extension of the Rocky Mountains. . . .'" page 29

Department of Geosciences, University of Wisconsin at Madison, Laramide Ranges: "The Laramide orogeny refers to a phase of mountain building that affected parts of Montana, Wyoming, Colorado, and lesser areas of nearby states. . . . The Laramide orogeny occurred from approximately 70 million years ago to 40 million years ago during a period when the Farallon oceanic plate was subducting rapidly beneath the western coast of the U.S. . . . The easternmost Laramide uplift is the Black Hills of western South Dakota. Immediately west of the Black Hills, the arc-shaped, isolated Big Horn Mountains are found in north-central Wyoming."

Mt Lion Foundation: "the Black Hills are part of the eastern most extension of the Rocky Mountains"

National Park Service for Wind Cave National Park: Under "Geology and Soils": "The Black Hills, which constitute the easternmost extension of the Rocky Mountains, are an isolated and unglaciated group of mountains that rise above the surrounding Plains"

South Dakota School of Mines and Technology: "The Black Hills uplift is within the northern Rocky Mountains and represents its easternmost extent."

Louisiana State University, masters thesis, page 5: "The Black Hills are considered the easternmost expression of the North American Laramide orogeny and represent the easternmost part of the Rocky Mountains."

US Geological Survey: "Black Hills of Wyoming and South Dakota, an isolated, easternmost extension of the Rocky Mountains"

US Geological Survey: "Geologically, the Black Hills represent the easternmost outpost of the Rocky Mountains."

Elevations and Distances in the United States, USGS, 1995 — This older USGS souce still lists the Black Hills as "East of the Rocky Mountains."

Joint Fire Science Program (USGS, Dept of the Interior, US Forest Service, National Park Service et al), page 7: "As the easternmost extension of the Rocky Mountains, the Black Hills. . . ."

South Dakota Department of Game, Fish & Parks, South Dakota Mountain Lion Management Plan 2010-2015, page 2: "The Black Hills, located in west-central South Dakota and northeastern Wyoming, represent the eastern most extension of the Rocky Mountains and represent the oldest mountains in North America (Froiland 1990)."

"Mountain" versus "Hill":

US Geological Survey: "There are no official definitions for generic terms as applied to geographic features. . . . ¶ the British Ordnance Survey once defined a mountain as having 1,000 feet of elevation, anything less was a hill, but the distinction was abandoned sometime in the 1920's. . . . The U.S. Board on Geographic Names once stated that the difference between a hill and a mountain in the U.S. was 1,000 feet of local relief, but even this was abandoned in the early 1970's."

 

 

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Copyright © 2013 Travis N. Wood