What They Are and How They Got Here

 

My latest book, Climbing Colorado’s Mountains, was edited from the original manuscript and more than 21,000 words had to be cut to fit the book format. Following is the original chapter I wrote about the geology of the mountains.

Rocky red towers of sandstone–remnants of an ancient seabed–frame Pikes Peak (14,110′) above Colorado Springs. [Photo: Stewart M. Green]

Three major provinces comprise Colorado’s topography: the plains or prairies of the east, plateaus of the west, and the mountains—specifically, the Rocky Mountains—that split the state from north to south, between the prairies and plateaus. The Rocky Mountain system is composed of many smaller mountain ranges and subranges, most running north-south, with a few running east-west. The mountains, or peaks, of Colorado are as varied as the forces that created and defined them. Our mountains began their slow development about 1.8 billion years ago, when the shifting of tectonic plates—sections of the earth’s outer crust or lithosphere—movement of molten rock or magma within the Earth’s crust, and volcanic eruptions all served to thrust the landscape of our state upward. At the same time, wind, rain, ice, lava, and rock fall pummeled the terrain, eroding the uplifted earth away to a lower, smoother playing ground. It is these two forces, uplift and erosion, that formed the mountains of Colorado we see and climb today.

Fishers Peak (9,627′) rises up more than 3,500 feet above Trinidad at the edge of the Great Plains near Raton Pass. [Photo: Susan Joy Paul]

Generally speaking, the major ranges of Colorado can trace their origins back to uplift in the form of batholiths and faulted anticlines. Batholiths formed when an igneous intrusion—molten rock that intruded the lithosphere but did not break through to the surface—solidified as a large mass beneath the earth, and was later exposed due to volcanic activity that pushed it up, and by erosion that cleared sediment from the surface. Underground pressure forced softer rock upward into tent-like folds or anticlines, and faults were created as the rock—under tremendous stress—split, and the sections shifted apart. Erupted volcanoes and layers of eroded and erupted rock, or sediments, topped some of the mountain ranges as well, forming newer ranges. Mountain building, or orogeny, is not isolated to Colorado, and in fact the Rocky Mountains extend north into Canada and south into New Mexico, and are part of a larger system known as the North American Cordillera, a subrange of the American Cordillera that stretches from Alaska to South America.

The many types of rocks created by uplift andberosion add to the variety in our peaks. Colorado’s mountains are composed mainly of igneous rocks like basalt, breccia, gabbro, granite, pegmatite, porphyry, and tuff; sedimentary rocks like conglomerate, dolomite, limestone, sandstone, and shale; and metamorphic rocks like gneiss,hornfels, migmatite, schist, and quartzite.

· Igneous rocks are formed by molten rock as it cools and hardens. Magma can rise and push through the surrounding rock, exploding above the surface as fine-grained volcanic rock, or it can solidify below the surface, as coarse-grained plutonic rock. Intrusions of plutonic rock may be exposed over time, as batholiths, dikes and plugs.

· Sedimentary rocks are made up of beds of material that have accumulated through erosion of older rocks, precipitated from water sources above or below the ground, or are the remains of plants and animals. The beds consolidate in layers, and the angle of the layers from the Earth’s surface present various slope and ledge systems, and challenges, for the mountain climber.

· Metamorphic rocks are created when rocks and minerals are subjected to intense heat and pressure, changing the mineral structure and forming a new type of rock. The type of new rock created varies, based on the original matter, the temperature and duration of the heating, and the amount of pressure. Contact metamorphism occurs in rock that’s heated due to proximity with superheated magma or a lava flow. Regional metamorphism is caused by the shifting of tectonic plates, when rock is forced deep into the Earth, and high temperatures and extreme pressure cause the rock to metamorphose.

The Crestone Mountains of the Sangre de Cristo Range form a striking backdrop above Great Sand Dunes National Park and Preserve in southern Colorado. [Photo: Stewart M. Green]

It may seem strange that the mountains of landlocked Colorado consist of such a wide variety of rocks and minerals, until you examine the geologic evolution of our state. The geologic record tells us the Earth’s crust stabilized four and a half billion years ago, and the uplift and erosion that formed Colorado’s landscape occurred within roughly the last 2 billion years. The mountains may not have been here since the beginning of time, but—compared to mere mortals, who appeared on Earth just 2 million years ago, and in Colorado a mere 15,000 years ago—they have been in development for a very, very, very long time. Here’s a brief summary of the development of our peaks:

· 1.8 billion years ago: The area on Earth we know as Colorado was a series of island chains off the coast of the ancient supercontinent of Laurentia. Tectonic plates, sections of the earth’s lithosphere, moved north and drove the islands under Laurentia.

· 1.7 billion years ago: During the Colorado Orogeny, magma beneath the lithosphere interacted with the island rock, forming igneous and metamorphic rock, the basement rock that emerged as the Colorado Province, and which forms the bases of our oldest mountain ranges.

· 1.4 billion years ago: The Berthoud Orogeny defined a period of tectonic plate shifts and batholith surges in Colorado, evidenced by—among others—the St. Vrain (Longs Peak) Batholith, Silver Plume Batholith, and the Mount Evans Batholith in the Front Range, the San Isabel Batholith in the Wet Mountains, and the St. Kevin Batholith in the Sawatch Range.

· 1.1 billion years ago: During the Grenville Orogeny the Pikes Peak Batholith intruded the outer crust of the earth as an irregular, elliptical mound of superheated magma, and cooled a mile or two beneath the surface.

· 1.1 billion to 500 million years ago: Erosion exposed and softened basement rock, forming low, rounded hills throughout the state.  Erosion also exposed the surfaces of the intruded batholiths, such as the 1,200-square-mile mass of the Pikes Peak Batholith that now makes up the Tarryall Mountains, Rampart Range, and the Pikes Peak Massif in south-central Colorado.

The Diamond Peaks at Cameron Pass form the southern terminus of the Rawah Range, offering views south to the Nokhu Crags of the Never Summer Range. [Photo: Susan Joy Paul]

·         320 million to 250 million years ago: A slow (very slow) collision between all the land masses formed the supercontinent of Pangaea. The collision created uplift, forcing large masses of metamorphic rock up through layers of limestone and dolomite—sediments of ancient seas—forming faulted anticlines in the basement rock of Colorado. As the rock was being uplifted, it was also being worn away by erosion. The Front Range Uplift in central Colorado and the Uncompahgre Uplift in western Colorado created two northwest-to-southeast trending ranges of about 10,000 feet, Frontrangia and Uncompahgria. These ranges comprised the major mountains of the Ancestral Rocky Mountains. The uplifts caused the complete erosion of surrounding sedimentary rock in some places, and today those areas are marked by an absence of old layers of rock, and an uncomformity exists where newer sedimentary rock lies directly on top of the ancient basement rock. The Great Unconformity refers to a great lapse of time in the physical, geologic history of the land, and is found in areas across Colorado.

·         250 million to 100 million years ago: The Ancestral Rockies eroded away, and their overlying sediment was swept down their slopes and deposited to the east and west in tumbled-down sediment.

·         75 million to 45 million years ago: Plate movement from the west increased, affecting a compression of the earth below, and buckling of the surface. A mountain-building episode of uplift known as the Laramide Orogeny occurred, where the area between what are now the cities of Grand Junction and Denver was shortened by as much as fifty miles, and the Laramide Mountains rose up, defining the areas of the major mountain ranges in today’s Colorado. At the same time, magma rose up in a diagonal line from the southwestern San Juan Mountains northeast to the Front Range. Much of the magma solidified below, forming Colorado’s Mineral Belt: great masses of igneous rock laced with deposits of gold, silver, lead, and zinc. Magma also made its way to the surface, feeding volcanoes. The Colorado River began to form at this time, west of the Laramide Mountains, eventually flowing southwest to carve out the Grand Canyon in Arizona.

· 45 million to 35 million years ago: Uplift slowed but erosion continued along the area of the Laramide Orogeny, and the peaks were gently reduced to low mountains and rolling hills rising from plains just a few thousand feet above sea level. At the same time, magma intrusion increased, exploding above the surface as volcanoes.

· 35 million to 26 million years ago: Volcanoes rose up in northern Colorado, spewing lava that hardened and was later eroded during periods of uplift, and by wind, water, and gravity. Volcanic activity in the southwest part of the state forced ash into the air which eventually settled, forming a thick layer of tuff throughout the area of the San Juan Mountains. That same activity formed volcanic rock still evident throughout the San Juans, West Elk Mountains, and the Never Summer Range. Igneous intrusion continued, forming more blocks of granite throughout the San Miguel, Sawatch, West Elk, and Elk Mountains, and the Front Range. This period also marks the beginnings of the Rio Grande Rift, when that same volcanic activity that pushed the land upward caused the lithosphere located between the uplifts to rise, thin out, spread apart, and fill with sediment eroded from nearby peaks, and blown in by wind from surrounding mountain passes. In south central Colorado, the Sangre de Cristo Fault and the Alvarado Fault began an active period of thrust, eventually defining the east and west borders of the Sangre de Cristo Range.

The sheer west face of Mount Zirkel (12,180’) is best viewed from Big Agnes Mountain (12,060’) in the Sawtooth Range. [Photo: Susan Joy Paul]

· 26 million years ago: Another tectonic plate shift pulled the land westward, toward the Pacific. Faulting occurred along the Colorado landscape, and the Rio Grande Rift grew, stretching from around Leadville in Colorado, to Chihuahua, Mexico. Great valleys were formed along the rift, such as the northern Arkansas Valley that split the Sawatch and Mosquito mountain ranges, and the San Luis Valley between the San Juan Mountains and Sangre de Cristo Range. Heat generated by plate movement deep within the earth caused a final, great uplift across the land, and much of Colorado was raised by about 6,000 feet.

· 26 million to 2.5 million years ago: Basalt flows capped the Grand Mesa, west.

· 2.5 million years ago: Temperatures dropped, glaciers moved in from the north, and the Ice Age began. Wind-driven ice and snow and grinding glaciers left their mark on 1.4 billion-year-old basement rock, witnessed by chiseled rock face and polished, alpine cirques, such as those on display in the Mummy Range. Subsequent freeze-thaw cycles severed rock from the mountains and cliffs into football-to-crate-sized chunks now seen as talus fields. Ice Age glaciers carved out valleys along the flanks of the Sierra Blanca Range, leaving behind alpine basins and loose moraines. Further pulverizing of the rock created scree fields, and the formation of talus and scree fields continues today.

· 170,000 to 120,000 years ago: A period of glacier activity occurred, witnessed by present-day moraines, polished rock, glacial cirques and enormous, stranded boulders, glacial erratics that were formed or carried by moving slabs of ice.

· 30,000 to 12,000 years ago: Another period of glacial activity continued to mark the land, and high basins were formed, the eventual settings for today’s alpine lakes. Humans first appeared in Colorado during this time.

· 12,000 to 5,000 years ago: Temperatures rose and the glaciers retreated.

· 5,000 years ago to the Present: Colorado’s current “glaciers” and perennial snowfields are not remnants of the Ice Age, but were formed in later years during short periods of cold, including the Little Ice Age that occurred just a few hundred years ago and ended in the late 1800s. Rock glaciers were also created, rocky remnants of ice glaciers seen along mountain slopes, their movement eased by bits of ice formed of precipitation caught and frozen beneath the surface. Rock glaciers are still found in Colorado, such as on Mount Mestas at La Veta Pass in south-central Colorado, and Engineer Mountain at Coal Bank Pass, in the southwest part of the state.

Mountain building did not come to an abrupt halt in the 21^st century, and Colorado’s mountains continue to evolve, shaped by forces of nature like uplift, erosion, precipitation, rockslides, mudslides, flashfloods, and changing temperatures; and by human intrusion with mining, road-building, and of course, mountaineering. This creates an ever-changing and unpredictable environment for the Colorado mountaineer, and a demand for vigilance on every outing.

Vermilion Peak (13,894′) tops San Juan county at Ice Lake Basin near Silverton. [Photo: Susan Joy Paul]

Climbing Colorado’s Mountains (October 2015, FalconGuides) features driving directions, route descriptions, maps, photos and GPS waypoints to 100 Colorado mountain adventures.