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  The Rocky Mountains

Natural History of the Rocky Mountains

The Rocky Mountains
    
The natural history of the Rocky Mountains began over 170 million years ago and has followed a repeating cycle of land upheaval followed by thousands of years of erosion. The western United States and the Rocky Mountains took shape during three major mountain building episodes between 170-40 million years ago (MYA). The Laramide Orogeny (70-40 MYA) was the last of these and formed the fundamental
structures of the modern Rocky Mountains.
    
     Today, the Rocky Mountains extend two thousand miles through two countries, from British Columbia and Alberta, Canada, to New Mexico. The Rockies also pass through the states of Idaho, Montana, Wyoming, Utah and Colorado and comprise over 40 distinct mountain ranges.
More information on individual ranges.

The Colorado Rocky Mountains
     
Long before today's Rockies began building, the Ancestral Rockies formed about fifty miles to the west around 320 million years ago (MYA). Considered at least as high as the current-day Himalayas, Colorado's Ancestral Rockies consisted of two ranges, Frontrangia and Uncompahgria. The mountains pushed upwards for 70 million years and then began eroding until the landscape was relatively flat again. Remnants of these ranges still can be seen in the Devil's Backbone west of Loveland, the Red Rocks in Morrison, the Maroon Bells near Aspen, and the Garden of the Gods in Colorado Springs.
    
    Around 85 MYA, seas spread across most of Colorado, forming white sandbars and beaches known today as the Dakota Sandstone layer. By 70 MYA, tectonic plates had begun to converge and clash under the Western U.S., causing the continental crust to buckle and fold like an accordion. As the land rose, so did molten magma which formed theColorado Mineral Belt that runs from the Front Range down through the San Juan Mountains and contains almost allthe gold, silver, lead and zinc deposits that fed the voracious Colorado mining industry. This period, knownas the Laramide Orogeny, lasted until about 40 MYA and was followed by another period of erosion which lowered the mountains to hills once again.
   
       Between 35 and 26 MYA, volcanoes erupted in the San Juans throwing hundreds of cubic miles of volcanic ash into the air. When it settled, the hot ash hardened to form a light colored glassy layer known as the San Juan Tuff. The Never Summer and West Elk ranges also saw volcanic activity between 27 and 21 MYA as well.
     
     Around 26 MYA, great faults creased the land, forming particularly the Rio Grande rift between the Sangre De Cristos and San Juan mountains and the upper Arkansas valley between the Sawatch and Mosquito ranges. The hills were thrust upwards over six thousand feet. Wind and water continued shaping the landscape, eroding away less resistant rock to form valleys and gorges. The final major mountain-shaping forces occurred during glacial episodes around 130,000 and 14,000 years ago. The glaciers scoured mountain valleys, carved out new ones, and left behind lakes and glacial formations like moraines and hanging valleys.

      Today, Colorado is topographically divided into three major geological zones: the Eastern Plains, the Rocky Mountains, and the Colorado Plateau. About 40% of the state isplains, 30% is mountains and 30% is plateau. The eastern plains and western plateau are primarily made up of sedimentary rock, while the rocky mountains are comprised of igneous, metamorphic, and sedimentary rock.

     The Eastern Plains rise from 3,500 feet above sea level at the eastern border to
6,000 feet at the eastern foothills of the Rockies. The plains are distinguished by two
shallow river valleys, the Arkansas and the South Platte, and by the rolling grasslands in
between.

    The Rocky Mountain zone lies in the center of the state and consists of six distinct mountain ranges (the Front Range, Wet Mountains, Sangre de Cristo, Park Range, Sawatch, San Juan) that vary from 6,000 to over 14,000 feet above sea level. Mount Elbert in the Sawatch Range is the highest mountain in the state at 14,431 feet. The RockyMountains are also distinguished by the Continental Divide, which winds its way through the mountains and separates rivers that flow down to the Pacific and Atlantic Oceans. All drainage west of the Divide flows into the Colorado River and out to the Gulf of California, with major tributaries including the Yampa, White, Gunnison, Dolores, and San Juan Rivers. East of the divide, water flows either via the South Platte and Arkansas rivers into the Gulf of Mexico or from the San Juan mountains into the Gulf of Mexico via the Rio Grande River.

     The Colorado Plateau marks the final major zone in the state and is located west
of the Rocky Mountains. These plateaus and mesas decline away from the mountains
with elevation variations between 11,000 feet down to 5,000 feet above sea level. The
major features of the region include the White River Basin, Grand Mesa, Uncompahgre
Plateau, Paradox Basin, and San Juan Basin.

Map of the major geologic regions of Colorado:

Telluride Region
    
As you stand in Telluride's box canyon and look up at the towering mountains where miners toiled for gold and other minerals, you might first want to know about all that gold – and whether or not any of it is still up there. But if you stare up at those mountains long enough, you might begin to ask some different questions about them thar hills.You may wonder, for instance, why are the peaks so jagged and why do they shoot up 4,500 feet from the valley floor like that? What's up with all that red rock, and how the heck did gold get up there, anyway?

   The Telluride region, located in the San Juan Mountain range, has been shaped over millions of years by both changes in the climate and the formation of various rock layers. Originally this region was flooded by a vast inland sea until a mountain building episode called the Laramide Orogeny began pushing up the land 70 million years ago. A period of volcanic activity followed about 5 million years later, which substantially added to the mass of these mountains."There were eruptions of volcanic activity in the area which shaped the mountains near us," explains local geologist Marcie Ryan.

      "These episodes capped the tops of the mountains with what is called the San Juan Tuff, a mixture of volcanic ash and glass shards welded together. The resulting deposit is identified by the colorful purple and green fragments cemented together." This mountain range, which is the youngest in the Rockies, looks jagged not because of these volcanic deposits but because it hasn't had as much time to erode as the others. The range went through another climactic change around 1.6 million years ago when a series of glaciers moved in, causing fundamental changes to the landscape. Ryan has documented evidence of at least 5 episodes of glaciation, while Rob Blair in The Western San Juan Mountains estimates that there could have been as many as 15 glacial advances in the last 2 million years.

     The Telluride valley shows distinct evidence of the effects of glaciation. The valley itself is a classic U-shape, indicating a glacier carved out its walls. Above the valley, Bridal Veil and Ingram basins are textbook examples of "hanging valleys" or valleys carved out by smaller glaciers that couldn't keep up with the main one. Other visible clues to glaciation are called "moraines." Society Turn is the site of a large terminal moraine."A terminal moraine is where the front of the glacier pushes debris in front of it," explains Ryan. "When the ice stops movingand starts melting, it leaves a ridge that extends across the valley. After the glacierstarted melting, the valley filled up with water. The valley floor is filled with 500 feet of lake sediment."

     Down valley from Society Turn, the profile changes from a U- to a V-shape, and according to Ryan, the land here was carved into a narrow valley by water rather than ice.On highway 145 between Placerville and Telluride, several layers of rock are visible on the hillside above. According to a chart provided by Ryan, there are 19 distinct rock layers or formations around Telluride, varying in thickness from 80 to 2,000 feet. Between mile markers 77 and 75, excellent examples of this stratification can be seen.The most visible layer in the lower canyon is the 1,150-foot thick Cutler formation, which was formed around 220 million years ago. Consisting of sandstone and shale deposited by streams, its rust-red color comes from the iron-oxide rich cement that binds the grains of sand together. Above this layer is a very distinctive and unusual black layer made of petroliferous limestone called "Pony Express." This layer is 155 million years old.

     Additional rock formations are visible around Telluride. By Society Turn, people oftenpractice climbing on an outcropping of rock that is part of the Dakota formation. Formed around 100 million years ago from sands deposited by streams, this 150-foot thick hard and tan-looking sandstone layer is the top rock surface holding up the nearby mesas.

     From the top of Lawson Hill, another outcropping is visible on the opposite side of the valley. Mancos Shale, a 2,000-foot layer of mudstone, was formed 90 million years ago of black and gray clays. This layer generally weathers easily, forms rounded slopes and, as its contents suggest, can shrink, expand, and shift horizontally or vertically depending on its exposure to moisture. The result is an unstable surface prone to movement and mudslides like the one that occurred in 1987 near the Telluride airport. The aftermath of this mudslide still is visible from the entrance to Mountain Village.

    Much higher up and more difficult to see is the 250-foot thick prominent cliff layer known asthe Telluride Conglomerate."Rock formations are named after the locality near which they were found," explains Ryan. " The Telluride Conglomerate was an old river deposit made up of older, rounded pebbles and cobbles that were lifted up and cemented together. It crops out just below the volcanic layer, and it is exposed well because the glacier eroded it."Climactic history and rock formations aside, the question still remains: how did all that gold get up there in the first place?

    "During the episode of mountain building, there was faulting and fracturing in the rock," said Ryan, who also leads mineral collecting and geology trips in the area. "Hydrothermal solutions filled the cracks and fissures and then started precipitating out minerals along with other precious metals that are soluble in hot water."Ryan goes on to describe how the miners found gold:"Sometimes it was really obvious. Other times it wasn't, and they looked for clues, such as if there was a vegetation anomaly where the vegetation looked stunted or different if it was near acidic rock. They also used geometry. If they saw a vein on one side, they would ask where it would come through on the other side. The richest mineral concentrations were where two veins intersected. A lot of times, of course, it was pure luck."

     Much easier than mining for gold ore in rock, which then had to be crushed, melted and separated, was panning for placer gold in the river. Ryan suggests that not only is there still plenty of gold in the San Miguel River, but that more gold still remains in the mountains thanwas ever taken out. Nowadays, however, it is either too dangerous or too costly to get to.

     George Cappis, a miner in the Telluride region for over 50 years, agrees with Ryan, recounting an intriguing example of how miners left gold behind."During World War II, the government gave out money to mine for lead and zinc because they wanted it for the war effort," Cappis recalls. "We were mining mostly for gold back then, and one night we were told to get allour tools and equipment out of this one tunnel. We never went back. There was still plenty of gold down there though."

    Gold wasn't the only mineral mined out of these mountains. Ryan lists the other major economic mineral deposits as copper, silver, lead, and zinc. And what about Telluridium, the ore that supposedly gave the town its name?Tellurium combines with other metals to form Telluride ore (Telluridium)," Ryan explains. "To be honest, there's not a lot of it here. Maybe they just liked the name."

(reprinted from an article by Allison Johnson in the 1998 Telluride Times Journal
Summer Examiner). Information for this article was taken from an interview with Marcie
Ryan, columns Ryan published in The Norwood Post, and from The Western San Juan
Mountains, edited by Rob Blair. For information on geology or mineral collecting trips,
call Ryan at (970) 728-3391.


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