Throughout time, people have been drawn to mountains for inspiration, recreation, and scientific exploration. Mountains are also vast warehouses of natural resources and libraries of geologic history.

Mountains form in response to the dynamic forces of our planet. The life spans of mountain belts, from initial uplifts to erosion to base levels, run from tens of millions to hundreds of millions of years. Ancient and active mountain belts are part of the fabric of continental crust and provide clues to events that build and reshape continents. The concept of plate tectonics (Condie 1989) provides a framework within which to investigate...

Andesitic to rhyolitic volcanic rocks of the San Juan Mountains, along with associated epithermal ores, were studied intensively during the twentieth century (e.g., Larsen and Cross 1956), culminating with detailed study of the Creede Mining District by Steven and Ratté (1965) as well as regional field and volcanological studies (Lipman, Steven, and Mehnert 1970; Steven and Lipman 1976; Steven et al. 1974). While much had previously been learned about the evolution of several complex caldera clusters from which at least twenty-two major ignimbrite (ash-flow) sheets (each 150–5,000 km³) were erupted at 30–26 Ma, research in support of the...

The eastern San Juan Mountains were less intensely mineralized than the western San Juans; in addition, the styles of mineralization were less diverse and the time interval of mineralization was shorter. Nevertheless, mines in the eastern San Juans produced more than $375 million of mineral wealth (more than a billion dollars at 2007 metal prices). The two most important mining districts, Creede and Summitville, have been the subjects of extensive scientific study and stand as classic examples, or archetypes, of their styles of mineralization.

All of the important mineralization in the eastern San Juans occurred during the caldera cycle of...

The San Juan Mountains encompass 6,000–7,000 mi² (15,000–18,000 km²) and thirteen peaks that rise above 14,000 feet (4,267 m). Some of these “fourteeners” are flanked by valley floors as low as 7,000 feet (2,133 m). The surface of the eastern San Juans is dominated by Tertiary volcaniclastic breccias and ignimbrites (ash-flow tuffs) derived from more than two dozen volcanic centers. The volcanic pile is hundreds to several thousand meters thick. At the surface, the western San Juans are composed mostly of resistant Precambrian metamorphic and plutonic rocks, Paleozoic through Cenozoic sedimentary layers, and smaller areas of Tertiary volcanic...

Knowledge of the occurrence, storage, and flow of groundwater in mountainous regions is limited by the lack of integrated data from wells, streams, springs, and climate. In his comprehensive treatment of the hydrogeology of the San Luis Valley, Huntley (1979) hypothesized that the underlying, fractured volcanic bedrock of the San Juan Mountains has relatively high bulk permeability and a regional-scale water table with a low hydraulic gradient. Other (some more recent) studies of fractured crystalline bedrock in mountainous terrain indicate that these rock units can act as aquifers (Kahn et al. 2008; Manning and Caine 2007; Robinson 1978; Stober and...

Temperatures in the San Juan Mountains are highly variable in both time and space. During a single day, temperatures in the high mountains can vary from well below freezing to above 15°C (60°F). Large extremes can also occur in the lower-elevation, more arid regions of southwestern Colorado. At a single instant in time, temperatures can range from well below freezing at the region’s highest elevations to well above freezing at the lower elevations. There are also significant seasonal changes, with extremes over 38°C (100°F) in summer and as low as –46°C (–50°F) in winter. Because of the complex terrain,...

Ecological processes and species occurrences within most ecosystems in the semiarid western United States are limited, in some way or another, by moisture. Aquatic ecosystems are obvious exceptions. The Western San Juan Mountains focused on flowing (or lotic) water—rivers, streams, and their associated riparian wetlands. In this volume, we devote a chapter to “still” (or lentic) water and itsecosystems, specifically lakes and reservoirs, and a second chapter to fen wetlands. This chapter investigates the physical, chemical, and biological nature of San Juan lakes and reservoirs and discuss human impacts on these ecosystems.

Mountain lakes and reservoirs are valued for...

Types of mountain wetlands include marshes, riparian areas, wet meadows, fens, bogs, and salt marshes (Carsey et al. 2002). However, fens are the dominant wetland type above 9,000 feet (2,700 m) in the San Juan Mountains (Chimner, Cooper, and Lemly 2010). The fen is a distinctive wetland type that accumulates organic soil, or “peat,” and can be termed a peatland. Peatlands are the most widespread wetland type in the world, occurring from the tropics to the arctic and covering an estimated area of 1.5 million mi² (4 million km²), or 3 percent of the Earth’s land surface (Maltby and Proctor...

The kingdom Eumycota is composed of fungi—heterotrophic, terrestrial organisms that are evolutionarily more closely related to the animal kingdom than to any of the other life forms on Earth. Fungi often appear similar to plants, but they are not photosynthetic; instead, they must obtain nourishment from organic substrates. Fungi enter into a wide variety of interactions with other organisms and with their waste products. Fungi may decompose animal or plant material by being saprophytes (living on dead materials), or they may be parasites (subsisting on living hosts and causing the hosts to suffer while the fungi benefit). Plants, with...

While driving from Pagosa Springs to South Fork over Wolf Creek Pass, we cannot help but notice changes in the vegetation that blankets the spectacular mountain scenery. The vegetation in the San Juan Mountains is dominated by different forest types, assemblages of species whose distributions are controlled by biological and physical factors (Spencer and Romme 1996). One factor rarely limits the distribution of a species; more often, an aggregate effect of many interacting factors—such as elevation, soil type, temperature, and precipitation—sets distribution limits. Natural disturbances—such as fire, insect outbreaks, wind, and avalanches—are the main influences that...

Hidden beneath the austere beauty of the rock, water, soil, trees, and plants of the San Juan Mountains and environs lies a complex community of insects whose lives have shaped the habitats of this architecturally diverse ecosystem. Insects and their life cycles, feeding habits, and life-history strategies provide the underlying frameworks of all terrestrial and freshwater communities (Johnson and Triplehorn 2004). Insects are adapted for survival in the harsh terrestrial environments of the San Juans, where cold temperatures, rarified atmosphere, low water availability, and increased ultraviolet radiation deter most animals.

High elevations of the San Juans do not deter the...

This chapter describes a few species of wild animals that occur in southwestern Colorado. What makes these species’ existence and natural histories in the San Juan Mountains noteworthy when any of hundreds of other species and their unique stories could have been chosen? These selected species are easier to portray than others because much has been documented about their life histories and interactions with humans. The puma has had a long antagonistic relationship with European settlers, elk and mountain sheep “fueled” the settlement of the region, and lynx and cutthroat trout have become the foci of recent reintroduction efforts.

Although...

The eastern San Juan Mountains include some of the most rugged terrain in the United States. Within these “Shining Mountains” lie Ute vision-quest sites, old trails and mines, stock driveways, and so few roads that, in 1975, Congress established the Weminuche Wilderness (in the San Juan and Rio Grande National Forests)—the largest in Colorado, with over 500,000 acres (200,000 hectares [ha]). This wilderness is home to the last authenticated grizzly bear trapped and killed in Colorado, in 1952. The bear was trapped close to the headwaters of the Rio Grande near a mountain named the Rio Grande Pyramid, not...

John Charles Frémont’s fourth expedition into La Garita Mountains at the eastern fringe of Colorado’s San Juan Mountain Range (1848–1849) bears all the intrigue found in classic Greek drama. The hero, Frémont, had become a popular champion among his countrymen. Dubbed the “Pathfinder” by contemporaries burning with the fever of Manifest Destiny, Frémont’s adventures fired the imaginations of young and old. His writings brought the worlds of science and exploration into the homes of average Americans and inspired dreams of venturing west to see the “Shining Mountains.” Among those not prone to read scientific jargon, dime novels—the literary...

“The difficult problem of our prosperity will then be solved, and we will then have to thank the enterprise and pluck of the much-maligned narrow gauge,” asserted Silverton’s San Juan Herald (April 27, 1882). The long-awaited arrival of the Denver and Rio Grande Western Railroad seemed only a short time away. Silverton could hardly wait for the day to arrive.

On Tuesday evening, June 27, excited townspeople distinctly heard the first engine whistle. Then it arrived. On July 13, the Herald proudly announced that the bridge across Mineral Creek, the last obstacle, was nearly completed. Said the editor, “We will...

This road log is meant for the visitor who wishes to further explore the geology, ecology, and human history, as seen from the highways that traverse and circumnavigate the eastern San Juan Mountains. This guide is designed so the traveler can enter a given highway at any point and travel in either direction. On the twenty relief maps herein, the points of interest (POI) are identified as black-filled boxes with numbers that correspond to write-ups in the text. The shaded relief maps come from the US Geological Survey website http://seamless.usgs.gov. Along each highway, green milepost (mp) signs will be...