“We ascended a high hill which lay south of our camp, from whence we had a view of all the prairie and rivers to the north of us. It was at the same time one of the most beautiful and sublime inland prospects ever presented to the eyes of man…. The main river, bursting out of the western mountains and meeting from the northeast a large branch which divides the chain of mountains, proceeds down the prairie, making many large and beautiful islands, one of which I judged contains 100,000 acres of land, all meadow ground, covered with innumerable herds of deer…. The great and lofty mountains, covered with eternal snows, seemed to surround the luxuriant vale, crowned with perennial flowers, like a terrestrial paradise, shut off from the view of man”. – Lieutenant Zebulon Montgomery Pike, 1807
The climate of the Sangre de Cristo National Heritage Area varies widely, depending on elevation. The San Luis Valley floor has a mean annual precipitation of just 7.5 inches . Much higher precipitation (with mean annual precipitation over 40 inches) is found in the highest mountain ranges. More than 67 percent of the annual precipitation occurs as scattered showers in spring and thunderstorms in summer. August is the wettest month with 50 percent of the total summer precipitation .
The average annual temperature is about 58 degrees Fahrenheit; summers and autumns are usually temperate. The average summer temperatures reach a high of 82 in July with average lows reaching the low 40s. Average winter temperatures dip below zero in December and January with average highs reaching into the low 30s.
The underlying geology of the San Luis Valley and associated mountain ranges provides both the literal and figurative foundation for understanding the landscape as we see it today. This history is a complex and fascinating one involving tectonic events, mountain uplifts, volcanism, glaciers, and erosion. The Sangre de Cristo and the San Juan mountain ranges illustrate two different types of geologic activity. Whereas the formation of the Sangre de Cristo Mountains was primarily influenced by faulting and lifting, the San Juan Mountains were mostly formed from volcanic activity. The resulting landscape and geologic formations narrate not only the physical history of the valley, but also tell the story of various cultures that made use of the resources it left behind.
The modern geologic history of the valley dates back to 65 million years ago when the great inland seas disappeared from the Rocky Mountains, leaving behind sedimentary rock (shale, limestone, sandstone, and siltstone) formed from marine deposits several hundred to several thousand feet thick in some places. A period of intense tectonic activity, known as the Laramide Orogeny, followed and produced mountain uplifts and corresponding basins from Montana to northern Mexico along the entire Rocky Mountain chain.
In southern Colorado, the Laramide uplift rose vertically as narrow, mostly north-south anticlines (or upfolds) and faults, which elevated the once flat-lying layers of sedimentary rock like rippled blankets. This action spawned the Sangre de Cristo Range. The synclines (or downfolds) formed the valleys and were rapidly filled with sediments eroding from the rising folds.
The Laramide uplift was followed by violent volcanic eruptions that occurred repeatedly throughout the Southern Rockies between 40 and 25 million years ago. Widely scattered volcanoes spewed lava and ash over hundreds of miles, with volcanic centers eventually coalescing to form a giant volcanic field that covered large portions of the southern and central Southern Rockies. The San Luis Valley is part of the Oligocene volcanic field, which covered all of south-central Colorado and adjacent New Mexico.
This volcanic activity brought to the surface an abundance of mineral-rich solutions from magma sources deep within the earth. This action resulted in the formation of the Colorado Mineral Belt, a narrow band of rocks containing rich deposits of gold, silver, and other precious metals stretching from Boulder southwest to Durango. While the San Luis Valley lies largely east of the core Mineral Belt, portions of Conejos County and the San Luis Mountains contain significant ore deposits. Platoro in Conejos County, for instance, was established in the 1870s as a mining camp when gold and silver were discovered around the nearby headwaters of the Conejos River. The community derived its name from the Spanish words for silver and gold (plata and oro).
Around 26 million years ago another uplift, known as the Miocene-Pliocene Uplift, triggered widespread faulting (shearing). The most important feature to form during this period, as it pertains to the formation of the San Luis Valley, was the Rio Grande Rift, where edges of the earth’s crust pulled away from each other along fault lines that ran through the valley. The San Luis Valley represents the deepest and broadest portion of the Rio Grande Rift, which extends from central Colorado southward to Chihuahua, Mexico.
Occurring simultaneously with the rift was another intense period of volcanic activity, which spawned the formation of several of the major landforms that now define the valley: the San Juan Mountains, the San Luis Hills, the Taos Plateau, and the San Pedro Mesa. These volcanoes produced lavas with a high proportion of quartz and feldspar, known as andesite, which underlies much of the Rio Grande National Forest in western Conejos County. Along the eastern side of the range between Capulin and Fox Creek, the mountains here are composed primarily of basalt (formed by lava) and ash-flow tuff (formed by ash), whereas the western edge of the county has a higher abundance of andesite.
The San Luis Hills and the San Pedro Mesa rise conspicuously 500 to 1,000 feet above the valley floor. The San Luis Hills actually comprise a chain of hills arcing northeast-southwest. They include the Brownie Hills, Fairy Hills, Piñon Hills, and South Piñon Hills. Flat Top rises the highest at 9,211 feet. The San Pedro Mesa, a separate geologic feature, extends north-south for about 15 miles east of San Luis from the Rio Costilla to the Rio Culebra. It rises to more than 8,800 feet and forms a prominent table-land, particularly when viewed from the west.
Basalt flows also formed the Taos Plateau in northern New Mexico, which is found south of the San Luis Hills and west of the Rio Grande. The Taos Plateau generally marks the southern boundary of the heritage area and extends for approximately 60 miles southward.
In areas along the eastern side of the valley, lava intrusions traveled upward along fault lines. For instance, the core of highest peak within the Sangre de Cristo range, Blanca Peak, is composed of gabbro – another form of volcanic rock. Its properties are different from basalt as it cooled below the earth’s surface rather than above it.
This additional volcanic activity also caused gold deposits to be emplaced along the fault lines, forming linear belts of precious metals that run the entire length of Costilla County, from the Battle Mountain Gold Mine just northeast of San Luis to the Independent Gold Mine in Saguache County. Some linear belts of gold also occur in Conejos County although they have not been developed.
Turquoise, which is produced by heating of the copper minerals in basalt, is found in several areas throughout the region. A historic turquoise mine exists near Manassa in Conejos County, which was known historically as the Lickskillet Turquoise Mine. On the west side of the valley, these faults have deposited belts of onyx, which were utilized in the Lime Kiln Creek area for the production of quick lime. Other types of minerals, including opal and agate, are formed from volcanic ash whose silica has been dissolved by water. One of the patterns in the agate found within the San Luis Valley (just northwest of the heritage are) is so distinctive that it is called Del Norte agate, named for the town of Del Norte in Rio Grande County.
Between the Sangre de Cristo Mountain Range and San Pedro Mesa lies the Culebra Re-entrant. The Culebra Reentrant, which occupies the curve of the mountain range east of San Luis between Blanca Peak and the New Mexico border, is defined by the gentler rise of the valley floor and the long spurs forested with piñon and juniper that merge into the foothills of the Sangre de Cristo range. Underlain by siltstone, this geologic feature is about 40 miles long.
Between the San Pedro Mesa and the San Luis Hills lies the Costilla Plains. This nearly level and featureless area extends southward from Blanca Peak through the border with New Mexico and continues southward to Taos. It is primarily underlain by gravel.
The Alamosa Basin occupies the northern and west-central parts of the San Luis Valley north of the San Luis Hills. A closed basin, it slopes gently toward the east from the Rio Grande’s alluvial fan. Recent research hypothesizes that about 3 to 3.5 million years ago, this basin was filled by an enormous high-altitude lake. Named Lake Alamosa, it is believed to have been one of the largest high-altitude lakes in North America, persisting for about 3 million years. During this time, it expanded and contracted, filling the valley with sediment until about 440,000 years ago when it is believed to have spilled out over the San Luis Hills, cut a deep gorge, flowed into the Rio Grande, and eventually receded due to climate change.
During the time that Lake Alamosa was in existence approximately 2 million years ago, the Ice Age caused extreme climatic fluctuations that resulted in the growth and retreat of enormous ice sheets and valley glaciers. In the Southern Rockies region, alpine glaciers formed in all major ranges, reaching as far south as south-central New Mexico. As glaciers moved down the mountains they stopped at just about the valley floor, leaving behind end-moraines such as the one seen at Zapata Falls. When the glaciers melted, they released large volumes of muddy water and a thick layer of mud formed at the bottom of the valley floor. Gravel and sand deposits from the surrounding mountain streams also flowed into the valley and settled in deposits that are estimated to be 4,000 to 7,000 feet thick in some places. This layering of eroded sediments (clay, silt, sand, and gravel) is what provides the basis for the large underground aquifers that exist today under the valley floor.
The enormous sand dunes of the Great Sand Dunes National Park and Preserve are believed to be derived from the sand deposits left over from Lake Alamosa. This sand blows with the predominant southwest winds toward a low curve in the Sangre de Cristo Mountains where it accumulates in a natural pocket. The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley. These opposing wind directions cause the dunes to grow vertically (NPS, Great Sand Dunes).
Remnants of smaller lakes that persisted after Lake Alamosa receded are still found today, in the form of sabkha wetlands. The sabkha forms where sand is seasonally saturated by rising groundwater. When the water evaporates away in late summer, minerals similar to baking soda cement sand grains together into a hard, white crust. Areas of sabkha can be found throughout western portions of the sand sheet, wherever the water table meets the surface (NPS, Great Sand Dunes).
Soils and Land Use
Mountain soils range from gently sloping to very steep. These soils are deep, highly porous, sandy and are underlain by gravelly subsoils. In some cases, there are rocky outcrops. The prevailing soils in the San Juan Mountains are derived mainly from weathering and erosion of volcanic rocks. Among others, these include the Seitz, Frisco, Granile, and Bendire complexes. These soils are used principally for livestock grazing and timber production. The most common native vegetation consists of western wheatgrass, blue grama, piñon and ponderosa pine, juniper, oak, and blue spruce with an understory of sideoats grama, and mountain muhly. While the foot slopes can be cultivated with irrigation, they are leachy and do not retain moisture.
In the Sangre de Cristo Mountains, the parent material is primarily outwash derived from granite, gneiss, mica schist, and sedimentary rock. Common soils include the Teewinot, Leadville, Stunner, Uracca, and Lakehelen complexes, which range from deep and well drained stony and sandy loams to bedrock outcrops in the higher elevations. Common uses include livestock grazing, timber production, and natural pastureland.
Valley soils are primarily classified as San Luis, Gunbarrel, Mosca, Hooper, Alamosa, and Travelers soil complexes. These are deep alluviums typically composed of sandy loams, loams, or clay loams underlain by gravelly subsoils that formed from igneous and metamorphic rock. Compared to the mountain soils, these are darker in color, and have a heavier texture and more compact structure.
Generally speaking, these soils are poorly drained, typically alkaline, very low in organic matter, and subject to waterlogging. In some cases, depth to water table ranges from 12 to 40 inches. These soils occur along the tributary stream bottoms and alluvial fans, and over extensive areas of the valley floor. They represent the most common soil types of the southern and western parts of the larger San Luis Valley. These soils are well adapted to grains, alfalfa, grasses, field peas, and vegetables, including the root crops. Principal native plants associated with these soil types include saltgrass, alkali sacaton, rabbitbrush, and greasewood, as well as sedges and rushes in the wetland and riparian areas. Ecoregions contain alluvial valleys, volcanic plateaus, alpine and subalpine forested mountains, shrubland-covered hills, sand dunes, sand sheets, salt flats, wetlands, and a variety of aquatic habitats. The ecological diversity of the Sangre de Cristo National Heritage Area is enormous. Its landscape comprises 12 of Colorado’s 35 Level IV Ecological Regions, the most detailed categorization of ecological regions defined by the US Environmental Protection Agency. Each ecoregion represents an area that is similar in geology, physiography, vegetation, climate, soils, land use, wildlife, and hydrology. They are designed to serve as a spatial framework for the research, assessment, management, and monitoring of ecosystems and ecosystem components, especially across federal agencies, state agencies, and nongovernmental organizations that are responsible for different types of resources within the same geographical areas.
Ecoregions within the Sangre de Cristo and San Luis mountain zones are principally determined by geology and elevation. They include the Alpine Zone, Crystalline Mid-Elevation and Subalpine Forests, Sedimentary Mid Elevation and Subalpine Forests, Volcanic Mid-Elevation and Subalpine Forests, and Foothill Shrublands.
The Alpine Zone makes up a small percentage of the heritage area. Found along the highest peaks of the Sangre de Cristo and San Luis mountain ranges, these are treeless glaciated areas with steep slopes and exposed rocky peaks that rise above the timberline at an elevation of 10,500 to 11,000 feet. The amount of precipitation received within this zone is the highest within the heritage area – between 35 and 70 inches per year – and its snowmelt serves as a water source to the lower elevations. Its principal land cover includes snowpack, ice, bare rock, and alpine meadows containing bistort, alpine timothy, alpine avens, alpine bluegrass, alpine clover, tufted hairgrass, and various sedges. The forest-tundra interface is sparsely colonized by stunted Englemann spruce, subalpine fir, and limber pine (krummholz vegetation). Some of the oldest recorded trees in North America, the Rocky Mountain bristlecone pine, can also found here.
The Crystalline and Sedimentary Subalpine Forests occupy most of the Sangre de Cristo Mountain Range within the heritage area, whereas the Volcanic Subalpine Forest occupies most of the San Luis Mountains. Distinguished by their bedrock, these are high mountain and steeply sloped, glaciated zones that range between 9,000 and 12,000 feet in elevation. Found below the Alpine Zone, they receive slightly less precipitation – between 28 and 50 inches per year (the Crystalline Subalpine Forest receiving slightly more), which persists as deep winter snowpack. Forests within these zones are dominated by Engelmann spruce and subalpine fir that are often interspersed with aspen groves, lodgepole pine, mountain meadows, and with Douglas fir at lower elevations. The Crystalline Subalpine Forest understory is dominated by dwarf huckleberry and grouse whortleberry, whereas the Sedimentary and Volcanic Subalpine Forests contain more kinnickinnick, snowberry, sedges, mountain brome, and forbs. Perennial streams are also found in this zone.
The Crystalline, Sedimentary, and Volcanic Mid-Elevation Forests are partially glaciated. The Crystalline and Sedimentary Mid-Elevation Forests occupy only a small portion of the heritage area east of Garland City in Costilla County, whereas the Volcanic Mid-Elevation Forest comprises a small zone west of Fox Creek and Centro in Conejos County. These ecoregions occupy an elevation ranging between 7,000 and 9,000 feet. Their forests are characterized by low mountain ridges, slopes, and outwash fans that receive between 20 and 32 inches of precipitation per year. The Crystalline Mid-Elevation Forest vegetation consists primarily of Ponderosa pine with areas of Douglas fir, and an understory of mountain mahogany, bitterbrush, wax currant, skunkbush, woods rose, mountain muhly, Junegrass, Arizona fescue, king spike-fescue, and various sedges.
The Sedimentary Mid-Elevation Forests contain more Gambel oak woodland, aspen forest, and two-needle piñon pine, as well as antelope bitterbrush, fringed sage, serviceberry, and snowberry. Volcanic Mid-Elevation Forests differ in that their understories contain more dwarf juniper, western wheatgrass, Oregon grape, blue grama, sideoats grama, and needlegrasses.
The Foothill Shrublands comprise a narrow zone at the foothills of both the San Luis and Sangre de Cristo mountain ranges. Mostly occupying an elevation range of 6,000 to 8,500 feet, they can extend up to 10,000 feet in small areas. This ecoregion is unglaciated and contains perennial as well as intermittent and ephemeral streams.
Receiving an average annual precipitation of 12 to 20 inches, it consists of mostly sagebrush, as well as some areas of piñonjuniper woodland that are interspersed with mountain mahogany shrubland, Gambel oak, mountain big sagebrush, skunkbush, serviceberry, fringed sage, and rabbitbrush, as well as such grasses as blue grama, Junegrass, western wheatgrass, Indian ricegrass, Scribner needlegrass, and muttongrass.
Ecoregions within the valley area include Salt Flats, Sand Dunes and Sand Sheets, San Luis Alluvial Flats and Wetlands, and San Luis Shrublands and Hills.
The San Luis Shrublands and Hills are found throughout much of the southern portion of the heritage area and encompass the San Luis Hills, Taos Plateau, and the lower foothills of both mountain ranges. Ranging between 7,900 to 9,100 feet in elevation, this ecoregion’s average annual precipitation averages between 10 and 14 inches per year. The lands are primarily used for rangeland and contain shrublands, grasslands, and piñon-juniper woodlands at their highest elevations. Species include big sagebrush, rubber rabbitbrush, winterfat, western wheatgrass, green needlegrass, blue grama, and needle-and-thread grass.
The San Luis Alluvial Flats and Wetlands ecoregion covers extensive areas of the San Luis Valley. In Alamosa County it extends along most of its western border and as far south as Antonito in Conejos County. Another large area is found in a stretch along Route 159A extending south from Blanca to the state line, and another extending southwest from San Luis to the Rio Grande. As its name suggests, it is a relatively flat area containing wetlands, springs, and areas with a high water table. It also hosts several large perennial streams that originate in the mountains. Ranging from 7,500 to 8,000 feet in elevation, it receives only 6 to 10 inches of precipitation per year. This ecoregion generally corresponds with irrigated cropland, which has replaced most of the natural vegetation (shadscale, fourwing saltbush, and greasewood). The most common crops include potatoes, alfalfa, barley, hay, and wheat, as well as small areas of vegetables such as lettuce, spinach, and carrots.
The Salt Flats comprise some of the lowest lying areas of the heritage area. They extend from the north boundary of the heritage area southward to the vicinity of La Sauses, making up approximately half of Alamosa County. This ecoregion receives only 6 to 8 inches of precipitation per year. Unlike the Alluvial Flats and Wetland ecoregion, however, most of these lands are not irrigated and remain in shrubland that are adapted to the alkaline soils (shadscale, fourwing saltbush, greasewood, horsebrush, spiny hopsage, rubber rabbitbrush, saltgrass, and alkali sacaton). Much of this region is used as low-density pastureland.
The Sand Dunes and Sand Sheets are located in and around the Great Sand Dunes National Park. This ecoregion is characterized by the dunes themselves, as well as the sandy grasslands that extend around three sides of the main dunefield, also known as the sand sheet. Almost 90 percent of the sand deposit is found in the sand sheet, while only about 10 percent is found in the main dunefield. The sand sheet is the primary source of sand for the Great Sand Dunes. Small dunes form here and then migrate into the main dunefield (NPS, Great Sand Dunes).
Comprising a unique ecosystem, this area has outstanding biodiversity significance. While the dune areas are mostly devoid of vegetation, some Indian ricegrass, blowout grass, and lemon scurfpea can be found here. The sand sheet plant communities are characterized by rabbitbrush, needle-and-thread grass, and rice grass, while scurf pea, skeleton weed, and blowout grass characterize the shifting sand component. Some of the sand sheet is used as native pastureland for bison and cattle.