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).