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Trans Pecos Main
Prehistoric Texas Main

Natural Setting: A Rocky Oasis in the Desert

The abundance and variety of resources at Hueco Tanks have attracted people to this desert setting throughout prehistoric and historic times. Within and at the base of the huge rock outcroppings—actually, four hills of stone—are special habitats, or econiches, where trees, plants, and animals normally found in more moist environs or at higher elevations, thrive. A variety of conditions foster this environmental oddity, but rocks and water are the key elements.

The rocks of Hueco Tanks were born millions of years ago as an underground mass of hot magma. The molten rock pushed upward and cooled beneath a layer of limestone. Over millions of years, wind and water wore away the limestone, exposing and sculpting the igneous rock to form the shapes we see today. Hollows in the rock (huecos) capture precious water during rainy periods—particularly the monsoons which typically occur in the late summer months. Rock crevices, seasonal ponds, wetlands, and man-made dams or catchments—some deriving from prehistoric times—store water as well. These " tanks" of water, so rare in the desert, have made Hueco Tanks a stopping point and haven for people and wildlife for thousands of years.

Hueco Tanks State Park and Historic Site is located in the southeast part of the Basin and Range physiographic province which is characterized by isolated, nearly parallel mountain ranges separated by broad flat basins, or bolsóns, as the Spanish called them, meaning large purse. The site is located near the north end of the Hueco Bolson, a basin that extends along the Rio Grande for about 130 miles from southeast to northwest.

The northern Hueco Bolson is flanked on the east by the Hueco Mountains, which actually are the dissected west edge of the Diablo Plateau. The main escarpment lies about 1 mile east of Hueco Tanks, and outlying foothills are located 2 to 3 miles to the northwest, west, and southwest of the state historic site. The west margin of the northern Hueco Bolson is defined by the Franklin Mountains, located around 24 miles west of Hueco Tanks.

Hueco Tanks State Park and Historic Site is situated on the sloping surface of accumulated sediments that have eroded from the west flank of the Hueco Mountains. Four rock masses—North Mountain, East Mountain, East Spur, and West Mountain—are the dominant topographic features in the park. Sheltered between the hills are basins on the south and west. Deep canyons cut into the northwest side of East Mountain (Mescalero Canyon) and the west side of North Mountain. The fractured and eroded rock hills also contain many smaller cracks, overhangs, and natural hollows, or huecos, for which this landmark is named.

Geology and Soils

Hueco Tanks is a laccolith-like (domed) igneous intrusion composed of porphyritic syenite, and was formed 35 million years ago. Between 30 and 24 million years ago, faulting formed the basin and range topography that characterizes the region today. Downthrown fault blocks underlie basins like the Hueco Bolson and are adjoined by uplifted fault blocks that make up ranges like the Hueco and Franklin Mountains. Over the last 25 million years, sediments were eroded from these mountain ranges and deposited in the Hueco Bolson. Known as the Santa Fe Group, they consist of colluvial, alluvial fan, water-laid, and lake deposits.

Recent deposits on the Hueco Bolson and Tularosa Basin are designated as Quaternary sedimentary units Q1 through Q4. The two deepest deposits—Q1 and Q2—pre-date human existence in the region. Archeological materials have been recovered from the upper part of Q3 and the base of Q4. The windblown sand that composes Q3 was laid down around 7,000 years ago; deposition ended around 1,100 years ago and the surface became stable. Archeological sites and features that are 1,000 years old or older may be buried in Q3 deposits but all materials younger than that age sit on or near the surface of the Q3 sands. For this reason, many of the archeological sites in the region are visible on the ground surface. Q4 consists of dune sand loosened when grasses were removed by overgrazing around 100 years ago and redeposited by wind.

In the Hueco Bolson, materials for making chipped stone tools, grinding tools, and pottery are available from a diverse array of sources. Rocks suitable for chipping can be found in bedrock formations, in alluvial fans, along arroyos and streams, and in basins. Chert is the most commonly available chippable rock near Hueco Tanks. Chert outcrops within 5 miles of the Tanks reportedly show evidence of prehistoric use as quarries. Rock for making chipped stone tools also is available from relatively distant primary sources such as in the foothills of the southern Sacramento Mountains. The Franklin Mountains 24 miles to the west contain chert as well as quartzite and rhyolite. Soledad rhyolite is available in the southern Organ Mountains. Obsidian is the only commonly chipped stone that does not occur in the rock formations of the Hueco Bolson region. The closest major obsidian outcrops are located in north-central New Mexico on Mount Taylor and Mount San Antonio, and in the Jemez Mountains.

Secondary sources of chippable stone in the Hueco Bolson are more widely available and include gravelly slopes, alluvial fans, arroyos, gravel bars on streams, and erosional basins. Present-day and ancient gravel deposits of the Rio Grande constitute major secondary sources. Where the original Rio Grande channel ran through the Hueco Bolson, gravel deposits of the Camp Rice Formation contain locally-derived rhyolite, chert, and quartzite, as well as obsidian and other rocks that originate in the upper Rio Grande drainage basin. These gravels are exposed along limited areas of arroyo walls, fault scarps, and eroded slopes. Gravels also are exposed on the basin floor in blowouts and other eroded areas, but are predominantly small.

Rock for making ground stone tools can be obtained from many geologic formations in the Hueco Bolson. Most accessible to the park is porphyritic syenite, which makes up the rock hills of Hueco Tanks. Limestone is available from the Hueco Mountains and outlying foothills, located 1 to 3 miles from the Tanks. Sandstone can be obtained from the Bliss Formation in the Hueco Mountains; the closest outcrop is around 12 miles south of Hueco Tanks. Granite is available from isolated outcrops that adjoin the south end of the Hueco Mountain range, ca. 18 miles south of Hueco Tanks.

Clay suitable for pottery making is available in a variety of settings across the Hueco Bolson, including playas, terraces of the modern and ancient Rio Grande, and distal alluvial fans. Although a number of clay samples have been submitted for chemical analysis, few have been directly matched to prehistoric pottery. The array of elements in the prehistoric ceramics of the Hueco Bolson suggests that they were made of clays derived from igneous rocks, however. Preliminary chemical analyses have indicated that clay from the igneous intrusion named Cerro Alto (located 4 miles northeast of Hueco Tanks) was used in ceramics on sites in the eastern Hueco Bolson, but this remains to be substantiated.

The soils in Hueco Tanks State Park and Historic Site are composed of porphyritic syenite, quartzose sand, and sediment derived from limestone. Mapped associations are Pajarito, Simona, Wink, and Mimbres; some nearby areas are identified to the Agustin association. These soils developed on alluvial fans and are friable, calcareous, and moderately alkaline. Mimbres association deep silt loam has developed in shallow watercourses and playas, and is mapped in three areas of the park: along the south boundary fence, on the east side of North Mountain, and on the northeast side of East Mountain. For desert farmers some 850 years ago, Mimbres soil, with its higher capacity for moisture retention, was critical to growing corn, beans, and other crops.

Water and Its Surface Distribution

Natural water sources in Hueco Tanks State Park and Historic Site consist of alluvial fan drainages, marshy areas where water accumulates, runoff from the rock hills, and natural rock reservoirs. Additional water sources that have been created by humans are discussed in the section, Weaving the Story.

Runoff water from the west flank of the Hueco Mountains is delivered to Hueco Tanks by a network of channels that drain the alluvial fan. The Tanks are positioned between large arroyos to the north and south, which carry runoff from major canyons in the range. A smaller stream once flowed between North and East Mountains, as indicated by freshwater snail shells recovered at depth in the present-day arroyo. Channels on alluvial fans carry runoff for some time after rains; moisture is retained in soils near them and in areas where the water ponds. Drainages originating on the west side of the Hueco Mountains flow toward the center of the Hueco Bolson, where the water eventually is lost to evaporation.


Marshy areas or cienegas were once present here, based on plant and mollusk remains found in those areas. The largest cienegas probably coincided with areas now identified to the relatively clayey Mimbres soil association; they still hold water after heavy rains, despite alterations from modern land use. Smaller seasonal wetlands are scattered through the rock hills, indicated by patches of plants growing in moist soils. Typically situated at drainage heads, many of these areas are currently dry as a result of land use, but would have been seasonally wet in prehistoric times.

The porphyritic syenite hills that comprise Hueco Tanks collect rainwater in cracks. Carried downward by gravity, the water carves shallow channels into the rocks, marked by dark stains. When channeled water reaches the base of the rock hills it creates patches of moist soil and small ponds that support mature trees and plants —water-lovers such as willow, cottonwoods, and ferns. During the rainy season the channels carry flows ranging from rivulets to waterfalls. Similar channels enclosed in the rock hills are evidenced by audible trickling and dripping after rains.


The myriad natural reservoirs for which Hueco Tanks is named are formed when rainfall and runoff are captured in cracks and hollows (huecos in Spanish) in the rock. Some of these features are open to the air, while others are sheltered by rocks. They hold water for periods ranging from several days to several months, depending on their size, depth, and exposure to evaporation. One notable group of huecos is called Honeycomb Pond, located on an outlying rock mass between East Mountain, East Spur, and West Mountain. Another large hueco may be represented by Laguna Prieta, a tank that was built to enclose what may have been a natural pond in a perched canyon on the west side of North Mountain.

Vegetation

The vegetation of the Hueco Bolson is classified generally as Chihuahuan desertscrub. Distribution of modern plant communities in this region is controlled by moisture, temperature, and elevation: desertscrub grows on the basin floor, grasslands on intermediate surfaces, and woodland communities at higher elevations. Hueco Tanks State Park and Historic Site is on an intermediate surface that is covered by desertscrub and degraded desert grassland, but because its igneous-derived soils hold water more effectively than soils in the surrounding area, the plant community in the state historic site is diverse and includes a number of woody and water-dependent species typical of desert, mountain, aquatic, and grassland habitats.

The level terrain surrounding the igneous outcrops is dominated by a desertscrub/grassland community including creosotebush, honey mesquite, tarbush, soaptree yucca, ocotillo, lechuguilla, sotol, prickly pear, and other cacti. Grama grasses, goosefoot, and amaranth grow near the rocks, while fourwing saltbush and threeawn and other grasses are widespread on the flats. Many of these plants would have been useful to native peoples.

Trees are concentrated along the base of the rock hills where runoff forms small water catchments, and in narrow canyons that provide greater moisture due to increased shade and lower evaporation. They include netleaf hackberry, Texas mulberry, Mexican buckeye, Arizona white oak, and rose-fruited juniper. The juniper, oak, and hackberry are relict components of the woodland that covered Hueco Tanks and similar locations in the late Pleistocene and early Holocene. Water-dependent plants are found in moist soils near dams and seeps, and in shaded fissures. These include leafy pondweed, hairy pepperwort, slender rush, Rio Grande cottonwood, and Goodding willow. .

Hueco Tanks contains several plants rarely found in the state, such as the only known population of Erect colubrina (Colubrina stricta) in the United States (although this shrub of the buckthorn family occurs in Mexico). A tall, spindly shrub of the mallow family (Abutilon mollicomum) also is found at two locations in the state historic site, which may represent its only occurrence in Texas. Mosquito plant (Agastache cana) of the mint family is confined primarily to rocky slopes, crevices, and ledges in the western Trans-Pecos and adjacent New Mexico. This perennial herb is quite limited in distribution but is fairly widespread at Hueco Tanks.

Animals

 
A visitor peers into one of the deep, natural tanks carved in the rock at Hueco Tanks. Photo by Susan Dial.
Hueco Tanks is located at the north end of the Hueco Bolson, one of a series of bolsons, or basins, lying between mountain ranges in the Trans Pecos. Hueco Bolson is flanked on the west by the Franklin Mountains and on the east by the Hueco Mountains. Map adapted from U.S. Geological Survey.
The Hueco Mountains rise in the distance above the Hueco Bolson covered by miles of desertscrub. Photo by Darrell Creel, TARL Archives.
Geology of Hueco Tanks and vicinity. Map by Edward Collins and Jay Raney, Bureau of Economic Geology, University of Texas at Austin.
View of Hueco Tanks from satellite, showing the four mountains of Hueco Tanks. The dark green areas are plants growing in the moisture-retaining soils characteristic of the Mimbres association. Alluvial soils such as these were important to Formative period farmers who grew corn and other crops. Photo courtesy Google Earth.
The rocky hills of Hueco Tanks are a granite-like, igneous material called porphyritic syenite. Cacti, sotol, and other desert plants thrive within the cracks and crevices in the rock as well as on the basin floor. Photo by Susan Dial.
Shady retreats like this oak glen are uncommon in the desert. Small econiches within Hueco Tanks support diverse plant and animal communities. Photo by Rupestrian Cyberservices, courtesy Texas Parks and Wildlife Department.
Water flows into a hueco following a period of rain. Photo courtesy Texas Parks and Wildlife Department.
Trees and lush grassy areas grow in the moist areas fed by water flowing from the rocky hills. Photo by Darrell Creel, TARL Archives.
Willows and other water-loving plants ring this water tank constructed by ranchers at the site in the early 20th century. During late spring, strong winds whip up dust and sand into the air, diminishing visibility. Photo by Susan Dial.




From out of the dust to life. So-called "fairy" shrimp, among the tiniest of creatures in the park, seasonally thrive in the huecos, providing food for other wildlife. Lying dormant in the dusty hollows during dry periods, these near-microscopic crustaceans burst back to life after periods of rainfall, attracting predators such as golden eagles and lizards. At left, a dry hueco, perhaps containing hundreds of dormant shrimp; center, a rain-filled hueco teeming with shrimp such as the Branchinecta-packardi, right. Photos credits: left, Susan Dial; center, Centennial Museum, El Paso; right, Wikipedia.

Due to its varied habitats and relatively abundant water sources, Hueco Tanks supports a diverse animal community including species from desert scrubland, grassland, and mountainous environments. The animals harvested by prehistoric occupants of Hueco Tanks also came from diverse habitats, as known through their remains in archeological sites. This animal population is much more abundant and diverse than that of the surrounding desert flats.

A number of mammal species have been documented at the state historic site through collection of live specimens and/or archeological remains. Ungulates are mule deer, bison, and pronghorn, the latter two from archeological collections. Although desert bighorn sheep are not included in live or archeological collections from the state historic site, these distinctive, curved-horn animals are depicted on at least eight rock art panels and bighorn sheep remains have been recovered from a cave in the Hueco Mountains. Carnivores are the black bear, bobcat, gray fox, coyote, badger, ringtail, skunk, raccoon, and mountain lion. Three rabbit species currently frequent the park and also were recovered from excavations: the black-tailed jackrabbit, desert cottontail, and eastern cottontail. Twenty rodents are known from live or archeological collections, ranging from porcupine to white-throated woodrat. Vole teeth were recovered from a late Pleistocene packrat midden in the Tanks; the nearest modern populations are in the Sacramento and Guadalupe Mountains. Six species of bats occupy cracks and crevices in the rock hills.

Hueco Tanks supports a diverse community of birds; more than 200 species have been recorded, including several rare birds. Some 44 birds are known or suspected to breed at the state historic site, including the prairie falcon, burrowing owl, white-throated swift, black-chinned hummingbird, ash-throated flycatcher, Crissal thrasher, blue grosbeak, Scott's oriole, and lesser goldfinch. Mourning dove and greater roadrunner have been documented in archeological collections. During migration periods, 31 species of wading birds, waterfowl, and shorebirds visit the site, attracted by the water sources. Many migratory songbirds pass through Hueco Tanks during spring and fall, including 19 warblers; in winter the park hosts up to 20 sparrow species.

Reptiles abound among the rocks and desertscrub. Among the 30 species recorded are 5 rattlesnakes native to the Trans Pecos: blacktail, Mojave, mottled rock, western diamondback, and prairie. The latter two have been found in archeological contexts, indicating they likely were eaten by prehistoric inhabitants. Some 17 species of lizards, 10 non-poisonous snakes, and 2 turtles round out the reptile inventory at the park. The Texas horned lizard is a candidate for federal listing as endangered or threatened, and has been listed as threatened at the state level. The most common non-poisonous snake is the western blackneck garter snake, which frequents moist areas. The remains of two turtles have been recovered from archeological excavations: desert box turtle and softshell turtle; the latter was almost certainly imported from a riverine habitat.

Seven amphibian species have been documented at the park: the barred tiger salamander, three toads, and three spadefoot toads. These species are associated with wetland habitats.

Modern Climate

The modern climate of the area is semiarid and characterized by significant seasonal and annual variations in temperature, precipitation, and wind. Less than 14 inches of rain fall annually in the Hueco Bolson, the basin stretching for 130 milesalong the Rio Grande. In El Paso, the average is even lower, with only 8 inches on average. Over 50 percent of the annual precipitation usually falls during the monsoon season which extends from July through October, when thunderstorms bring heavy rain and occasional flooding.

Winter rainfall is less abundant but is not as susceptible to loss through evaporation. Winter/spring rainfall has a critical impact on the effective length of the growing season, because the soil must be sufficiently hydrated to support plant germination in the spring. In general, much of the precipitation falls so rapidly that it does not infiltrate the soil, resulting in high-energy runoff events.

Summers generally are long and hot, with around 104 days reaching a maximum temperature of 90°F or higher. Winters typically are short and cool, with 65 days dropping to a minimum temperature of 32°F or lower on average.

The low humidity and abundant sunshine of this region result in an evaporation rate that is up to 12 times greater than the annual precipitation. Lake basins in El Paso County experience evaporation rates of around 72 inches per year on average. March and April are the driest months and also have the highest and most sustained winds, causing dust and sand storms that blanket the basins with windblown deposits. Prevailing winds are from the west except during the summer monsoon season, when they originate from the east to southeast.

The climatic regime of this area has fluctuated over time, with attendant repercussions in the plant and animal communities. In the section following, we take a look at the paleoclimate of the site, as evidenced by data ranging from fossil packrat middens to rock imagery.

A mountain lion pauses for a drink after a rainstorm at one of the shallow huecos in the rocks. Photo from exhibit, Centennial Museum, El Paso.
Rattlesnakes, fearsome pests to modern visitors, were on the dinner plate of prehistoric peoples at the site. Shown is a Western Diamondback. Photo courtesy of U.S. Fish and Wildlife.
A rock squirrel nibbles on a bud of claret-cup cacti. Photo by Rupestrian Cyberservices, courtesy Texas Parks and Wildlife Department.
Doves perch on a yucca bloom stalk in a thorny thicket. One of some 200 bird species in the park, dove were sought by prehistoric hunters, as judged by remains in archeological sites. Photo by Susan Dial.
The red flowers of the thorny ocotillo plant stand out in contrast to the dark juniper growing within a high crevice. Photo by Rupestrian Cyberservices, courtesy Texas Parks and Wildlife Department.