Tag: Amargosa drainage

Geoglyphs & Rock Alignments

Fort Irwin & Beyond

Juduth Reed, archaeologist – photo Russell Kaldenberg

A geoglyph is a ground design created by arranging or removing surface materials so the figure appears when viewed from above. In desert settings, this usually means placing or clearing pavement stones, exposing lighter soil, or scraping shallow lines that catch low-angle light. Mojave examples tend to occupy quiet, stable surfaces such as old lake margins, bajadas, ridgelines, and mesa tops. Their age is difficult to determine without stratified artifacts, and they usually appear in liminal settings that suggest signaling, marking, ceremony, or boundary use.

Mojave Desert geoglyphs are scattered and subtle, blending with the surface rather than dominating it. They are created by repositioning varnished stones or removing surface layers, forming sinuous lines, circles, meanders, keyhole forms, and occasionally serpentine figures. Most notable examples can be found in the eastern and central Mojave, where travel corridors, ancient water sources, and basin edges converge. Documented sites are located at Fort Irwin, along the Amargosa drainage, near the Lower Colorado River region, and within ancient lake basins such as Cronese, Soda, and Silver. These figures are commonly twenty to sixty feet long or wide. They are not dramatic from the ground; they reveal their form from oblique or aerial views. Many alignments appear to mark direction, vantage, or symbolic forms rooted in local cultural landscapes. Research is limited by erosion, restricted access to lands, and the scarcity of datable material.

Geoglyphs at Fort Irwin became known only after archaeologists expanded survey work into newly added training lands. Earlier work on the site documented petroglyphs, pictographs, and small rock circles, but newer surveys revealed another category of rock art: broad surface alignments set directly into the desert pavement. These geoglyphs consist of fist-sized stones arranged into straight lines, curves, swirls, and branching patterns covering portions of pavement roughly a quarter of an acre in size. They sit so low and blend so closely in tone with the surrounding ground that they remain almost invisible until someone familiar with desert varnish and pavement structure points them out. Artifacts and oxidation patterns provide relative age clues, though no firm dates are given.

Archaeologists describe the Mojave landscape as highly readable, with scars, signals, and surface changes preserved by aridity. In this setting, rock alignments are found on stable pavements, old lake margins, and gentle rises where water once flowed across the ground. Fort Irwin sits within that framework: ancient lake basins, remnant shorelines, and corridors that once linked seasonal camps. Nearby lithic scatters suggest long-term movement associated with water, game, and travel. Interpretations of the geoglyphs remain limited. Some broken quartzite fragments hint at possible ceremonial use, but the exact meaning remains unknown. Cultural memory tied to such features has not survived, and researchers avoid overreaching beyond what the land itself reveals.

Photo by Russell Kaldenberg

Within the broader Goldstone basin sector of the installation, survey data also note a low ridge with surface materials arranged into a curving alignment that may represent a stylized serpent or directional form. Its placement on a quiet slope between pavement and basin edge fits a familiar Mojave pattern in which subtle figures mark routes, thresholds, or vantage points without leaving associated domestic remains. Features of this kind are typically visible only from an angled view, where dark varnished stones contrast with lighter soil. Because the land is part of an active training area, precise locations are protected, and access is restricted to guided visits. As with other prehistoric sites on the post, Fort Irwin treats these alignments as resources to be safeguarded.

Together, the abstract pavement figures and the additional curving alignment illustrate how ancient travelers marked the basin edges and crossings of the central Mojave. They show that even in a landscape that seems empty at first glance, the ground carries the record of movement, gathering, and intention shaped into the surface itself.

Core Bibliography: Mojave Geoglyphs and Rock Alignments

Allen, Mark W. 1991. Archaeological Investigations at Fort Irwin. Fort Irwin Cultural Resources Program.

Basgall, Mark E. 1993. Chronometric Studies in the Mojave Desert. Publications in California Prehistory 34.

Clewlow, C. William Jr. 1976. Prehistoric Trails of the Lake Mojave Region. UC Archaeological Research Facility Report 30.

Davis, Emma Lou. 1978. The Ancient Californians: Rancholabrean Hunters of the Mojave Desert. Ballena Press.

Fort Irwin Cultural Resources Program. Various Survey Reports and Inventory Summaries, 1980s to present.

Goldstone Deep Space Communications Complex. Cultural Resources Overview Studies, 1990s–2000s.

Heizer, Robert F., and Martin A. Baumhoff. 1962. Prehistoric Rock Art of Nevada and Eastern California. University of California Press.

Minor, Rick. 1987. Intaglios and Ground Figures of the American Southwest. American Rock Art Research Association.

Schaefer, Jerry. 1995. Cultural Resource Management Studies at Fort Irwin, California. ASM Affiliates.

  1. U.S. Army, Fort Irwin Cultural Resources Program. Survey reports and site documentation for expanded training lands, various years.
  2. Sutton, Mark Q., and Jill K. Gardner. Patterns of Mojave Desert Prehistory. Nevada State Museum Anthropological Papers, 1997.
  3. Warren, Claude N., and Robert H. Crabtree. Prehistory of the Southwest and Great Basin. In Handbook of North American Indians, Vol. 11, Great Basin. Smithsonian Institution, 1986.
  4. Draut, Amy E., et al. Late Pleistocene lake histories in the Mojave River and Amargosa Basin region. USGS Professional Papers and Open-File Reports, various years.
  5. McCarthy, Daniel. Ground figures of the Mojave and Colorado Deserts. In Rock Art Papers, San Diego Museum of Man, various volumes.
  6. GSA and USGS publications on desert pavement formation, varnish development, and surface stability relevant to geoglyph preservation.
  7. California Department of Parks and Recreation. Archaeological surveys within the Mojave Desert region, assorted site records.

Special thanks to Russell Kaldenburg

Lake Tecopa Geology

Lake Tecopa was a large lake in southeastern California during the Ice Age. Due to changes in climate, earthquakes, and shifting rivers, it formed and disappeared several times over the past two million years.

How Lake Tecopa Changed Over Time

Early Lakes (~2 million to 765,000 years ago):

  • Before the Amargosa River reached the area, smaller lakes from local rainfall and runoff filled the basin.
  • These lakes could not drain, so water stayed in the basin until it evaporated.

Amargosa River Connection (~765,000 to 580,000 years ago):

  • Around 765,000 years ago, the Amargosa River started flowing into the Tecopa Basin.
  • This caused the lake to grow much larger, filling the basin several times over the next 200,000 years.

Final Overflow (~185,000 years ago):

  • The lake reached its highest level and spilled over, creating a channel that drained into Death Valley.
  • Once this happened, Tecopa was no longer a closed lake. Water from the Amargosa River could now flow toward Death Valley instead of staying in the basin.
  • This spillover permanently changed the region’s water system.

How the Spillover Happened

  • The lake overflowed because of heavy rainfall, rising water levels, and erosion of the basin’s southern part.
  • As water broke through, it carved a channel that deepened over time, allowing the Amargosa River to connect with Death Valley.
  • After this, Lake Tecopa could no longer reform as a long-lasting lake.

Evidence Left Behind

  • Ancient Shorelines: Rings around the basin show where the lake once stood at different times.
  • Fossils: Scientists have found tiny fossils of algae and freshwater mollusks, proving that the lake supported life.
  • Rock Deposits: Layers of calcium carbonate and tufa formed in the lake, marking its highest water levels.
  • Spillover Channel: The area where the lake drained remains a landform today, showing where the water escaped.

Comparing Lake Tecopa to Other Ice Age Lakes

  • Lake Tecopa vs. Lake Manly (Death Valley):
    • Lake Tecopa’s overflow helped form Lake Manly in Death Valley.
    • Lake Manly was filled and dried up multiple times, depending on the climate conditions.
    • Unlike Tecopa, Lake Manly was always part of a larger river system.
  • Lake Tecopa vs. Lake Mojave (Mojave Desert):
    • Lake Mojave (which included Silver and Soda Lakes) was fed by the Mojave River instead of the Amargosa.
    • Unlike Tecopa, Lake Mojave did not spill over into another basin. Instead, it dried up when the climate changed.
    • Both lakes grew and shrank over time due to changes in rainfall and temperature.

Why This Matters

The story of Lake Tecopa helps scientists understand how Ice Age lakes formed, changed, and disappeared. The lake’s final overflow was an important event because it changed how water flowed across the region. Studying Tecopa’s past gives us clues about ancient climates, shifting landscapes, and the history of water in the Mojave Desert.