Quaternary Geology and Hydrogeology of Colorado Mountain Watersheds

The basin's bedrock framework is equally important. The Laramide Orogeny, a mountain-building event roughly 70 to 40 million years ago, uplifted the Southern Rockies, and later caldera volcanism in the San Juans blanketed much of southwestern Colorado in ash and lava. Older sedimentary rocks, including the Cretaceous Mancos Shale and Mesaverde and Williams Fork formations, record ancient seaways through transgressive-regressive cycles. Different lithologic compositions — shale, sandstone, volcanic rock, granite — produce different stream chemistries through chemical weathering, the slow dissolution of minerals that delivers solutes to streams and consumes atmospheric CO2. Surface geomorphic features like landslides, moraines, and rock glaciers all create distinctive flowpaths that influence both water chemistry and surface runoff.

Foundational work

Early geological characterization of the region began with broad reconnaissance mapping. Cross and Larsen described the volcanic and sedimentary framework of the San Juan region (Cross & Larsen, 1935), while Miller and colleagues mapped the Precambrian basement and Paleozoic cover of the southern Sangre de Cristo Mountains (Miller et al., 1963). Rankin's stratigraphic work on the Upper Cretaceous Colorado Group established that formations such as the Greenhorn limestone and Carlile shale could be traced across northern New Mexico and southern Colorado, providing a stratigraphic backbone for later studies (Rankin, 1944).

Quaternary research in the Elk Mountains and adjacent ranges took shape later. Retzer documented at least three glacial advances on Grand Mesa, recognizing that ice sheets had overtopped the mesa rim and that successive moraines differed strikingly in soil development — evidence of very different ages (Retzer, 1954). Decades later, Brugger applied cosmogenic 10Be and 36Cl dating to terminal moraines in the Taylor River drainage, providing the first numerical ages for late Pleistocene glaciation in the central Colorado Rockies (Brugger, 2007).

Key findings

Across the Elk Mountains and Sawatch Range, paleoglacier reconstructions converge on a striking picture of Last Glacial Maximum climate. Using equilibrium-line altitudes from 22 reconstructed paleoglaciers and a degree-day mass-balance model, Brugger concluded that mean summer temperatures during the LGM were about 7°C cooler than present, and that this estimate is robust even allowing for modest precipitation changes (Brugger, 2010). This finding is consistent with independent reconstructions from across the Southern and Central Rockies. Schweinsberg and colleagues refined the timing of these events in the upper Arkansas River valley, reporting Pinedale terminal moraine ages averaging 21.8 ± 0.7 ka and outburst-flood terraces dating to roughly 19–21 ka, sharpening understanding of how mountain glaciers responded to global climate forcing (Schweinsberg et al., 2016).

Glacial signals continued into the Holocene at smaller scales. Refsnider and Brugger used lichen growth on rock glaciers in the Elk Mountains and Sawatch Range to identify three statistically distinct episodes of rock glacier activity at approximately 3080, 2070, and 1150 years before present, each interpreted as a discrete cooler interval (Refsnider & Brugger, 2007). The oldest episode aligns with widely documented Neoglacial cooling across the southern Rockies, while the younger two correspond to regional Audubon-age activity not always captured in other climate proxies.

The bedrock and surficial framework also controls modern water and slope behavior. Stratigraphic work demonstrated that key Cretaceous units such as the Greenhorn limestone can be mapped continuously across northern New Mexico, while overlying Mancos shales cannot be reliably subdivided in the field (Rankin, 1944). Sedimentologic analysis of the Dakota Sandstone documented dominant southeasterly paleocurrents sourced from the San Luis and Apishapa uplifts, helping reconstruct ancient drainage networks that influenced today's aquifer architecture (Campbell, 1976). On modern landscapes, avalanche modeling by Mears showed that tree removal and wind loading can extend a 300-year avalanche runout by roughly 50 meters — far enough to reach structures previously considered safe — a direct reminder that surface processes and human modification interact on geologic templates (Mears, 2006).

Current frontier

Early work from the 1930s through the 1970s established the stratigraphic and structural framework, while cosmogenic dating studies in the 2000s and 2010s pinned numerical ages onto glacial features. Research since 2020 has shifted toward integrating these long-term records with the modern critical zone — the thin layer where rock, water, and life interact. Quirk and colleagues recently extended the cosmogenic chronology directly into the East River watershed at Gothic, dating the retreat of the East River glacier from its maximum position to roughly 17–18 ka, with recessional stands between 13 and 15 ka (Quirk et al., 2024). Their paleoclimate reconstruction provides the first rigorous local glacial timeline for the watershed that anchors much of RMBL's contemporary ecohydrology research.

A second emerging direction connects geology directly to stream chemistry and the carbon cycle. Slosson and colleagues measured solute concentrations across Rocky Mountain watersheds with contrasting bedrock and land cover, demonstrating that landforms like landslides, moraines, and rock glaciers each generate distinct flowpaths and weathering signatures that modify the CO2 budget of mountain rivers (Slosson et al., 2025). New paleontological work on Cretaceous formations in northwestern Colorado is simultaneously refining understanding of the deeper sedimentary record (Eberle et al., 2024); (Crothers et al., 2026).

Open questions

Many questions remain. How precisely did glacier retreat in the East River watershed track regional climate transitions such as Heinrich Event 1 and the Bølling-Allerød warming, and can the timing be refined enough to test ecological responses on the landscape that RMBL scientists study today? How do solute generation and CO2 drawdown vary between rock-glacier-dominated and moraine-dominated subcatchments, and how will these contributions change as alpine permafrost degrades? How will reductions in winter snowpack and changes to montane forest cover alter avalanche runout, slope stability, and floodplain aquifer recharge in coming decades? Answering these questions will require continued integration of geochronology, geomorphic mapping, and high-frequency hydrochemistry — the kind of cross-disciplinary work the Knowledge Fabric is designed to support.

References

Brugger, K. A. (2007). Cosmogenic 10Be and 36Cl ages from late Pleistocene terminal moraine complexes in the Taylor River drainage basin, central Colorado, USA. Quaternary Science Reviews. →

Brugger, K. A. (2010). Climate in the southern Sawatch Range and Elk Mountains, Colorado, U.S.A., during the last glacial maximum: inferences using a simple degree-day model. Arctic, Antarctic, and Alpine Research. →

Campbell, C. V. (1976). Sedimentology of braided alluvial interval of Dakota Sandstone, northeastern New Mexico. →

Cross, W., Larsen, E. S. (1935). A brief review of the geology of the San Juan region of southwestern Colorado. →

Crothers, R., et al. (2026). An actinopterygian-dominated fish fauna from the Upper Cretaceous Williams Fork Formation, northwestern Colorado. Cretaceous Research. →

Eberle, J. J., et al. (2024). A new Late Cretaceous metatherian from the Williams Fork Formation, Colorado. PLOS ONE. →

Mears, A. I. (2006). Avalanche size increase resulting from tree removal and wind loading — a case study from central Colorado using Aval-1D. →

Miller, J. P., Montgomery, A., Sutherland, P. K. (1963). Geology of part of the southern Sangre de Cristo Mountains, New Mexico. →

Quirk, B. J., et al. (2024). Latest Pleistocene glacial chronology and paleoclimate reconstruction for the East River watershed, Colorado, USA. Quaternary Research. →

Rankin, C. H. (1944). Stratigraphy of the Colorado group, Upper Cretaceous, in northern New Mexico. →

Refsnider, K. A., Brugger, K. A. (2007). Rock glaciers in Central Colorado, U.S.A., as indicators of Holocene climate change. Arctic, Antarctic and Alpine Research. →

Retzer, J. L. (1954). Glacial advances and soil development, Grand Mesa, Colorado. American Journal of Science. →

Schweinsberg, A. D., et al. (2016). Pinedale glacial history of the upper Arkansas River valley. Geological Society of America eBooks. →

Slosson, J., et al. (2025). Linking surface processes, solute generation, and CO2 budgets across lithological and land cover gradients in Rocky Mountain watersheds. Water Resources Research. →

Keith A. Brugger