5.3 Ma 

26 Ma 

23.8 Ma 

26–29 Ma 

33.7 Ma 

10 ka 

1,800 ka 

780 ka 

130 ka 

Qaa 

Qla Qa 

Qc Qrf 

Qes Qfp 

Qed 

Qay 

Qai 

Qao 

Qpy 

Qp 

QTla 

Qlag 

Th 

Tcu 

Qed 

Qes 

Qla 

Qlag 

QTla 

Qpy 

Qp 

Qa 

Qfp 

Qaa 

Qay 

Qai 

Qao 

Qc 

Qrf 

Th 

Tc 

CORRELATION OF SURFICIAL AND BEDROCK MAP UNITS

Eolian Alluvial Mass Wasting 

Rockfall 

and talus Piedmont Fluvial Colluvial 

Holocene Pleistocene Pliocene 

late middle early 

QUATERNARY TERTIARY 

Lacustrine 

1.8 Ma 

Volcanic 

Miocene Oligocene 

Major 

unconformity

late late 

Alamosa W 

Alamosa E 

La Jara 

Pikes 

Stockade 

37°30ʼ

37°15ʼ 

106° 105° 

106°00ʼ 105°45ʼ 

Index to 7.5-minute Topographic Quadrangles 

REFERENCES CITED

Machette, M.N., 1985, Calcic soils of the southwestern United 

States, in Weide, D.L., ed., Soils and Quaternary geology of 

the southwestern United States: Geological Society of 

America Special Paper 203, p. 1–21.

Machette, M.N., 2004, New evidence for ancient Lake Alamosa 

in the San Luis Basin of Colorado: Geological Society of 

America Abstracts with Program, v. 36, no. 5, p. 530.

Rogers, K.L., Repenning, C.A., Forester, R.M., Larson, E.E., 

Hall, S.A., Smith, G.A., Anderson, Elaine, and Browen, T.J., 

1985, Middle Pleistocene (Late Irvingtonion: Nebraskan) 

climatic changes in south-central Colorado: National 

Geographic Research, v. 1, no. 4, p. 535–563.

Siebenthal, C.E.,1910, Geology and water resources of the San 

Luis Valley, Colorado: U.S. Geological Survey Water Supply 

Paper 240, 128 p.

Thompson, R.A., and Machette, M.N., 1989, Geologic map of 

the San Luis Hills area, Conejos and Costilla Counties, 

Colorado: U.S. Geological Survey Miscellaneous 

Investigations Series Map I–1906, scale 1:50,000.

 DESCRIPTION OF MAP UNITS

[Map-unit names and stratigraphic associations are based on mapping over the entire Alamosa 1/2° x 1° sheet, which contains about 

60 units. Combined map units, such as Qay/Qai, indicate thin deposits (Qay) over older deposits (Qai) as seen in limited exposures. In 

addition, some map units of questionable age or origin are shown with a query (?) after the map-unit name (for example, QTla?)]

EOLIAN DEPOSITS

Eolian dune sand (Holocene)—Eolian sand that forms small pronounced dunes as much as 5 m high. Commonly interspersed 

with interdune playas that are comprised of organic silts and clays. Some dunes are active; others have indistinct buried soils 

(A horizons) indicating multiple episodes of stability and redeposition

Eolian cover sand (Holocene)—Eolian sand that forms thin (1–3 m) sheets and small coppice (plant anchored) dunes

LACUSTRINE DEPOSITS

Lake and pond deposits (Holocene)—Lacustrine deposits in small local lakes and ponds, commonly impounded by alluvial fans or 

eolian sand

Shoreline deposits of Lake Alamosa (middle? Pleistocene)—Coarse-grained, locally derived sand and gravel in barrier bars, spits, 

and shorelines of ancient Lake Alamosa (see Machette, 2004), and fine-grained sand and silt in lagoons and ponds 

impounded by barrier bars and spits formed by the ancient lake. Mainly preserved at elevations of 7,620–7,650 ft 

(2,323–2,332 m) on the northern margin of the San Luis Hills, in the southeastern part of the map area. Constitutes the 

uppermost part of the Alamosa Formation of Siebenthal (1910)

Alamosa Formation of Siebenthal (1910) (middle? Pleistocene to Pliocene)—Closed-basin lacustrine deposits (mainly clay, silt, 

and sand) and near-shore deposits (sand and fine gravel) related to alternating, climatically(?) driven shallow- and 

deep-water phases of ancient Lake Alamosa. The best exposed section of the Alamosa Formation is along Hansen Bluff 

(east margin of map area), where Rogers and others (1985) have made an extensive collection of fossils (flora and fauna) 

and volcanic ashes, and have analyzed the paleomagnetic record in the sediments. The maximum elevation of the lake was 

about 7,650–7,670 ft (2,332–2,338 m) as recorded by unit Qlag elsewhere in the San Luis Basin. Strong calcic soils (stage 

III; Machette, 1985) on the shoreline features suggest that the lake overflowed through the San Luis Hills about 300–400 ka

ALLUVIAL DEPOSITS—PIEDMONT FACIES

Younger piedmont alluvium (upper Pleistocene)—Fine- to coarse-grained alluvium and debris flows that form moderate to steeply 

sloping surfaces adjacent to bedrock-cored hills (San Luis Hills for this map). Alluvium is typically silty sand to pebbly sand. 

Contains zonal soils with A, Bw or Bt, and (or) Bk horizons. Surface of deposits typically not dissected

Undifferentiated piedmont alluvium (middle Pleistocene)—Fine- to coarse-grained alluvium and debris flows that form moderately 

to steeply sloping surfaces adjacent to bedrock-cored hills (San Luis Hills on this map). Alluvium is typically silty sand to 

pebbly or cobbly sand. Contains zonal soils with A, Bw or Bt, and Bk horizons, the latter of which locally have stage II–III 

morphologies (K horizons). Depositional surfaces typically dissected

ALLUVIAL DEPOSITS—FLUVIAL FACIES 

Undifferentiated stream alluvium (Holocene)—Fine-grained silt and clay to medium-grained sand and gravel that form active 

stream channels and floodplains along most drainage courses too small to map separately as either unit Qaa or Qfp. Grain 

size and lithology variable depending on material being eroded in stream headwaters. Gravel clasts generally <2 cm in 

diameter, slightly coarse (5–8 cm) along Rio Grande

Active stream alluvium (upper Holocene)—Fluvial sand and gravel in modern to recently active stream channels that retain fresh 

expression (that is, oxbow loops, abrupt terrrace escarpments). Gravel clasts generally 5–10 cm in diameter along the Rio 

Grande, <3 cm in diameter along other streams

Floodplain alluvium (upper Holocene)—Fine-grained silt and clay to medium-grained sand and pebble gravel that form floodplains 

adjacent to active channels. Floodplains commonly have muted depositional morphology (that is, infilled oxbow loops, subtle 

terrrace margins). Gravel clasts generally <2 cm in diameter, slightly coarse (5–8 cm) along Rio Grande

Younger stream alluvium (upper Pleistocene)—Fluvial sand and gravel in former floodplains that are now terraces elevated 1–2 m 

above modern stream level. Commonly contains weak soils with Bw and (or) Bk horizons 

Intermediate stream alluvium (upper middle Pleistocene)—Fluvial sand and gravel in former floodplains that are now at 

intermediate elevation (2–5 m) above modern stream level. Many of the elongate, northeast-trending deposits fill alluvial 

channels that were incised into lake deposits (unit QTla) following draining of Lake Alamosa (see description of units Qlag 

and QTla). These channels are now topographically high as a result of erosion of the much softer lake deposits (that is, 

topographic reversal). The relict surface of the intermediate stream alluvium has moderately developed soil (Bt and stage 

II–III Bk horizons). Locally, thin deposits of unit Qay overlie unit Qai

Older stream alluvium (middle Pleistocene)—Fluvial sand and gravel in former floodplains that are now elevated 10–15 m above 

modern streams. Forms major high-level surfaces east of the Rio Grande, such as on Hansen Bluff (east margin of map 

area). Commonly has strongly developed calcic soil (Bk and stage III–IV K horizons) 

MASS-WASTING DEPOSITS—Gravitationally driven, locally derived deposits; only mapped in the San Luis Hills, which are in the 

southeastern part of the map area

Colluvium, undivided (Holocene to middle Pleistocene)—Poorly sorted, nonstratified, sandy to gravelly deposits; typically on 

moderate to steep slopes of resistant volcanic rocks. Typically includes some debris-flow deposits and locally derived 

alluvium

Rockfall and talus (Holocene to middle Pleistocene)—Unsorted, nonstratified deposits of angular rock debris; typically 

accumulates at the base of bedrock escarpments 

VOLCANIC ROCKS, OLIGOCENE

Hinsdale Formation (upper Oligocene)—Basaltic lava, associated breccia, and near-vent pyroclastic deposits. Typically forms 

flat-topped mesas of the San Luis Hills in the southeastern part of the map area. Previously dated at about 26 Ma by 

whole-rock K/Ar methods (Thompson and Machette, 1989)

 

SYMBOLS

Contact

Terrace riser—Back edge of mappable fluvial 

terrace within unit Qfp

Lacustrine spit or barrier bar of Lake Alamosa— 

Highest shoreline in map area is at about 

2,332 m (7,650 ft) (Machette, 2004) along 

the northern margin of the San Luis Hills in 

the southeastern part of the map area 

l l l l

Conejos Formation (Oligocene) as mapped by Thompson 

and Machette (1989)—Includes mafic to 

intermediate-composition vent-facies rocks consisting 

of lava flows, flow breccias, explosion breccias, and 

mudflow breccias, all from local sources. Previously 

bracketed at 26–29 Ma by whole-rock K/Ar dates 

(Thompson and Machette, 1989). Separated into an 

upper and lower sequence based on observed 

stratigraphy and variations in flow morphology and 

mineralogy. In this area, only the lower sequence is 

locally preserved beneath rocks of the Hinsdale 

Formation (unit Th). See Thompson and Machette 

(1989) for details

 

 

l l l l l l

l l l l

l l l l l l l

l l l l l l l l l l

l l l l

l l l

l l l

l l l l l l l l

l l l l l

l l l l l l l l

l l l l l l

l

l l l l l

l l l l

Qay 

Qfp 

Qfp 

Qfp 

Qfp 

Qfp 

Qfp 

Qfp 

Qaa 

Qa 

Qa 

Qa 

Qaa 

Qaa 

Qed 

Qed 

Qes 

Qed 

Qed 

Qed 

Qed 

Qed 

Qa 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qai 

Qai 

Qai 

Qai 

Qla 

Qai 

Qai 

Qai 

Qai Qai 

Qai 

Qai 

Qay 

Qay 

Qay 

Qay 

Qay/Qai 

Qay/Qai 

Qay 

Qfp 

Qfp 

(upper) 

Qfp (lower) 

Qed 

Qfp 

Qes 

Qa 

Qa 

Qaa 

Qed 

Qed 

Qed 

Qed 

Qed 

Qed 

Qed Qed 

Qed 

Qfp 

Qa 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qaa 

Qaa 

Qfp 

Qfp 

Qao 

Qfp 

Qed 

Qao 

Qay? 

Qfp 

QTla 

QTla 

QTla 

Qai 

Qai Qai 

Qai 

Qai? 

Qai 

Qai 

Qai 

Qai 

Qai 

 Qay 

 Qai 

 Qay/Qai? 

 Qay 

 Qa 

 Qa 

 Qaa 

 Qaa 

 Qa 

 Qa 

 Qa 

 Qa 

 Qa 

 Qaa 

 Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

Qay 

QTla? 

Qlag 

Qlag 

QTla 

Qaa 

Qfp Qla 

Qaa 

Qaa 

Qay 

Qay 

Qpy 

Qay 

Qay 

Qa 

Qa 

Qaa 

QTla 

Qfp 

Qa 

Qa 

Qa 

Qa 

Qai 

QTla? 

QTla? 

Qfp 

Qfp 

Qay? 

Qfp 

Qa 

Qa 

Qfp 

Qes? 

Qes 

Qay 

Qa 

Qa 

Qlag 

Qlag 

Qrf 

Qrf 

Qrf 

Qla 

Th Tc 

Tc 

Tc 

Tc 

Th 

Th 

Qes 

QTla 

QTla 

QTla 

Qc 

Qc 

Qc 

QTla

QTla 

QTla 

Qfp 

Qfp 

Th 

Th 

 

Qp 

Qp 

Qp 

Qpy 

Qpy 

Qpy 

Qpy 

QTla? 

QTla 

Qay Qrf 

Qa 

Qaa 

Qay 

Qai 

U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY 

OPEN-FILE REPORT 2005-1392 

Preliminary Geologic Map of the Northwestern Part of the Alamosa 30 X 60 Minute 

Quadrangle, Alamosa and Conejos Counties, Colorado 

By

Michael N. Machette and Ren A. Thompson 

2005 

 COLORADO 

ALAMOSA 30 X 60

MINUTE SHEET

Map area

Base enlarged from 1:100,000-scale metric topographic map (1/2° by 1° sheet) 

of Alamosa, Colorado, 1983; Universal Transverse Mercator projection, 1927 

North American datum.

Geology mapped in 2004–2005 using aerial photographs, U.S. Dept. of 

Agriculture county soil survey maps, and field reconnaissance. Geology 

compiled on 1:24,000-scale topographic base maps (see index map below) 

using a photogrammetric plotter (Kern PG-2). 

 SAN LUIS HILLS 

37°00ʼ 

37°30ʼ Any use of trade names in this publication is for

descriptive purposes only and does not imply

endorsement by the U.S. Geological Survey

SCALE 1: 50 000

1 1/ 2 0 1 2 3 4 MILES

1 .5 0 1 2 3 4 5 KILOMETERS