Surficial Geologic Map of the Red Rock Lakes Area, 

Southwest Montana

By Kenneth L. Pierce, Tara L. Chesley-Preston, and Richard L. Sojda

Pamphlet to accompany

Open-File Report 2014–1157

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

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Suggested citation:

Pierce, K.L., Chesley-Preston, T.L., and Sojda, R.L., 2014, Surficial geologic map of the Red Rock

Lakes area, southwest Montana: U.S. Geological Survey Open-File Report 2014–1157, 22 p., scale 

1:24,000, http://dx.doi.org/10.3133/ofr20141157.

ISSN 2331-1258 (online)
iii

Contents

Abstract ......................................................................................................................................................... 1

Surficial Geologic History of the Red Rock Lakes Area ................................................................................. 1

Tectonic Setting and Active Faulting.......................................................................................................... 3

Present-Day Lakes and Lake Centennial................................................................................................... 4

Present-day Lakes and Wetlands........................................................................................................... 4

Lake Centennial ..................................................................................................................................... 4

Surficial Deposits in Western Centennial Valley................................................................................... 10

Alluvial Deposits....................................................................................................................................... 11

Glacial Deposits....................................................................................................................................... 11

Eolian Deposits........................................................................................................................................ 12

Landsliding............................................................................................................................................... 12

Surface and Groundwater........................................................................................................................ 12

Description of Surficial Geologic Map Units, Red Rock Lakes Area, Southwest Montana........................... 13

Unconsolidated Surficial Deposits............................................................................................................ 13

Volcanic Rocks ........................................................................................................................................ 18

Sedimentary Rocks.................................................................................................................................. 19

Conclusion................................................................................................................................................... 19

Acknowledgments ....................................................................................................................................... 20

References .................................................................................................................................................. 20
iv

Sheet

Surficial geologic map of the Red Rock Lakes area, southwest Montana ..................................................link

Figures

1. Location of the Red Rock Lakes map area .............................................................................................2

2. Diagrammatic cross section portraying flow of groundwater................................................................. 10

Tables

1. Radiocarbon and calibrated ages, Red Rock Lakes area, Centennial Valley, 

southwest Montana........................................................................................................................................6

2. Optically stimulated luminescence (OSL) ages of sands associated with Lake Centennial, 

southwest Montana........................................................................................................................................7

3. Location by latitude and longitude of samples with ages shown in tables 1 and 2..................................8

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Divisions of Quaternary and Neogene time used in this report

Period or Epoch or Subperiod Age

Holocene 0–11.7 ka (about last 10,000 radiocarbon years)

Quaternary

late 11.7–126 ka 

Pleistocene middle 126–781 ka

early 781 ka–2.588 Ma

Neogene 2.588–23 Ma

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2010). Ages are 

expressed in ka for kilo-annum (thousand years) and Ma for mega-annum (million years).
1

Surficial Geologic Map of the Red Rock Lakes Area, 

Southwest Montana

By Kenneth L. Pierce, Tara L. Chesley-Preston, and Richard L. Sojda

Abstract

The Centennial Valley and Centennial Range continue to be formed by ongoing 

displacement on the Centennial fault. The dominant fault movement is downward, creating space 

in the valley for lakes and the deposition of sediment. The Centennial Valley originally drained 

to the northeast through a canyon now represented by a chain of lakes starting with Elk Lake. 

Subsequently, large landslides blocked and dammed the drainage, which created Lake 

Centennial, in the Centennial Valley. Sediments deposited in this late Pleistocene lake underlie 

much of the valley floor and rest on permeable sand and gravel deposited when the valley 

drained to the northeast. Cold Pleistocene climates enhanced colluvial supply of gravelly 

sediment to mountain streams and high peak flows carried gravelly sediment into the valley. 

There, the lower gradient of the streams resulted in deposition of alluvial fans peripheral to Lake 

Centennial as the lake lowered through time to the level of the two present lakes. Pleistocene 

glaciers formed in the high Centennial Range, built glacial moraines, and also supplied glacial 

outwash to the alluvial fans. Winds from the west and south blew sand to the northeast side of 

the valley building up high dunes.

The central part of the map area is flat, sloping to the west by only 0.6 meters in 13 

kilometers (2 feet in 8 miles) to form a watery lowland. This lowland contains Upper and Lower 

Red Rock Lakes, many ponds, and peat lands inside the “water plane,” above which are 

somewhat steeper slopes. The permeable sands and gravels beneath Lake Centennial sediments 

provide a path for groundwater recharged from the adjacent uplands. This groundwater leaks 

upward through Lake Centennial sediments and sustains wetland vegetation into late summer. 

Upper and Lower Red Rock Lakes are formed by alluvial-fan dams. Alluvial fans converge from 

both the south and the north to form outlet thresholds that dam the two shallow lakes upstream. 

The surficial geology aids in understanding how the landscapes in and around the Red Rock 

Lakes Wildlife Refuge were formed and how they transmit water.

This report uses metric units except for altitudes that are also given in feet because 

contours on the base map are in feet and the reader would have to convert from metric units to 

feet to understand the map relationships.

Surficial Geologic History of the Red Rock Lakes Area

The Red Rock Lakes map area covers the eastern part of the Centennial Valley located in 

southwestern Montana (fig. 1) and centers on the Red Rock Lakes National Wildlife Refuge. 

This refuge has served as a key recovery site for trumpeter swans since 1935 (U.S. Fish and 

Wildlife Service, 2014). The area is known for cold, snowy winters and extensive wetlands. The 
2

area is drained by the Red Rock River and is downstream from the uppermost headwaters of the 

Missouri River system. A lake once occupied the area, Lake Centennial, which was created by 

landslide-damming of a northern paleo-canyon of the Red Rock River (fig. 1). This late 

Pleistocene lake was not a typical glacial or pluvial lake, although it has been so described 

(Witkind, 1976; Mannick, 1980; Witkind and Prostka, 1980; Sonderegger and others, 1982b). 

The Centennial Mountains flank the map area to the south. Beyond the mountains lies the 

Eastern Snake River Plain, which is also the track of the Yellowstone hotspot (fig. 1). The

hotspot track becomes younger to the northeast and is associated with late Cenozoic volcanism, 

faulting, and uplift that have greatly affected the geology and landscapes of the map area (Pierce 

and Morgan, 1992). Knowledge of the complex geologic history of this area is important to 

understanding the wetlands and other landscapes of the area.

Figure 1. Location of the Red Rock Lakes map area. Note the string of lakes (Elk, Hidden, Cliff, and 

Wade Lakes) define a paleo-canyon of the Red Rock River northward into the Madison Valley. The 

eastern Snake River Plain is also the track of the Yellowstone hotspot, which is associated with late 

Cenozoic volcanism, faulting, and uplift (Pierce and Morgan, 1992). The two youngest caldera 

groupings along the hotspot track are calderas of the Heise volcanic field (6.6–4.45 Ma; Morgan and 

McIntosh, 2005) and calderas of the Yellowstone volcanic field (2.05–0.64 Ma; Christiansen, 2001). 

Quaternary faults are shown in red and faults other than Centennial fault and the Lima Reservoir fault 

are from U.S. Geological Survey and Montana Bureau of Mines and Geology (2006). The Centennial 
3

fault extends, with right-stepping en echelon faults, from east of Henrys Lake (Myers and Hamilton, 

1964; Morgan and others, 2008) through Red Rock Pass (Witkind, 1975) to the bold front of the 

Centennial Range east of Odell Creek. Westward from Odell Creek, the fault is poorly expressed for 15 

km and then becomes well defined for 18 km to Mud Lake (Witkind, 1975; Petrik, 2008). At Mud Lake, 

fault offset switches to the north side of the valley as the Lima Reservoir fault (Myers and Hamilton, 

1964; Bartholomew and others, 2002). This fault continues west to beneath the Lima Dam (Anastasio 

and others, 2010).

Tectonic Setting and Active Faulting

The Centennial Mountains and Valley are an active basin-and-range structure formed by 

the Centennial normal fault as a result of north-south extension. The valley continues to be 

created by down-faulting along the Centennial normal fault and the half graben nature of the 

structure results in southward tilting of the valley. Topographic surveys across the Lost River 

fault before and after the 1983 Borah Peak, Idaho, earthquake find that about 85 percent of the 

vertical fault offset is downward movement of the basin block and only about 15 percent is a 

result of upward movement of the mountain block (Stein and Barrientos, 1985). The map area is 

in the middle part of Centennial fault (fig. 1), which extends from Henrys Lake to Mud Lake 

(Witkind, 1975, 1976; Witkind and Prostka, 1980; Petrik, 2008). 

In the map area east of Odell Creek, the Centennial Mountains consist mainly of resistant 

Paleozoic limestones. Along this section of the range, surface offset on the Centennial fault is 

clearly observed to form scarps and locally offsets Pinedale moraines about 10 meters (m). Near 

the base of the cliff-like range front, gently sloping Pinedale moraines intersect the fault scarp—

the resulting 40° or steeper slopes create very difficult footing. However, Witkind (1975, 1976) 

did not recognize what this study clearly identifies to be the main fault scarp; Witkind mapped 

only a minor fault scarp in the fan gravels (Qpfg) 700 m north of the main post-glacial fault 

scarp herein mapped. Right lateral offset was observed along two subsidiary fault scarps north of 

the main fault. The two subsidiary faults are shown on the map sheet, 1.7 and 1.8 kilometers 

(km) north-northeast of Taylor Mountain where the crest of a lateral moraine is offset in a right 

lateral direction by 4.2 and 5.1 m respectively.

Throughout nearly all the map area west of Odell Creek, fault scarps were recognized in 

only two places even though the Centennial Mountains are as high here as east of Odell Creek. It 

is likely that Quaternary faulting continues through this area, but is here obscured by landsliding 

and mountain-scale gravitational spreading of the range in the incompetent mudstones of the 

Upper Cretaceous Frontier Formation. West of the map area, along the Centennial Mountains 

(fig. 1), fault scarps offset latest Pleistocene fan gravels about 10 m (Witkind, 1975; Petrik, 

2008). 

Dip projections of the Quaternary and Neogene tuffs (QNt) from the north side of the 

Centennial Valley to the south side of the Centennial Mountains indicate offset on the Centennial 

fault of about 1,800 m (Mannick, 1980; Morgan and others, 2008). Offset of 1,800 m of these 

2.05–4.45 Ma tuffs yields a slip rate between 0.4 and 0.9 millimeters per year (mm/yr). 

Displacement of Pinedale moraines and fan gravels by roughly 10 m over the last 14,000 years 

(age of deglaciation) yields a slip rate of about 0.7 mm/yr, which is intermediate between the 

estimates of the long-term rates.

On the valley floor, north of the Centennial fault, a minor fault offsets shorelines at the 

edge of the “water plane” (green line on map sheet) 6 km north of Lakeview and extends 1.5 km 

to the northwest. Traversing southeast along this alignment, a possible fault may be responsible 

for the northwest strike of the straight north shore of Upper Red Rock Lake. Between these 
4

aligned features, Sonderegger and others (1982b, p. 19) noted a series of thermal anomalies 

(warm zones). 

The term “fan form” is used for an alluvial fan that may be buried by lake sediments but 

still retains the conical form. On the Odell Creek alluvial fan-form, the following northwest 

trending minor faults were observed: (1) 2.7 km north-northeast of Lakeview, a fault scarp is 

noted in lidar data (Rebecca Levine, Univ. of New Mexico, written commun., 2012), and (2) 4.5 

km northeast of Lakeview, a step-up to the northeast occurs in the opposite direction of the fan 

slope. In the western part of the map area, the lowland area with the label “subsided area” is 

lower in elevation than the lake floor of Lake Centennial and is as low or lower than the presentday Red Rock River—tectonic subsidence seems to be the only explanation for this low area at 

and below the base level of the river. 

Present-Day Lakes and Lake Centennial

Present-day Lakes and Wetlands

Upper and Lower Red Rock Lakes are shallow lakes less than 2 m deep. The term “fan 

form” is used here to denote conical landscape units whose main shape (shown on the map by 

dashed dark blue arrows) was built by alluvial fan deposition, in large part prior to the deposition 

of Lake Centennial sediments. The threshold of the two lakes is formed by the convergence of 

alluvial fan-forms to make a low dam or “fan dam.” The fan forms of Tepee and Odell Creeks 

converge from the north to south and form a threshold for Upper Red Rock Lake, and the fan 

forms of Metzel and Curry–Matsingale–Collins Creeks converge to make a threshold for Lower 

Red Rock Lake. Both lakes are located just upstream of these north-south convergences formed 

by large alluvial fan-forms. The modern lakes occur upstream from these “fan dams” of the 

Centennial Valley floor, and these constrictions act as a low threshold, or “fan dams” for the 

outlets of the present lakes.

The central part of the map area is a flat valley floor containing Upper and Lower Red 

Rock Lakes and extensive peaty marshes (Qhpc) with many small ponds, all enclosed by what is 

termed here as the “water plane.” The “water plane” encloses a nearly horizontal wetted area that 

slopes west at 0.6 m over 12 km (2 feet in 8 miles) as measured by the altitudes of the Upper and 

Lower Lakes (2,015 and 2,014 m; 6,610 and 6,608 ft, respectively). This low, poorly drained 

area is a relic of the central part of the poorly drained Pleistocene Lake Centennial.

Lake Centennial

Lake Centennial covered a large part of the map area during the late Pleistocene (Qpl and 

Qpla; Witkind, 1976; Witkind and Prostka, 1980; Sonderegger and others, 1982a, 1982b; 

O’Neill and Christiansen, 2004; Mumma and others, 2012). A delta (Qhd) on the north side of 

Red Rock Creek enclosed by the 6,680-foot contour indicates the highest, well-expressed level 

of Lake Centennial about 20 m above the two modern, shallow lakes. An origin of Lake 

Centennial by sudden damming is inferred from an exposure (04P37, table 1) along the Red 

Rock River 3.5 km upstream from Upper Red Rock Lake. This exposure shows rhythmically

bedded, deep-water lake sediments abruptly overlying alluvial gravels where no soil developed. 

Additionally, cores and auger holes in the map area also reveal five or more meters of lake 

sediments abruptly overlying either gravel or coarse sand, both inferred to be of alluvial origin. 

This abrupt change also supports a quick transition from a subaerial to a lake environment. One 
5

example is the core taken from Lower Red Rock Lake where lake sediments abruptly overlie 

coarse alluvial sand (Mumma and others, 2012).

The Red Rock River once flowed northeast into the Madison Valley through a paleocanyon (fig. 1) that is now highlighted by a string of steep-walled lakes beginning with Elk Lake 

at the south and including Hidden, Cliff, and Wade Lakes (Mansfield, 1911; Myers and 

Hamilton, 1964, fig. 49; Mannick, 1980; Sonderegger and others, 1982b). An extensive landslide 

(Sonderegger and others, 1982a) just northeast of Elk Lake blocked this northeast drainage and 

was responsible for damming of the Red Rock River to form Lake Centennial. Landslides 

presumably occurred during the late Pleistocene when climates were cooler and allowed for 

greater effective moisture to facilitate mass movement. Pre-Lake Centennial drainage to the 

northeast seems reasonable given the following altitudes. The bottom of Elk Lake (fig. 1) has an 

altitude of 2,013 m (6,605 ft; Lund, 1974), which combined with an estimated sediment fill of 10 

m yields a paleo-canyon floor altitude of less than 2,004 m, an altitude more than 11 m lower 

than Upper Red Rock Lake. 

Upon damming of the drainage, Lake Centennial developed and subsequently filled high 

enough to release water across the paleo-divide at the western end of the Centennial Valley, 

which is the present location of the Lima Dam, 40 km west of Lower Red Rock Lake (fig. 1). 

The Lima dam site exposes a previous fill of alluvial fan and other sediment that projects to a 

water level above the dam height. Projections of the surface of the now eroded alluvial fans 

above the Lima Dam yield a threshold level of 0 to 20 m above the highest level of eastern Lake 

Centennial of 2,036 m (6,680 ft). The very low carbonate content of the lower 3 m clayey 

sediment in the core from Lake Centennial (location 06P49, table 3; Mumma and others, 2012) 

suggests the lake was not a closed basin and thus was draining. The hanging wall of the 

Quaternary Lima Reservoir fault is beneath the Lima Dam. The last offset on the Lima Reservoir 

fault is estimated to have occurred ~13 ka by Anastasio and others (2010). Three faulting events 

are estimated to have occurred in the last 20,000 years by Bartholomew and others (2002). 

During the existence of Lake Centennial, down-faulting of the Lima Reservoir fault would have 

lowered the outlet threshold and provided a reasonable mechanism for at least some of the 

decrease in water level of Lake Centennial. 

Lake Centennial was dammed sometime between 30,000 and 20,000 years ago. The older 

age is based on ages from a core from the southern part of Lower Red Rock Lake. At that 

location, the lowest Lake Centennial sediments have a projected age of ~28,800 cal yr B.P. based 

on organic sediment 18 centimeters (cm) above the base of the lake sediments that have a mean 

calibrated age of ~28,135 cal yr B.P. (table 1, location 06P49, 964 cm, sample altitude ~2,005 m 

(6,577 ft); Mumma and others, 2012). An age of about 20,000 years derived from a very small 

sample of organic sediment, with a median calibrated age of 19,783 cal yr B.P., located 16 cm 

above the base of Lake Centennial clayey silts that rest on alluvial gravel [(table 1, location 

04P37, altitude ~2,022 m (6,635 ft)]. Thirty meters downstream from this site, a channel-fill of 

yellow sand that apparently cut into the lower part of lake sediments yielded an optically 

stimulated luminescence (OSL) age of 16.8 ± 0.76 ka (table 2, 07P49); this supports a lake 

margin near an altitude of ~2,023 m (~6,636 ft) at this time.
6

Table 1. Radiocarbon and calibrated ages, Red Rock Lakes area, Centennial Valley, southwest Montana1

[cm, centimeters; mg, milligram; ft, feet; ~, approximately; cal yr BP, calibrated years before present]

Sample field ID 

and depth (cm) WW Lab # Material dated 14C age ± Delta 13C Median Calibage (cal yr BP)

2-sigma range

(cal yr BP) Remarks

Core, Lower Red Rock Lake2 Core through 8.5 m (28 ft) of lake sediments to alluvial gravel at altitude (alt.) of 6,580 ft.

06P49, 191–192 6741 Wood 3,055 30 -25 3,264 3,170–3,359 Core in southern part of Lower Red Rock 

Lake also contained Mazama and Glacier 

Peak ashes (Mumma and others, 2012). 

Cored 8.5 m of lake sediments overlying 

alluvial sand.2

06P49, 470–472 6157 Organic material 10,570 25 -10.25 12,549 12,426–12,625

06P49, 687–690 6158 Plant material 13,795 45 -14.32 16,433 16,079–16,814

06P49, 735–739 6172 Organic sediment 15,125 45 -25.06 18,559 18,133–18,708

06P49, 964 6173 Organic sediment 23,280 90 -26.91 28,135 27,812–28,490

Base of well-bedded Lake Centennial sediments on alluvial gravel

04P37, 500 5487 Organic sediment 16,660 120 -25 19,783 19,512–20,053

Base of lake sediments overlying gravel is 

at 6,635 ft altitude. Sample very small 

(0.08 mg).

Samples indicating lake levels near or below present

03P20, 68–69 4745 Peat 735 30 NA 692 657–726 West shore, Lower RR Lake. Buried A 

03P20, 75–77 4746 Humic A-horizon 12,210 40 NA 14,069 13,952–14,186 soil horizon at altitude ~6,606 ft

05P37, 85 5510 Peat 2,165 35 NA 2184 2,057–2,311 Core, SE shore Upper RR Lake. Present 

altitude of dated shallow lake samples 

now at @ 6,608, 6,600 and 6,596 ft

05P37, 346–349 5511 Peaty 11,560 35 NA 13,389 13,287–13,491

05P37, 455 5512 Plant filaments 12,675 52 NA 15,019 14,620–15,429

05P132, 310–314 5488 Plant fragments 10,220 25 NA 11,942 11,749–12,134 NW of airstrip, sample from shallow lake

Lake sediments? (silt, sand and clay) on gravel at 4.7 m Top of auger hole @ 6,660 ft

03P14, 301 4750 Organic sediment 12,260 35 NA 14,118 13,933–14,534 SW of Lower RR Lake. Sample alt.

~6,650 ft

Holocene alluvium and peat accumulation

05P104, 140 5514 Charcoal 3,685 35 NA 4,027 3,909–4,145 Beneath 140 cm of overbank clayey silt 

with weak soil along Odell Creek 

03P23, 56–58 4744 Peat 320 40 NA 389 301–483 Small valley SE of Lower RR lake began 

accumulating peat at this time

Buried humic A-horizons in dune sand

05P123, 35 5515 Humic soil 255 35 NA 300 (-4)–435 Buried A soil horizon 

05P123, 95–100 5516 A-horizon charcoal 510 35 NA ~531 502–627 Buried A soil horizon

1 All dates calibrated with Calib online calibration program (M. Stuiver, P.J. Reimer, and R. Reimer; http://intcal.qub.ac.uk/calib/). 2 Depth is below coring platform, which is 141 cm above lake bottom.
7

Table 2. Optically stimulated luminescence (OSL) ages of sands associated with Lake Centennial, southwest Montana.

[%, percent; ppm, parts per million; K, potassium; U, uranium; Th, thorium; Gy, Gray or absorbed radiation energy per unit mass; ka, thousands of years; yrs, years; NaI, sodium iodide; Ge, germanium]

Sample

information

% Water

contenta K (%)b U (ppm)b Th (ppm)b Cosmic doseadditions (Gy/ka)c

Total Dose

Rate (Gy/ka)

Equivalent

Dose (Gy) nd Age(ka)e

12P15 4 (28) 1.01 ± 0.03 1.63 ± 0.04 5.15 ± 0.23 0.13 ± 0.01 1.78 ± 0.06 37.4 ± 2.37 28 (30) 20.8 ± 1.53

Planar bedded fine beach sand [altitude (alt.) 6,610–6,615 feet (ft)] exposed along Red Rock River 500 meters (m) downstream of Brundage Bridge

12P17 16 (43) 2.03 ± 0.06 2.65 ± 0.09 9.45 ± 0.38 0.26 ± 0.02 3.22 ± 0.11 53.4 ± 1.91 34 (40) 16.6 ± 0.82

Pebbly alluvial sand (alt. ~ 6,624 ft) above lake sediments exposed along Odell Creek 200 m downstream from airstrip bridge

07P49 17 (22) 2.25 ± 0.10 2.52 ± 0.12 10.5 ± 0.32 0.17 ± 0.01 3.23 ± 0.07 54.1 ± 2.27 24 (24) 16.8 ± 0.76

Yellows alluvial sand and gravel lens in lowest part of lake sediments (alt. 6,636 ft) in bank of Red Rock Creek 200 m upstream from Elk Road bridge

07P50 9 (36) 1.83 ± 0.13 2.28 ± 0.08 8.77 ± 0.19 0.25 ± 0.02 3.02 ± 0.06 16.9 ± 1.22 9 (9) 5.60 ± 0.42

Horizontally bedded sand (alt. ~6,647 ft) near top of same section as 07P49 and unconformably beneath gravelly alluvium

12P43 18 (43) 3.25 ± 0.14 3.87 ± 0.16 14.1 ± 0.61 0.28 ± 0.02 4.87 ± 0.18 28.1 ± 1.62 19 (24) 5.78 ± 0.39

Massive fine sandy silt (loess or recycled loess?, alt. ~ 6,745 ft) on top of well-bedded sediments of Lake Centennial 50 m upstream from 07P50

a Field moisture, with figures in parentheses indicating the complete sample saturation %. Ages calculated using 25% of saturation values, except for 07P49 (75% saturation). b

Analyses obtained using laboratory Gamma Spectrometry (low-resolution NaI detector for the 2007 samples and high-resolution Ge detector for the 2012 samples). c Cosmic doses and attenuation with depth were calculated using the methods of Prescott and Hutton (1994). See text for details. d Number of replicated equivalent dose (De) estimates used to calculate the mean. Figures in parentheses indicate total number of measurements made including failed runs with unusable data. e

Dose rate and age for fine-grained 150–125 μm quartz sand. Linear + exponential fit used on age, errors to one sigma.
8

Table 3. Location by latitude and longitude of samples with ages shown in tables 1 and 2.

Sample Latitude Longitude

03P14 44.6135 -111.8593

03P20 44.6379 -111.8601

03P23 44.6124 -111.8207

04P37 44.6174 -111.6536

05P104 44.6104 -111.8031

05P123 44.6990 -111.8149

05P132 44.6292 -111.8013

05P37 44.5981 -111.6979

06P49 44.6226 -111.8425

07P45 44.6528 -111.9819

07P49 44.6174 -111.6540

07P50 44.6174 -111.6540

12P15 44.6528 -111.9819

12P17 44.6128 -111.8059

12P43 44.6174 -111.6537

Near the west map boundary, planar cross-bedded beach sands have an OSL age of 20.8 

± 1.53 ka, which is correlated to an early, low water phase of Lake Centennial at an altitude of 

2,015–2,016 m (6,610–6,614 ft; table 2, locality 12P15; Shannon Mahan, U.S. Geological 

Survey, written commun., July 16, 2013). The beach sand is at least 2.4 m thick, extending to 

near the level of Red Rock River. This beach sand is buried by eolian sand with weak calcic soils 

consistent with an age no older than late Pleistocene.

In a section exposed in the bank of Odell Creek, interbedded sands and pink, silty clay 

that are interpreted to be a Lake Centennial delta yielded an OSL age of 16.6 ± 0.82 ka (table 2, 

location 12P17; altitude of 2,019 m, 6,624 ft). Creek-bank exposures combined with augering 

indicate 3.7 m of well-bedded Lake Centennial sediments below the OSL sample that lies on preLake Centennial alluvial gravel and sand.

Pollen obtained from the core of Lower Red Rock Lake was analyzed to determine the 

climate and vegetation history (location 06P49; Mumma and others, 2012). The upper 3.7 m of 

the core that spans the interval 0 to 11,000 years ago is composed of marl, rich in organics (marl 

is used here for carbonate-rich mud), and probably post-dates Lake Centennial. Pollen from this 

interval indicates: (1) open pine-Douglas-fir-juniper forest from present to 2,400 years ago; (2) 

pine-Douglas-fir parkland steppe from 2,400 to 7,100 years ago; (3) Douglas-fir-pine forest 

steppe from 7,100 to 8,500 years ago; and (4) pine-forest steppe from 8,500 to 10,500 years ago. 

The 3.7 to 6.6 m depth spans an interval from 11,000 to 17,000 years ago and contains pollen 

that indicates spruce-pine-fir parkland. It has intermediate amounts of organics and carbonate 

that probably date from Lake Centennial. The 6.6 to 8.4 m depth spans an interval 17,000 to

28,000(?) years ago and has pollen indicating tundra vegetation. The sediment of this interval is 

silty clay and clayey silt, and is poor in both calcium carbonate and organic material, suggesting 

Lake Centennial was cold and deep at this time.

After a maximum lake height at an altitude of ~2,036 m (~6,680 ft, northeast shore) the 

lake lowered, presumably by erosion of its outlet. On the north side of the Centennial Valley, 

shorelines are particularly apparent on aerial photographs (blue lines on map sheet). These 

shorelines may be ice-thrust shorelines because beach sands were not observed, and appear 
9

similar to modern ice-thrust ridges along the margin of Upper Red Rock Lake (see map sheet). 

Lowering of Lake Centennial is also evident by decreasing levels of delta fronts, most notably of 

Tepee Creek (north side of valley) and Red Rock Creek (near east side of map). 

After progressive lowering of Lake Centennial to about the present lake levels (altitudes 

~2,014–2,015 m; 6,608–6,610 ft), lake level history is complex during the 14 to 9 ka interval, as 

dated by radiocarbon and OSL (tables 1 and 2). A complicating factor is southward tilting of the 

basin associated with the Centennial fault as well as the left lateral stepping of the fault in the 

eastern part of the map area (fig. 1). At the present altitude level of Lower Red Rock Lake along 

its west shore, a buried, dark A soil horizon yields an age of ~14,000 cal. yr B.P. (table 1, 

locality 03P20, 75–77cm, WW4746). This soil formed subaerially before the lake rose, which

led to its burial by 68 cm of peat with a basal age of ~700 cal yr B.P. (table 1, 03P20, 68–69 cm, 

W4745). Additionally, along the south shore of Lower Red Rock Lake, a calcareous soil on 

pebbly beach deposits (Qpb) about 3 m above the present lake is consistent with lake levels

being 3 m above present in latest Pleistocene time (~15–12 ka).

Near the eastern map boundary, two samples just above the top of well-bedded Lake 

Centennial sediments yielded mid-Holocene ages and provide a minimum age for Lake 

Centennial at an altitude near 2,025 m (6,645 ft). A massive, very fine sandy silt 55 cm thick that 

is likely primary or recycled loess at the top of well-bedded Lake Centennial sediments has an 

OSL age of 5.78 ± 0.39 ka. About 50 m downstream and above the well-bedded Lake Centennial 

sediments, planar bedded sands yielded an OSL age of 5.60 ± 0.42 ka (table 2, samples 12P43 

and 07P50 respectively; Shannon Mahan, U.S. Geological Survey, written commun., July 16, 

2013). Although these two ages suggest that Lake Centennial was near an altitude of 2,026 m 

(6,646 ft) in mid Holocene time, an OSL age from the center of the map area indicates that Lake 

Centennial lowered to 2,019 m (6,624 ft) at 16 ka. Given these conflicting OSL ages as well as

radiocarbon ages indicating that lake levels were near the present-day lakes in the latest 

Pleistocene (discussed below), it remains uncertain if Lake Centennial extended into the 

Holocene and therefore queries were placed along the Pleistocene–Holocene boundary in the 

Correlation of Map Units for Lake Centennial related deposits. 

On the southeast shore of Upper Red Rock Lake, peaty organics with calibrated 

radiocarbon ages of ~2 ka and ~13 ka and plant fibers dating 14 ka (05P37, 68–69, 346–349, 455 

cm; table 1) indicate a shallow lake during these times. Down-faulting of the Centennial Valley 

along the Centennial fault may explain the submergence of peat and fibrous vegetation to well 

below the present level of the modern lakes. The Centennial fault has about 10 m of vertical 

stratigraphic offset in post-glacial time (~14–12 ka). Such faulting tilts the basin into the fault

and deepens the basin towards the fault. The outflow location of Red Rock River downstream 

from Lower Red Rock Lake is less affected by this tilting because the river is farther away from 

the fault than the southern parts of both lakes. Thus the typical half-graben tilting towards the 

Centennial fault would result in several meters of submergence of the southern parts of the lakes 

after the latest Pleistocene. 

Radiocarbon dating of the lake history may be complicated by two sources of error. First, 

aquatic vegetation growing entirely underwater can incorporate bicarbonate dissolved from old 

limestone. Based on the ash-controlled age-depth plot for the Lower Red Rock Lake core 

(06P49), the magnitude of the error for submergent aquatic plant material could be ~1,700 years 

too old (sample 06P49, 470–472 cm; Mumma and others, 2012, fig. 2). Second, coal pebbles 

have been observed in deposits along Odell Creek derived from the Cretaceous sedimentary 

rocks of the Centennial Mountains west of Odell Creek. Thus, finely powdered coal may be 

present in Lower Red Rock Lake deposits and result in radiocarbon ages too old for dark 

“organic sediment” (table 1). Although numerical dating of Lake Centennial remains poorly 
10

resolved, this study concludes that the lake existed broadly between about 22,000 and 12,000 

years ago.

Figure 2. Diagrammatic cross section portraying flow of groundwater. Low permeability Lake Centennial 

sediments floor the central part of the valley and confine the groundwater in more permeable sands 

and gravels beneath them (see text). This semi-confined groundwater percolates upward and thus 

helps sustain the wetlands late in the growing season.

Surficial Deposits in Western Centennial Valley

Deposits that are correlative with Lake Centennial deposits described here are considered 

to be present in the western Centennial Valley (fig. 1; Myers and Hamilton, 1964; Hill and 

others, 2005; Anastasio and others, 2010). Along the hanging wall of the trench across the Lima 

Reservoir fault scarp (see fig. 1), Bartholomew and others (2002) interpreted the upper 4 m of 

fine-grained sand and silt units as loess with mostly stage II and III soil carbonate. The hanging 

wall of the trench is at an altitude of about 2,018 m (6,620 ft) and geomorphically the same as 

the floor of Lake Centennial elsewhere in the area. Loess thicker than half a meter is not 

described elsewhere on late Pleistocene deposits in the Centennial Valley (this report; Anastasio 

and others, 2010). We suggest that the four meters of “loess” with soil carbonate might also be 

interpreted as marl of lacustrine origin. 
11

Quaternary deposits that pre-date Lake Centennial are recognized at the Merrell

vertebrate locality 0.8 km southeast of the Lima Dam (fig. 1). Wave-cut cliffs of the Lima 

Reservoir expose a rich vertebrate fauna (Hill and others, 2005). This includes a lower Stratum A 

of largely, poorly sorted, fine sand and sandy, silty clay of possible lake origin with ages of about 

60 ka by OSL dating and 50–45 ka by radiocarbon dating. Stratum B occurs above Stratum A

and includes sediment deposited in a marsh with radiocarbon ages of >44 ka to 32 ka (Hill and 

others, 2005). 

Alluvial Deposits

Deposits of alluvial fan gravel (Qpfg) have accumulated where drainages decrease 

gradient and spread out from confined bedrock valleys onto the open valley floor. Transport and 

deposition of alluvial fan gravels were more active under Pleistocene glacial climates than under 

the Holocene interglacial climate (Ritter and others, 1993: Hanson and others, 2006). Compared 

to the present Holocene climate, the longer cold-season climate under Pleistocene conditions led 

to deeper accumulation of snow. Seasonal melting of these snows occurred later when solar 

insolation was stronger resulting in greater peak discharge (Pierce and Scott, 1982). Also, the 

colder Pleistocene climate enhanced colluvial movement of detritus from slopes to valley floors. 

Geomorphic evidence of streams draining into Lake Centennial is recorded by subtle but

steeper slopes of delta fronts (Qpd), and commonly associated alluvial fan gravels graded to 

these delta fronts (Qprg). These are best displayed at Red Rock, Odell, and Tepee Creeks. Such 

delta fronts, as well as other shoreline features, are scarce to absent on the Curry–Matsingale 

Creek “fan form” west of Lower Red Rock Lake.

Stream activity during Holocene time is generally restricted to present stream courses 

(Qal) and has built sandy deltas (Qhd) into Upper and Lower Red Rock Lakes. Along the 

Holocene meander belt of Odell Creek, alluvial clayey silt at a depth of 1.4 m has a median 

calibrated radiocarbon age of 4,027 years (table 1, 05P104, 140 cm; 3,685 ± 35 yr BP). 

Glacial Deposits

Glacial moraines of Pinedale age form bulky accumulations on the hanging wall of the 

Centennial fault in the southeast part of the map area (Qym). These moraines occur at the lower 

end of about eight very steep valleys that head in amphitheater-shaped glacial cirques on the 

northern front of the Centennial Range. The moraines correlate with the last glaciation, known as 

the Pinedale, which culminated in the northern Rocky Mountains about 18,000–15,000 years ago 

(Licciardi and Pierce, 2008; Thackray, 2008). These Pinedale moraines are likely piled on top of 

Bull Lake moraines (Qom) contributing to their bulk and large apparent volume. Glacial 

outwash extends northward from these moraines forming Pinedale alluvial fans (Qpfg). As 

diagrammed in figure 2, water from the mountain slopes infiltrates into the moraines to become 

groundwater flow and surface runoff, which now seldom flows across the dry outwash fans 

(Qpfg). The groundwater flow then reemerges as nearly continuous springs in the wetlands 

above the south shore of Upper Red Rock Lake. 

West of Odell Creek, the terrain reaches altitudes as high as the glaciated terrain east of 

Odell Creek suggesting the extent of glaciation is likely to be similar, although Pinedale 

moraines were identified only along Collins Creek (Qym) and Odell Creek (see fig. 1). Pinedale 

glaciers probably also occupied valleys that head as high as Collins and Odell Creeks, but 

moraines are not shown on the map because they either: (1) transition to landslides farther down 
12

valley, or (2) are obscured by landsliding and forest cover. Possible older moraines of Bull Lake 

age (Qom) are mapped just east of the Odell Creek fan and along Collins Creek.

Eolian Deposits

An extensive tract of sand dunes occurs on the northern side of the Centennial Valley 

(Qd). The dunes stand as high as 30 m and form longitudinal dunes aligned southwest to 

northeast. This orientation appears to be the combined effect of winds from both the south and 

west as shown by wind rose diagrams of winds stronger than 30 km per hour 

(http://www.raws.dri.edu/cgi-bin/rawMAIN.pl?inMRED, period 1988–2010, accessed 2011). The 

dunes appear to be largely stable and the bulk of the sand accumulation may date from the late 

Pleistocene and have been deflated from the floor and beaches of Lake Centennial. Local dune 

areas about 10 m across exhibit active blowout hollows and contain rare plant species (U.S. Fish 

and Wildlife Service, 2009). Late Holocene eolian sand activity is indicated by dark gray, A soil 

horizons buried by additional eolian sand. In the northern dune area, a humic A horizon located 

35 cm below the surface dates to about 300 years old and another A horizon located about 50 cm 

deeper dates about 500 years old (table 1, locality 05P123). A linear tract of sand dunes is also 

located in the west-central part of the map area in the area occupied by Lake Centennial and 

therefore post-dates the lake. This tract trends southwest to northeast, intersects the Red Rock 

River, and may act as a source of sediment building up the river level (William Locke, oral 

commun., 2006).

On the north side of the valley, a sheet of sand (Qsd) occurs between the highest level of 

Lake Centennial and dune sand (Qd). The sand may be distributed by both wind transport to the 

northeast and alluvial transport to the south.

Wind-blown silt (loess) is present on alluvial (Qpfg) and lacustrine (Qpl, Qpla) deposits, 

but is typically only a quarter- to half-meter thick and is mixed into the underlying deposits. 

Landsliding

Large landslides (Qls, Qef) are mapped on the north slope of the Centennial Range, west 

of Odell Creek, and are developed by incompetent mudstone and shale beds from the Cretaceous 

Frontier Formation. Earthflows (Qef) are mapped where lower part of landslide has lobe-like 

form. Landslides also are found in the northeast part of the map area and farther east, where they 

block an ancestral northeast drainage of the Centennial Valley that is now marked by a chain of 

lakes that include Elk, Hidden, and Cliff Lakes. 

Surface and Groundwater

The flow of groundwater is diagrammed in figure 2. With high surface runoff during the 

spring snowmelt, surface water from Red Rock, Odell, Elk, Tom, and other creeks flow through 

the central wetland inside the “water plane” in a manner somewhat comparable to that of a 

freshwater lake. On a much slower time scale, water largely from snowmelt percolates down in 

the earth and joins the groundwater. Groundwater recharge is probably large in the high, snowy, 

Centennial Mountains and adjacent alluvial fans on the south side of the valley, and moderate in 

the lower, less snowy, upland terrains of sand dunes, alluvial fans, and rhyolite tuffs on the north 

side of the valley (fig. 2). Groundwater flows from the uplands towards the low, central part of 

the Centennial Valley. Shallow groundwater flows downslope through permeable alluvial fan 

gravels and then continues through fan gravels and valley-center sands that are beneath low-
13

permeability Lake Centennial sediments (fig. 2). At several sites cored through Centennial Lake 

sediments, permeable sand and gravel were found beneath the lake sediments and groundwater 

commonly rose in auger holes when this aquifer was encountered. 

This slow-moving groundwater dissolves soluble minerals as it percolates through the 

ground. In places where groundwater is at or near the surface in this semiarid landscape, 

evaporation and evapotranspiration concentrates the dissolved material from this groundwater. 

Springs and green vegetation, particularly in mid-summer, indicate where the groundwater table 

approaches the surface; these areas are commonly located between the upper limit of Lake 

Centennial sediments and the “water plane.” Evaporation of groundwater is indicated by the 

saline-alkaline soils mapped by Nielson and Farnsworth (1965) on low beach ridges and adjacent 

swales that are commonly just above the “water plane” in most places except south and east of 

Upper Red Rock Lake.

Late in the growing season, surface water runoff diminishes significantly throughout the 

wetland area inside the “water plane” and groundwater works its way up through the Lake 

Centennial sediments and provides moisture needed for plant life. Beneath ponds and lakes in the 

central wetland, driving open tubes down into the upper part of the lake sediments and 

measurement of groundwater head commonly show upward gradients later in the growing season 

but locally may show downward gradients early in the growing season (Greenwood and others, 

2011). Head pressures appear to be dynamic both seasonally and annually.

Description of Surficial Geologic Map Units, Red Rock Lakes Area, 

Southwest Montana

Unconsolidated Surficial Deposits

Qal Alluvium (Holocene and late? Pleistocene)—Poorly consolidated alluvium of sand, 

gravel, and silt adjacent to and near the level of modern streams. Upper part of unit 

commonly fine grained (clay, silt, and sand), and rich in organic material. Lower part 

of the unit contains sand and gravel and may include late Pleistocene deposits, 

particularly at depth. Locally more than 3 m thick along Odell Creek, charcoal at 1.4 

m below floodplain surface has an age of about 4,000 years (table 1, 05P104, 140

cm, WW 5514). Generally no thicker than a few meters 

Qhd Delta deposits (Holocene)—Tan sand to silt, moderately well sorted. Deltas deposited 

by Odell Creek and Red Rock Creek and extend into present Upper and Lower Red 

Rock Lakes. Odell Creek delta has “bird foot” form. Probably a few meters thick 

Qhss Silty shoreline complex (Holocene)—Silty shorelines along north shore of Upper Red 

Rock Lake. Built by the lake at present level with crests no more than a half meter 

above Upper Red Rock Lake. Complex consists of about five linear shorelines that 

trend southeast-northwest and were built successively to the southwest. The southern 

face of the shorelines is commonly water saturated and can act like a “quick-silt” if 

walked on. Shorelines of muddy silt were probably created by the wind driving lake 

ice and shallow-lake sediments northeastward into the shore and are formed by a 

similar process to peaty thrust ridges shown on the south and southwest shore of 

Upper Red Rock Lake. The linear trend of the shorelines over several kilometers 
14

may have been controlled by offset along a postulated WNW-trending fault at the 

northern margin of the shoreline complex. Thickness probably a few meters 

Qsd Alluvial and eolian sand undivided (Holocene and late Pleistocene)—Tan sand, 

mostly fine and medium grained, with a well-drained soil. Forms gently sloping sand 

sheet that contrasts with higher local relief of nearby sand dunes. Mapped east of 

Teepee Creek where deposit has an overall gentle south slope as well as low slopes 

parallel to local drainages from the northeast. Mapped west of Teepee Creek between 

dune sands and lake sediments at and above upper level of the late Pleistocene lake 

(altitude ~6,670 feet). Unit may be a combination of sandy alluvium washed south 

from dune and eolian sand deflated from Pleistocene lake sediments 

Qd Dune sand (Holocene and late Pleistocene)—Tan, fine to medium grained, well-sorted 

sand, with a well-drained soil. Sand grains rounded and frosted. Forms linear sand 

dunes with northeasterly orientation that is the combined result of westerly and 

southerly winds, as shown by wind rose diagrams (http://www.raws.dri.edu/cgibin/rawMAIN.pl?inMRED, period 1988–2010, accessed 2011). The main dune 

accumulation was probably in late Pleistocene time. Local continued activity 

indicated by shallow buried soil horizons that at 35 cm deep, dates about 300 years 

old and at 95–100 cm deep dates about 530 years old (location 05P123; table 1). 

Also includes blow-outs with active, poorly vegetated sand. Thickness 1 m to ~30 m 

Qls Landslide deposits, undifferentiated (Holocene and late Pleistocene)—Coarse rock 

fragments in a muddy matrix. Irregular surface topography with undrained or poorly 

drained depressions. Landslides with less definitive topographic expression are 

probably more extensive than mapped in the irregular mountain slopes south of 

Lower Red Rock Lake. In northeast part of map area, the main landslide area was 

mapped by Sonderegger and others (1982a) as “Huckleberry Ridge Tuff—displaced 

by mass wasting” and smaller area near northern boundary as “Tertiary basalt—

displaced by mass wasting.” In northeast corner of map area and farther east, 

landslides of Huckleberry Ridge Tuff dammed an old drainage of Red Rock River 

now partly occupied by Elk Lake. Irregular topography mapped near Matsingale 

Creek southwest of Lower Red Rock Lake mapped as “Qls?” is questioned because 

it may also include Pinedale glacial moraines

Qef Earthflow deposits (Holocene and late Pleistocene)—Coarse, angular rock fragments 

in a muddy matrix. Deposit has flow-like form with levee-like marginal ridges, lobelike form in lower part, and generally a steep front at its lower end. Irregular surfaces

include undrained depressions. Probably mostly active in the late Pleistocene, but 

locally steep fronts near angle of repose suggest some Holocene activity. A waterwell drilled into and through the earthflow west of Lakeview encountered waterbearing gravel that is probably the same gravel mapped as “thrust gravel” (see Qtg) 

beyond the toe of the earthflow

Qhpc Peat of central wetland (Holocene)—Peat, dark brown, fibrous. Occurs inside “water 

plane” denoted by green line, where peaty soils support extensive beaked sedge 

(Carex utriculata) vegetation. “Water plane” is nearly a flat surface in the central 

part of valley that slopes very gently to the west (0.6 m in 12 km; 2 feet in 8 miles) 

and is wetted during much of the growing season. The peat is 0.5 m or more thick 

and locally overlies a buried humic paleosol. Two ages from the base of the peat 
15

located in the western part of the central lowland peat are ~700 years ago (03P20) 

and ~2,200 years ago (05P37; table 1). Below peat of the central wetland are lake 

sediments, ranging from pale-tan, shelly marl, silty clay, to fine sand. Location of 

“water plane” is locally marked by areas of rush (Juncus balticus) that is nearly 

black on aerial photographs and is just above and outside the “water plane.” Lack of 

oxidation of peat over time indicates long-term saturated conditions probably due to 

upward percolation of groundwater (fig. 2). Unit surrounds Upper and Lower Red 

Rock Lakes and many shallow ponds that are shown by blue color. May locally 

contain alluvial sand and silt 

Qhpa Peat above water plane (Holocene)—Fibrous, dark brownish-gray peat formed mostly 

from sedge (Carex) and willows. Separately mapped above the “water plane” in only 

two areas—(1) southeast of Lower Lake, and (2) east of Upper Lake, although local 

peat occurs elsewhere. Peat one-half to several meters thick. Associated with 

emergent spring water and spongy ground on terrain that slopes gently towards the 

lakes

Qpfg Alluvial fan gravel (late Pleistocene)—Gravel with interbeds of silt, sand, and mud. For 

alluvial fans of larger drainages, gravel is subrounded. For smaller drainages gravel 

is subangular and platy. Upper meter is commonly mixed with loess and other fine 

material. Exposures deeper than a meter are very rare but at depth are probably clast 

supported gravels with a well-washed sand matrix as indicated by exposures into the 

older but similar gravel (thrust gravel, Qotg) near Lakeview. Lithology of gravel 

reflects the bedrock upslope. For fan gravel derived from the Centennial Range east 

of Odell Creek, gravel clasts mostly limestone and deposited as outwash associated 

with glacial moraines at foot of the range. Locally adjacent to glacial moraines, 

includes bouldery mounds suggestive of debris flows. For fan gravels west of Odell 

Creek, gravel clasts mostly basalt, rhyolite, quartzite, and sandstone. For fan gravel 

north of Upper and Lower Red Rock Lakes, pebble gravel is rich in rhyolite tuff. 

Alluvial fan gravel (Qpfg) generally has weak calcic soil horizon 50–75 cm thick 

and only thin (1 mm) carbonate coats on underside of stones. Probably overlies 

similar but older alluvial fan gravels deposited as the Centennial Valley aggraded 

during Quaternary time. Water from Centennial Range percolates through alluvial 

fan gravel (Qpfg) and nourishes band of darker vegetation inferred to be associated 

with less permeable lake sediments (Qpla; fig. 2). Although alluvium (Qal) is 

mapped along larger drainages where topographically inset into this map unit 

(Qpfg), smaller, narrow bands of Holocene alluvium are not mapped separately

Qfgu Undifferentiated fan gravel (late Pleistocene)—Small alluvial and colluvial fans of 

small, intermittent drainages. Mapped along range front between the larger fans and

larger drainages. Sorting less than that of fans from larger drainages and may include 

debris flow deposits. Clasts are typically subangular. The fan surface is generally of 

late Pleistocene age, but probably overlies older fan gravel

Qprg Recessional alluvial fan gravel and sand (late Pleistocene)—Forms pie-shaped sectors 

of alluvium graded to deltaic lake-margin deposits at and below the upper level 

(2,036 m, 6,680 ft) of Lake Centennial. Similar to alluvial fan gravel (Qpfg), and the 

unit formed as Lake Centennial receded and the larger streams (Odell, Red Rock, 

Tom, and Tepee Creeks) prograded to increasingly lower altitudes. Thickness 
16

roughly 1–2 m. Gravel and sand composition reflect the source of sediment carried 

by the streams, as described for Pleistocene alluvial fan gravel (Qpfg)

Qpla Lake Centennial deposits and alluvium (late Pleistocene)—Complex of fine-grained, 

bedded lake sediments and alluvium. Alluvium consists of gravelly sand to sandy 

mud and becomes thinner and finer towards the valley axis. Mapped on south side of 

the Centennial Valley where unit has morphology of low gradient “fans” but the 

surface slope of deposit is commonly much less than that of the alluvial fan gravel 

(Qpfg) farther upslope. Upper limit of unit inferred from dark vegetation indicates

groundwater emergence thought to be associated with low permeability lacustrine 

sediment. The conical “fan” form of the complex is considered to be largely 

inherited from that of older Pleistocene alluvial fans buried by Lake Centennial 

sediments (Qpl). Map unit commonly capped with several decimeters of loess. 

Recognizable shorelines are only locally mapped on south side of the Centennial 

Valley in contrast to shorelines being common on north side of the valley. South and 

east of Upper Red Rock Lake, the upper limit of unit is quite uncertain but mapped 

to include wet, peaty ground. Along Odell Creek 1.5 km north of Lakeview, a 

combined bluff and auger section displays the following units: 0–30 cm pebbly 

sandy loam, (alluvial), grayish brown A and B soil horizons; 30–40 cm pebbly sand, 

white (dry) Bk horizon; 40–75 cm silt and fine sand, Bk horizon; 75–130 cm silt and 

very fine sand, beds 5–10 cm thick, effervesces weakly; 130–180 cm three couplets 

of tan sand grading upward into pink clayey silt, interpreted to be deltaic alluvial 

sand capped by lake sediment (middle sand has OSL age of 16.6 ± 0.82 ka, locality 

12P17, table 2); 180–270 cm bedded dark gray silt, pink clayey silt, and very fine 

sand, effervesces strongly (lacustrine); 270–430 cm well-bedded dark gray silt and 

pink clayey silt, varved(?), effervesces weakly; 430–450 cm sandy interval, coarse to 

fine sand beds; 450–520 cm well-bedded dark gray silt and pink clayey silt, 

effervesces weakly; 520–550 cm gravel and sand, water rises in hole (aquifer). 

Section contains small coal pebbles 

Qpl Lake Centennial deposits (late Pleistocene)—Well-bedded sand, silt, and clay. Sandy 

near surface generally grading downward to silt and silty clay. Oxidizes to tan but 

fresh auger and core samples are greenish-gray below water table. On north side of 

valley, upper lake margin at ~6,670–6,680 ft altitude. Upper part of lake sediments

generally effervesces strongly in hydrochloric acid whereas lower part effervesces 

moderately to quite weakly. On north side of Centennial Valley and in western part 

of map area, unit occurs adjacent to extensive sand dunes and lake sediments that

may in part result from lacustrine reworking of old eolian sand that predates Lake 

Centennial. Surface of deposit commonly shows low horizontal shoreline ridges 

(shown by blue lines on map), notably apparent on aerial photographs and may be 

either beach deposits or ridges formed by wind-driven thrusting of lake ice into 

shore. Deposit mantled with 20–50 cm of loess. Lake sediments may be locally 

overlain by thin alluvium and eolian sand, and silt. Adjacent to Culver Pond dam, 

lacustrine deposits are 5 m thick that grade upward from clayey silt to silt to 

interbedded silt and fine sand and to fine sand. Above the lacustrine deposits is 

pebbly, alluvial sand in which a soil with a calcic horizon 75 cm thick is developed. 

Below the lake sediment is >1 m of sandy gravel, which makes a good aquifer (fig.

2). Along Red Rock River near the west margin of the map, well-sorted beach sand 

with low-angle planar cross beds yielded optically stimulated luminescence (OSL) 
17

age of 20.8 ± 1.53 ka (locality 12P15, altitude ~6,610–6,615 ft, table 2; Shannon 

Mahan, U.S. Geological Survey, written commun., July 16, 2013). This deposit 

apparently represents a low level of Lake Centennial in its early history. Along Red 

Rock Creek near east margin of map, the base of varved(?) Lake Centennial 

sediments have a median calibrated radiocarbon age of 19,783 years (locality 04P37, 

altitude ~6,635 ft, table 1, John McGeehin, written commun., 2006) and an alluvial 

sand nearby in the lower part of the Lake Centennial sediment has as OSL age of 

16.8 ± 0.76 ka (locality 07P49, altitude ~6,636 ft, table 2). 

Qpd Lake Centennial deltaic deposits (late Pleistocene)—Sand and gravel deposits of larger 

streams at and below the upper level of Lake Centennial. Generally recognized by: 

(1) steeper slope of inferred delta front, (2) horizontal trend of inferred shoreline at 

lower margin of deposit, and (3) location downslope from pie-shaped sectors of 

alluvial fan sediments. Along east map boundary on north side of Red Rock Creek, 

map unit includes delta top at 6,680 foot contour, which is the highest Lake 

Centennial delta mapped 

Qpb Low beach and shoreline ridges (late Pleistocene?)—Sand, silt, and pebble gravel 

forming shoreline(s) within 3 m elevation of Upper and Lower Red Rock Lakes.

Mapped only where topographically expressed. On south shore of Lower Red Rock 

Lake, sand and pebbly sand has an alkaline calcic soil with 50-cm-thick, light tan Bk 

horizon that probably indicates late Pleistocene age. On north shores of Lower and 

Upper Red Rock Lakes, locally forms low ridges of silt and sandy silt that may be of 

ice-thrust origin and younger than the Pleistocene 

Qym Glacial moraines, younger (late Pleistocene, Pinedale glaciation)—Unsorted deposits 

of gravel, sand, silt, and clay. East of Odell Creek on north side of Centennial Range, 

forms bouldery morainal ridges rich in limestone and some Precambrian green 

schist. Moraines at the five valley mouths that were examined have steep distal 

slopes (>25°) and undrained depressions indicating a Pinedale rather than Bull Lake 

age. Moraines mostly forested with small meadows. Sharp-crested moraines are 

offset ~10 m by the Centennial fault. South of the map area, Pinedale moraines occur 

along Odell Creek (fig. 1). West of Odell Creek, the only Pinedale moraine mapped 

is along the valley of Collins Creek (the drainage east of Curry Creek) where glacial 

deposits contain clasts of basalt, quartzite (some striated), sandstone, and rhyolite. 

Deposits support a mosaic of forest and meadows. Farther down Collins Creek 

valley, material mapped as questionable landslide (Qls?) probably includes Pinedale 

morainal material. Centennial Mountains west of Odell Creek are as high as the 

glaciated range east of Odell Creek and probably supported unmapped Pleistocene 

glaciers, but moraines were not observed in this rugged, forested, extensively 

landslid terrain 

Qofg Older fan gravel (middle? Pleistocene, Bull Lake glaciation?)—Gravel, poorly sorted 

with boulders up to a meter across. Only deposit mapped is just south of Upper Red 

Rock Lake Campground. Well-developed Bk soil horizon with continuous carbonate 

in matrix and carbonate coats on stones 2–4 mm thick. Forms low mound with 

younger fan gravel inset on west and east sides. Limestone boulders on surface have 

5–10 cm deep weathering pits 
18

Qtg Thrust gravel (middle? Pleistocene, Bull Lake glaciation?)—Coarse, rounded gravel, 

clast supported with a sandy matrix. Located just north of Lakeview at leading 

margin of large earthflow. Surface of gravel forms ridges inferred to form by 

imbricate thrusting of alluvial gravel driven by the weight of the earthflow upslope. 

Soil Bk horizon more than half a meter thick with carbonate coats ~2 mm thick 

indicates a Pinedale or older age for the original gravel whereas the inferred 

landslide-driven thrusting occurred later; perhaps continuing into the Holocene. In 

well drilled for Lakeview bunkhouse, this gravel deposit overlies clayey sediment 

and thus the gravel may have been thrust along the top of this clayey sediment 

Qom Glacial moraine, Older (middle Pleistocene, Bull Lake age?)—East of Odell Creek, 

mapped where Alden (1953, p. 175) distinguished an older moraine into which a 

valley was eroded by “an interglacial stage of stream erosion” prior to deposition of 

the younger (Pinedale) moraines. West of Odell Creek and down valley from 

Pinedale moraines of Collins Creek (the stream east of Curry Creek), mapped as 

“Qom?” for gentle, arcuate ridge and swale deposit studded with very large, 

scattered boulders. An alternate interpretation is that the deposit is an old landslide 

with subdued arcuate ridges that resemble end moraines 

Qbd Upland bench deposits (early Pleistocene?)—Bouldery bench deposits in southwest 

part of map area. An 8-m-thick section exposes mostly subangular to angular basalt 

boulders in matrix-supported, debris-flow deposit. Rounded reworked quartzite 

clasts are locally common as well as some clasts of porous limestone. Although 

considered by Witkind and Prostka (1980) to be a pediment deposit, local thickness, 

boulders, and matrix-supported fabric indicate at least part of unit is a thick debris 

flow deposit. Carbonate coats, 0.5 cm thick, and deep incision indicate an age 

considerably older than the last glaciation (Pinedale) 

Volcanic Rocks

Qba Basalt, (early Pleistocene?)—Dark gray, dense basalt with sparse olivine phenocrysts. 

Mapped only south of Elk Lake where it has K-Ar age of 1.95 ± 0.35 Ma and 

considered to be younger than Quaternary Huckleberry Ridge Tuff (Sonderegger and 

others, 1982a, 1982b) 

QNt Welded tuffs (early Pleistocene and Neogene)—Siliceous rhyolite with phenocrysts of 

quartz and feldspar. Pinkish-gray, gray to brown welded tuff. Forms much of the 

terrain on north side of map area where it dips gently (~5 degrees) to the south as a 

result of tilting toward the active Centennial normal fault. Although mapped as the 

Huckleberry Ridge Tuff by Sonderegger and others (1982a), and O’Neill and 

Christiansen (2004), much of this unit probably includes older tuffs. The Kilgore 

Tuff (4.45 ± 0.05 Ma) is recognized in this area (Morgan and McIntosh, 2005, figure 

2A). Robert Christiansen (2001, plate 3, and written commun., 2012) mapped only 

the southern part of the area between Elk Lake and Tepee Creek as Huckleberry 

Ridge Tuff and mapped the larger, remaining northern part of the map area as tuff

that predated the Huckleberry Ridge Tuff. The 2.05 Ma Huckleberry Ridge Tuff is 

the first and largest major eruption from the Yellowstone volcanic field to the 

southeast of the map area (fig. 1) (Christiansen, 2001), whereas the older tuffs 
19

originated from the Heise volcanic field (4.4 to 6.6 Ma) to the south and southwest of 

the map area (fig. 1; Morgan and McIntosh, 2005)

Nba Basalt (Neogene)—Dark gray basalt. Upper part of flow is vesicular and middle part is 

commonly columnar jointed. Commonly contains small olivine phenocrysts. 

Weathers to reddish iron-oxide colors. Mapping after Sonderegger and others 

(1982a) where classified as “Pliocene”

Sedimentary Rocks

Ns Sedimentary rocks (Neogene)—Light gray to cream, dense limestone and light gray to 

off-white, fine- to medium-grained sandstone. This unit includes both Tertiary 

sandstone and Tertiary limestone of Sonderegger and others (1982a, their units Ts

and Tls, respectively) and compiled as Sixmile Creek Formation by O’Neill and 

Christiansen (2004). Occurs on slopes on the north side of map where it is overlain 

by upper Cenozoic tuffs. Mapping after Sonderegger and others (1982a) where unit 

classified as lower Miocene(?) 

Conclusion

Ongoing offset on the Centennial fault has created the Centennial Valley and Centennial 

Range. Offset is dominantly (~85 percent) by downward movement of the valley side of the 

fault. This downward movement of the Centennial Valley creates space for sediment to 

accumulate as sediment is carried by streams by erosion of the adjacent bedrock uplands. The 

valley once drained north through a paleo-canyon into the Madison Valley, and the Centennial 

Valley was drained by an axial stream depositing sand and gravel and flanked by gravelly 

alluvial fans. This canyon was then blocked by landslides to create Lake Centennial. Sediments 

of this late Pleistocene Lake Centennial compose the floor of the central part of the valley and 

are flanked on the north and south by late Pleistocene alluvial fans (Qpfg). Cold Pleistocene 

climates enhanced colluvial supply of gravelly sediment to mountain streams and high peak 

flows carried gravelly sediment into the valley where the lower gradient of the streams resulted 

in deposition of alluvial fans. At this time, Pleistocene glaciers built glacial moraines (Qym) and 

also supplied glacial outwash to the alluvial fans. As Lake Centennial lowered, presumably by 

erosion of its outlet near Lima Dam, lower shorelines and deltas (Qpd) formed. During and after 

the drainage of Lake Centennial, winds from the southwest blew sand to the northeast side of the 

valley creating high sand dunes (Qd). 

The central part of the map area is nearly horizontal, sloping to the west only 0.6 m in 13 

km (2 feet in 8 miles). This forms a flat, watery, lowland that contains upper and lower lakes, 

many ponds, and peat lands included inside the nearly flat “water plane” that is bounded by 

somewhat steeper slopes. Beneath the sediments of Lake Centennial are permeable sands and 

gravels that host groundwater recharged by surface and subsurface flows from the adjacent 

mountains and uplands (fig. 2). Groundwater from these sands and gravels leaks upward to 

sustain local vegetation adjacent to and within the central wetland (fig. 2). Alluvial fans converge 

from both the south and the north to form thresholds that serve as outlets for Upper and Lower 

Red Rock Lakes held upstream. Understanding of this surficial geology enhances understanding 

of how the landscapes operate in and around the Red Rock Lakes National Wildlife Refuge.
20

Acknowledgments

The radiocarbon ages were provided by Jack McGeehin of the U.S. Geological Survey 

(USGS). Shannon Mahan of the USGS graciously provided and discussed the OSL ages. Jim 

Schmidt offered expertise on interpretation of some sediment exposures. Steve Custer critiqued 

the section on groundwater. Stephanie Mumma and Cathy Whitlock studied the core from Lower 

Red Rock Lake. Bill West, Mike Parker, Jeff Warren, and Jackie Vann of the Red Rock Lakes 

National Wildlife Refuge provided help in many ways. Lois Walsh graciously granted access to 

the moraines and the Centennial fault in the eastern part of the map area. Malvina Adolph, 

Ashley Yearous (Jaquez), Aurelie Dufour, Matt Trebesch, R. Hayes Buxton, and Don

Rosenberry helped with the field and office work. We thank Marith Reheis, Bill Locke, and 

Linda Pierce for very helpful reviews of this report. Christine Chan and Lisa Ramirez Rukstales 

edited this report and prepared the map and text for release.

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ISSN 2331-1258 (online)

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