Bent’s Old Fort National Historic Site 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2005/005 

National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 
THIS PAGE: 

Inside Bent’s Old Fort, Bent’s Old Fort NHS. 

ON THE COVER: 

Bent’s Old Fort from the surrounding area, Bent’s Old Fort 

NHS 

Photos by: NPS 
Bent’s Old Fort National Historic Site 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2005/005 

Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, Colorado 80225 

September 2005 

U.S. Department of the Interior 

Washington, D.C.
The Natural Resource Publication series addresses natural resource topics that are of interest and 

applicability to a broad readership in the National Park Service and to others in the management of 

natural resources, including the scientific community, the public, and the NPS conservation and 

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designed and published in a professional manner. 

Natural Resource Reports are the designated medium for disseminating high priority, current natural 

resource management information with managerial application. The series targets a general, diverse 

audience, and may contain NPS policy considerations or address sensitive issues of management 

applicability. Examples of the diverse array of reports published in this series include vital signs 

monitoring plans; "how to" resource management papers; proceedings of resource management 

workshops or conferences; annual reports of resource programs or divisions of the Natural Resource 

Program Center; resource action plans; fact sheets; and regularly-published newsletters. 

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Printed copies of reports in these series may be produced in a limited quantity and they are only 

available as long as the supply lasts. This report is also available from the Geologic Resource Evaluation 

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sending a request to the address on the back cover. 

Please cite this publication as: 

KellerLynn, K. 2005. Bent’s Old Fort National Historic Site, Geologic Resource Evaluation Report. 

Natural Resource Report NPS/NRPC/GRD/NRR—2005/005. National Park Service, Denver, Colorado. 

NPS D-74, September 2005 
BEOL Geologic Resource Evaluation Report iii 

Table of Contents 

Executive Summary ...................................................................................................... 1

Introduction ................................................................................................................... 3

Purpose of the Geologic Resource Evaluation Program ............................................................................................3

Geologic Setting .........................................................................................................................................................3

Geologic Issues............................................................................................................. 8

Eolian Deposits, Climate Change, and Wind Erosion.................................................................................................8

Flooding......................................................................................................................................................................8

Groundwater.............................................................................................................................................................10

Mineral Resources....................................................................................................................................................11

Paleontological Resources .......................................................................................................................................12

Soil Compaction .......................................................................................................................................................12

Streamflow and Stream Channel Morphology ..........................................................................................................13

Surface Water Quality and Level..............................................................................................................................13

Wetlands ..................................................................................................................................................................13

Geologic Features and Processes............................................................................. 15

Arkansas River .........................................................................................................................................................15

Eolian Deposits.........................................................................................................................................................16

River Terraces..........................................................................................................................................................16

Structural Features...................................................................................................................................................17

Map Units and Properties........................................................................................... 18 

Map Unit Properties Table........................................................................................................................................20

Regional Geologic Setting.......................................................................................... 21

References Cited......................................................................................................... 24

Appendix A: Geologic Map......................................................................................... 26

Attachment 1: Digital Geologic Map CD 
iv NPS Geologic Resources Division 

List of Figures 

Figure 1: Location Map for Bent’s Old Fort National Historic Site...............................................................................5

Figure 2: Physiographic Provinces of the Conterminous United States......................................................................6

Figure 3: The Great Plains Physiographic Province. ..................................................................................................7

Figure 4: Topographic Map of the Vicinity of Bent’s Old Fort National Historic Site. ................................................14

Figure 5: Rock Units in the Vicinity of Bent’s Old Fort. .............................................................................................19

Figure 6: Geologic Time Scale. ................................................................................................................................23
BEOL Geologic Resource Evaluation Report 1 

Executive Summary 

This report has been developed to accompany the digital geologic map produced by Geologic 

Resource Evaluation staff for Bent’s Old Fort National Historic Site. It contains 

information relevant to resource management and scientific research. 

Bent’s Old Fort National Historic Site was not 

established as a “geologic” park; however, its location in 

the Colorado Piedmont makes it part of both the human 

history and the geologic history of the Great Plains 

(Fig 1). 

The focus of the resource management program at Bent’s 

Old Fort is to maintain the historic character of the park 

(National Park Service, 1993). Understanding geologic 

processes and features that are part of the site will help 

resource managers reconstruct the historic landscape 

and anticipate natural hazards that could damage 

cultural resources. 

At first “geologic glance,” the national historic site seems 

simply to highlight the geologic processes that occur on a 

floodplain in a fluvial setting. However, in addition to 

flooding of the Arkansas River, many other geologic 

processes and features occur in the vicinity of Bent’s Old 

Fort: 

• Eolian Deposits and Wind Erosion—Windblown sand 

and loess (windblown dust) blanket large parts of 

eastern Colorado, including the south side of the 

Arkansas River near La Junta eastward to Kansas. In 

recent studies of Quaternary climate change, 

investigators have shown an interest in loess and eolian 

sand, because long loess sequences contain detailed 

records of Quaternary glacial- interglacial cycles. Loess 

is a direct record of atmospheric circulation and 

identification of loess representing paleowind patterns 

in the geologic record can test general atmospheric 

circulation models. In addition, widespread eolian 

deposits in eastern Colorado and adjacent parts of the 

Great Plains are important sources of information for 

reconstructing Quaternary aridification in the interior 

of North America 

• Groundwater—Groundwater is the principal source of 

water for domestic use in the Arkansas River valley. 

River terrace deposits are sources of large supplies of 

water, and eolian deposits are major catchment areas 

for recharging underlying aquifers. 

• Mineral Resources—The principal mineral resources 

in Otero and Crowley Counties are sand and gravel, 

which are quarried extensively for road construction. 

Locally some building stone has been quarried from 

the Dakota Sandstone, the Greenhorn Limestone, and 

the Fort Hays Limestone Member of the Niobrara 

Formation. The hearth in the kitchen of the historic 

site, which is original, is made of uncut pieces of 

limestone [probably the Greenhorn Limestone]. Clay 

and silt, which range in thickness from the east side to 

the west side of the fort, are possible sources for the 

manufacture of adobe blocks. Oil and mineral 

exploration does not take place within the local region 

of Bent’s Old Fort, but the potential remains. Mineral 

rights remain outstanding on nine parcels within the 

historic site. 

• Paleontological Resources—Potential for fossils within 

the boundaries of Bent’s Old Fort exists in the Bridge 

Creek Member of the Greenhorn Limestone (Upper 

Cretaceous). This formation is known to contain 

Durania cornupastoris, an extinct group of bivalved 

(pelecypods) mollusks that are a rare find in the 

Western Interior. Additionally, the Pleistocene- age 

Louviers Alluvium has produced a mammoth 

(Mammothus columbi) tusk and a horse (Equus sp.) 

tooth in the vicinity of the national historic site. This 

stream- deposited sediment hosts fossils of other 

vertebrates, including camels and great bison, and 

fossil mollusks in other areas of eastern Colorado. 

Hence, these paleontological resources are also likely 

to occur within the boundaries of Bent’s Old Fort 

National Historic Site. 

• River Terraces—Alluvium (i.e., river terraces) upon 

which the fort sits, and in the vicinity of the site, serves 

as a record of climatic changes throughout the 

Quaternary. A major climatic cooling occurred during 

the Quaternary and brought on glaciation. Episodic 

base- level changes, primarily caused by cyclic changes 

in climate, are recorded by these alluvial deposits. 

Terrace deposits also form the most important 

unconsolidated groundwater aquifer in the area. 

• Soil Compaction—Soils immediately around the 

parking lot at Bent’s Old Fort National Historic Site 

are continuously compacted by vehicles parking 

outside the paved parking area. Compaction of soils 

reduces vegetation and increases runoff, potentially 

changing species characteristics. However, the area 

disturbed by soil compaction is small and does not 

significantly impact the overall vegetative system. 

• Streamflow and Stream Channel Morphology—

Human activities, such as removing water for irrigation 

during low flows of the summer, have probably 

impacted the geologic processes of streamflow and 

stream channel morphology more than any other 

process in the vicinity of Bent’s Old Fort. Such 

activities have altered the character of the Arkansas 

River—from meandering to braided—in a relatively 

short period of time (i.e., 1936–1947) and alterations 

continue. 
2 NPS Geologic Resources Division 

• Surface Water Quality and Level—The Arkansas River 

provided the original supply of water to the fort. 

Today, however, the river is polluted by particulate 

matter from runoff, solid waste from grazing animals, 

and sewage effluent from several upriver towns with 

marginal treatment facilities (National Park Service, 

1975a). Also, the volume of the river changes rapidly in 

relation to spring runoff, rainfall over the watershed, 

and the amount of water diverted for irrigation. 

• Wetlands—Wetlands occur in the area surrounding 

the fort, for example, the Arch wetland, the Casebolt 

wetland, and Day Pond. Wetlands at Bent’s Old Fort 

National Historic Site could be serving a significant 

ecological purpose by acting as natural filters of 

pollutants. Wetlands also have the potential to serve as 

flood buffers. 
BEOL Geologic Resource Evaluation Report 3 

Introduction 

The following section briefly describes the regional geologic setting of Bent’s Old Fort 

National Historic Site and the National Park Service Geologic Resource Evaluation 

Program. 

This report serves as a starting point for understanding 

the geologic resources at Bent’s Old Fort National 

Historic Site and is meant to bridge potential gaps 

between resource management and the historic site’s 

geologic map. 

Purpose of the Geologic Resource Evaluation Program 

Geologic resources serve as the foundation of 

ecosystems and yield important information needed for 

decision making in National Park System units. The 

National Park Service (NPS) Natural Resource 

Challenge, an action plan to advance the management 

and protection of resources, has focused efforts to 

inventory the natural resources under the stewardship of 

the National Park Service. The geologic component is 

carried out through the Geologic Resource Evaluation 

(GRE) Program administered by the NPS Geologic 

Resources Division. The goal of the GRE Program is to 

provide each of the identified 274 “natural area parks” 

with a digital geologic map, a geologic evaluation report, 

and a geologic bibliography. Each product is a tool to 

support the stewardship of National Park System 

resources and each is designed to be user friendly to 

non- geoscientists. In preparing products the GRE team 

works closely with park staff and partners (e.g., USGS, 

state geologic surveys, and academics). 

The GRE teams hold scoping meetings to review 

available data on the geology of a particular unit and to 

discuss associated geologic issues. Park staff are afforded 

the opportunity to meet with experts on the geology of 

their unit. Scoping meetings are usually held in each unit 

individually to expedite the process although some 

scoping meetings are multi- unit meetings for an entire 

Vital Signs Monitoring Network. 

Please visit the NPS Geologic Resources Division website 

(http://www.nature.nps.gov/geology) for additional 

information regarding this report or for up- to- date 

contact information. 

Geologic Setting 

The Great Plains—the words alone create a sense of 

space, a feeling of destiny, and a challenge. Bent’s Old 

Fort National Historic Site was an integral part of this 

destiny, and the geographic backdrop was, and still is, an 

immense sweep of country, reaching north and south 

from Canada to Mexico and spreading eastward from 

the Rocky Mountains. 

So often maligned as a drab, featureless area, the Great 

Plains is actually a land of contrasts and variety: canyons 

carved into solid rock by the water of the Pecos and Rio 

Grande, the seemingly endless grain fields of Kansas, the 

desolation of the Badlands, and the beauty of the Black 

Hills. The Great Plains physiographic province hosts 

many interesting, even spectacular, geologic features, 

many of which are part of the National Park System. For 

example, in the Black Hills of South Dakota at Mount 

Rushmore National Memorial, uplifted Precambrian 

rocks have been sculpted into the faces of four of the 

nation’s most significant presidents. Also in South 

Dakota, Paleozoic limestones contain the remarkable 

Wind and Jewel cave systems. Magma generated during 

and after the Laramide uplift of the Black Hills produced 

a number of localized intrusions, including the 

spectacular Devils Tower in Wyoming. The scenic 

badlands of Theodore Roosevelt National Park in North 

Dakota are developed in Tertiary strata (see Fig 6). 

Oligocene sediments and a fossilized collection of early 

mammals that lived in the vast Serengeti- like plains of 

western North America are found at Badlands National 

Park in South Dakota, and fluvial sediments containing 

fossil mammal bones at Agate Fossil Beds National 

Monument in Nebraska record the story of mammal 

evolution into Miocene time. Additionally, the volcanic 

outpourings during post- Pleistocene time at Capulin 

Mountain are part of the Great Plains province. 

Although generally not considered a “geologic” National 

Park System unit, Bent’s Old Fort National Historic Site 

is indeed part of the geologic heritage of the Great Plains. 

The national historic site lies in the Colorado Piedmont 

section of the Great Plains province (figs. 2 and 3). The 

area ranges in elevation from 3,975 feet (1,212 m) at the 

point where the Arkansas River enters Bent County to 

5,200 feet (1,585 m) north of Delhi, Colorado—a span of 

more than 1,200 feet (366 m). Generally speaking, the 

area around Bent’s Old Fort consists of a series of flat to 

gently rolling surfaces with steep intervening slopes. The 

southeastern part of Otero County has been deeply cut 

by the Purgatoire River and its tributaries. The relief 

along the drainage in the rest of the area is generally 

much less (Weist and others, 1965). 

The Colorado Piedmont is situated at the foot of the 

Rockies, largely between the South Platte and the 

Arkansas Rivers. After leaving the mountains, the South 

Platte, to the north, and the Arkansas, to the south, have 

excavated the Tertiary (65.5- to 1.81- million years old) 

sedimentary rock layers removing great volumes of 

sediment. These Tertiary rock units were originally 

deposited by shifting stream channels, floodplains, 

swamps, and occasional volcanic ash (Kiver and Harris, 

1999). At Denver, the South Platte River has cut 

downward 1,500 to 2,000 feet (457 to 610 m) to the 

present level; no Tertiary rock layers remain in the 

vicinity of Bent’s Old Fort. Three well- formed terrace 
4 NPS Geologic Resources Division 

levels flank the river’s floodplain, and remnants of a 

number of well- formed higher land surfaces are 

preserved between the river and the mountains. Around 

the national historic site these Quaternary terraces rest 

unconformably upon Cretaceous shales and limestones. 

Along the western margin of the Colorado Piedmont, the 

layers of older sedimentary rock have been sharply 

upturned by the rise of the mountains. The edges of 

these upturned layers have been eroded differentially, so 

that the hard sandstone and limestone layers form 

conspicuous and continuous hogback ridges. North of 

the South Platte River, near the Wyoming border, a scarp 

that has been cut on the rock of the High Plains marks 

the northern boundary of the Colorado Piedmont 

(Trimble, 1980). 

Extending eastward from the mountain front at Palmer 

Lake, a high divide separates the drainage of the South 

Platte River from that of the Arkansas River. The crest of 

the divide north of Colorado Springs is generally 

between 7,400 and 7,600 feet (2,256 and 2,316 m) above 

sea level, nearly 1,500 feet (457 m) higher than Colorado 

Springs and more than 2,000 feet (610 m) higher than 

Denver (Trimble, 1980). 

Much of the terrain in the two rivers valleys has been 

smoothed by windblown sand and silt. Northwesterly 

winds, which frequently blow with near- hurricane 

velocities, have whipped fine material from the 

floodplains of the streams and spread it eastward and 

southeastward over much of the Colorado Piedmont. 

Blowout dunes are the most common dune type in the 

vicinity of Bent’s Old Fort. 

In the Colorado Piedmont the erosional effects of 

streams are the most conspicuous features of the 

landscape. These fluvial features are enhanced by the 

steep tilting of layered rocks along the western margin of 

the Colorado Piedmont and modified by wind action, 

which has softened the landscape with a widespread 

cover of windblown sand and silt (Trimble, 1980). 
BEOL Geologic Resource Evaluation Report 5 

Figure 1: Location Map for Bent’s Old Fort National Historic Site. Bent’s Old Fort has been a crossway of travel throughout its history. 

Today, the national historic site is bordered on the north by Colorado 194 and on the south by U.S. Route 50. The Atchison, Topeka 

and Santa Fe Railroad parallels U.S. 50 and has an easement along the national historic site’s southern boundary. 
6 NPS Geologic Resources Division 

Figure 2: Physiographic Provinces of the Conterminous United States. Bent’s Old Fort National Historic Site is part of the Great Plains 

physiographic province—an immense sweep of country reaching north and south from Canada to Mexico and spreading eastward 

from the Rocky Mountains. 
BEOL Geologic Resource Evaluation Report 7 

Figure 3: The Great Plains Physiographic Province. Bent’s Old Fort National Historic Site is located in the Colorado Piedmont section 

of the Great Plains physiographic province. Other National Park System units in the Great Plains province are Agate Fossil Beds

National Monument (Nebraska), Badlands National Park (South Dakota), Capulin Volcano National Monument (New Mexico), Devils 

Tower National Monument (Wyoming), Jewel Cave National Monument (South Dakota), Mount Rushmore National Memorial (South 

Dakota), Theodore Roosevelt National Park (North Dakota), and Wind Cave National Park (South Dakota). 
8 NPS Geologic Resources Division 

Geologic Issues 

This section identifies issues related to geology that may warrant attention from 

resource managers at Bent’s Old Fort National Historic Site. These issues are relevant 

for maintenance of facilities, mitigation of hazardous conditions, and protection of 

resources. 

Eolian Deposits, Climate Change, and Wind Erosion 

Eolian (windblown) deposits blanket large parts of the 

west- central Great Plains. In eastern Colorado, situated 

at the higher and drier western margin of the Great 

Plains, eolian sediment covers about 60% of the area east 

of the Rocky Mountains. Of this eolian cover, about 30% 

is sand and 70% is loess (windblown dust) (Madole, 

1995). The wide distribution of eolian deposits in this 

region was recognized almost as soon as geologic 

exploration began in the late 19th century (Emmons and 

others, 1896; Gilbert, 1896). However, the deposits were 

largely ignored for most of the following century, except 

for areas of dune sand that were mapped in connection 

with water- resource investigations. Loess received even 

less attention than eolian sand and is, therefore, 

underrepresented on most geologic maps, especially 

where it is thin (less than 5 feet [1.5 m]) or overlies rocks 

that produce residuum that is similar to loess, which is 

the case in much of the region (Madole, 1995). 

In recent studies of Quaternary climate change, however, 

investigators have shown a renewed interest in loess and 

eolian sand. Much of the attention now given to loess 

stems from new studies of long loess sequences that 

contain detailed records of Quaternary glacialinterglacial cycles. Such loess sequences are thought to 

be a terrestrial equivalent to the foraminiferal oxygen 

isotope record in deep- sea sediments (Muhs and others, 

1999). Loess is also a direct record of atmospheric 

circulation and identification of loess paleowinds in the 

geologic record can test atmospheric general circulation 

models (Muhs and others, 1999). In addition, the 

widespread eolian deposits of eastern Colorado and 

adjacent parts of the Great Plains are important sources 

of information for reconstructing the history of 

aridification in the interior of North America during the 

Quaternary (Madole, 1995). 

Eolian sands are attractive deposits for paleoclimate 

reconstruction on the Great Plains for a number of 

reasons. First, eolian sand is highly sensitive to climatic 

change because stabilizing vegetation on such deposits in 

semiarid regions cannot tolerate significant reductions in 

precipitation. Second, research indicates that the eolian 

deposits of the Great Plains record a history of repeated 

windblown sand during the Holocene. Third, dune 

morphology and stratification, and isotopic and 

geochemical fingerprinting of sand source areas allow 

reconstruction of paleowind patterns, which in turn 

provide the basis for atmospheric circulation models and 

air mass distributions (Muhs and others, 1993). 

A better understanding of how climate has varied in 

drought- prone regions like eastern Colorado is 

potentially useful for predicting and assessing potential 

effects of future climatic change (Madole, 1995). Shifts to 

a slightly drier climate could have serious socioeconomic 

consequences in this as well as other semiarid and arid 

regions. Population centers in these regions are 

dependent on water from river systems or on 

groundwater pumped from aquifers. Demands on these 

water resources are great and increasing. Also, drought in 

these regions sets the stage for massive wind erosion of 

agricultural land, and generation of dust storms that 

could adversely impact areas far downwind of source 

areas (McCauley and others, 1981; Péwé, 1981). 

The location of Bent’s Old Fort National Historic Site 

places it at center stage for climate change studies. In 

addition, land use practices in the region play an 

anthropogenic role in dust storms and wind erosion. For 

example, the primary land use in the area surrounding 

Bent’s Old Fort is irrigated agriculture (Woods and 

MacDonald, 2002). However, a sale of water rights from 

the Fort Lyon Canal, as was under negotiation in 1992, 

would cause changes in land use around the national 

historic site. Such a sale would transfer water shares from 

the Arkansas River valley to Front Range cities and 

change the pattern of land use around Bent’s Old Fort 

National Historic Site from agricultural to grazing (Bent’s 

Old Fort National Historic Site, 1992). Potential 

outcomes for the historic site could be positive, leading 

to an improvement of the historic scene, or negative, 

resulting in incomplete restoration that acts as a seed bed 

for exotic plants and a source of dust. Water sales have 

occurred in the valley in the past with minimal success at 

prairie restoration. This external threat periodically 

surfaces as the possibility of selling water rights to urban 

areas is proposed (Bent’s Old Fort National Historic Site, 

1992). 

Air quality is diminished primarily by dust from 

agricultural activities, and high winds. Traffic on gravel 

and dirt roads and agricultural burning are lesser 

contributors to poor air quality (National Park Service, 

1993). 

Bent’s Old Fort National Historic Site has been free of 

commercial grazing since 1966. In 1975, when the fort was 

reconstructed, the National Park Service purchased a 

few horses, a pair of oxen, and a mule for use in support 

of the living history interpretive program. Federal land 

on the north side of the Arkansas River within the 

historic site’s boundaries is used for the grazing of 

federally owned animals. At present Bent’s Old Fort 
BEOL Geologic Resource Evaluation Report 9 

National Historic Site has three horses, two oxen, a mule, 

and assorted fowl (Fran Pannebaker, Bent’s Old Fort 

National Historic Site, written communication, May 25, 

2005), which help set the historic scene at the 

reconstructed fort. Grazing is carefully controlled under 

a plan worked out in conjunction with the local soil 

conservation service (National Park Service, 1975a). 

Flooding 

Bent’s Old Fort was built of adobe on the edge of a low 

terrace projecting southeastward into the floodplain of 

the Arkansas River (Swenson, 1970). Figures showing the 

river course and floodplain in Swenson (1970) plot Bent’s 

Old Fort as lying at the northern edge of—but not 

actually in—the floodplain. However, in 1921 the “Great 

Pueblo Flood” crested above the level of the fort grounds 

causing most of the remaining original adobe walls to 

collapse (National Park Service, 1975a). The site also was 

threatened by flooding in 1965; however, floodwaters did 

not reach the ruins or cause any property damage 

(National Park Service, 1975a). 

The floodplain north of the Arkansas River is about 1 

mile (1.6 km) in width, very flat, and bordered on the 

north by low bluffs or hills. The floodplain south of the 

river varies from 0.25 to 0.5 mile (0.4 to 0.8 km) in width. 

Hills on the south side rise more gradually than on the 

north, with an average rate of rise about 125 feet (38 m) 

per mile (1.6 km) (Fig 4). Along the floodplain, 

groundwater is generally within 10 feet (3 m) of the land 

surface (Weist and others, 1965). 

The water table in the area has risen in recent years 

(Woods and MacDonald, 2002), which is a factor in the 

basement flooding at Bent’s Old Fort. Investigators have 

attributed the rising water table to a combination of 

factors: (1) a decline in the amount of groundwater being 

pumped from the aquifer, (2) increased seepage from the 

Fort Lyon Canal, (3) increased deep percolation of 

irrigation water, and (4) aggradation of the bed of the 

Arkansas River leading to higher river stage and greater 

leakage from the river alluvial aquifer. The relative 

importance of these factors varies with proximity to the 

Arkansas River, the canal, and areas of groundwater 

pumping and surface irrigation (Woods and MacDonald, 

2002). 

Storms over the upper Arkansas River basin are generally 

either low- intensity, long- duration rainfall events over a 

large area or short duration rainfall events with high 

intensity over a small area. The former are most 

prevalent during the autumn, winter, and spring seasons. 

Thunderstorms are most active during July and August, 

although the two largest and most destructive floods on 

record occurred during the month of June when general 

storms concentrated in a series of intense cloudbursts 

over the foothills and plains areas (Army Corps of 

Engineers, 1977). 

Floods on the Arkansas River are of two general types. 

One type, “spring floods,” results from melting snow and 

is often augmented by storm runoff. The other, “summer 

floods,” results entirely from storm runoff. Spring floods 

are characterized by comparatively moderate rates of 

flow of long duration with large volumes of runoff. 

Summer floods are characterized by high peak rates of 

discharge with relatively smaller volumes of runoff 

(Army Corps of Engineers, 1977). 

Hazards to life and extent of damage caused by any flood 

depend on the topography of the area flooded, depth 

and duration, velocity of flow, rate of rise, and 

developments on the floodplains (Army Corps of 

Engineers, 1977). Velocities greater than 3 feet (0.9 m) per 

second are generally considered hazardous to life and 

property. Water flowing in excess of 4 feet (1.2 m) per 

second is capable of transporting sediment and causing 

severe erosion of stream banks and fill around bridge 

abutments. Where velocities drop below 2 feet (0.6 m) 

per second, debris and silt deposits can build up (Army 

Corps of Engineers, 1977). The main line of the Atchison, 

Topeka and Santa Fe Railway, and the branch line from 

La Junta to Denver, transverse the floodplain. Numerous 

county roads, Colorado Highway 109, and U.S. 50 and 

350 also cross the floodplain (Army Corps of Engineers, 

1977) (Fig 1 and 4). 

Obstructions, natural and anthropogenic, within 

floodways impede flood flows, creating backwater and 

increased heights. Debris washing downstream during 

floods often collects against bridges or within restricted 

flow areas, reducing waterway openings and impeding 

flood flow. This creates a damming effect and, depending 

on the degree of clogging, causes greater backwater 

depths with increased overbank flooding. Also, 

pronounced increases in flow velocity usually occur 

downstream from an obstruction, thus extending the 

flood damage potential. Anthropogenic obstructions on 

or over the floodway such as dams, levees, bridges, and 

culverts can create more extensive flooding than would 

otherwise occur (Army Corps of Engineers, 1977). Along 

the Arkansas River levees have been constructed by local 

interests. These levees are generally ineffective in 

preventing large floods and in some instances, such as 

during the June 1965 flood, may increase damage done by 

flood waters (Army Corps of Engineers, 1977). 

Additionally, certain materials stored in areas subject to 

flooding and high velocity flood flows pose potentially 

dangerous situations. For example, toxic, explosive, or 

flammable materials are usually stored in tanks and 

similar floatable containers that, if unrestrained, could be 

carried downstream threatening distant areas. Timber, 

lumber, and other similar floatable material may 

accumulate in substantial quantities during floods 

clogging downstream bridge openings and causing 

serious hazards and damage (Army Corps of Engineers, 

1977). 

Historical records reference many floods in the Arkansas 

River valley above John Martin Reservoir, located about 

20 miles (32 km) east of Bent’s Old Fort. The earliest 

known in the area occurred in 1826. The next notable 

flood was in 1844. Other reported floods occurred in 

1859, 1864, 1869, and 1894. These floods were generally 

confined between Pueblo and the present John Martin 

Dam, an area which includes Bent’s Old Fort. Major 
10 NPS Geologic Resources Division 

floods were experienced at various localities in the 

subbasin in 1921, 1935, 1942, 1955, and 1965. The flood of 

1921 was the greatest flood of record on the Arkansas 

River at La Junta (Army Corps of Engineers, 1977) (Fig 1). 

Descriptions of the 1921 and 1965 floods, which caused 

damage to or threatened the fort, may be enlightening to 

resource managers at Bent’s Old Fort National Historic 

Site and useful in planning for future threats from 

flooding at the fort. Only in rare instances has a stream 

experienced the largest flood possible. Severe as the 

maximum known flood may have been on a given 

stream, it is generally accepted that a larger flood will 

occur. Streams with similar geographic and 

physiographic characteristics have experienced larger 

floods than those in the La Junta area. Severe storms 

undoubtedly will occur in the area causing future floods 

that will equal or surpass known historic floods (Army 

Corps of Engineers, 1977). 

Flood of June 1921 

Two areas of intense rainfall were observed above 

Pueblo. One of these was mostly north of the towns of 

Florence and Canon City. The other was fairly well 

distributed on both banks of the Arkansas River between 

Portland and Pueblo. Precipitation in this latter area is 

reported to have been as much as 11 inches (28 cm) in six 

hours while in the other area reports of as much as 9 

inches (23 cm) in six hours were made (Army Corps of 

Engineers, 1977). The rainfall in the upper area was 

reported to have occurred first so that runoff combined 

with that below to produce the largest flood ever 

recorded at Pueblo. The peak discharge of 103,000 cubic 

feet per second (2,917 m3

/s) occurred at midnight on June 

3, 1921. Above La Junta the effect of valley storage on 

peak attrition was obscured by the inflow from tributary 

streams and the peak discharge at La Junta was 200,000 

cubic feet per second (5,664 m3

/s). North La Junta was 

flooded; water was 4 to 6 feet (1.2 to 1.8 m) deep on 

Second Street and reached the Otero County Jail. 

Citizens reported that 13 persons were drowned, though 

some of the bodies were not found until June 9. Areas in 

Huerfano and Purgatoire Rivers and in Timpas, Adobe, 

and Horse Creeks were flooded. Heavy rains occurred 

on the main stem of the Arkansas River from La Junta to 

Lamar. Below La Junta tributary inflow was small (Army 

Corps of Engineers, 1977). 

Flood of June 1965 

Flooding is often caused by a single event. The ensuing 

damage is caused by the downstream progression of the 

flood wave. For the June 1965 floods, especially on the 

main river course, a series of flood waves caused 

progressive flooding. 

Steady rains began in the Arkansas River basin in 

Colorado on May 22, 1965, and as much as 2.7 inches (6.8 

cm) of rain was reported for a single day. The days of 

June 4 and 5 also received relatively high precipitation, 

setting the stage for “rivers to roll” beginning June 14. 

Colorado in general received fairly heavy rain on June 14; 

hail damaged some areas, particularly around Colorado 

Springs. Rainfall was light on June 15, torrential on June 

16, and of tremendous proportions on June 17. The June 

16 and 17 storms covered practically all areas in 

southeastern Colorado and northeastern New Mexico, 

with the greatest concentration of rainfall near Holly, 

Lamar, and Two Buttes, Colorado (Snipes, 1974). 

The highest peak discharge determined on the main stem 

of the Arkansas River above John Martin Reservoir was 

104,000 cubic feet per second (2,945 m3

/s) at a site 1.2 

miles (1.9 km) downstream from Chico Creek. This peak 

flow resulted from nearly coincidental timing of the 

flood crest moving down the Arkansas River and the 

peak discharge of 52,200 cubic feet per second (1,478 

m3

/s) from Chico Creek (Snipes, 1974). 

The total runoff in the Arkansas River basin in Colorado 

from the June 1965 storms exceeded that of any previous 

flood for which records exist. John Martin Reservoir 

stored 320,000 acre- feet (3.9 × 108

 m3) during June 14–26, 

and an additional 485,000 acre- feet (5.98 × 108

 m3) of 

inflow downstream from the dam flowed past the station 

near Coolidge, Kansas, during June 17–27. Thus, the total 

June 1965 storm runoff at the state line, adjusted for 

storage in the reservoir, would be 805,000 acre- feet (9.9 

× 108

 m3). Records are not available at the Coolidge 

station, but at Holly, only 6 miles (10 km) upstream, the 

total storm runoff in 1921 was 410,000 acre- feet (5.1 × 108

m3

) (Snipes, 1974.) The high runoff downstream from 

Lamar caused the large difference between 1965 and 1921. 

At Lamar the total storm runoff in 1921 was about 

450,000 acre- feet (5.6 × 108

 m3), compared with 400,000 

acre- feet (4.9 × 108

 m3) (adjusted for storage in John 

Martin Reservoir) in 1965 (Snipes, 1974). 

Flood water drowned 14 people, and at least two other 

deaths were attributed to the storms and activities related 

to the 1965 floods. Damage amounted to more than $60 

million, about $56 million in Colorado and Kansas, and 

the remainder in New Mexico (Snipes, 1974). 

Groundwater 

Otero County and the southern part of Crowley County 

include an area of about 1,500 square miles (3,885 km2

) in 

the Arkansas River valley. Groundwater is the principal 

source of water for domestic use by local residents, most 

of whom are farmers or ranchers. 

Rocks in the area, which range from Late Jurassic to 

Holocene in age (see Fig 6), influence the availability of 

groundwater. Five of the rock units are major aquifers: 

the Wisconsin- age (see table 1) terrace deposits, the 

valley- fill deposits, the Nebraskan deposits, the Dakota 

Sandstone, and the Cheyenne Sandstone Member of the 

Purgatoire Formation. The Wisconsin terrace deposits 

are the principal source of groundwater for irrigation 

and municipal supplies, yielding as much as 2,000 gallons 

(7,570 L) per minute; they also supply water for stock and 

domestic use. The valley- fill deposits supply water to 

domestic and stock wells and in places yield as much as 

400 gallons (1,514 L) per minute to a few irrigation wells. 

The Nebraskan- age deposits supply water to stock and 

domestic wells and feed springs that supply water to two 

towns and a private water association. The Dakota 

Sandstone and Cheyenne Sandstone Member of the 
BEOL Geologic Resource Evaluation Report 11 

Purgatoire Formation are sources of artesian water for 

many stock and domestic wells and for some industrial, 

municipal, and private water- association wells. They 

may yield as much as 80 gallons (303 L) per minute 

(Weist and others, 1965). 

The Wisconsin terrace deposits are the best source of 

large supplies of water; however, they are so heavily 

developed in most of the area that some wells interfere 

with each other (see Fig 4). Moderate supplies of water 

for irrigation can be obtained from the valley- fill 

deposits. The rest of the unconsolidated deposits yield 

only small amounts of water for stock and domestic 

purposes. Water in the Dakota Sandstone and the 

Cheyenne Sandstone Member of the Purgatoire 

Formation is under artesian pressure (Weist and others, 

1965). 

Groundwater in the unconsolidated materials moves 

generally eastward over an irregular bedrock surface. 

Waters within the Dakota Sandstone and Cheyenne 

Sandstone Member of the Purgatoire Formation move 

northeastward (Weist and others, 1965). The saturated 

zone is usually less than 40 feet (12 m) thick. Along the 

floodplain, groundwater is generally within 10 feet (3 m) 

of the land surface. In 1973, groundwater levels were 

recorded in the gravel deposits at depths of 

approximately 11 feet (3.4 m) beneath the surface (Ken R. 

White Company, 1973). In the immediate vicinity of the 

proposed seepage beds for the historic site’s sewage 

disposal system, the water table was measured at a depth 

of 13 feet (4 m) (National Park Service, 1975b). These 

levels may rise with an increased flow of water in the 

Arkansas River (Ken R. White Company, 1973). 

Groundwater reservoirs in the area are recharged by 

subsurface and surface in- flow, including precipitation 

(which averages 11 to 14 inches [28 to 36 cm] per year) 

(Weist and others, 1965), seepage from streams and 

irrigation canals, and spreading of water for irrigation. 

They are discharged by subsurface outflow, 

evapotranspiration, seepage into streams, and pumping 

from wells (Weist and others, 1965). 

All groundwater in the Bents Old Fort area can be classed 

as sodium calcium sulfate bicarbonate in character. It is 

generally of fair to good quality for most uses. Water 

from the unconsolidated formations is rated as hard to 

very hard, whereas water from the two artesian aquifers 

is generally rated as soft to moderately hard (Weist and 

others, 1965). 

Under conditions of low to average flow in the Arkansas 

River, groundwater levels in the vicinity of Bent’s Old 

Fort are controlled primarily by the rate of surface water 

irrigation and leakage from the Fort Lyon Canal (see Fig 

4). Under high streamflow conditions, such as those that 

occurred during the 1999 peak flow, a local reversal of 

the hydraulic gradient between the alluvial aquifer and 

the Arkansas River can occur. The flow of water from the 

river, combined with increased leakage from the Fort 

Lyon Canal, are a likely cause of the high water- table 

levels observed in the vicinity of the fort and the 

consequent basement flooding that occurred in 1999 

(Woods and MacDonald, 2002). Proposed reductions in 

the flow rate of the Fort Lyon Canal may slow or even 

reverse the upward trend in water- table levels in the 

vicinity of the national historic site (Woods and 

MacDonald, 2002). 

Potential Impacts to Groundwater 

A number of factors threaten groundwater quality at 

Bents Old Fort National Historic Site. The national 

historic site is bordered on the north by Colorado 194 

and on the south by U.S. Route 50. The Atchison, 

Topeka, and Santa Fe Railroad parallels U.S. 50 and has 

an easement along the site’s southern boundary (Bent’s 

Old Fort National Historic Site, 1992) (see Fig 4). 

Hazardous wastes are transported by truck and rail 

without historic site staff’s knowledge. Adjacent 

landowners use aerial spraying and ground spraying of 

fertilizer and pesticides on their fields (Bent’s Old Fort 

National Historic Site, 1992). A radioactive waste dump, 

called the Thatcher Dome Project, has been proposed 44 

miles (71 km) southeast of La Junta. This project was 

shelved in 1992 and, although determining the potential 

economic and environmental impacts of the radioactive 

waste dump on groundwater quality is difficult, it 

presents a concern for park managers (Bent’s Old Fort 

National Historic Site, 1992). In addition, the waste water 

irrigation ditch of the Fort Lyon Canal Company 

parallels the south side of Colorado 194, across the 

entrance of the historic site within the highway right- ofway. This unlined ditch contributes to surface water and 

groundwater supplies and appears to be modifying 

vegetation east of the picnic area. The tail end of the 

ditch crosses NPS property perpendicular to Colorado 

194 in the northeast corner of the national historic site 

(Bent’s Old Fort National Historic Site, 1992). As a result 

of these factors, significant potential exists for 

misapplication of fertilizers and pesticides that could 

affect NPS property or hazardous waste accidents that 

could affect the groundwater system in the area. 

Mineral Resources 

The principal mineral resources in Otero and Crowley 

Counties are sand and gravel, which are quarried 

extensively for road construction. In addition, such 

deposits of alluvium may be suitable for use as aggregate 

for concrete and as riprap (Scott, 1963). These deposits 

are found chiefly on top of mesas and ridges along the 

Arkansas River (Weist and others, 1965). Clay and silt, 

which range in thickness from 3 feet (0.9 m) on the east 

side of the fort to 7 feet (2.1 m) on the west side, are 

possible sources for the manufacture of adobe blocks 

(Ken R. White Company, 1973), which is of historic 

interest and potentially useful for future repairs to the 

fort. 

Locally some building stone has been quarried from the 

Dakota Sandstone, the Greenhorn Limestone, and the 

Fort Hays Limestone Member of the Niobrara 

Formation (Weist and others, 1965) (see Fig 5). With 

respect to the national historic site, the hearth in the 

kitchen, which is original, is made of uncut pieces of 

limestone [probably Greenhorn Limestone] (Bent’s Old 

Fort Web site, http://www.nps.gov/beol/kitchen.htm). 
12 NPS Geologic Resources Division 

Oil and mineral exploration does not currently take place 

within the local region of Bent’s Old Fort; however, the 

potential for future exploration and development 

remains (Bent’s Old Fort National Historic Site, 1992). 

For instance, most of the limestone and dolomite 

reservoir beds of the central Kansas oil fields persist 

westward into eastern Colorado, particularly into 

Prowers and Bent Counties. Therefore, oil and gas 

reservoirs may exist in that area (Maher, 1947). Oil and 

gas production occurs 25–30 miles (40–48 km) east and 

northeast of the historic site. Three wells have been 

drilled within 3 miles (5 km) of the historic site, but were 

plugged and abandoned because they were not 

economically viable (Bent’s Old Fort National Historic 

Site, 1992). 

Privately held mineral rights may exist within a National 

Park System unit when the government acquires the 

surface without acquiring the mineral rights, or when 

private, fee- simple parcels (privately held surface and 

mineral rights) exist within the unit. Private mineral 

rights exist on nine parcels within the historic site (Bent’s 

Old Fort National Historic Site, 1992). Should an 

individual or company propose exploration or 

development for oil and gas within the historic site, park 

managers would implement 36 Code of Federal 

Regulations Part 9, Subpart B to permit and control 

operations. If exploration and development of sand, 

rock, or gravel is proposed on split estate land (federally 

held surface rights and privately held mineral rights), 

park managers would issue a special use permit under 36 

Code of Federal Regulations Part 5 to permit and control 

operations. In either case, if the NPS deems the impacts 

of proposed exploration or development as 

unacceptable, federal acquisition of the mineral rights 

may be pursued (National Park Service, 1993). 

Paleontological Resources 

Durania cornupastoris 

In 1991 a group of at least 28 conjoined individuals of the 

rudist Durania cornupastoris, an extinct group of 

bivalved (pelecypods) mollusks, were discovered and 

collected from the uppermost part of the Bridge Creek 

Member of the Greenhorn Limestone in a road cut along 

U.S. 50 about 5 miles (8 km) northeast of La Junta, 

outside the boundaries of Bent’s Old Fort National 

Historic Site. The rudist- producing bed in the Bridge 

Creek Member extends into the national historic site; 

thus, the potential for this fossil within the boundaries 

exists (Scott and others, 2001). 

The discovery of the cluster of rudists is an unusual find 

of a species that was originally described in France (Des 

Moulins, 1826). Aside from fairly abundant occurrences 

in the Niobrara Formation of Kansas, rudists are rare 

fossils in the Upper Cretaceous rocks of the Western 

Interior. The 1991 find near Bents Old Fort was the first 

report of this species in the area (Cobban and others, 

1991). This particular find falls within the conventionally 

recognized range of the species but extends its 

geographic occurrence (Cobban and others, 1991). 

Investigators envision the species living in a low- to 

moderate- energy environment, marked by only 

occasional influxes of sediment that suffered little or no 

subsequent reworking by currents (Cobban and others, 

1991). They suggest that the demise of this Durania

“bouquet” was caused by increasing turbidity (or other 

environmental change) that inhibited effective feeding by 

mantle marginal particle entrapment (Skelton, 1979). 

Mammothus columbi 

Jackson Moore, a NPS archaeologist, discovered a 

fragmentary mammoth tusk at Bent’s Old Fort. He 

collected tusk fragments between 1963 and 1966. The 

remains were found in a gravel bed overlying a white 

limestone unit (Moore, 1973). The fragments have been 

tentatively identified as Mammothus columbi (N. Russel, 

personal communication, 2000). According to the 

historic site’s museum records, archaeologist Jerry 

Dawson collected an additional three mammoth tusk 

fragments in 1992 (Scott and others, 2001). 

Louviers Alluvium 

According to Scott (1963), the Louviers Alluvium in the 

Kassler quadrangle (Colorado) contains a large 

assemblage of fossil mollusks and vertebrates, including 

mammoths, camel, and great bison. Louviers Alluvium is 

an early Wisconsin- age terrace deposit. Weist and others 

(1965) mapped two Wisconsin- age terrace deposits (Qtw1

and Qtw2

) in the vicinity of the national historic site, and 

Tweto (1979) mapped a gravel and alluvium deposit that 

he described as consisting of Broadway and Louviers 

alluvium (Qg) near Bent’s Old Fort. The lower 

Wisconsin- age terrace deposit (Qtw2

) is the potentially 

fossiliferous unit in the national historic site. For 

example, during mapping, investigators found a horse 

(Equus sp.) tooth in a gravel pit (Weist and others, 1965). 

Although no research has been performed or inventory 

made, the paleontological resources found in this deposit 

in the Kassler quadrangle also could be in the vicinity of 

Bent’s Old Fort (K. McKinney, U.S. Geological Survey, 

written communication, April 2004). In addition, the 

fragments of mammoth tusk now in the national historic 

site’s collection are likely from the Louviers alluvium (K. 

McKinney, U.S. Geological Survey, written 

communication, April 2004). 

Soil Compaction 

Physical degradation of soils results from activities such 

as land clearing, erosion, and compaction by machinery. 

Soil structure may be altered so that infiltration capacity 

and porosity are decreased, and bulk density and 

resistance to root penetration are increased. Such soils 

have impeded drainage and are quickly saturated: the 

resultant runoff can cause accelerated erosion and 

transport of pollutants such as pesticides (Berger and 

Iams, 1996). At Bent’s Old Fort National Historic Site 

soils immediately around the parking lot are 

continuously compacted by vehicles parking outside the 

paved parking area. Compaction of soils reduces 

vegetation and increases runoff, possibly changing 

species characteristics (National Park Service, 1993). 

However, the area disturbed by soil compaction is small 

(about 0.25 acres [0.1 ha]) and does not significantly 
BEOL Geologic Resource Evaluation Report 13 

impact the overall vegetative system (National Park 

Service, 1993). Existing developed areas (i.e., roads, 

buildings, paving, and paths) cover approximately 8 acres 

(3.2 ha) of the national historic site (National Park 

Service, 1993). 

Streamflow and Stream Channel Morphology 

The Arkansas River is a perennial but highly regulated 

river; the flow of its tributaries depends on precipitation 

and return flow from irrigation. Horse Creek is the only 

tributary north of the Arkansas River. South of the river, 

the Apishapa and Purgatoire Rivers, Timpas Creek, and 

Crooked Arroyo are the principal tributaries (Weist and 

others, 1965). 

The flow of the Arkansas River is affected by numerous 

upstream diversions and the Pueblo Dam. The dam lies 

approximately 62 miles (100 km) upstream from the 

historic site and effectively separates this section of the 

Arkansas River from its headwaters in the Rocky 

Mountains. The mean annual discharge at La Junta (U.S. 

Geological Survey gauge no. 07123000) is 261 cubic feet 

per second (7.4 m3

/s), and the mean annual flow for the 

period following the closure of the Pueblo Dam in 1974 is 

7,203 cubic feet per second (204 m3/s) (Woods and 

MacDonald, 2002). 

Since 1936 the amount of water diverted from the 

Arkansas River for irrigation has increased dramatically 

(Swenson, 1970). The irrigation season typically extends 

from March through September. Irrigation water is 

obtained primarily from the Fort Lyon Canal, which lies 

1.5 miles (2.5 km) north of Bent’s Old Fort and flows from 

west to east approximately parallel to the Arkansas River 

(see Fig 4). The diversion point for the canal lies 

approximately 12 miles (20 km) upstream from the 

historic site. Some irrigation water also is obtained by 

pumping groundwater from the alluvial aquifer (Woods 

and MacDonald, 2002). Investigators believe that 

removing water from the Arkansas River during the low 

flows of summer is responsible for the river becoming 

more braided in character (Swenson, 1970). 

A study in 1970 shows that human activities altered the 

character of the Arkansas River in a relatively short 

period of time (i.e., 1936–1947), and alterations continue. 

In the early 1990s, the Colorado Department of 

Transportation requested assistance from the U.S. Army 

Corps of Engineers to stabilize the banks of the Arkansas 

River in the reach of river that parallels Colorado 194, at 

the northeast corner of Bent’s Old Fort National Historic 

Site. Flooding in the past caused closure of the road, 

which is a significant issue for visitation and access to the 

site because Colorado 194 is the primary transportation 

corridor. The project used rock groins on the north bank 

to force flow onto the south side. Project groins were 

designed to have vegetated slopes. Final drawings 

showed all the groins outside the historic site’s boundary 

and not within the fort’s view shed. The final project 

proposal was reviewed by staff at Bent’s Old Fort 

National Historic Site (Bent’s Old Fort National Historic 

Site, 1992) and the project was completed by 1994. 

In addition, the banks of the Arkansas River are eroding 

northward along the Fort Lyon Canal. Assistance from 

the NPS Water Resources Division and engineering 

assistance will be necessary to determine and reduce the 

environmental effects of this waste water ditch in terms 

of water level and impacts of chemicals (Bent’s Old Fort 

National Historic Site, 1992). 

Special Note: On the frequently asked questions (FAQ) 

page of the Bent’s Old Fort’s Web site 

(http://www.nps.gov/beol/parkinfo.htm), question 6 asks 

“Has the river changed course?” The answer: “It changed 

course every year during the time of the fort due to 

annual flooding” is misleading and should probably be 

changed to reflect historical records. “During the time of 

the fort” (i.e., 1833–1849) records state that the Arkansas 

River was about 300 yards from the fort, but they do not 

say if it was this close on the south or east, or describe 

any changes over time. Also, no maps or charts showing 

the relative position of the fort and the Arkansas River 

are known (Swenson, 1970). 

An appropriate answer should incorporate the fact that 

the first survey of the area was not made until July and 

August 1869. Swenson (1970) evaluated the data gathered 

at that time and concluded that “Despite the fact that the 

Arkansas River, in the reach near Bent’s Old Fort, has 

well- developed meanders and the river is flowing on 

easily erodible material, there has been relatively little 

shift of the stream since 1869. Little progressive 

migration of meanders downstream has occurred.” 

In addition, the use of “annual flooding” in the FAQ 

answer is misleading because annual floods have not 

occurred, as noted from the previous discussion of 

flooding and the floods of 1921 and 1965. 

Surface Water Quality and Level 

The Arkansas River provided the original supply of water 

at the fort (Bent’s Old Fort National Historic Site Web 

site, http://www.nps.gov/beol/swbastion.htm). Today, 

however, the river is polluted by particulate matter from 

runoff, solid waste from grazing animals, and sewage 

effluent from several upriver towns with marginal 

treatment facilities (National Park Service, 1975a). Also, 

the volume of the river changes rapidly in relation to 

spring runoff, rainfall over the watershed, and the 

amount of water diverted for irrigation. 

Wetlands 

The area surrounding the fort includes several wetlands 

(Woods and MacDonald, 2002). The 52- acre (21- ha) 

Arch wetland lies on the Arkansas River floodplain 

immediately north of the fort. National Park Service 

personnel report that the Arch wetland has gradually 

increased in extent over the last 15–20 years. The 1- acre 

(0.4- ha) Casebolt wetland lies approximately 1,476 feet 

(450 m) southwest of the fort in a meander bend of the 

Arkansas River, and the 0.5- acre (0.2- ha) Day Pond is 

located adjacent to the Arkansas River about 984 feet 

(300 m) southwest of the fort (Woods and MacDonald, 

2002). 
14 NPS Geologic Resources Division 

The Arch wetland is supported by groundwater inflows 

from the adjacent upland. Reductions in the flow rate of 

the Fort Lyon Canal would probably reduce 

groundwater inflows and may therefore lower water 

levels and cause a decline in the size of Arch wetland 

(Woods and MacDonald, 2002). 

Although analysis has not been conducted, wetlands at 

Bent’s Old Fort National Historic Site have the potential 

to serve a significant ecological purpose by acting as 

natural filters of pollutants (National Park Service, 1993). 

Fertilizers, organic products, and other chemicals used 

on the agricultural lands surrounding the national 

historic site may be reducing the quality of the 

subsurface waters, and the wetlands could be 

sequestering and filtering these pollutants. Wetlands also 

have the potential to serve as flood buffers (Berger and 

Iams, 1996). 

Figure 4: Topographic Map of the Vicinity of Bent’s Old Fort National Historic Site. Bent’s Old Fort lies on the northern edge of the Arkansas 

River floodplain. Note the number of wells on the 1953 map and the location of the Fort Lyon Canal (northwest corner of the map area). Also 

note the locations of highways 50 and 194 and the Atchison, Topeka and Santa Fe Railway in relation to the fort “ruins.” This topographic 

map was published before Bent’s Old Fort was authorized as a national historical site. Source: USGS Hadley, Colorado, quadrangle (1953). 
BEOL Geologic Resource Evaluation Report 15 

Geologic Features and Processes 

This section provides descriptions of the most prominent and distinctive geologic features 

and processes at Bent’s Old Fort National Historic Site. 

The Great Plains contains thick layers of rock that 

formed in oceans, and younger layers of rocks deposited 

by streams. These rocks have been affected by tectonic 

movements, carved by streams, dissolved by 

groundwater, and blown by winds. All of these agents 

have played important roles in determining the 

landscape and the landforms of the Great Plains, but 

streams were the most influential. 

Arkansas River 

The Arkansas River is a broadly meandering stream in 

the vicinity of Bent’s Old Fort National Historic Site. It 

flows approximately 6 miles (10 km) to cover a linear 

distance of 3 miles (6 km). The river’s meanders are 

developed on a floodplain that, in general, is less than 

0.75 mile (1.2 km) wide. The floodplain is entirely 

developed in unconsolidated materials, underlain by 30 

to 40 feet (9 to 12 km) of fill. The river gradient is about 5 

feet per mile (1.5 m/1.6 km), and the sediment load 

carried is moderately heavy, consisting largely of silt and 

sand in this reach of the river. During the period of 

stream gauging recorded at La Junta, the flow of the 

Arkansas River ranged from no flow to 200,000 cubic 

feet per second (5,664 m3

/s) (Swenson, 1970). Large 

diversions of water are made for irrigation, which 

accounts in part for the wide range in discharge of the 

stream. Discharge is also affected by rapid snowmelt in 

the mountains and cloudbursts within the basin, which 

can cause heavy flooding in the area. The maximum 

flood recorded occurred on June 4, 1921; it resulted from 

a cloudburst during a period of heavy snowmelt 

(Swenson, 1970). 

Meandering Streams 

Streams that flow upon floodplains, whether erosional or 

depositional, move in sweeping bends called meanders. 

Meanders change position by eroding sideways and 

slightly downstream. The sideways movement occurs 

because the maximum velocity of the stream shifts 

toward the outside of the bend, causing erosion of the 

outer bank. At the same time the reduced current at the 

inside of the meander results in the deposition of coarse 

sediment, especially sand. Thus by eroding its outer bank 

and depositing material along its inner bank, a stream 

moves sideways without changing its channel size. Due 

to the slope of the channel, erosion is more effective on 

the downstream side of a meander. Therefore, in 

addition to growing laterally, the bends also gradually 

migrate down the valley. 

Changes have occurred in the river’s channel 

morphology over time. The presence of meander 

scallops on the edges of the terraces bordering the 

floodplain and more or less filled oxbow lakes confirm 

meandering in the past (Swenson, 1970). Evaluation of 

available data—i.e., surveys from 1869 and 1882 and aerial 

photographs from November 20, 1936—indicate that 

little significant lateral migration of the meanders or 

shifting of the Arkansas River occurred between 1869 

and 1936. Furthermore, very little shifting of the river’s 

course occurred in the reach of the river that runs past 

the fort or downstream from the fort between November 

1936 and July 1947. However, major changes in the stream 

took place in upstream reaches: the rather straight course 

of 1869 through 1882—more or less maintained through 

1936—became very sinuous by 1947. 

Another major change also took place during the period 

between 1936 and 1947: the Arkansas River became a 

braided stream. Islands were few and of small size in 

1936, but they became numerous and of large size by 

1947; this tendency has remained. Since 1936 the amount 

of water diverted from the river for irrigation has 

increased significantly. Investigators believe that removal 

of water during the low flows of summer is responsible 

for the development of the more braided character of the 

Arkansas River (Swenson, 1970). 

Oxbow Lakes 

Sometimes the downstream migration of a meander is 

slowed when it reaches a more resistant portion of the 

floodplain. This allows the next meander upstream to 

“catch up.” Gradually the neck of land between adjacent 

meanders is narrowed. When they get close enough, the 

river may erode through the narrow neck of land to the 

next loop. Also, when at flood stage the stream may have 

the energy to break through the divide between two 

loops, making a shorter channel segment. The new 

segment is called a cutoff. Because of its shape, the 

abandoned bend is referred to as an oxbow lake. 

Standing water in an oxbow that was isolated by a cutoff 

then sealed with alluvium is called an oxbow lake. Its 

surface is the water table. 

Cutoffs occur not only on the floodplains but in incised 

meanders, where, by penetrating divides, they make the 

tunnel from which natural bridges evolve (Wyckoff, 

1999). 

Braided Streams 

The development of braided channels is favored by 

several factors. The most important factors leading to 

their development are large bed load and readily erodible 

bank material, which enable channel shifts to occur with 

relative ease. Once formed, bars in braided channels can 

become rapidly vegetated and thereby stabilized as 

islands. Highly variable discharge is typical of many 

braided rivers. By promoting alternating channel 

degradation and aggradation, large fluctuations in 
16 NPS Geologic Resources Division 

discharge often suppress the establishment of vegetation 

on braided- channel bars (Summerfield, 1991). 

As discharge declines after a flood, the coarse bed load is 

the first to be deposited in the channel. This material 

forms the nucleus of bars that grow downstream as the 

flow velocity is reduced and finer sediment accumulates. 

With further decreases in discharge the water level 

progressively falls and the bars are gradually exposed. 

During subsequent floods, some or all of the bars in a 

braided channel may be submerged depending on the 

discharge attained. During large floods braided channels 

can experience major diversions of flow (Summerfield, 

1991). 

Eolian Deposits 

Eolian (windblown) sediment covers 60% of Colorado 

east of the Rocky Mountains; about 30% of the sediment 

is sand and 70% is loess (windblown dust). The Arkansas 

River eolian sand area, which covers 220 square miles 

(570 km2

), consists mainly of a belt of dunes, generally 0.2 

to 0.4 miles (0.3 to 0.6 km) wide, along the south side of 

the river from near La Junta, Colorado, eastward to 

Kansas (Madole, 1995). Voegeli and Hershey (1965) 

described the eolian sand as flanking the Arkansas River 

from just west of the confluence of Big Sandy Creek and 

the Arkansas River eastward to the Kansas state line. 

They noted that the dune sand in this area has a 

maximum thickness of about 82 feet (25 m) but averages 

33 feet (10 m). Blowout dunes are the most common dune 

type in the area (Madole, 1995). Blowout dunes are large 

accumulations of sand derived from a “blowout”—a 

saucer- or trough- shaped hollow formed by wind 

erosion in a preexisting sand deposit. 

Gilbert (1896), working along the Arkansas River and the 

Black Squirrel sand area, was the first to conclude that 

streams were the primary source of eolian sand in the 

area. Since then, many studies have reached this 

conclusion. However, although streams are the principal 

source of eolian sand, they are not the only source; in 

places, eolian sediment apparently is derived from 

residuum of Upper Cretaceous and Tertiary sedimentary 

rocks (Madole, 1995). 

Quartz is the dominant mineral in the area’s eolian sand; 

feldspar is also present in amounts that appear to vary 

with sediment source (Madole, 1995). Although very fine 

sand tends to be dominant in dunes, most eolian- sand 

deposits contain all sizes of sand and some silt and clay. 

Few data are available with which to evaluate how sand 

size varies with local source area or deposit age. 

However, sand size does tend to decrease systematically 

downwind away from the South Platte River (Shroba, 

1980; Muhs, 1985). 

The roundness and sorting of sand grains appear to vary 

with sediment source. Grains derived from the 

floodplains and channels of large streams, such as the 

South Platte and Arkansas Rivers, tend to be rounded 

and well sorted, whereas a high percentage of the grains 

derived from small tributaries to these rivers are 

commonly sub- angular and not well sorted (Madole, 

1995). 

Presently, most dune sand in the vicinity of Bents Old 

Fort is stable and covered with vegetation (see Fig 5). 

Whereas blowout dunes are the most common type in 

areas south of the Arkansas River, parabolic dunes are 

dominant throughout northeast and central Colorado 

(Madole, 1995). Parabolic dunes are sand dunes with 

long, scoop- shaped forms, convex in the downwind 

direction so that their horns point upwind. When 

perfectly developed, parabolic dunes viewed from above 

approximate the form of a parabola. Except for the dune 

belt flanking the south side of the Arkansas River, dune 

fields are few, small, and widely scattered in southeastern 

Colorado (Madole, 1995). 

River Terraces 

Changes in channel gradient, discharge, or sediment load 

can lead to a river channel incising its floodplain. The 

original floodplain is thereby abandoned and is left as a 

relatively flat bench, known as a river terrace, which is 

separated from the new floodplain below by a steep 

slope. The fort at Bent’s Old Fort National Historic Site 

sits on such a terrace. River terraces can also be cut into 

bedrock, in which case they are covered by only a thin 

sediment veneer. 

River terraces are inclined downstream but not always at 

the same inclination as the active floodplain. A valley side 

may contain a vertical sequence of terraces. The lowest is 

the youngest and may retain traces of floodplain 

morphology, while the highest is the oldest and is usually 

partly degraded. Terraces can be either paired—forming 

when vertical incision is rapid in comparison with the 

lateral migration of the river channel, or unpaired—

forming where lateral shifting of the channel is relatively 

rapid; this results in the river cutting terraces alternately 

on each side of the valley floor. 

Terraces can be formed under a wide range of 

circumstances. Many episodes of channel entrenchment 

and terrace formation probably arise from changes in 

base level or fluctuations in climate, although tectonic 

deformation along river courses can also lead to terrace 

development (Summerfield, 1991). (Base level is the 

theoretical limit or lowest level toward which erosion of 

the Earth’s surface constantly progresses but seldom, if 

ever, reaches. In particular, base level refers to the level 

below which a stream cannot erode. The general or 

ultimate base level for the land surface is sea level, but 

temporary base level may exist locally.) 

During the Quaternary Period (see Fig 6) a major climatic 

cooling caused glaciation. Episodic base- level changes 

are recorded by alluvial deposits, such as terraces, in the 

Front Range and eastern Colorado. These changes are 

thought to be caused mainly by cyclic changes in climate, 

but could partly be a result of uplift that continued into 

Holocene time (Epis and others, 1980). 

The alluvium in the vicinity of Bent’s Old Fort National 

Historic Site serves as a record of climatic changes 

throughout the Quaternary Period. Geologists have 

studied, described, and named these deposits. The ages 

of these deposits correspond to the glacial episodes that 

occurred on continental and regional scales. Depending 
BEOL Geologic Resource Evaluation Report 17 

on the scale a particular investigator applied during his or 

her study, the names of the deposits vary (Table 1). In 

addition, during the 1960s investigators conducted much 

of the work for the glacial chronology of Colorado. 

Reports from studies of the early 1960s typically retain 

continental terminology because the glacial chronology 

for Colorado had not been worked out yet. 

The occurrence of caliche—gravel and sand cemented 

with calcium carbonate—in the various Pleistocene 

deposits is diagnostic for determining the age of terraces. 

Generally, the youngest deposits contain the least caliche 

(Weist and others, 1965). 

Structural Features 

Otero and Crowley Counties lie near the southern end of 

the Denver basin, a large syncline that extends 

northward to Wyoming. The area surrounding Bent’s 

Old Fort National Historic Site is bounded on the east 

and southeast by the Las Animas arch and on the south 

and southwest by the Apishapa uplift. The Las Animas 

arch, also termed the Sierra Grande arch, is one of the 

largest structural features in the vicinity of the historic 

site. Rich (1921) describes the arch as “a probable buried 

mountain range of early Permian age east of the present 

Rocky Mountains in New Mexico and Colorado.” Both 

of these regional anticlines start at the Sierra Grande 

uplift near the New Mexico state line. 

These major structural features combine to give the 

general northward or northwestward dip to the 

consolidated beds throughout most of Otero and 

Crowley Counties. In the southwestern part of Otero 

County the effects of the Apishapa uplift are more 

pronounced, and the beds generally dip northeastward. 

Dips are generally between 1° and 3° but locally may be as 

much as 36° (Patton, 1923). Dips greater than 3° are more 

common south of the Arkansas River. Locally, abrupt 

changes in both the amount and direction of dip occur 

(Weist and others, 1965). 

Small faults in the area are numerous; most of them are in 

the Codell Sandstone Member of the Charlile Shale and 

the Fort Hays Limestone Member of the Niobrara 

Formation (see Fig 5). These faults are usually nearly 

vertical and have a displacement of less than 40 feet (12 

m). Few of the faults can be traced for as much as a mile 

(1.6 km) before they lose their identity in less competent 

beds (Weist and others, 1965). 

Duce (1924) defined two local structural features in 

southern Otero County. The first is the Ayer flexure, a 

small monoclinal fold—a local steepening in an 

otherwise uniform gentle dip—just east of Ayer siding, 

which he described as an unclosed dome with the 

southern end open. It has no topographic expression and 

appears as an inlier—an area or group of rocks 

surrounded by rocks of younger age—of Greenhorn 

Limestone (Weist and others, 1965). The other feature is 

the Black Hill dome southeast of Bloom siding. It is a 

small dome imposed on a monocline. Most of the dome 

is south of Otero County (Weist and others, 1965). 

Table 1. Sequence of Quaternary Deposits 

Epoch Continental glaciation Regional glaciation Deposit 

Loess and eolian sand 

Piney Creek Alluvium 

Holocene 

Pre- Piney Creek 

Alluvium 

Wisconsin Pinedale Broadway Alluvium 

Bull Lake Louviers Alluvium 

Illinoian Slocum Alluvium 

Kansan Verdos Alluvium 

Pleistocene 

Nebraskan 

Pre- Bull Lake 

Rocky Flats Alluvium 

 Nussbaum Alluvium 

Sources: Scott (1963) and Tweto (1979). 

Note: Nussbaum alluvium—also considered a Quaternary deposit (Epis and others, 1980)—formed through pedimentation rather than 

terrace formation. That is, the surface formed through backwearing and removal of debris by rill wash and unconcentrated flow rather 

than lateral planation by steep-gradient streams. Not all geologists agree with this description of pediment genesis, however. 
18 NPS Geologic Resources Division 

Map Units and Properties 

This section provides a description for and identifies many characteristics of the rock 

units in the vicinity of Bent’s Old Fort National Historic Site. The following table is 

highly generalized and is provided for informational purposes only. Ground disturbing 

activities should not be permitted or denied on the basis of information contained in this 

table. 

The table that comprises this section is meant to 

compliment the digital geologic map of Bent’s Old Fort 

National Historic Site. The data files associated with the 

digital map provide the names of deposits and map 

symbols (see included CD); however, no descriptions of 

the deposits or rocks are included. The digital geologic 

map shows the floodplain area in the immediate vicinity 

of the fort. Therefore, this section of the report also 

includes rocks mapped in T23S, R54W of Otero County 

during a 1965 water- supply study (Fig 5). 

The table provides readily available information for 

resource managers about the rock formations and 

deposits in the vicinity of the historic site. The table 

provides descriptions and correlates rock units from 

three sources: 

Linn, S., 1999, Geologic map of Bent’s Old Fort National 

Historic Site and vicinity, Otero and Bent Counties, 

Colorado [unpublished map]: Lakewood, Colorado, 

National Park Service, scale 1:24,000. 

Tweto, O., 1979, Geologic map of Colorado: Reston, 

Virginia, U.S. Geological Survey, scale 1:500,000. 

Weist, W.G., Jenkins, E.D., and Horr, A., 1965, Geology 

and occurrence of ground water in Otero County and 

southern part of Crowley County, Colorado: U.S. 

Geological Survey Water- Supply Paper 1799, 85 p., 4 

plates, scale 1:125,000.
BEOL Geologic Resource Evaluation Report 19 

Figure 5: Rock Units in the Vicinity of Bent’s Old Fort. Bent’s Old Fort National Historic Site (Otero County, Colorado) is in T23S, R54 W 

(area shown in figure). During a water-supply survey, Weist and others mapped the rocks in the vicinity of Bent’s Old Fort. These rocks 

are discussed in the rock units table of this section. Source: Weist and others (1965). 

BEOL Geologic Resource Evaluation Report 21 

Regional Geologic Setting 

Geographers have subdivided the United States into 

physiographic regions that, although they have great 

diversity within themselves, are distinctly different from 

one another. 

From the Rocky Mountains on the west to the 

Appalachians on the east, the interior of our country is a 

vast lowland known as the Interior Plains. These plains 

are bounded on the south by a region of Interior 

Highlands, consisting of the Ozark Plateaus and the 

Ouachita province, and by the Coastal Plain. In the Great 

Lakes region, the Interior Plains lap onto the most 

ancient part of the continent, the Superior Upland. West 

of the Great Lakes it extends far to the north into 

Canada. The Rocky Mountains are distinctly different 

from the region to the east, which is the Great Plains. The 

Great Plains, then, is the western part of the great 

Interior Plains; the Rocky Mountains form its western 

margin and past glacial activity defines its eastern margin. 

During the Pleistocene Epoch, huge glaciers formed in 

Canada and advanced southward into the great, central, 

low- lying Interior Plains of the United States. These 

glaciers and their deposits modified the surface of the 

land they covered, mostly between the Missouri and the 

Ohio Rivers; they smoothed the contours and gave the 

land a more subdued aspect. This glacially smoothed and 

modified land is called the Central Lowland. Although 

the ice sheets flowed into the northern part, the Great 

Plains is the largely unglaciated region that extends from 

the Gulf Coastal Plain in Texas northward into Canada 

between the Central Lowland and the foot of the Rocky 

Mountains. Its eastern margin in Texas and Oklahoma is 

marked by a prominent escarpment, the Caprock 

escarpment. Its southern margin, where it abuts the 

Coastal Plain in Texas, is at another abrupt rise or scarp 

along the Balcones fault zone (Trimble, 1980). 

Within the Great Plains are many large areas that differ 

greatly from adjoining areas. The Black Hills stand out 

distinctively from the surrounding lower land; its dark, 

forested prominence can be seen for miles from any 

direction. At the southern end of the Great Plains is 

another, less imposing, forested prominence—the 

Central Texas uplift. Most impressive, perhaps, is the 

huge, nearly flat plateau known as the High Plains, which 

extends southward from the northern border of 

Nebraska through the panhandle of Texas and forms the 

central part of the Great Plains. The eastern and western 

rims of the southern High Plains are high, cliffed, 

erosional escarpments—the Caprock escarpment on the 

east and the Mescalero escarpment on the west. The 

north edge of the High Plains is defined by another 

escarpment, the Pine Ridge escarpment, which separates 

the High Plains from a region that has been greatly 

dissected by the Missouri River and its tributaries. There, 

several levels of rolling upland are surmounted by small 

mountainous masses and flat- topped buttes that are 

entrenched by streams. This region is the Missouri 

Plateau. The continental glacier covered the 

northeastern part of the Missouri Plateau and altered its 

surface (Trimble, 1980). 

The South Platte and Arkansas Rivers and their 

tributaries have similarly dissected an area along the 

mountain front called the Colorado Piedmont. The 

Pecos River has excavated a broad valley trending 

southward from the Sangre de Cristo Range in New 

Mexico into Texas. The Mescalero escarpment separates 

the Pecos Valley from the southern High Plains. South 

and east of the Pecos Valley, extending to the Rio 

Grande and the Coastal Plain, is the broad plateau of 

bare, stripped, flat- lying limestone layers bearing little 

but cactus that is called the Edwards Plateau. Green, 

crop- filled valleys with gently sloping valley walls and 

rounded stream divides trend eastward from the High 

Plains of western Kansas and characterize a Plains 

Border section. And finally, between the Colorado 

Piedmont on the north and the Pecos Valley on the 

south, volcanic vents, cinder cones, and lava fields form 

another distinctive terrain in the part of the Great Plains 

called the Ration section (Trimble, 1980). 

Early geologic events and the later shaping processes 

determined the distinctive character of the landscape in 

each section. All parts of this region have a similar early 

history, and the differences are the results of local 

dominance of certain processes in the ultimate shaping 

of the landscape, mostly during the last few million years 

(Trimble, 1980). Precambrian mountains were reduced to 

low- relief by the time shallow Cambrian (early 

Paleozoic) (see Fig 6) seas began to creep across and 

cover much of the North American continent. 

Fluctuating shallow seaways and relatively thin layers of 

Paleozoic and Mesozoic strata characterize the history of 

the craton until Late Cretaceous time when the Laramide 

Orogeny began. Uplift of the Rockies produced a 

corresponding downwarp in the Great Plains where 

extensive shallow basins formed. In particular, the 

Williston Basin formed in the northern Great Plains in 

the Montana- Dakota area, the Denver- Julesburg Basin 

formed in eastern Colorado, and Midland Basin formed 

in Texas (Thornbury, 1965). 

Alluvium from the eroding mountains filled these 

shallow basins to overflowing. Shifting stream channels, 

floodplains, swamps, and occasional volcanic ash 

(especially in Oligocene and Miocene time) mostly 

characterize the Tertiary- age Great Plains sediments. 

Some 6–8 million years ago, the landscape was quite 

monotonous as the vast fluviatile plain sloped gradually 

eastward from the mountains—with only the Black Hills 

and a few other local mountains rising about them. The 

depositional surface of the Great Plains ramped up to the 

mountain edges where erosional plains were cut across 

the hard Precambrian and younger rocks to form the 

high- level erosion surfaces found in Rocky Mountain 

National Park and other Rocky Mountain areas (Kiver 

and Harris, 1999). 
22 NPS Geologic Resources Division 

Uplift of the Rocky Mountains and eastward tilting of 

the Great Plains during the Pliocene Epoch initiated a 

new regime of downcutting and removal of sedimentary 

debris. Tertiary sediments were completely removed 

from the Colorado Piedmont and certain other areas. 

However, the High Plains—from South Dakota to the 

Texas border—form an extensive area where the late 

Miocene- Pliocene surface has been little eroded. A 

narrow remnant of this old surface extends westward in 

southeastern Wyoming, north of the Colorado 

Piedmont; it is called the Gangplank. Modern- day 

Union Pacific tracks and Interstate 80 make use of this 

natural sedimentary rock ramp to cross the Laramie 

Mountains (Kiver and Harris, 1999). 

Pleistocene ice sheets pushed southwestward out of 

Canada and covered the northern part of the Great 

Plains, the area north of the Missouri River. Prior to 

glaciation, the Missouri flowed northeastward into 

Hudson Bay. Blocked off by ice during the Wisconsin 

advance, the river followed a new course along the front 

of the glacier. Once established, the Missouri maintained 

essentially this course after the glacier retreated (Kiver 

and Harris, 1999). 

The contrast in landforms on the two sides of the glacial 

boundary is striking. The glaciated section is noted for its 

hummocky topography made of irregular moraines in 

which kettles, kettle lakes, and morainal lakes are 

abundant. The streams wander aimlessly through the 

glacial deposits, into and out of lakes and swamps. In the 

areas south of the glacial boundary, streams have 

developed regular drainage patterns and typical fluvial 

landforms (Kiver and Harris, 1999). 

Dry climatic episodes during the past 1,000 years have 

generated desert conditions—conditions that would 

make modern agriculture impossible (Kiver and Harris, 

1999). This semiarid region hangs close to the threshold 

where windblown sediment will again move and convert 

thousands of square miles into dunes and desert (Muhs 

and others, 1993).
BEOL Geologic Resource Evaluation Report 23 

Eon Era Period Epoch Dates Age of 

Holocene 0.01 

Quaternary Pleistocene 1.81 

Pliocene 5.32 

Neogene Miocene 23.8 

Oligocene 33.7 

Eocene 55.0 

Cenozoic 

Tertiary 

Paleogene 

Paleocene 65.5 

Mammals 

Cretaceous 142.0 

Mesozoic Jurassic 205.1 

Triassic 250 

Reptiles 

Permian 292 

Pennsylvanian 320 

Carboniferous Mississippian 354 

Amphibians 

Devonian 417 

Silurian 440 Fishes 

Ordovician 495 

Phanerozoic 

Paleozoic 

Cambrian 545 Invertebrates 

Proterozoic 2,500 

Archean 2,500–3,800? 

Hadean 

Also known as Precambrian 

3,800–4,600? 

Figure 6: Geologic Time Scale. Dates (in millions of years) reflect the International Union of Geological Sciences International Stratigraphic 

Chart, except for the boundary between Archean and Hadean, which is not listed (http://www.stratigraphy.org/chus.pdf). The U.S. Geological 

Survey lists this boundary and the formation of Earth (at approximately 4,600 million years ago), which are used here. 
24 NPS Geologic Resources Division 

References Cited 

This section provides a listing of references cited in this report. A more complete geologic 

bibliography is available and can be obtained through the NPS Geologic Resources 

Division. 

Army Corps of Engineers, 1977, Special flood hazard 

information, Arkansas River, Anderson and King 

Arroyos, La Junta, Otero, County, Colorado: 

Albuquerque, New Mexico, Department of the Army, 

Albuquerque District, Corps of Engineers, 29 p. 

Bent’s Old Fort National Historic Site, 1992, Resource 

management plan: La Junta, Colorado, Bent’s Old Fort 

National Historic Site, 126 p. 

Berger, A.R., and Iams, W.J., 1996, Geoindicators—

assessing rapid environmental changes in Earth 

systems: Rotterdam, A.A. Balkema, 480 p., 

http://www.gcrio.org/geo/toc.html (December 2003). 

Cobban, W.A., Skelton, P.W., and Kennedy, W.J., 1991, 

Occurrence of the rudistid Durania cornupastoris (Des 

Moines, 1826) in the Upper Cretaceous Greenhorn 

Limestone in Colorado, in Sando, W.J., ed., Shorter 

contributions to paleontology and stratigraphy: U.S. 

Geological Survey Bulletin 1985, p. D1–D8. 

Des Moulins, C., 1826, Essai sur les spherulites qui 

existent dans the collections de MM. F. Jouannet et C. 

Des Moulins, et considerations sur la famille à laquelle 

ces fossils appartiennent: Bulletin de l’Histoire 

Naturelle Société Linneenne de Bordeaux, v. 1, p. 141–

143. 

Duce, J.T., 1924, Geology of parts of Las Animas, Otero, 

and Bent Counties: Colorado Geological Survey 

Bulletin 27, part 3, p. 73–102. 

Emmons, S.F., Cross, W., and Eldridge, G.H., 1896, 

Geology of the Denver Basin in Colorado: U.S. 

Geological Survey Monograph 27, 556 p. 

Epis, R.C., Scott, G.R., Taylor, R.B., and Chapin, C.E., 

1980, Summary of Cenozoic geomorphic, volcanic and 

tectonic features of central Colorado and adjoining 

areas, in Kent, H.C., and Porter, K.W., eds., Colorado 

geology: Denver, Colorado, Rocky Mountain 

Association of Geologists, p. 135–156. 

Gilbert, G.K., 1896, The underground water of the 

Arkansas Valley in eastern Colorado: U.S. Geological 

Survey 17th Annual Report, part 2, p. 551–601. 

Ken R. White Company, 1973, Soils investigation, 

National Park Service, Bents Old Fort, Colorado [job 

no. 3106]: Denver, Colorado, The Ken R. White 

Company, 6 p. 

Kiver, E.P., and Harris, D.V., 1999, Great Plains province, 

in Geology of U.S. parklands (5th ed.): New York, John 

Wiley & Sons, Inc., p. 665–671. 

Linn, S., 1999, Geologic map of Bent’s Old Fort National 

Historic Site and vicinity, Otero and Bent Counties, 

Colorado [unpublished map]: Lakewood, Colorado, 

National Park Service, scale 1:24,000. 

Madole, R.F., 1995, Spatial and temporal patterns of late 

Quaternary eolian deposition, eastern Colorado, 

U.S.A.: Quaternary Science Reviews, v. 14, p. 155–177. 

Maher, J.C., 1947, Subsurface geologic cross section from 

Scott County, Kansas, to Otero County, Colorado 

[State Geological Survey of Kansas, Oil and Gas 

Investigations, Preliminary Cross Section No. 4]: 

Lawrence, Kansas, University of Kansas Publications, 

11 p. 

McCauley, J.F., Breed, C.S., Grolier, M.J., and 

MacKinnon, D.J., 1981, The U.S. dust storm of 

February 1977, in Péwé, T.L., ed., Desert dust—origin, 

characteristics, and effect on man: Boulder, Colorado, 

Geological Society of America Special Paper 186, p. 

123–147. 

Moore, J.W., Jr., 1973, Bent’s Old Fort—an archeological 

study: Denver, Colorado, State Historical Society of 

Colorado, Pruett Publishing Company, 144 p. 

Muhs, D.R., 1985, Age and paleoclimatic significance of 

Holocene sand dunes in northeastern Colorado: 

Annals of the Association of American Geographers, v. 

75, p. 566–582. 

Muhs, D.R., Millard, H.T., Jr., Madole, R.F., and Schenk, 

C.J., 1993, History of desertification on the Great 

Plains—a Holocene history of eolian sand movement, 

in Kelmelis, J.A., and Snow, K.M., eds., Proceedings of 

the U.S. Geological Survey global change research 

forum, Herndon, Virginia, March 18–20, 1991 [U.S. 

Geological Survey Circular 1086]: Reston, Virginia, 

U.S. Geological Survey, p. 102–103. 

Muhs, D.R., Swinehart, J.B., Loope, D.B., Aleinikoff, J.N., 

and Been, J., 1999, 200,000 years of climate change 

recorded in eolian sediments of the High Plains of 

eastern Colorado and western Nebraska, in Lageson, 

D.R., Lester, A.P., and Trudgill, B.D., eds., Colorado 

and adjacent areas: Boulder, Colorado, Geological 

Society of America Field Guide 1, p. 71–91. 
BEOL Geologic Resource Evaluation Report 25 

National Park Service, 1975a, Final environmental 

assessment/final master plan, Bent’s Old Fort National 

Historic Site, Colorado [NPS- 866]: Denver, Colorado, 

National Park Service, Denver Service Center, 51 p. 

National Park Service, 1975b, Site improvement, Bent’s 

Old Fort [D- 19]: Denver, Colorado, National Park 

Service, Denver Service Center, 62 p. 

National Park Service, 1993, Draft environmental impact 

statement and general management plan/development 

concept plan, Bent’s Old Fort National Historic Site, 

Otero, County, Colorado [DES 93- 41]: La Junta, 

Colorado, Bent’s Old Fort National Historic Site, 163 p. 

National Park Service, 2001, Management Policies 2001: 

Washington, D.C., National Park Service, 137 p. 

Patton, H.B., 1923, Underground water possibilities for 

stock and domestic purposes in the La Junta area, 

Colorado: Colorado Geological Survey Bulletin 27, 

part 1, p. 1–52. 

Péwé, T.L., 1981, Desert dust—an overview, in Péwé, T.L., 

ed., Desert dust—origin, characteristics, and effect on 

man: Boulder, Colorado, Geological Society of 

America Special Paper 186, p. 1–10. 

Rich, J.L., 1921, A probable buried mountain range of 

early Permian age east of the present Rocky Mountains 

in New Mexico and Colorado: American Association 

of Petroleum Geologists Bulletin, v. 5, no. 5, p. 605–

608. 

Scott, G.R., 1963, Quaternary Geology and geomorphic 

history of the Kassler quadrangle, Colorado: U.S. 

Geological Survey Professional Paper 421- A, p. 1–67. 

Scott, R., Santucci, V.L., and Connors, T., 2001, An 

inventory of paleontological resources from the 

national parks and monuments in Colorado, in

Santucci, V.L., and McClelland, L., eds., Proceedings 

of the 6th fossil resource conference 

[NPS/NRGRD/GRDTR- 01/01]: Lakewood, Colorado, 

National Park Service, Geologic Resources Division, p. 

178–202. 

Shroba, R.R., 1980, Geologic map and physical properties 

of the surficial and bedrock units of the Englewood 

quadrangle, Denver, Arapahoe, and Adams Counties, 

Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ- 1524, scale 1:24,000. 

Skelton, P.W., 1979, Preserved ligament in a radiolitid 

rudist bivalve and its implication of mantle marginal 

feeding in the group: Paleobiology, v. 5, p. 90–106. 

Snipes, R.J., 1974, Floods of June 1965 in Arkansas River 

basin, Colorado, Kansas, and New Mexico: U.S. 

Geological Survey Water- Supply Paper 1850- D, p. D1–

D97. 

Summerfield, M.A., 1991, Global geomorphology—an 

introduction to the study of landforms: Essex, 

England, Longman Scientific & Technical 

[copublished in the United States with John Wiley & 

Sons, Inc., New York], 537 p. 

Swenson, F.A., 1970, Meandering of the Arkansas River 

since 1833 near Bent’s Old Fort, Colorado: U.S. 

Geological Survey Professional Paper 700- B, p. B210–

B213. 

Thornsbury, W.D., 1965, Regional geomorphology of the 

United States: New York, Wiley, 609 p. 

Trimble, D.E., 1980, The geologic story of the Great 

Plains—a nontechnical description of the origin and 

evolution of the landscape of the Great Plains: U.S. 

Geologic Survey Bulletin 1493, 54 p. [Reprinted 1990 

and 1993, Medora, North Dakota, Theodore Roosevelt 

Nature and History Association]. 

Tweto, O., 1979, Geologic map of Colorado: Reston, 

Virginia, U.S. Geological Survey, scale 1:500,000. 

Voegeli, P.T., and Hershey, L.A., 1965, Geology and 

ground- water resources of Prowers County, 

Colorado: U.S. Geological Survey Water- Supply Paper 

1772, 97 p. 

Wayne, W.J., Abet, J.S., Agard, S.S., Bergantino, R.N., 

Bluemle, J.P., Coates, D.A., Cooley, M.E., Madole, 

R.F., Martin, J.E., Mears, B., Jr., Morrison, R.B., and 

Sutherland, W.M., 1991, Quaternary geology of the 

Northern Great Plains, in Morrison, R.B., ed., 

Quaternary nonglacial geology, conterminous U.S.: 

Boulder, Colorado, Geological Society of America, 

The Geology of North America, v. K- 2, p. 441–476. 

Weist, W.G., Jenkins, E.D., and Horr, A., 1965, Geology 

and occurrence of ground water in Otero County and 

southern part of Crowley County, Colorado: U.S. 

Geological Survey Water- Supply Paper 1799, 85 p., 4 

plates, scale 1:125,000. 

Woods, S.W., and MacDonald, L.H., 2002, The cause of 

basement flooding at Bent’s Old Fort National Historic 

Site, Colorado [final report for contract CA 1200- 99-

009 CSU- 23]: La Junta, Colorado, Bent’s Old Fort 

National Historic Site, 35 p. 

Wyckoff, J., 1999, Reading the Earth—landforms in the 

making: Mahwah, New Jersey, Adastra West, Inc., 352 

p.
26 NPS Geologic Resources Division 

Appendix A: Geologic Map Graphic 

This image provides a preview or “snapshot” of the geologic map for Bent’s Old Fort 

National Historic Site. For the digital geologic map, see included CD. 

The original map digitized by NPS staff to create this product was Linn, Sarah, 1999, Surficial geologic map of Bent's Old Fort NHS and 

Vicinity, Colorado, NPS, unpublished, 1:24000 scale. For a detailed digital geologic map and cross sections, see included CD. 
The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific and other information about 

those resources; and honors its special responsibilities to American Indians, Alaska Natives, and affiliated Island Communities. 

Bent’s Old Fort National Historic Site 

Geologic Resource Evaluation Report 

Natural Resource Report NPS/NRPC/GRD/NRR—2005/005 

NPS D-74, September 2005 

National Park Service 

Director • Fran P. Mainella 

Natural Resource Stewardship and Science 

Associate Director • Michael A. Soukup 

Natural Resource Program Center 

The Natural Resource Program Center (NRPC) is the core of the NPS Natural Resource Stewardship and Science 

Directorate. The Center Director is located in Fort Collins, with staff located principally in Lakewood and Fort 

Collins, Colorado and in Washington, D.C. The NRPC has five divisions: Air Resources Division, Biological 

Resource Management Division, Environmental Quality Division, Geologic Resources Division, and Water 

Resources Division. NRPC also includes three offices: The Office of Education and Outreach, the Office of 

Inventory, Monitoring and Evaluation, and the Office of Natural Resource Information Systems. In addition, Natural 

Resource Web Management and Partnership Coordination are cross-cutting disciplines under the Center Director. The 

multidisciplinary staff of NRPC is dedicated to resolving park resource management challenges originating in and 

outside units of the national park system. 

Geologic Resources Division 

Chief • David B. Shaver 

Planning Evaluation and Permits Branch Chief • Carol McCoy 

Credits 

Author • Katie KellerLynn

Editing • Sid Covington and Melanie Ransmeier 

Digital Map Production • Stephanie O’Meara 

Map Layout Design • Melanie Ransmeier
Geologic Resources Division 

Natural Resource Program Center 

P.O. Box 25287 

Denver, CO 80225 

http://www.nature.nps.gov/geology/inventory/ 

(303) 969-2090 

EXPERIENCE YOUR AMERICATM 

National Park Service 

U.S. Department of the Interior