U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2005–5072

Sedimentology and Stratigraphy of the Palisades, Lower 

Comanche, and Arroyo Grande Areas of the Colorado River 

Corridor, Grand Canyon, Arizona 
Sedimentology and Stratigraphy of the Palisades, 

Lower Comanche, and Arroyo Grande Areas of the 

Colorado River Corridor, Grand Canyon, Arizona

By Amy E. Draut, David M. Rubin, Jennifer L. Dierker, Helen C. Fairley, Ronald E. Griffiths, Joseph 

E. Hazel, Jr, Ralph E. Hunter, Keith Kohl, Lisa M. Leap, Fred L. Nials, David J. Topping, and 

Michael Yeatts

Scientific Investigations Report 2005–5072

U.S. Department of the Interior

U.S. Geological Survey 

This report analyzes various depositional environments in three archaeologically significant areas of the Colorado River corridor 

in Grand Canyon.

 Archaeological features are built on and buried by fluvial, aeolian, and locally derived sediment, representing a complex interaction between geologic and cultural history. These analyses provide a basis for determining the potential influence of Glen 

Canyon Dam operations on selected archaeological sites and thus for guiding dam operations in order to facilitate preservation 

of cultural resources.
U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government.

United States Government Printing Office: 2004

For additional copies please contact:

USGS Information Services

Box 25286

Denver, CO 80225

This report and any updates to it are available at:

http://pubs.usgs.gov/sir/2005/5072

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Manuscript approved for publication, April 5, 2005

Text edited by Peter H. Stauffer

Layout and design by Stephen L. Scott 

Cataloging-in-Publication data are on file with the Library of Congress
Contents

Abstract ------------------------------------------------------------------------------------------------------ 1

Introduction -------------------------------------------------------------------------------------------------- 1

Site selection ------------------------------------------------------------------------------------------ 3

Previous work -----------------------------------------------------------------------------------------3

Methods ------------------------------------------------------------------------------------------------------ 4

Identification of depositional environments ------------------------------------------------------- 5

Grain-size analyses ----------------------------------------------------------------------------------- 6

Palisades area ----------------------------------------------------------------------------------------------- 6

Geomorphic setting ----------------------------------------------------------------------------------- 6

Archaeological significance ------------------------------------------------------------------------ 10

Stratigraphic and geomorphic analyses ---------------------------------------------------------- 10

Grain-size analyses ---------------------------------------------------------------------------------- 10

Driftwood elevation survey: Palisades stage-discharge relationship ------------------------- 10

Palisades site summary ----------------------------------------------------------------------------- 14

Lower Comanche area ------------------------------------------------------------------------------------ 14

Geomorphic setting ---------------------------------------------------------------------------------- 14

The Lower Comanche “white layer” --------------------------------------------------------------- 16

Archaeological significance ------------------------------------------------------------------------ 18

Stratigraphic and geomorphic analyses ---------------------------------------------------------- 18

Grain-size analyses ---------------------------------------------------------------------------------- 18

Driftwood elevation survey: Lower Comanche stage-discharge relationship ---------------- 18

Lower Comanche site summary -------------------------------------------------------------------- 20

Arroyo Grande area ---------------------------------------------------------------------------------------- 21

Geomorphic setting ---------------------------------------------------------------------------------- 21

Archaeological significance ------------------------------------------------------------------------ 23

Stratigraphic and geomorphic analyses ---------------------------------------------------------- 23

Grain-size analyses ---------------------------------------------------------------------------------- 24

Arroyo Grande site summary ----------------------------------------------------------------------- 26

Discussion and conclusions ------------------------------------------------------------------------------ 28

Observations related to aeolian sediment -------------------------------------------------------- 28

Potential implications for management objectives ---------------------------------------------- 31

Future research directions -------------------------------------------------------------------------- 31

Acknowledgments ----------------------------------------------------------------------------------------- 32

References -------------------------------------------------------------------------------------------------- 32

Appendix 1: Descriptions of stratigraphic sections and geomorphology at

 archaeological site locations, Palisades area ------------------------------------------------------ 37

Appendix 2: Descriptions of stratigraphic sections and geomorphology at 

 archaeological site locations, Lower Comanche area --------------------------------------------- 43

Appendix 3: Descriptions of stratigraphic sections and geomorphology 

 at archaeological site locations, Arroyo Grande area --------------------------------------------- 51

III
Figures

 1. Location map of Grand Canyon, with study areas indicated --------------------------------------- 3

 2. Aerial photograph (2002) of the Palisades study area ---------------------------------------------- 7

 3. Photograph of aeolian dunes at Palisades ----------------------------------------------------------- 8 

 4. Photographs of playa deposits at the eastern edge of the Palisades study area --------------- 9

 5. Aerial photograph (1965) of the Palisades study area ---------------------------------------------- 9

 6. Grain-size plots for units F and G of section 1, Palisades ----------------------------------------- 11

 7. Grain-size plot for unit H of section 3, Palisades --------------------------------------------------- 11

 8. Grain-size data from section 4, Palisades ----------------------------------------------------------- 11

 9. Grain-size data for samples of a flood deposit, unit B of section 4, Palisades ----------------- 11

10. Photographs of driftwood logs at Palisades -------------------------------------------------------- 12

11. Graph of stage-discharge relationship for the Palisades area, based on

 elevations of driftwood logs -------------------------------------------------------------------------- 13

12. Graph showing proportions of sediments of different depositional 

 environments in sections at Palisades -------------------------------------------------------------- 13

13. Aerial photograph (2002) of the Lower Comanche study area ------------------------------------ 15

14. Aerial photograph of the arroyo system at Lower Comanche ------------------------------------ 15

15. Aerial photograph (1965) of the Lower Comanche area ------------------------------------------- 16

16. Photograph of the white-layer horizon at Lower Comanche ------------------------------------- 16

17. Grain-size plots for units C and F of section 2, Lower Comanche -------------------------------- 19

18. Grain-size plots for units B, E, and F of section 6, Lower Comanche ---------------------------- 19

19. Graph of stage-discharge relationship for the Lower Comanche area, 

 based on elevations of driftwood logs --------------------------------------------------------------- 20

20. Graph showing proportions of sediments of different depositional 

 environments in sections at Lower Comanche ----------------------------------------------------- 20

21. Aerial photographs (2002) of the Arroyo Grande study area -------------------------------------- 22

22. Photographs of the main arroyo network at Arroyo Grande -------------------------------------- 22

23. Aerial photograph (1965) of the Arroyo Grande area ---------------------------------------------- 23

24. Photograph showing a wide drainage at the downstream end of the 

 Arroyo Grande area ------------------------------------------------------------------------------------ 23

25. Grain-size plots for sections 1a and 1b, Arroyo Grande ------------------------------------------- 24

26. Grain-size plots for two flood units (units C and H) in section 5, Arroyo Grande --------------- 25

27. Grain-size plots for subunit G3 of section 6, Arroyo Grande -------------------------------------- 25

28. Grain-size plots for section 8, Arroyo Grande ------------------------------------------------------ 25

29. Grain-size plots for section 10, Arroyo Grande ----------------------------------------------------- 25

30. Graphs showing grain-size trends within four flood units at Arroyo Grande ------------------- 26

31. Graph showing proportions of sediments of different depositional

 environments in sections at Arroyo Grande -------------------------------------------------------- 27

32. Stratigraphic diagram for section 1, Palisades ----------------------------------------------------- 37

33. Stratigraphic diagram for section 2, Palisades ----------------------------------------------------- 38

34. Stratigraphic diagram for section 3, Palisades ----------------------------------------------------- 39

35. Photograph showing units A, B, and C of section 4, Palisades ----------------------------------- 40

36. Stratigraphic diagram for section 4, Palisades ----------------------------------------------------- 40

IV
37. Photograph showing aeolian and fluvial sediment at archaeological

 site C:13:99, Palisades --------------------------------------------------------------------------------- 41

38. Photograph showing migrating sand dune at archaeological

 site C:13:100, Palisades -------------------------------------------------------------------------------- 42

39. Photograph showing thin sedimentary cover at archaeological 

 site C:13:272, Palisades -------------------------------------------------------------------------------- 42

40. Photograph of section 2, Lower Comanche --------------------------------------------------------- 44

41. Stratigraphic diagram for section 2, Lower Comanche ------------------------------------------- 44

42. Photograph of section 3, Lower Comanche, showing the white layer --------------------------- 46

43. Stratigraphic diagram for section 3, Lower Comanche ------------------------------------------- 46

44. Stratigraphic diagram for section 4, Lower Comanche ------------------------------------------- 48

45. Stratigraphic diagram for section 5, Lower Comanche ------------------------------------------- 48

46. Stratigraphic diagram for section 6, Lower Comanche ------------------------------------------- 49

47. Photograph of one of the roasting features, feature 3, at archaeological

 site C:13:273, Lower Comanche ---------------------------------------------------------------------- 50

48. Photograph of a roasting feature at archaeological site C:13:333, 

 Lower Comanche -------------------------------------------------------------------------------------- 50

49. Photograph of archaeological site C:13:337, Lower Comanche ---------------------------------- 51

50. Stratigraphic diagram for sections 1a and 1b, Arroyo Grande ----------------------------------- 52

51. Stratigraphic diagram for sections 2a and 2b, Arroyo Grande ----------------------------------- 53

52. Field sketch of section 3, Arroyo Grande ------------------------------------------------------------ 55

53. Photograph of section 4, Arroyo Grande ------------------------------------------------------------ 55

54. Stratigraphic diagram for section 4, Arroyo Grande ----------------------------------------------- 56

55. Photograph of fluvial climbing ripples in unit C of section 5, Arroyo Grande ------------------- 57

56. Close-up photograph of one ripple crest in unit C of section 5, Arroyo Grande ---------------- 58

57. Stratigraphic diagram for section 5, Arroyo Grande ----------------------------------------------- 58

58. Uninterpreted and interpreted photographs of sedimentary couplets in unit G 

 of section 6, Arroyo Grande --------------------------------------------------------------------------- 59

59. Uninterpreted and interpreted photographs of sedimentary couplets in

 upper part of unit I of section 6, Arroyo Grande ---------------------------------------------------- 60

60. Stratigraphic diagram for section 6, Arroyo Grande ----------------------------------------------- 60

61. Stratigraphic diagram for section 7, Arroyo Grande ----------------------------------------------- 61

62. Stratigraphic diagram for section 8, Arroyo Grande ----------------------------------------------- 62

63. Stratigraphic diagram for section 9, Arroyo Grande ----------------------------------------------- 64

64. Stratigraphic diagram and field sketch of section 10, Arroyo Grande --------------------------- 66

65. Stratigraphic diagram for section 11, Arroyo Grande --------------------------------------------- 67

66. Photograph of dune forms on the fluvial terrace at Arroyo Grande ----------------------------- 68

67. Photograph of feature 1 at archaeological site G:03:064, Arroyo Grande ----------------------- 68

68. Photograph of feature 12 at archaeological site G:03:064, Arroyo Grande ---------------------- 68

V
1

Abstract

This report presents initial results of a joint effort 

between geologists and archaeologists to evaluate the significance of various depositional processes and environments in the prehistoric formation and modern preservation 

of archaeological sites along the Colorado River corridor in 

Grand Canyon National Park. Stratigraphic investigations of 

the Palisades, Lower Comanche, and Arroyo Grande areas 

of Grand Canyon yield detailed information regarding the 

sedimentary history at these locations. Reconstruction of past 

depositional settings is critical to a thorough understanding 

of the geomorphic and stratigraphic evolution of these three 

archaeologically significant areas. This examination of past 

sedimentary environments allows the relative significance 

of fluvial, aeolian, debris-fan, and slope-wash sedimentary 

deposits to be identified at each site. In general the proportion of fluvial sediment (number and thickness of flood 

deposits) is shown to decrease away from the river, and 

locally derived sediment becomes more significant. Flood 

sequences often occur as ‘couplets’ that contain a fluvial 

deposit overlain by an interflood unit that reflects reworking of fluvial sediment at the land surface by wind and local 

runoff. Archaeological features are built on and buried by 

sediment of various depositional environments, implying 

a complex interaction between geologic and cultural history. Such field analysis, which combines geological and 

archaeological information and techniques, can provide a 

basis for future determination of the effects of Glen Canyon 

Dam operations on selected areas of the river corridor. This 

knowledge is essential to the development of preservation 

strategies for cultural resources in Grand Canyon.

Introduction

Since the closure of Glen Canyon Dam in 1963, drastic 

changes in the natural hydrologic and sedimentologic regime 

have resulted along the Colorado River environment in 

Grand Canyon (see, for example, Andrews, 1986; Webb and 

others, 1999). The dam has reduced the supply of sediment 

at the upstream boundary of Grand Canyon National Park 

by about 95 percent, with the Paria River remaining the only 

major supplier of sediment at the upstream park boundary. 

The predam mean annual sediment load at the upstream park 

boundary was ~60 million metric tons; the postdam mean 

annual sediment load is ~3 million metric tons (Topping and 

others, 2000a). The other substantial postdam supplier of 

sediment is the Little Colorado River, which enters the Colorado 98 km downstream from the Paria River and supplies an 

additional 5–10 percent of the predam sediment load (Topping and others, 2000a). 

Operation of the dam smoothes natural variations in the 

river’s hydrograph that occurred in response to snow melt 

and other seasonal climatic fluctuations. Predam discharge 

records from the Colorado River gage at Lees Ferry, Arizona 

(US Geological Survey station number 09380000), 24.5 km 

below Glen Canyon Dam, indicate that discharge was typically below 226 m3

/s (7,980 ft3/s) for approximately half of 

the year, while the mean annual spring snow-melt flood was 

2,410 m3/s (85,000 ft3/s; Topping and others, 2003). The 

discharge range released from the dam is substantially less 

than this natural variability; maximum power-plant capacity 

calls for flows no greater than 940 m3/s (33,200 ft3/s), and 

Sedimentology and Stratigraphy of the Palisades, 

Lower Comanche, and Arroyo Grande Areas of the 

Colorado River Corridor, Grand Canyon, Arizona

By Amy E. Draut1, David M. Rubin1, Jennifer L. Dierker2, Helen C. Fairley3, Ronald E. Griffiths3, 

Joseph E. Hazel, Jr4, Ralph E. Hunter1, Keith Kohl3, Lisa M. Leap2, Fred L. Nials5, David J. Topping3, 

and Michael Yeatts6

1

 U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, CA 95060

2

 National Park Service, 823 San Francisco St., Suite B., Flagstaff, AZ 

86001

3

 U.S. Geological Survey, Grand Canyon Monitoring and Research Center, 

2255 N. Gemini Drive, Flagstaff, AZ 86001

4

 Department of Geology, Northern Arizona University, Flagstaff, AZ 86011

5

 GeoArch, 10450 W. 8th Place, Lakewood, CO 80215

6

 Hopi Cultural Preservation Office, Northern Arizona University Department of Anthropology, Flagstaff, AZ 86011
2 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

flows above 708 m3/s (25,000 ft3/s) occur under the current 

operating regime only for isolated experimental purposes. 

Although seasonal variations in the hydrograph have been 

flattened, daily discharge fluctuations generally occur over 

a much greater range than in the predam state (Topping and 

others, 2003). Control of river discharge by dam operations 

has important implications for the storage and redistribution 

potential of sediment in the river corridor. In the absence 

of flood events, the relatively little sediment that the river 

now carries cannot be deposited at the higher elevations 

that received sediment regularly prior to dam closure. In the 

absence of lower flows (< 142 m3/s [5000 ft3/s]), which are 

no longer allowed to be released from the dam under the 

1996 Record of Decision signed by the Secretary of the Interior, the sediment-storage capability in the main channel is 

greatly reduced (Topping and others, 2000a, 2003). Sediment 

supplied by tributaries below the dam is exported from the 

canyon on time scales of weeks to months under the present 

flow-operation regime (Topping and others, 2000a, b; Rubin 

and others, 2002).

Numerous studies have identified consequences of the 

altered hydrograph and sediment content in the Colorado 

River. The size and number of subaerial sand deposits, 

many of which are used as camping beaches by river runners, have shown a systemwide decrease over the past four 

decades, punctuated by episodic aggradation during the 

1983 flood of 2,700 m3/s (97,000 ft3/s), the 1996 and 2004 

beach/habitat building flow (BHBF) experiments (1,270 m3/s 

[45,000 ft3/s] and 1,160 m3/s [41,000 ft3/s], respectively), and 

sediment input from occasional tributary floods (Beus and 

others, 1985; Schmidt and Graf, 1987; Budhu and Gobin, 

1994; Kearsley and others, 1994; Kaplinski and others, 

1995; Schmidt and Leschin, 1995; Wiele and others, 1996; 

Hazel and others, 1999; Schmidt and others, 2004). Riparian vegetation has colonized areas at lower elevations than 

would have been possible in predam time when annual floods 

removed young vegetation; vegetation encroachment has 

contributed significantly to the loss of open sand area along 

the river (for example, Turner and Karpiscak, 1980). The 

reduction in open sand-bar area has affected recreational use 

of the river corridor; sand-bar erosion and vegetation growth 

have altered the biomass composition and riparian habitat in 

complex ways (for example, Dolan and others, 1977; Carothers and Brown, 1991; Webb and others, 1999). The biological community within the postdam river has undergone major 

changes as a result of colder water temperatures (as subsurface reservoir water is released from the dam), increased 

light penetration, and alterations in food sources associated 

with the low sediment input. Such conditions are believed to 

have contributed to the decline in native fish populations in 

Grand Canyon, among other effects (Carothers and Brown, 

1991; Valdez and Ryel, 1995; Douglas and Marsh, 1996). 

The effect of Glen Canyon Dam operations on archaeological resources in Grand Canyon has received little 

research attention to date. At the time that the Bureau of 

Reclamation sponsored the creation of the Glen Canyon 

Environmental Studies (GCES) research initiative in 1982, 

primary research objectives included physical and biological resources, while effects on cultural resources were not 

addressed (Fairley and others, 1994; Fairley, 2003). The 

relative inattention paid to potential dam effects on archaeological sites largely stemmed from the perception that, 

because few archaeological remains were preserved within 

the river’s annual flood limit, cultural features would not 

be greatly affected by dam operations. It has more recently 

been suggested (Hereford and others, 1993, Yeatts, 1996, 

1997; Thompson and Potochnik, 2000) that postdam alterations in the sediment load and flow regime of the Colorado 

River may significantly affect the preservation potential of 

archaeological sites within the river corridor, even above the 

annual flood zone. 

Of the nearly 500 archaeological sites that have been 

recorded in the river corridor between Glen Canyon Dam 

and Separation Canyon (255 river miles), more than 250 are 

considered to be within the Area of Potential Effect (APE) 

of dam operations designated by the National Park Service 

(NPS; Fairley and others, 1994; Neal and others, 2000). 

Repeated monitoring visits have shown that many of the sites 

located in or on poorly consolidated sediment deposits are 

actively eroding due to incision by gullies, aeolian deflation 

processes, and visitor impact (for example, Leap and others, 

2000; Pederson and others, 2003, Fairley, 2003). The process 

of gully incision, and the base level to which small, ephemeral drainage systems respond, were first proposed to be 

linked to dam operations by Hereford and others (1993). In 

a study of surficial geology and archaeological site erosion, 

Hereford and others (1993) documented increased erosion of 

predam fluvial terrace deposits, and associated archaeological sites, by gully incision that accompanied unusually high 

precipitation in the mid-1970s. From these observations followed the hypothesis that rapid rates of incision were caused 

by the lowering of base level at the mouths of ephemeral 

drainages to meet the new, postdam elevation of high-flow 

sediment deposition. Hereford and others (1993) proposed 

that incision rates would remain high until gully morphology 

has equilibrated to the postdam base level, as much as 3–4 m 

below the lowest predam alluvial terraces.

Thompson and Potochnik (2000) revisited this hypothesis in a comparative study of alluvial terrace evolution in 

Grand and Cataract Canyons. These authors modified the 

base-level concept to include the restorative effects of fluvial 

deposition in the mouths of gullies and arroyos (thus raising base level), and the potential for aggradation of predam 

fluvial terrace deposits by aeolian deposition of reworked 

flood sand. Thompson and Potochnik (2000) concluded 

that sediment deprivation and lack of floods, caused by 

dam operations, have removed the potential for sediment 

deposition that could ameliorate precipitation-induced gully 

development. They developed a predictive geomorphic 

model designed to identify areas prone to gully incision, a 

concept later modified by Pederson and others (2003). The 

emphasis of Thompson and Potochnik (2000) on the poten-
3

tial for aeolian deposition to protect fluvial terraces, and, by 

extension, archaeological sites, led to the conception of the 

present study; it has been further suggested elsewhere that 

aeolian deposition in incipient gullies is “one of the strongest 

restorative forces operating at archaeological sites” (Neal 

and others, 2000, p. 77). Repeated high-resolution mapping 

conducted after the 1996 BHBF experiment confirmed that 

such floods can deposit sediment in arroyo mouths and that 

such deposits can serve as a source for wind-blown sand 

that accumulates at higher elevation, leading to net deposition above the flood stage elevation (Yeatts, 1997; Hazel and 

others, 2000).

In order to evaluate the degree to which the presence and 

operation regime of Glen Canyon Dam may influence the 

stability of archaeological features, site-specific stratigraphic 

and geomorphic knowledge is essential. Establishing the 

local importance of fluvial, aeolian, and other processes in 

predam time is an important precursor to any future assessments of dam-operation effects on these and other areas of 

the river corridor. Detailed field investigations of the stratigraphic record at three locations along the Colorado River 

corridor in Grand Canyon were initiated for the purpose of 

determining the relative importance of various depositional 

environments in areas known to have been occupied by prehistoric cultures. This study focuses on the Palisades, Lower 

Comanche, and Arroyo Grande areas of Grand Canyon 

(fig. 1). Locations of study sites are not specified explicitly 

because of the need to keep archaeological-site locations 

confidential.

This work is intended to provide examples of applicable 

methods and relevant data that can facilitate similar studies 

in other areas; stratigraphic interpretations from these three 

sites are not intended to be extrapolated to other regions of 

the river corridor. Although the depositional processes that 

control the sedimentary history at these locations are certainly important in other areas, the relative importance of 

each in these three particular locations as determined by this 

study should not be assumed for any other area without sitespecific investigation.

Site Selection

In May 2003, reconnaissance work was done to assess 

the sedimentary and geomorphic setting of approximately 40 

archaeologically significant areas along the Colorado River 

corridor. The purpose of this initial survey was to observe 

interaction between aeolian sediment and archaeological 

sites, as a basis for ongoing investigations studying the connection between aeolian deposition and archaeological site 

preservation. The three sites discussed in this report were 

identified for further study as a result of observations made 

during the 2003 reconnaissance field work. In selecting the 

Palisades, Lower Comanche, and Arroyo Grande areas for 

detailed stratigraphic analysis, the research group chose to 

focus on areas where aeolian deposition was suspected to 

form an important component of the sedimentary history. 

Operating on the hypothesis that aeolian deposition might 

form a substantial component of the stratigraphy studied at 

these three sites, members of this research group spent 3–5 

days at each site in May 2004 studying stratigraphy in sufficient detail that the relative importance of aeolian sediment 

deposition could be more thoroughly assessed. 

Each of the three sites targeted for this study has experienced reduction in a source of aeolian sand since the closure of Glen Canyon Dam in 1963. Multiple sets of aerial 

photographs that span the time period from 1965 to 2002 

(available from the Grand Canyon Monitoring and Research 

Center) indicate that the area of open sand exposed near river 

level has, at each of the three locations, decreased because of 

both loss of sand (erosion of sand bars) and increased vegetation cover. Low-elevation areas of nearby sand bars spend 

more time submerged as a result of flow fluctuations, reducing the potential source area for wind-blown sand because 

sand must be dry to be entrained by the wind. Two of the 

three sites (Palisades and Lower Comanche) experienced 

some sediment deposition during the 1996 controlled flood 

experiment, indicating that future flood experiments could 

potentially be used to alleviate erosion that has occurred in 

those areas (Grand Canyon Monitoring and Research Center, 

aerial photographs; Yeatts and others, 1996, 1997). Instrument stations have been established at Palisades and Lower 

Comanche in November 2003 and April 2004, respectively, 

to quantify aeolian sand transport; results of stratigraphic 

analysis at those sites presented here are intended to supplement the concurrent aeolian sediment-transport data that will 

be discussed in a separate publication.

Previous Work

A number of previous studies have presented sedimentary and geomorphic data from Grand Canyon that are relevant and complementary to this investigation. Geomorphic 

mapping by Hereford (1993) and by Hereford and others 

(1993, 1996) in the Palisades area generated detailed interpretations of the surficial geology that are discussed and 

0 km 50

113 ° 112° 37°

36°

Palisades Lower

Comanche

Arroyo

Grand

Lake Mead

Lake Powell

Glen

Canyon

Dam

Paria River

ARIZONA

Grand Canyon National Park

Li

t

t

l

e

Colorado

R

i

v

er

Lees Ferry

Colorado

River

Figure 1. Location map of Grand Canyon with the three study areas 

indicated.

Introduction
4 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

expanded upon in this present study. Radiocarbon dating 

work (Hereford, 1993; Hereford and others, 1996) has also 

provided much of the available information on the timing of 

various alluvial terraces and debris-fan deposits at Palisades. 

It is noteworthy that the Palisades area was one location 

used by Hereford and others (1993) to outline the baselevel hypothesis discussed above. Thompson and Potochnik 

(2000) studied the Palisades area during the formulation of 

their restorative base-level hypothesis, and they concluded 

that drainage catchment areas at Palisades did support the 

idea that incision of alluvial terraces could be attributed to 

dam-induced lowering of base level. Thompson and Potochnik (2000) list Palisades and Arroyo Grande as two areas 

where, during their geomorphic survey and modeling study, 

they identified stratigraphic evidence for paleogullies that 

had filled with aeolian sand. 

Grams and Schmidt (1999) used ground (oblique) and 

aerial photography of the Palisades area, among other study 

sites, to evaluate changes in the extent of surficial sand 

deposits since 1890. Repeated high-resolution mapping by 

Yeatts (1996, 1997) and Hazel and others (2000) demonstrated erosion and aggradation of sand deposits at Palisades 

as a result of the 1996 BHBF experiment. Notably, Yeatts 

(1997) and Hazel and others (2000) documented the migration of sediment deposited in the main arroyo mouth at Palisades to higher elevation over the year following the flood, 

attributed this migration to aeolian processes, and identified 

such consequences of the BHBF as potentially beneficial 

for preservation of cultural resources above the 1,270-m3/s 

(45,000-ft3/s) level. Gellis (1994) reported field observations 

of erosion at archaeological sites during work with the Zuni 

Tribe, including parts of the Palisades and Lower Comanche 

areas discussed here. Pederson and others (2003) monitored 

arroyo incision at both Palisades and Arroyo Grande, among 

other sites, in a geomorphic study of gullying processes and 

an assessment of the effectiveness of erosion-control structures such as checkdams. 

Mapping of surficial deposits at Granite Park (approximately 2 km downstream of the Arroyo Grande site discussed here) by Hereford and others (2000) provides 

information applicable to geomorphic interpretations at 

Arroyo Grande. Several radiocarbon dates listed in Hereford 

and others (2000) were for samples from stratigraphic units 

at Arroyo Grande, aiding age determination of deposits in 

that area.

Fairley and others (1994) completed the first comprehensive survey of archaeological sites along the river corridor in Grand Canyon. This survey provided baseline data 

for defining the depositional context of many archaeological 

sites, and it generated the first data set against which later 

monitoring surveys could be compared. Subsequent monitoring summaries by NPS have provided repeated geomorphic 

observations related to archaeological site location, condition, and preservation (Coder and others, 1994a, b, 1995; 

Leap and others, 1996, 1997, 1998, 2000, 2003; Leap and 

Kunde, 2000; Dierker and others, 2001, 2002). Pederson and 

others (2003) used repeated aerial surveys to evaluate rates 

of gully incision in areas known to be archaeologically significant, including Arroyo Grande.

Sedimentology and stratigraphy of flood deposits in 

Grand Canyon have been addressed by several previous studies. McKee (1938) first presented facies descriptions of Colorado River flood strata at a number of exposures at tributary 

mouths in the canyon. Rubin and others (1990) and Schmidt 

(1990) used stratigraphic exposures in active river-level 

sand bars to describe the formation and evolution of separation and reattachment bars in zones of flow recirculation in 

eddies. Rubin and others (1994) analyzed stratigraphy within 

deposits from the 1983 2,700 m3/s (97,000 ft3/s) flood and 

weaker flood events in 1984–1986, using the data to estimate 

deposition rates and to evaluate the potential impact of various dam-controlled flow regimes on the erosion and accumulation of sediment on camping beaches. O’Connor and others 

(1994) used paleoflood deposits, dated by radiocarbon techniques, to estimate discharge levels for floods as old as 4,500 

years BP. The peak discharges of the paleofloods studied by 

O’Connor and others (1994) were subsequently revised by 

Topping and others (2003) using archived US Geological 

Survey gaging data that were not readily available for previous analysis.

Grain-size analyses of sediment sampled from flood 

deposits in Glen Canyon (below the dam) and Grand Canyon 

have demonstrated sediment-supply limitation in both 

predam and postdam flood events (Rubin and others, 1998; 

Topping and others, 2000a, b; see also Rubin and Topping, 

2001). Supply limitation in flood deposits was shown by 

those studies to be manifested by a coarsening upward of 

grain size within individual flood deposits, in contrast to the 

fining-coarsening-fining or fining-upward fluvial sequences 

that result from supply-unlimited flows. Upward coarsening 

of deposits from supply-limited floods occurs as suspended 

sediment becomes progressively depleted of fine-grained 

material in a supply-limited system (the finer sizes having 

been transported downstream first and farthest). Fine sediment can then be winnowed from the stream bed during 

the later stages of the flood, increasing the median grain 

size of the sediment that remains exposed at the bed (Rubin 

and others, 1998). The relationships between sedimentsupply limitation in the Colorado River and the resulting 

sedimentology of flood deposits discussed by Rubin and 

others (1998) and Topping and others (2000a, b) are further 

developed in this study of predam fluvial sequences.

Methods

Data collected from the three sites discussed in this 

report were obtained primarily through detailed examination 

of vertical stratigraphic sections. At all three locations, the 

presence of arroyo networks facilitated the exposure of vertical faces for this purpose; at Palisades and Lower Comanche, 

several additional sections were exposed by digging small 
5

pits. Sedimentary characteristics were described, and unit 

thicknesses were measured in detail, for most of the sections 

discussed. At selected locations, sediment samples were collected for subsequent grain size analysis using a Coulter laser 

particle-size analyzer at the Grand Canyon Monitoring and 

Research Center laboratory in Flagstaff, Arizona. Geomorphic observations, supplemented by shallow test-pits, were 

made in the general area of archaeological sites in order to 

evaluate sedimentary and geomorphic processes directly 

relevant to each of the sites of interest. To standardize stratigraphic sections, all sections in this report are described 

beginning with the uppermost unit and progressing downsection. Unless otherwise specified, measurements of stratigraphic depth refer to depth below the land surface.

Surveying equipment was used to document the locations of stratigraphic sections, as well as the thalweg location 

and headward extent of major arroyo branches at Palisades 

and Arroyo Grande. At Palisades and Lower Comanche, the 

locations and elevations of logs and milled timber interpreted 

as driftwood were also surveyed; these data are used to place 

flood events that occur in the stratigraphic record within the 

context of historic flood discharge levels.

Stratigraphic work, including sediment sampling, has 

been permitted at these locations under National Park Service Research and Collecting Permit #GRCA-2003-SCI0101 issued by Grand Canyon National Park. Research that 

took place at Arroyo Grande, located in the Area of Cooperation (AOC) between the Hualapai Nation and the National 

Park Service, was also authorized by the Hualapai Office 

of Cultural Resources. All stratigraphic investigations have 

been conducted in close collaboration between geologists 

and archaeologists to ensure that culturally significant areas 

are treated with extreme care. Sediment pits and arroyo wall 

exposures that were logged and studied during this work 

were located away from the immediate vicinity of archaeological features. No cultural artifacts were exposed or collected during this work. All artificial pits were filled in at the 

completion of the field work to conceal the work areas.

Identification of Depositional Environments 

Within a vertical stratigraphic exposure, small-scale 

sedimentary structures are used as the best diagnostic indicator of depositional environment. Structures that are characteristic of fluvial and aeolian environments, respectively, 

can often be identified by the dimensions, scale, grain-size 

sorting, and spatial orientation of bedding apparent in outcrop exposure. Diagnostic features of fluvial and aeolian 

stratification have been described thoroughly elsewhere (for 

example, Walker, 1963; Stokes, 1968; Harms and others, 

1975; Hunter, 1977a, b; McKee, 1979; Rubin and Hunter, 

1982, 1987; Rubin, 1987) and will not be discussed in detail 

here. Some manifestations of fluvial and aeolian sedimentary 

structures appear similar and can be difficult to distinguish 

(for example, aeolian ripples with low climb angles and fluvial structures that represent upper plane-bed conditions). 

In such cases, depositional environment may sometimes be 

inferred by observation of lateral gradation into other, more 

diagnostic structures. 

Numerous stratigraphic horizons studied in these and 

other areas of Grand Canyon do not contain well preserved 

sedimentary structures. In most cases, absence of sedimentary structures in fine-grained surficial sediment is attributable to biologic effects. The presence of vegetation on the 

land surface interferes with the development of sedimentary 

structures as aeolian sediment is deposited; wind ripples 

forming on a sparsely vegetated surface have shorter crests 

and less regular trough elevation than on unvegetated surfaces, while denser vegetation cover reduces or prevents 

aeolian entrainment and transport of sand (for example, 

Olson, 1958; Bressolier and Thomas, 1977; Ash and Wasson, 

1983; Buckley, 1987). Organic debris on the land surface 

can also interfere with the development of wind ripples, as 

can a variable wind direction during deposition. Bioturbation by trampling, animal burrowing, and the growth of plant 

roots can disturb or destroy sedimentary structures after they 

have formed. Particularly in cases where the rate of sediment deposition is slow, bioturbation in both subaerial and 

subaqueous environments can quickly obscure sedimentary 

structures (for example, Dott and Bourgeois, 1982; Wheatcroft, 1990; Boudreau, 1994).

Textural differences (grain size analysis) may in some 

cases distinguish fluvial and aeolian material if sedimentary 

structures are indistinct. However, the range of grain sizes 

observed to occur within Colorado River fluvial deposits (a 

function of sediment availability, discharge, and proximity 

to flooding tributaries) is great enough that there is some 

overlap between textural characteristics of fluvial and aeolian 

deposits in Grand Canyon. Textural analysis as a diagnostic 

tool should therefore be used with caution. If sedimentary 

structures are not distinct, reworking of a primary sedimentary deposit by wind may be inferred on the basis of the 

morphology of the deposit (for example, presence of sand 

shadows or coppice dune accumulation).

Identification of slope-wash sedimentation is commonly 

based on the presence of locally derived lithic clasts in a 

deposit; slope-wash materials are commonly more poorly 

sorted and immature (with respect to mineralogy and weathering of sediment grains) than primary fluvial or aeolian 

deposits (for example, Benito and others, 2003). Surficial 

runoff events may result in the development of channel features; channels filled with locally derived sand and gravel 

were observed in a number of the stratigraphic sections 

described during this work. In some locations studied, notably Arroyo Grande, subaerial reworking of surficial sediment may be identified by the presence of charcoal and ash 

concentrations in the soil. Horizons that contain such burned 

material are readily distinguishable from isolated cultural 

hearth features; the more widespread, diffuse charcoal occurrences that are common in Arroyo Grande imply a regional 

burning event at the land surface, such as a grass fire. 

Methods
6 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

Grain-Size Analyses 

Grain-size analyses of sediment samples provide information about sedimentary dynamics that cannot necessarily be 

determined in the field. Grain-size data can, in some cases, 

also facilitate identification of the depositional environment 

of certain stratigraphic units. Although variation may occur 

between the grain-size distributions of sedimentary units 

deposited by the same type of event, and these data are therefore not necessarily representative of every deposit with a 

similar event history, 36 samples with known depositional 

environments that were analyzed during this study yielded 

distributions with little enough overlap between, for example, 

fluvial and aeolian fields that this technique may be considered 

useful in identifying a likely depositional environment for 

samples without diagnostic sedimentary structures. 

The grain-size distribution of flood deposits is determined 

by the concentration and grain size of sediment available in 

the flow at various stages of the flood event; concentration and 

grain size are in turn determined by the nature and intensity of 

the event that initiated the flood (main-stem river snow-melt 

flood vs. tributary flood, for example) and the source area from 

which the sediment is derived. The latter is influenced in part 

by channel morphology; in areas where a river flows over an 

alluvial floodplain at high discharge, the flow may entrain finegrained floodplain sediment that is not available as a sediment 

source when the river is confined to its main channel at lower 

discharge. 

Within aeolian deposits, grain-size distribution depends 

upon such factors as the intensity and duration of the wind, 

which determine the degree of winnowing and sorting that 

occur, as well as the composition and grain size of sand in the 

source region that provides sediment to the aeolian deposits. 

Slope-wash deposits contain probably the greatest range of 

possible sedimentary characteristics; their local derivation 

leads to substantially less ‘maturity’ of the resulting deposit 

compared to fluvial or aeolian strata, in which grains have 

typically undergone extensive transport before deposition. The 

grain-size distribution and sediment composition of slopewash deposits depend on the intensity, location, and duration 

of the precipitation event that generated them, as well as the 

lithology of source rocks that produce the sediment. Local 

precipitation often transforms the landscape in ways that alter 

characteristics of the stratigraphic record where such deposits 

are found; rill and gully formation and subsequent filling can 

result in channel-fill structures and lens-shaped slope-wash 

deposits. 

Sediment samples were analyzed for grain-size distribution using a Beckman Coulter LS 100Q laser particle-size 

analyzer linked to a LS variable-speed fluid module. The 

Beckman Coulter LS 100Q computes particle-size distributions based on the Fraunhofer laser diffraction theory, which 

states that small particles in the path of a laser will diffract the 

light in a known, symmetrical pattern. The angle of diffraction 

inversely corresponds to the size of the particle. This instrument uses 126 detector rings to measure the pattern of diffracted light intensities as a function of the angle from the axis 

of the original laser beam in order to compute the particle-size 

distribution. Particles from 0.375 μm to 948.2 μm in diameter 

can be measured in this way (http://www.coulter.com/products/instrument/partChar/pc_ls230.asp).

Sample preparation was minimal because of the overall lack of organic material and relative coarseness of the 

samples, negating the need for hydrogen peroxide (H2O2) and 

sodium hexametaphosphate ((NaPO3)6) treatments. Deionized water was added to the samples in a small beaker, and 

the beaker placed on a stir plate for agitation. A representative 

subsample was then extracted from suspension in the beaker 

using a pipette. The sediment and water mixture was added 

into the Coulter until the optimum obscuration range (8–12 

percent) was reached. This process was repeated up to four 

times for each sample; the average of the 4 runs was used in 

subsequent data analysis to minimize potential subsampling 

errors. Grain-size distribution is reported as percent by volume 

and assumes spherical particles.

Palisades Area

Geomorphic Setting

The Palisades (Palisades of the Desert) area (fig. 2) lies 

within the fluvial geomorphic reach described by Schmidt 

and Graf (1987) as the Furnace Flats reach. Bedrock at river 

level in this area is the Dox Formation, a series of interbedded fine-grained sandstones and shales of Precambrian age 

that commonly display a characteristic red to nearly purple 

color (originally named the Dox Sandstone7 by Noble, 1914, 

this unit was redescribed by Stevenson and Beus, 1982, as the 

Dox Formation on the basis of its varied lithology). Sandstone 

beds of the Dox Formation are locally offset by major fault 

displacement along the Palisades segment of the Butte Fault, a 

northwest-trending structure related to the east Kaibab Monocline that crosses the river in this area (Huntoon and others, 

1996). 

Various surficial deposits are present on the wide, relatively flat expanse downstream of Palisades Creek. At least 

three episodes of Holocene debris-fan activity have been 

inferred for the Palisades Creek debris fan on the basis of 

stratigraphic relationships and repeat photography (Hereford, 1993; Hereford and others, 1996), the youngest having 

occurred between 1965 and 1984. Additional, undated recent 

debris-flow activity is apparent in leveed, unvarnished debrisfan deposits on the tributary north of Palisades Creek. Terrace 

morphology in the central Palisades area is attributed to fluvial 

deposition. Hereford (1993) mapped four alluvial terrace 

7

 Dox Sandstone remains the official USGS name of this unit. This report 

follows the use of ‘Dox Formation’ introduced by Stevenson and Beus (1982), 

based on the shale-rich lithology of this unit in the study area.
7

Figure 2. Aerial view (May 2002) of the Palisades study area. Locations of stratigraphic sections 1 through 4 are indicated by numbered dots. The dashed line marks the approximate 

boundary of playa deposits, which show a pink-red color because of derivation of sediment 

from the Dox Formation (Stevenson and Beus, 1982) in the adjacent canyon wall. Boxes 

indicate locations of photographs in figures 3 and 4. The location of the arroyo mouth is 

indicated, though the arroyo network is not readily visible in this image. River discharge in 

this photograph is 226 m3/s.

Palisades Area
8 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

units that could be dated to predam fluvial activity, in addition to postdam flood deposits present at lower elevation. The 

predam terrace deposits described on river left at Palisades 

by Hereford and others (1993; 1996) include (from oldest to 

youngest): (1) Alluvium of Pueblo-II age (map unit ‘ap’), a 

unit 1–3 m thick that was inferred, on the basis of archaeological artifacts, to have a probable age range of before A.D. 

950 to A.D. 1075–1200. (2) Upper mesquite terrace (map unit 

‘umt’), a terrace-forming very fine to fine sand unit as much 

as 2 m thick that contains multiple fluvial deposits within 

it; radiocarbon ages and archaeological remains suggested 

deposition of this unit between A.D. 1270–1470 and A.D. 

1882–83, though analysis of driftwood location and elevation 

made during this study implies that the 1884 flood (5,940 m3/

s, 210,000 ft3/s; Topping and others, 2003) apparently covered 

most of the ‘umt’ area. (3) Lower mesquite terrace (map unit 

‘lmt’), fluvial sediments within the elevation limit of the 1921 

flood (4,800 m3/s, 170,000 ft3/s; Topping and others, 2003) 

based on the location of driftwood lines; Hereford (1993) 

attributed an age of A.D. 1884 to the 1930s to this unit. (4) 

Predam alluvium (map unit ‘pda’), an exposure of sediment 

1–2 m thick at lower elevation than ‘lmt’ but above the 1983 

flood sand; topographic position suggested a date of early 

1930s to 1957. An additional alluvial package, which Hereford (1993) named the “striped alluvium” (map unit ‘sa’) 

on the basis of interbedding of alluvium with locally derived 

Dox Formation sediment, is present on river right in this area 

but not on river left where this study focused; the striped 

alluvium is the oldest and highest-elevation alluvial deposit 

recognized in this part of the river corridor. Dates obtained 

from exposures believed to be correlative with this ‘sa’ unit 

range from 2500 B.C. to A.D. 300 (2568 to 2289 B.C. in 

upper Marble Canyon, O’Connor and others, 1994; older 

than 1300 B.C. at Granite Park, river mile 209, Hereford and 

others, 2000; 800 B.C. to A.D. 300 in eastern Grand Canyon, 

Hereford, 1993).

A large, river-based arroyo network (one whose outlet, or 

base level, is at the elevation of the main river channel) has 

incised these alluvial terraces; the age of the arroyo incision is 

uncertain, but it appears to have begun after 1890, on the basis 

of historical photograph analysis by Hereford (1993). Incision has deepened since 1965, judging from aerial photograph 

comparison (Hereford, 1993; Hereford and others, 1993). 

Photographic evidence discussed by Thompson and Potochnik 

(2000) suggests that in 1973, a partial arroyo network was 

present that was blocked by the ‘pda’ alluvium and associated 

aeolian dunes. By 1980, the arroyo channel had breached the 

dunes and drained into the Colorado River (Thompson and 

Potochnik, 2000). Repeated survey work by Pederson and 

others (2003) documented incision of this arroyo related to 

precipitation events during the 2002 monsoon season.

Aeolian dune forms are present over a widespread area 

atop the fluvial terraces described by Hereford (1993); that 

study mapped an area of coppice dune accumulations. Recent 

active dune migration is apparent in this area, with milled 

timber and an historic-era horse skull having been partially 

buried by aeolian dunes; much of the land surface today is 

cryptogamic and colonized by grasses, and it appears deflated 

over much of its extent (fig. 3). 

The dune field is bordered at its eastern (landward) 

margin by playa deposits that show evidence of local sediment derivation (receiving slope-wash from the red Dox 

Formation immediately above) and recent ponding and desiccation (fig. 4). Two drainage divides are evident within the 

playa zone. Most of its area drains to the north, feeding the 

main arroyo network. A second drainage basin collects water 

that evaporates in a ponding area west of the main playa 

expanse, immediately east of the Beamer hiking trail, with no 

outlet to the river. A third, southern drainage network brings 

runoff from the eastern playa area to the small arroyo developed at the extreme southern end of the Palisades area. 

The river in the Palisades area is in a zone of flow expansion below two debris fans, where the river corridor widens 

to more than 400 m, in part because of drainage control by 

the Palisades segment of the Butte Fault. A separation bar and 

reattachment bar form in the long, narrow eddy downstream 

of this rapid (Grams and Schmidt, 1999), with a second lowelevation sand bar sometimes present that is episodically 

exposed subaerially (fig. 2). Examination of ground and aerial 

photographs by Grams and Schmidt (1999) has shown that the 

extent of open (unvegetated) sand area on these sand bars has 

greatly decreased relative to the predam exposure. Reduction 

of open sand area at Palisades has been due both to a loss of 

high-elevation sand (Grams and Schmidt, 1999) and to a substantial increase in colonization by riparian vegetation (figure 

47 of Turner and Karpiscak, 1980). Recent survey of a large 

Figure 3. Aeolian dunes at Palisades (location shown on figure 2). 

The dune at the left side of the photograph is one of the more active 

dunes in this area, with exposed, unvegetated loose sand at its crest. 

Although dune forms are present on the terrace surface and reach 

heights of 2 m (relative to the surrounding terrace elevation), much 

of the landscape shows evidence of deflation and recent inactivity. Vegetation cover is abundant though typically thin (for example, 

cheat grass, shrubs, and occasional mesquite); cryptogamic soil and 

pedestal development are common.
9

boulder that appears partly buried in sand in a 1941 groundbased photograph indicates that a large part of the eddy sand 

bar at Palisades deflated by at least 2 m between 1941 and 

2000 (D. J. Topping, unpub. data, 2000); aerial-photograph 

analysis suggests that most of this deflation was due to erosion 

that occurred after the closure of Glen Canyon Dam (Grams 

and Schmidt, 1999). An aerial view of the Palisades area as it 

looked in 1965 is shown in figure 5. Modeling work by Wiele 

and Franseen (1999, 2001) indicated that flood stages exceeding 1,270 m3

/s (45,000 ft3/s) could result in substantial deposition on the sand bars immediately below Palisades Creek 

(with greater deposition predicted at flows in excess of 2,800 

m3/s [~100,000 ft3/s], even assuming a relatively low sediment 

concentration in the flow). On the basis of these hydrographic 

models, such flows have been suggested as a means by which 

to improve preservation of archaeological sites in the Palisades 

area (Wiele and Franseen, 1999).

The 1,270 m3/s (45,000 ft3/s) controlled flood experiment in 1996 caused scour and erosion of sand deposits in 

the Palisades eddy on river left. Although a net loss of sediment occurred in this area during the experiment, the flood 

deposited sand in the mouth of the main arroyo (Yeatts, 

1996). Repeated high-resolution surveys in the year following the 1996 flood demonstrated a slight net gain of sediment 

(17 m3 in the survey area) above the 1,270 m3/s line (45,000 

ft3/s; surveys at Palisades were conducted two months and 

13 months after the flood). This accretion indicated that sediment deposited in and around the arroyo mouth had apparently 

been transported to higher elevation by wind action (Yeatts, 

1997), consistent with the ‘restorative’ processes proposed by 

Thompson and Potochnik (2000). Survey mapping conducted 

Figure 4. Playa deposits at the eastern edge of the Palisades study 

area. A, The drainage channel in the northern (upstream) playa 

deposits is the source for the main arroyo network, which affects several archaeological sites (photograph taken in April 2004). B, Standing 

water in the central part of the playa (photograph taken in November 

2003). The reddish color of the playa sediment reflects local derivation from the Dox Formation (Stevenson and Beus, 1982), in contrast 

to the typically buff-colored Colorado River flood deposits. Analysis of 

photographs by Thompson and Potochnik (2000) indicates that, prior 

to 1980, drainage of the playa area was blocked by aeolian dunes, and 

had not yet reached the river as the northern part of it does today.

Figure 5. Aerial photograph (1965) of the Palisades study area (photograph from the Grand Canyon Monitoring and Research Center). River 

discharge is between 680 and 792 m3/s.

Palisades Area
10 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

in 1998 and 1999 showed that aeolian sediment deposited 

after the 1996 BHBF flow had been retained at Palisades, 

and that arroyo incision in the 3.5 years following the flood 

had not yet reduced the arroyo flood elevation to its preflood 

level (Hazel and others, 2000). Medium-format photographic 

stations were established before the 1996 BHBF flood at 

locations where the arroyo showed filling and erosion after 

the flow. NPS archaeologists continue to take photographs 

at those locations, as part of the site-monitoring program, at 

selected areas to monitor these processes near the mouths of 

significant arroyos such as the one at Palisades (for example, 

Balsom and Larralde, 1996).

Archaeological Significance

The wide, relatively flat land in the Palisades area contains abundant evidence of prehistoric human occupation. 

Nine separate prehistoric archaeological sites have been identified, several of which contain multiple habitation and artifact 

features (National Park Service, 2004). Sites have been dated 

by radiocarbon methods and artifact identification to Pueblo I 

and Pueblo II (PI and PII) periods (A.D. 800–1000 and A.D. 

1000–1150 respectively; Fairley and others, 1994). Several of 

the sites are affected by the development of an arroyo system 

in the central Palisades area, which has contributed to the loss 

of artifacts in some areas. Checkdams have been placed in 

several branches of the arroyo by the National Park Service 

(NPS) and representatives from the Zuni Tribe to reduce the 

erosion of artifacts by arroyo incision (Leap and Coder, 1995). 

To limit damage to archaeological sites by visitor impact, NPS 

relocated a section of the Beamer Trail through the Palisades 

area in 1992.

In addition to prehistoric cultural features, historical artifacts are present at Palisades dating back to mining operations 

in this area during the late 1800s and early 1900s. Two mine 

shafts, with tailing piles, are left as evidence of copper ore 

extraction at the Tanner-McCormick mine, which ceased to be 

actively mined in 1920. Historical features include the remains 

of a cabin (used between 1890 and 1910) with associated 

metal and glass artifacts; many items have been removed by 

visitors in the last few decades (Coder, 1994). 

Stratigraphic and Geomorphic Analyses

Locations of stratigraphic sections described below are 

shown on figure 2. Detailed stratigraphic logs are described 

in appendix 1 (see figs. 32 through 36). Section 1 is a vertical face that forms the north wall of the main arroyo trunk 

at low elevation. Section 2 is located 78 m landward (east; 

up-arroyo) from section 1, and was compiled from sediment 

exposures in an arroyo wall as well as a shallow pit excavated 

in the arroyo floor at the base of the naturally exposed wall. 

Section 3 is located farther up the arroyo, 40 m from section 

2, and is a pit dug into the surface at the northern (upstream, 

with respect to the Colorado River) margin of the extensive 

playa deposits behind the dune field. Section 4, 85 m southwest of section 3, is a pit dug into the surface of a small playa 

exposure west of the main playa area.

Geomorphic characteristics of the area surrounding 

each archaeological site studied at Palisades are discussed in 

appendix 1 (figs. 37 through 39). Stratigraphic data and geomorphic observations are summarized in the section below 

titled “Palisades Site Summary”.

Grain-Size Analyses

Figure 6 shows the results of grain-size analyses on two 

horizons from section 1. Unit F, an aeolian unit, is shown to be 

coarser than unit G, a fluvial deposit within this stratigraphic 

section. Figure 7 shows the grain-size distribution for unit H 

of section 3 at Palisades; this horizon contained no diagnostic 

sedimentary structures, and its depositional environment could 

not be definitively ascertained in the field. The grain-size plot 

is ambiguous also; this sample plots between the fields defined 

by known fluvial and aeolian deposits (fig. 7). The most likely 

explanation for this horizon is that it is a fluvial deposit that 

has undergone some degree of winnowing by wind, but not 

enough to remove all of the fine grains that would leave a 

mature aeolian lag deposit.

Grain-size analyses for three horizons within section 4 

are shown in figure 8. Three of these samples were collected 

from unit B, a flood deposit with well-defined fluvial climbing ripples. This flood deposit coarsens upward slightly (fig. 

9). Upward coarsening of deposits has been attributed by 

Rubin and others (1998) and Topping and others (2000a) to 

sediment-supply limitation during the flood that deposited a 

particular stratum. This inverse grading occurs as fine-grained 

suspended sediment becomes progressively depleted throughout the duration of the flood (the finer sizes having been 

transported downstream earliest and farthest); given unlimited sediment supply, constant bed conditions, and sufficient 

accommodation space, a flood deposit should first coarsen and 

then fine upward, mimicking suspended-sediment response to 

the hydrograph. Unit E of section 4, the poorly sorted distal 

debris flow sediment, exhibits a coarser overall grain-size 

profile than the fluvial units (fig. 8). The one sample analyzed 

from the fluvial deposit unit F in section 4 is finer than any of 

the three samples from unit B (fig. 8).

Driftwood Elevation Survey: Palisades StageDischarge Relationship

In order to establish an approximate stage-discharge 

relationship for flood elevations in the Palisades area, the location and elevation of driftwood logs were surveyed during 

field work in May 2004 by D. J. Topping and J. E. Hazel. 

Horizontal coordinates of survey data are referenced to the 

NAD83(1999) epoch and were projected into NAD83 Ari-
11

Cumulative percent finer

Grain size (mm)

0

20

40

60

80

100

0.001 0.01 0.1 1

PalStrat1UnitF

PalStrat1UnitG

Figure 6. Grain-size plots for units F (an aeolian deposit) 

and G (a fluvial deposit) of section 1, Palisades. See appendix 1 for sample descriptions. 

0

20

40

60

80

100

0.001 0.01 0.1 1

Fluvial

Aeolian

Grain size (mm) 

Cumulative percent finer

0

20

40

60

80

100

0.001 0.01 0.1 1

PalStrat4UnitB1

PalStrat4UnitB2

PalStrat4UnitB3

PalStrat4UnitE

PalStrat4UnitF

Grain size (mm)

Cumulative percent finer

Figure 7. Grain-size plot for unit H of section 3 at Palisades, a 

horizon with no depositional environment clearly identifiable in 

the field. The gray fields show the range of grain-size distributions obtained from known fluvial (n=27) and aeolian (n=9) 

deposits (inferred from sedimentary structures and/or geomorphic features). The grain-size distribution of unit H suggests 

some degree of aeolian reworking that has partially winnowed 

a fluvial deposit. See appendix 1 for sample descriptions.

Figure 8. Grain-size data from section 4 at Palisades. Unit B is a 

Colorado River flood deposit; within unit B, sample #1 was collected 1 cm above the base of the flood unit, sample #2 from 9 cm 

above the base of the flood unit, and sample #3 from 28 cm above 

the base of the flood. Unit E is a distal debris-flow deposit. Unit F 

is a fluvial deposit. See appendix 1 for sample descriptions.

D50 (microns)

Normalized height within flood deposit

Silt + Clay content (percent)

Normalized height within flood deposit

0

0.2

0.4

0.6

0.8

1

10 100

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

�

�

Figure 9. Grain-size data for three samples taken within a 

41-cm-thick flood deposit, unit B of section 4 at Palisades. The 

deposit coarsens upsection with respect to median grain size 

(A), a minor change, and with respect to the proportion of silt 

and clay (B). See appendix 1 for sample descriptions.

Palisades Area
12 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

zona state plane central zone grid (FIPS zone 0202) in meters. 

Point coordinates include a vertical component referenced to 

the NAD83(1999) ellipsoid.8 Driftwood in the Palisades area 

occurs in several elevation groups, interpreted as deposits from 

the most recent historical flood events (fig. 10). Though flood 

waters can deposit wood and other debris during both the peak 

and the receding limb of the flood, the uppermost elevation 

(within 10–20 cm) of each ‘cluster’ of driftwood is inferred to 

represent the high water mark for historical flood events with 

known discharge (Topping and others, 2003). Some overlap 

occurs between driftwood lines deposited by floods of similar 

magnitude (such as the 1957 and 1958 floods). Some of this 

overlap is due to the fact that driftwood logs are emplaced 

during both the peak and the receding limb of a flood and also 

due to postflood natural deflation of the local ground surface 

by slope-wash and aeolian processes. Recognizing that some 

disruption of original log placement has also occurred in the 

past because of mining activity, camp-site arrangement, and 

trail maintenance, logs near camp sites, trails, and areas associated with the Tanner-McCormick mine and cabin were not 

included in this study. Each survey point was assigned a quality value of excellent, good, fair, or poor, based on whether 

evidence could be found for the logs occurring approximately 

in their original place of deposition (for example, forming 

part of a continuous log line, or having been partly buried by 

dune sand or debris-fan sediment; fig. 10) and on how closely 

the point corresponded to the elevations of other points in its 

elevation cluster. 

Driftwood elevation data were used to compile a stagedischarge relationship, shown in figure 11. Maximum flood 

discharge values used in this calculation are those of Topping 

and others (2003): 5,940 m3/s (210,000 ft3/s) for the 1884 

flood, 4,810 m3/s (170,000 ft3/s) for the 1921 flood, 3,540 m3/s 

(125,000 ft3/s) for the 1957 flood, 3,060 m3/s (108,000 ft3/s) 

for the 1958 flood, and 2,700 m3/s (97,000 ft3/s) for the 1983 

flood. The plotted relationship in figure 11 uses only those data 

points that had been assigned ‘good’ or ‘excellent’ quality. 

Because the water surface during a flood will not be exactly 

horizontal, this stage-discharge curve, while approximating 

maximum flood-stage elevations for the Palisades area in general, may not accurately reflect exact maximum flood elevations at every specific location. 

From the curve in figure 11, it is apparent that a flood with 

a magnitude comparable to the 1921 event would probably 

Figure 10. Examples of driftwood logs surveyed at Palisades. A, Logs 

(indicated by arrows) from the 1884 flood are underlain and overlain 

by locally derived fan sediment. B, 1921 driftwood (with metal bolt 

indicated by arrow) near the downstream end of the study area. 1921 

logs are also overlain and underlain by fan sediment at this location. 

8

 Heights given by this coordinate system, referenced to the NAD83(1999) 

ellipsoid, differ from “above sea level” orthometric heights, or NAVD88 

elevations. Heights are referenced to the NAD83 ellipsoid instead of to 

sea level because the current national geoid model (GEOID03) is not sufficiently refined to model gravity anomalies in Grand Canyon to the required 

accuracy. As a result, the magnitude of gravitational effects on positional 

accuracy cannot be accurately estimated without experimental evaluation of 

a best-fit equipotential surface. The NAD83(1999) ellipsoid reference in the 

Grand Canyon area yields heights that are 23–25 m above heights referenced 

to sea level.
13

inundate all four of the detailed stratigraphic profile locations studied at this site (land surface elevations of 798.898, 

800.742, 800.972, and 800.612 m for sections 1, 2, 3, and 4 

respectively; fig. 12). At 798.898 m, the modern land surface 

at the location of section 1 would be inundated by a discharge 

of 3,300 m3/s (117,000 ft3/s), indicating that the fluvial sediment in unit E of section 1 (at a depth of 29–40 cm below the 

modern land surface) could have been deposited by the 1958 

flood event, the most recent flood to reach a level above this 

elevation. Fluvial deposits that occur at lower elevation in 

section 1 lie within a zone that would have been inundated frequently (possibly annually) in predam time and so cannot be 

confidently attributed to flood events in specific years.

The highest occurrence of fluvial ripples in any of the 

measured stratigraphic sections is at an elevation of 800.652 

m (the top of unit B within section 2). This is comparable to 

the uppermost elevation range of the 1921 flood inferred from 

driftwood locations in the vicinity of section 2 (fig. 11). If unit 

B was deposited by the 1921 flood, this would imply that unit 

D of section 2, the next-highest flood in that profile, would 

be the 1884 flood deposit. Flood deposits that underlie unit D 

in the stratigraphic record (none observed in this study at the 

location of section 2) would then be attributed to prehistoric 

floods. 

In section 4, the uppermost fluvial deposit has a top elevation of 800.262 m (unit B of section 4). Given this elevation 

(below both the 1921 and 1884 flood maxima), the 1921 flood 

is inferred to be the most recent event that could have left 

this deposit. If this is the case, the next-lowest flood deposit 

in section 4 would be attributed to the 1884 flood: unit F, at a 

depth of 107–114 cm below the land surface. If this interpretation is correct, it implies that units C, D, and E, consisting of 

playa and aeolian sediment, had accumulated at the location 

of section 4 in the 37 years between the 1884 and 1921 floods, 

equivalent to an accumulation rate of 0.84 cm/yr. While such 

a rate may seem high, it is not unreasonable given the great 

local variability that may occur in an environment dominated 

Discharge (m3/s)

Elevation (m)

794

795

796

797

798

799

800

801

802

803

0 1,000 2,000 3,000 4,000 5,000 6,000 7,000

Palisades

1921

1957

1958

1983

1996-2004

1884

Figure 11. Stage-discharge relationship for the Palisades area, based 

on surveyed elevations of driftwood logs inferred to represent historical floods. Only points considered to be of ‘good’ or ‘excellent’ quality 

(that is, with evidence for driftwood occurring in its original position) 

were used to establish this relationship.

1

2

4

800

46 106 117 137

 Fluvial sediment

Aeolian sediment

Locally derived sediment

Sediment of unknown origin

798

799

801

3

Elevation (m)

Distance from river (m)

798.898

800.742

800.972

800.619

Figure 12. Proportions of fluvial, aeolian, and locally derived sediment, and sediment for which a depositional environment could 

not be confidently identified, in stratigraphic sections at Palisades 

(aeolian and fluvial sediment indicated only where confirmed by 

sedimentary structures). Stratigraphic section numbers are shown 

below the stacked-column graph for each; upper elevation (in m) of 

each section is shown above the column. The vertical axis indicates 

elevation referenced to the NAD83(1999) datum for the stratigraphic 

sections (shown actual height). Elevations of colored bars within each 

section indicate proportion of that sediment type only, and do not 

represent actual stratigraphic position of individual horizons within 

that section. 

Palisades Area
14 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

by dune migration and playa accumulation; a small number 

of local precipitation or high-wind events could have resulted 

in episodically rapid accumulation of sediment in some locations. If, alternatively, unit B of section 4 is interpreted to be a 

deposit resulting from the higher-stage 1884 flood instead of 

the 1921 flood, this implies that unit F must be related to a prehistoric flood event and that erosion has removed the record of 

the 1921 flood at the location of this profile.

Palisades Site Summary 

Stratigraphic sections documented at Palisades indicate 

repeated inundation of this area by Colorado River floods 

in predam time. Fluvial deposits containing climbing ripple 

structures were present in three of the four vertical sections 

studied in detail, and the fourth (section 3) contained sediment 

for which fluvial deposition was suspected but could not be 

confirmed by sedimentary structures. The presence of spatially 

extensive fluvial deposits indicates that virtually all of the area 

between canyon walls in the Palisades region was submerged 

episodically during predam high flows; the upstream (northward) migration direction of fluvial ripples indicates that an 

eddy existed on river left in this area (compare the slackwater 

deposits of Kochel and Baker, 1982, and channel-widening 

deposits of Benito and others, 2003). The upstream-tilted 

water-surface slope apparent in driftwood lines from both 

the 1884 and 1921 floods provides additional evidence for a 

large eddy existing on river left in this area at discharges at 

or above 4,810 m3/s (170,000 ft3/s). It is possible that fluvial 

deposits that underlie modern playa deposits on the eastern 

edge of the Palisades study area may have formed in relation 

to ponding of water upstream of ancient debris flows. Several 

active debris fans exist close to and downstream of this area, 

including Espejo (river mile 67.4), Comanche (mile 67.7), and 

Tanner (mile 69.0) Creeks. Alternatively, these fluvial deposits 

may have been emplaced in an eddy return channel during 

flood recession. Figure 12 illustrates the decreasing influence 

of fluvial deposition with distance from the river.

Reworking of fluvial sediment by wind in this area 

appears to be common. In several of the stratigraphic sections, aeolian deposits were observed between or in close 

association with fluvial deposits (compare Benito and others, 

2003); aeolian material also was found commonly interbedded with locally derived playa sediment in sections 3, and 4. 

Playa sediment formed a volumetrically minor component of 

these stratigraphic sections, but the number of playa horizons 

indicates that such deposits frequently interrupt the record 

of aeolian deposition. The present-day surface geomorphology includes aeolian dunes that have been covered to varying 

degrees by vegetation and cryptogamic crust. Burial of driftwood logs associated with the 1884 and 1921 Colorado River 

floods by aeolian dunes indicates active dune migration as 

recently as the early 20th century. Present morphology of most 

dunes in the Palisades area is consistent with relative inactivity; cryptogamic crust formation, colonization by grasses, and 

deflation features suggest that most dunes are relatively inactive. One exception is a dune ~2 m high that is migrating in a 

northward direction and filling in a shallow branch of the main 

arroyo network at the location of archaeological site C:13:100 

(appendix 1; fig. 38).

The influence of locally derived sediment increases landward and toward the debris fan (fig. 12). Slope-wash and/or 

playa sediments are readily apparent in sections 2, 3, and 4. 

Driftwood logs associated with the 1884 and 1921 floods are 

also partly buried by debris-fan and colluvial sediments in the 

downstream part of the study area. Three of the nine Palisades 

archaeological sites (C:13:98, C:13:272, and C:13:355) are 

located within the range of influence of the Palisades Creek 

debris fan, while a fourth (C:13:33) is built on sediment 

derived from a second debris fan. Of the nine sites, six were 

apparently constructed in or on fluvial sediment at least in 

part, with or without aeolian reworking (C:13:99, C:13:100, 

C:13:272, C:13:334, C:13:336, and C:13:355). Four of the 

nine archaeological sites (C:13:99, C:13:100, C:13:272, and 

C:13:336) have been preserved at least in part by a cover 

of aeolian sediment, with minor contributions from aeolian 

sedimentary cover at two additional sites (C:13:101 and 

C:13:335). Erosion associated with arroyo cutting is affecting two of the sites (the large C:13:99 site in the area of the 

main arroyo network, and C:13:355). Exposure of the site can 

be attributed at least in part to aeolian deflation at five of the 

sites (C:13:99, C:13:100, C:13:101, C:13:272, and C:13:336). 

Gellis (1994) reported that camping activity may have accelerated erosion in the Palisades area; because of relocation of 

trails and other mitigation measures implemented by the NPS 

since 1994, this is not believed to be a major impact agent at 

these archaeological sites currently (National Park Service, 

2004).

Lower Comanche Area

Geomorphic Setting

The Lower Comanche study area is shown in figure 13, 

with a detailed view of the arroyo in which the majority of the 

stratigraphic sections were located shown in figure 14. Like 

Palisades, Lower Comanche falls within the Furnace Flats 

geomorphic reach defined by Schmidt and Graf (1987). The 

region studied for this project lies downstream of the Comanche area discussed by Thompson and Potochnik (2000) in their 

survey of drainage catchment areas and alluvial terrace incision. 

The canyon in this area remains wide, and Precambrian 

Dox Formation is exposed at the river’s edge on river right. 

On river left in the vicinity of the study area, Pleistocene 

debris-flow deposits tens of meters thick are present (Machette 

and Rosholt, 1989; Pederson and others, 2004). Riverward 

of the relict debris fans, large (>10 m high) sand dunes form 

a large dune field (fig. 13). Many of the dunes in this field 

show evidence of active aeolian transport, though grasses 
15

and sparse mesquite have colonized the area. Several small 

interdune/playa areas indicate the sporadic presence of standing water, with desiccation cracks present. A shallow (0.3 m) 

test pit dug into an interdune deposit in May 2003 revealed at 

least two episodes of fine-grained interdune/playa sedimentation, separated by coarser aeolian sand, indicating migration 

of dunes over time. East of the dune field, on either side of the 

Beamer hiking trail, is a flat surface that shows evidence of 

recent standing water (abundant desiccation cracks). Precipitation appears to pond locally in the flat region, which is isolated 

from the well-developed arroyo drainage to the south.

The arroyo network at the southern end of the Lower 

Comanche area drains two minor tributaries on river left that 

release abundant red-colored sediment from the Dox Formation. Vertical exposures of sediment in the arroyo walls were 

used for five of six stratigraphic sections studied at this site 

(discussed below).

The left margin of the river channel in this region is lined 

predominantly by fluvial cobble bars, with local thin sand 

cover. A narrow sand beach (fig. 13) is present as the river 

widens into an eddy at the southern end of the Lower Comanche area; the area of open sand appears to have been reduced 

relative to its size in 1965 aerial photos (fig. 15). One possible 

source of sediment reaching this channel-margin sand bar, in 

addition to any fluvial deposition, is aeolian transport of sand 

from a large sand-covered cobble bar on river right. Wind 

directions recorded at Lower Comanche in Spring 2004 are 

consistent with that possibility. 

Comparison of recent (2002) aerial photographs with 

those taken in 1965 reveal a significant increase in vegetative 

Figure 13. Large-scale aerial view of the Lower Comanche area 

(May 2002; Grand Canyon Monitoring and Research Center). Locations of stratigraphic sections 1 through 6 are indicated by numbered 

dots. The box indicates the area enlarged in figure 14. River discharge is 226 m3

/s.

Figure 14. Aerial view of the arroyo system at Lower Comanche. The 

dark red sediment in the arroyo floors is locally derived material from 

the Dox Formation. The buff-colored, higher elevation deposits into 

which the arroyo has cut are dominated by fluvial and aeolian sediment. Locations of stratigraphic sections 1, 2, 4, 5, and 6 are shown 

with numbered dots. Small black circles (points A, B, C, D1, D2, E1, 

and E2) represent datums used for topographic mapping of the white 

layer discussed in the text.

Lower Comanche Area
16 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

cover along the channel margin (figs. 13 and 15). Mesquite 

trees are abundant in the lower part of the main arroyo mouth. 

It is possible that flow of the Colorado River in the Lower 

Comanche area has been affected numerous times in the past 

by debris flows on the Tanner debris fan, approximately 2 km 

downstream of the study region. Debris flows on the Tanner 

fan would be expected to cause temporary damming of the 

river behind it, ponding water at Lower Comanche and inundating land on river left that is included in the study area. 

However, activity on the Tanner debris fan that generated the 

largest and highest-elevation debris-flow deposits there (tens 

of meters above modern river level) dates from early to middle 

Pleistocene time (Pederson and others, 2004) and so did not 

affect development of the Holocene terraces discussed here.

The Lower Comanche “White Layer”

During reconnaissance work at Lower Comanche in May 

2003, a resistant 0.5- to 1.5-cm-thick fine-grained white horizon of undetermined origin was noted to occur in an extensive 

area (>2,000 m2) of the arroyo-wall sedimentary exposure 

and in the dune field several hundred meters to the northwest. 

Because its origin is unknown, this horizon is referred to by the 

informal descriptive term “white layer” in this report. Possible 

explanations proposed at that time for the occurrence of this 

horizon included volcanic ash deposition, prehistoric cultural 

ash material, or a caliche-type substance precipitated in place. 

A sample of this layer was collected in May 2003 for analysis 

under a petrographic microscope. Disaggregated and cemented 

in epoxy, the material was then thin-sectioned. In thin section, 

the white-layer material appeared crystalline, ruling out ash 

of organic origin, but its crystal character did not resemble 

volcanic ash under the microscope. During field work in May 

2004, a sample of white-layer sediment was immersed in dilute 

hydrochloric acid, which produced a strong reaction. The 

majority of the sample was destroyed by the acid, implying a 

high proportion of calcium carbonate in its composition, leaving only a small amount of silt and sand behind. 

This white layer consists primarily of clean, white silt and 

varies in thickness from 0.5 to 1.5 cm in all exposures where 

it has been observed (fig. 16). The unit commonly comprises 

three phases: a lowermost, seemingly purest, very white layer 

about 0.5 cm in thickness; a middle layer (frequently missing 

Figure 15. Aerial photograph of the Lower Comanche area taken in 

1965 (Grand Canyon Monitoring and Research Center). River discharge is between 680 and 792 m3

/s.

Figure 16. The white-layer horizon (center of photograph) at Lower 

Comanche. Several round burrows have disturbed this exposure. See 

text for facies description.
17

and/or discontinuous) that has what appears to be bioturbated 

sands, apparently occurring in voids resembling insect burrows; and an uppermost, mixed and impure layer not as white 

as the lowermost phase. In the general area of the mouth of 

the unnamed tributary south of Comanche Creek, this layer 

appears to be restricted to low places in the topography, such 

as interdune areas and valley bottoms within the arroyo network. In the valley bottoms, beds pinch out toward the edges 

of the arroyo branches. Where present, the white layer commonly undulates with the topography of the paleosurface on 

which it occurs; if depositional, this would indicate that it had 

been deposited at a time when gullies that had previously dissected the valley floors were essentially completely healed. 

The texture and composition of underlying sediments does not 

seem to significantly influence the character or thickness of the 

layer. Typically, the layer unconformably overlies fine-grained 

alluvial and/or slope-wash deposits and is overlain by similar 

sediments, but locally it is overlain and underlain by relatively 

coarse, high-energy alluvial gravels. Its lower contact is concordant with underlying topography and is micro-irregular; 

the upper contact is slightly more gradational and less abrupt, 

but is still quite distinct. This upper contact appears to have 

undergone minor degrees of mixing and bioturbation. The 

white layer lacks desiccation cracks, oxidation, salt crystals or 

pseudomorphs, indications of salt-induced turbation of sediment, or root structures of obviously halophytic plants that are 

commonly associated with evaporite deposits. The material 

is significantly whiter in color than nearby relatively pure silt 

deposits. It is found over a wide enough range of elevation and 

variety of topographic settings to imply that a groundwaterrelated origin is unlikely. In at least one locality (interdune 

area), a number of discontinuous, very white laminae, presumably of the same material, are found within several cm above 

the primary layer.

The elevation of the top of the white layer at selected 

outcrop points was mapped by R. E. Hunter during this study 

by hand-leveling relative to the closest survey data points (at 

stratigraphic sections 2 and 4). The locations of these outcrop 

points (shown as small black circles on figure 14) were determined by tape-and-compass mapping relative to the surveyed 

points. The distribution of the white layer is considered to 

represent the topography of the surface at the time the layer 

was emplaced. The gully system at Lower Comanche is not 

sufficiently three-dimensional for a contour map of the original 

topography to be made, but it is apparent from this mapping 

exercise that the upper elevation of the white layer descends 

southwestward from 795.52 m to 794.13 m (points A and B 

on figure 14, respectively) over a distance of 37 m (a slope of 

2.15o). This southwestward slope is the general direction in 

which the modern gullied land surface slopes and is probably 

the approximate slope direction of the original surface at the 

time of white-layer emplacement. In addition to the overall 

slope of the white layer, the exposures revealed several smaller 

features of its topography. 

In the smallest notable topographic irregularity (point C 

on figure 14), the white layer was observed to be present in a 

preexisting scour deposit that measured 5 cm deep and 40 cm 

wide. The layer maintains its uniform thickness throughout the 

depression, as well as on both rims. The white layer is directly 

overlain by locally derived gravel that fills the scour. Although 

the white layer in this depression is directly underlain by silt 

or fine sand instead of gravel, gravel-filled scour depressions 

separated from one another by silt or sand occur at several 

horizons in the half-meter of section vertically above and 

below this depression. The location of point C was apparently 

a site for episodically running tributary flow, localized channel cutting, and gravel transport, while at intervening times the 

channel was covered by finer sediment. The white layer at this 

location was emplaced at a time after a channel had been cut 

but before the channel had been filled by gravel.

Between points D1 and D2 (fig. 14), the white layer 

descends eastward into a large depression filled partly by 

locally derived gravel. The depression, like the one at point C, 

was apparently a stream channel, although the opposite side 

of the channel is no longer visible because it has been eroded 

by the present gully system. The channel was >1 m deep and 

several meters wide. In contrast to the small channel at point 

C, this larger channel was not completely filled by gravel but 

instead migrated eastward as sand and silt beds prograded eastward into the channel at dips that locally approach the angle of 

repose. The white layer is both overlain and underlain by silt or 

fine sand on the flat west side of the channel margin (point D1) 

and on the upper slope of the channel. Near the channel center 

(at point D2 and northwest) the white layer is both overlain and 

underlain by gravel.

Between points E1 and E2 (and for some distance north 

and south of those points) the white layer dips westward as 

steeply as 25o, descending nearly 0.5 m in a distance of 1.0 m. 

If the exposures had extended farther in an east-west direction, 

even greater elevation change might have been documented, as 

the dip angles were not observed to flatten out in either direction. In this area, the white layer is both overlain and underlain 

by silt or fine sand. It could not be determined whether the topographic irregularity in this area was due to a channel to the west 

or a dune form to the east. This site is located at the eastern edge 

of the present arroyo, which may have existed for a long time.

In all of these topographic irregularities (points C, D1-D2, 

and E1-E2), the uniform thickness of the white-layer horizon is 

suggestive of deposition of suspended sediment in a relatively 

quiescent fluid (water or air) whose vertical extent was great 

compared to the vertical extent of the topographic irregularities. Although recent studies have not shown major post-Pleistocene activity of the Tanner debris fan, 2 km downstream of 

Lower Comanche (Pederson and others, 2004), it is possible 

that a relatively minor Tanner debris flow caused short-lived 

impoundment of water in this area; otherwise, an airfall origin 

of the white layer seems more likely. One problem with an 

airfall explanation is that the relations at point C would require 

that the small channel was first cut by water, then dried out and 

coated by the airfall layer and finally filled with tributary gravel 

without disruption of the fine-grained white-layer material 

below.

Lower Comanche Area
18 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

At this time, the origin of the white-layer horizon is 

undetermined. Deposition as a pedogenic horizon is considered unlikely, because of the uniformity of thickness, the 

consistency of its characteristics in sediments of significantly 

variable textures and depths, the abrupt character of its lower 

boundary, and the topography of the bed. Although pedogenic 

CaCO3 can occur in lenticular masses having abrupt upper 

and lower contacts, this typically is not found in early-stage 

pedogenic carbonate accumulation. Likewise, thickness and 

degree of cementation typically vary over the catena; this 

horizon does not resemble pedogenic horizons observed by 

members of this research group in other soils or Holocene 

deposits from desert environments. Similarly, deposition in an 

evaporitic environment can likely be eliminated on the basis of 

the absence of desiccation features, oxidation, soil weathering, 

and groundwater phenomena, and the relation to topography. 

The wide distribution, topography, and elevational range of the 

deposit, and the powdery character of the (apparently CaCO3) 

material indicate that deposition in standing water is unlikely. 

Likewise, the fine grain size and physical characteristics of 

the deposit appear to eliminate deposition by running water 

(other than, perhaps, minor redeposition in a slopewash environment). Physical attributes of this horizon resemble those of 

air-fall deposits such as volcanic ash (including the fact that it 

is significantly purer at the bottom), though its dominantly carbonate composition does not. One major difference between 

the white layer and known volcanic ash layers, however, is the 

apparent degree of reworking and redeposition near the toes of 

alluvial slopes and alluvial fans that is usually observed in ash 

deposits and is absent in the white layer.

Archaeological Significance

Multiple cultural features on river left in the Lower 

Comanche region have been dated to the Late PI – Early PII 

Formative period (Fairley and others, 1994; National Park 

Service, 2004). Additional sites contain artifacts that are 

likely related to prehistoric and early historic Hopi habitation 

(Fairley and others, 1994), and at least one site is of unknown 

cultural affiliation. This area is significant in that it contains a 

higher concentration of roasting features than any other area 

in the eastern canyon. The sites are devoid of any Puebloantype structures. Archaeologists infer that this area could have 

been occupied by non-Puebloan people; the features present 

at Lower Comanche are more representative of Cohonina or 

prehistoric Puebloan occupation. This area appears to have 

been used for processing of food and not for direct habitation; 

living areas were perhaps located across the river, upstream, or 

downstream of this region.

Monitoring reports from NPS that date back to the 1980s 

have identified aeolian deflation, surface erosion, and gully 

incision as the greatest impact agents at archaeological sites 

in the Lower Comanche area (National Park Service, 2004). 

Because the majority of the sites are located in the large dune 

field, aeolian erosion by deflation and dune migration are particularly influential. The condition of archaeological sites at 

Lower Comanche has also been impacted to a lesser degree by 

visitors; the Beamer hiking trail passes directly through this 

area. 

Stratigraphic and Geomorphic Analyses

Detailed stratigraphic logs for the Lower Comanche area 

are described in appendix 2 (figs. 40 through 46). Stratigraphic 

sections 1, 2, 4, 5, and 6 are located in vertical exposures of 

sediment that form the walls of the arroyo network upstream 

of the tributary drainage. Section 3 is a pit dug into a flat land 

surface ~270 m north of the arroyo area where the other five 

sections were recorded. Geomorphic characteristics in the 

immediate area surrounding each archaeological site are discussed in appendix 2 (figs. 47 through 49). Stratigraphic data 

and geomorphic observations are summarized in the section 

below titled “Lower Comanche Site Summary.”

Grain-Size Analyses

The results of grain-size analyses from sediment samples 

collected from section 2 at Lower Comanche are shown in 

figure 17. When plotted superimposed on fields that represent 

the range of grain-size distributions for fluvial and aeolian 

samples whose depositional environment could be ascertained 

by field observation, both samples from section 2 (units C 

and F) plot within the fluvial field. Unit C was interpreted in 

the field as dominated by tributary alluvium, and unit F as 

the edge of a channel-fill horizon. The similarity in particlesize characteristics between these samples and known fluvial 

deposits indicates that these units may also include (and may 

actually be dominated by) interbedded or reworked Colorado 

River sediment, a result of the complex interaction between 

fan and fluvial processes. Two of the three samples from section 6 at Lower Comanche whose depositional environment 

was uncertain in the field also fall into the ‘fluvial’ range (fig. 

18). A sample collected from unit B of section 6, a horizon 

where faint fluvial climbing ripples appeared to be present, 

and a sample from unit F of section 6, where faint fluvial 

climbing ripples appeared to be present in the upper part of 

the deposit, both plot within the fluvial field. These samples 

are finer grained than a sample from unit E of section 6, which 

was determined in the field to be a flood deposit on the basis 

of readily apparent fluvial climbing ripples. 

Driftwood Elevation Survey: Lower Comanche 

Stage-Discharge Relationship

The location and elevation of driftwood logs were surveyed in the Lower Comanche area on both banks of the river. 

In the lower elevation areas on river left at Lower Comanche 

(near sections 5 and 6 described above), dense vegetation 
19

limited the extent of the driftwood survey work. Data were 

not collected in the vicinity of the popular camp on river right 

known as the upper Tanner camp, to avoid sampling an area 

where logs had been rearranged to form camp sites and to 

avoid sampling logs that have moved downslope due to aeolian deflation. The uppermost 10–20 cm within each surveyed 

‘cluster’ of driftwood were used to compile a stage-discharge 

relationship (fig. 19). Maximum flood discharge values used in 

this calculation are those of Topping and others (2003): 5,940 

m3/s (210,000 ft3/s) for the 1884 flood, 4,810 m3/s (170,000 

ft3/s) for the 1921 flood, 3,540 m3/s (125,000 ft3/s) for the 1957 

flood, 3,060 m3/s (108,000 ft3/s) for the 1958 flood, and 2,700 

m3/s (97,000 ft3/s) for the 1983 flood. As with the Palisades 

data (fig. 11), the plotted relationship for Lower Comanche 

shown in figure 19 uses only those data points that had been 

assigned ‘good’ or ‘excellent’ quality in terms of the apparent original placement of driftwood logs and the agreement 

between that point and the elevation ‘cluster’ for that flood. 

Because the water surface during a flood is unlikely to be 

horizontal, this stage-discharge curve represents approximate 

flood elevation maxima for the Lower Comanche region but 

is unlikely to accurately represent local conditions in specific 

areas within this region.

Of the six stratigraphic sections studied at Lower Comanche, only sections 5 and 6 contained unambiguous fluvial 

deposits (with distinct fluvial sedimentary structures). Using 

the stage-discharge relationship from figure 19, it is apparent that the highest elevation fluvial deposits observed in this 

area (~795 m in unit C of section 6, and possibly unit B of this 

same profile) lie above the inferred 1921 flood limit. These 

uppermost fluvial deposits in section 6 could therefore be 

consistent with deposition during the 1884 flood. This would 

imply that other possible flood deposits lower in the section 6 

stratigraphy (units E, F, H, and J) were left by prehistoric flood 

events. These stage-discharge relationships assume that the 

bed of the river (and surface elevation of river flows, for any 

given discharge) were the same as the modern state; it is possible that paleoenvironmental factors (for example, short-lived 

ponding behind debris fans, channel migration) could have 

0

20

40

60

80

100

0.001 0.01 0.1 1

ComStrat2UnitF

ComStrat2UnitC

Fluvial Aeolian

Grain size (mm)

Cumulative percent finer

Figure 17. Grain-size analyses for units C and F in section 2, Lower 

Comanche, superimposed on fields that show the range of grain-size 

distribution for samples of known fluvial (n=27, light gray field) and 

aeolian (n=9, dark gray field) origin (from the presence of diagnostic sedimentary structures and/or geomorphic features). Although 

these two samples appeared in the field to be largely composed of 

locally derived sediment, the fact that both plot within the fluvial field 

indicates that fluvial sediment may form a substantial amount of these 

units (by interaction between fluvial and fan processes, leading to interbedding of sediment from each source). See appendix 2 for sample 

descriptions.

0

20

40

60

80

100

0.001 0.01 0.1 1

ComStrat6UnitB

ComStrat6UnitE

ComStrat6UnitF

Fluvial Aeolian

Cumulative percent finer

Grain size (mm)

Figure 18. Grain-size analyses for units B, E, and F in section 6 at 

Lower Comanche. See appendix 2 for sample descriptions.

Lower Comanche Area
20 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

dictated a stage-discharge relationship at the time of deposition that is different from that of today.

The elevation of the land surface at section 5 (793.395 

m) is low enough that this location could be inundated by 

a flow comparable to or slightly less than that of the 1921 

flood, and possibly by the peak of the 1957 flood. At the other 

stratigraphic sections, much greater floods (with discharge 

higher than can be accurately estimated using this stage-discharge relationship) would be required to deposit sediment on 

the existing land surface (surface elevations are 796.710 m 

at section 1, 796.147 m at section 2, 797.480 m at section 3, 

795.874 m at section 4, and 795.525 m at section 6). The floor 

of the arroyo, in contrast, would be inundated by a discharge 

of ~5,100 m3/s (180,000 ft3/s) at the location of section 6, 

assuming that the regional average conditions represented by 

this stage-discharge relationship would hold. At four times the 

level of the 1996 BHBF flood experiment, such a discharge (or 

greater) would be needed to promote restorative fluvial sediment deposition in this arroyo network.

Lower Comanche Site Summary

Stratigraphic sections described in the southern, arroyoincised Lower Comanche area indicate multiple episodes 

of fluvial sedimentation followed by reworking at the land 

surface. Aeolian sedimentary structures are present in some 

of the fine-grained deposits. The abundant buff-colored silt 

and sand material in sections 1, 2, and 4 could be interpreted 

as tributary alluvium derived from aeolian deposits upstream 

and on valley slopes during local runoff events, or as sediment derived from the Colorado River (fluvial or reworked 

aeolian) that may have interbedded with tributary sediment 

as fluvial and fan processes intersect. Results of grain-size 

analyses of two samples from section 2 favor the interpretation of reworked Colorado River sediment. The importance of 

Elevation (m)

Discharge (m3/s)

786

787

788

789

790

791

792

793

794

795

796

797

0 1,000 2,000 3,000 4,000 5,000 7,000

Lower Comanche

1921

1957

1958

1983

1996-2004

1884

Figure 19. Stage-discharge relationship for the Lower Comanche 

area, based on surveyed elevation of driftwood logs inferred to 

represent historical floods. Only points considered to be of ‘good’ or 

‘excellent’ quality were used to establish this relationship. The 1921 

point that was surveyed comes from a cluster of logs ~1.2 m above 

a well-defined 1957 driftwood assemblage. No wood was identified 

that could confidently be identified as an 1884 deposit; the dashed line 

shows the 1884 flood discharge as it would intersect with the extrapolated stage-discharge curve.

797

796

795

60 72 85 96 175

794

793

792

 Fluvial sediment

Aeolian sediment

Locally derived sediment

Sediment of unknown origin

Elevation (m)

Distance from river (m)

795.874

4

5

6

796.147

797.480

793.395

795.525

3

2

Figure 20. Proportions of fluvial, aeolian, and locally 

derived sediment, and sediment for which a depositional environment could not be confidently identified, in stratigraphic sections at Lower Comanche 

(aeolian and fluvial sediment indicated only where 

confirmed by sedimentary structures). Stratigraphic 

section numbers are shown below the stacked-column graph for each; upper elevation (in m) of each 

section is shown above the column. The vertical axis 

indicates elevation referenced to the NAD83(1999) 

datum for the stratigraphic sections (shown actual 

height). Elevations of colored bars within each section indicate proportion of that sediment type only, 

and do not represent actual stratigraphic position of 

individual horizons within that section.
21

various depositional environments does not show a clear trend 

with distance from the river (fig. 20) because much of the total 

stratigraphic thickness studied had no diagnostic sedimentary 

structures. Well-defined Colorado River flood deposits (those 

that contain fluvial sedimentary structures) were observed 

only in sections 5 and 6, the two closest to the river. Locally 

derived sediment, predominantly Dox sandstone and shale 

clasts, is present in all of the sections described (fig. 20). 

Numerous channel-fill structures are apparent in arroyo wall 

exposures (sections 1, 2, 4, and surrounding area), indicating 

paleogully development similar to that in the modern arroyo 

network. Data from section 3 indicate that repeated wetting 

episodes have occurred on the large flat area west of the main 

dune field, interpreted as ponding due to precipitation runoff 

that carries locally derived sediment. 

The majority of archaeological sites at Lower Comanche are affected to a great degree by aeolian processes in the 

main dune field. Five of the six cultural sites in this area are 

located within this dune field (C:13:274, C:13:333, C:13:335, 

C:13:337, and C:13:373). Four of those five were originally 

situated on aeolian sediment (C:13274, C:13:333, C:13:335, 

and C:13:373), with the fifth (C:13:337) built on an interdune/playa surface within the dune field. Four of the six sites 

(C:13:274, C:13:333, C:13:335, and C:13:373) show evidence 

of at least partial burial by aeolian sediment; two of those four 

(C:13:274 and C:13:373) have only minor sediment cover. 

Three of the five sites within the dune field are affected by 

aeolian deflation (C:13:274, C:13:333, and C:13:335), and 

one of those (C:13:333) is exposed as a result of dune migration. One of the sites in the dune field, C:13:333, has begun 

to be affected by gully incision that reaches into the upper 

elevations of a dune ~10 m high. Dune migration and aeolian 

sediment transport are expected to continue to affect these 

five sites; it is likely that new artifacts will become exposed 

as dune migration continues, while others are reburied by 

wind-blown sand. The one site not located in the dune field 

(C:13:273) is affected significantly by arroyo incision and 

local runoff (Gellis, 1994; National Park Service, 2004). 

Arroyo-related erosion is severe at Feature 3 of that site and is 

predicted to undercut more of Features 1 and 3 as backwasting of the gully walls progresses. The site is also impacted to a 

lesser degree by visitor use of the Beamer hiking trail.

Much of the fine-grained (silt to fine sand) material in 

stratigraphic sections at Lower Comanche does not contain 

preserved diagnostic sedimentary structures. Degradation of 

original structures is attributed primarily to bioturbation, in 

the form of animal burrowing and the growth of plant roots. 

In the absence of lithic grains that imply a local sediment 

source, such fine-grained deposits are generally interpreted as 

having been originally derived from Colorado River deposits 

(and later having been subject to aeolian and tributary-flood 

reworking). Without sedimentary structures, it is not possible 

to confidently ascertain whether such deposits remain in their 

original position as fluvial layers or have been transported 

and redeposited by wind. Although grain-size analysis may 

in some cases distinguish between these options, the range of 

grain sizes present within verifiable fluvial deposits is great 

enough that caution should be used in assigning depositional 

environment solely on this basis (see dicussion in section titled 

“Methods”).

It is worthwhile to consider the possibility that debris 

flows on the large Tanner debris fan (~2 km downstream) 

could have affected river discharge levels at Lower Comanche (R. H. Webb, written commun. with H. C. Fairley, 2004). 

While none of the strata identified during this work explicitly 

suggest quiescent ponding or lacustrine sedimentation, it 

is possible that elevations of observed fluvial strata may be 

higher than comparable discharge would reach today, because 

of prehistoric damming behind the Tanner fan. This possibility 

could be addressed more fully if a suite of dates were available 

from fluvial deposits at this site and from debris flow events at 

Tanner.

The origin of the so-called white layer remains to be 

determined. It would be advantageous to analyze a sample by 

x-ray diffraction, which could provide more information on its 

composition, and consequently on its emplacement.

Arroyo Grande Area

Geomorphic Setting

The left bank of the Colorado River in the area known 

as Arroyo Grande is part of the Area of Cooperation (AOC) 

between the Hualapai Nation and the National Park Service in 

western Grand Canyon (Memorandum of Understanding by 

and among the Hualapai Tribe, Grand Canyon National Park, 

and Lake Mead National Recreation Area, October 2000). The 

river in this reach is located along the trend of the major Hurricane Fault system (Huntoon and others, 1981), which locally 

offsets Precambrian schist and gneiss along river left. Schist 

and amphibolite-grade gneiss dominate locally derived sediment in the study area.

The study site discussed here (fig. 21) is approximately 

1 km north of the limit of mapping work done by Hereford 

and others (2000) at Granite Park. Many of the alluvial terraces and other geomorphic features identified at Granite Park 

by Hereford and others (2000) are present at Arroyo Grande, 

including designations of the upper mesquite terrace (map unit 

‘umt’), lower mesquite terrace (map unit ‘lmt’) and predam 

alluvium (map unit ‘pda’) deposits considered by Hereford 

and others (2000) to be equivalent to those mapped by Hereford (1993) and Hereford and others (1996) at Palisades. In 

the Arroyo Grande region, the oldest two alluvial terrace units 

defined by Hereford (1993) and Hereford and others (1996, 

2000), the ‘pda’ map unit and the local equivalent of what was 

called the striped alluvium (‘sa’) unit at Palisades by Hereford 

(1993), are both truncated and overlain by the ‘umt’ alluvium. 

This younger ‘umt’ deposit typically occurs at lower elevation 

than the older ‘sa’ and ‘pda’ in the eastern canyon; in this area 

of the western canyon, where ‘umt’ forms a cap over the older 

alluvial deposits, the covering of Pueblo II age cultural sites 

Arroyo Grande Area
22 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

by subsequent flood deposits may explain the relative rarity of 

exposed PII sites in the western canyon (Fairley, 2003, p. 112).

As the name suggests, the Arroyo Grande area is characterized by a wide, deep arroyo network (figure 22) that has incised 

these terrace deposits, exposing cultural material and stratigraphy in the arroyo walls. The arroyo network is as much as 5 m 

deep in places. A smaller, secondary arroyo is present downstream of the primary system (figure 21), which has incised <0.5 

m into the uppermost alluvial terrace and more than 1 m into the 

lower terrace units. A nickpoint near the location of section 5 

described at this site suggests that incision of the uppermost terrace will soon increase. 

The main arroyo at Arroyo Grande is considered to be a 

predam feature, because it is present in aerial photographs from 

1965 (fig. 23). The extent of arroyo development appears (at 

least qualitatively) to have increased since 1965, from examination of stereo pairs of 1965 photographs, but the scale of those 

images is such that more detailed measurements of arroyo features in 1965 would not be reliable. Repeated ground-based and 

photogrammetric mapping by Pederson and others (2003) found 

little change in the depth and configuration of arroyos at Arroyo 

Grande before and after the 2002 monsoon season, the only 

exception being the partial filling of a minor rill with aeolian 

sediment between February and October 2002. 

The mouth of the primary arroyo drains into the river near 

the center of a large eddy on river left (fig. 21). Comparison of 

recent aerial photographs with those from 1965 shows that the 

extent of riparian vegetation has greatly increased during the 

past 40 years, and that the area of open sand in the main eddy 

and along the channel margin has decreased (figs. 21 and 23). 

Aerial photographs taken immediately before the 1996 flood 

experiment and again six months after the flood show that a 

sand bar ~5 m wide (measured at a river discharge of 220 m3/s 

[8,000 ft3/s]) formed during the flood at the arroyo mouth in the 

eddy on river left.

The terrace surfaces into which the arroyos cut have undergone deflation by wind, evidenced by pedestal development of 

pebbles and cultural artifacts of up to 5 cm above the surrounding land surface. Coppice dunes are common on the terraces, 

although well developed cryptogamic crust and colonization by 

grasses, mesquite, and other vegetation indicate relatively inactive dunes at this time. A large, active sand dune is present at 

the extreme downstream end of the Arroyo Grande area, 280 m 

downstream of the center of the main arroyo network (fig. 24). 

Figure 21. Aerial view of the Arroyo Grande area (photographs from 

the Grand Canyon Monitoring and Research Center, taken in May 

2002). Larger-scale view (A) shows locations of aerial views in B, and 

of the dune shown in an oblique view in figure 24. Locations of stratigraphic sections 1 through 11 are shown in B. In B, a smaller, secondary arroyo network is visible to the east of the main arroyo (section 5 

is in the secondary arroyo. River discharge is 226 m3/s.

Figure 22. The main arroyo network at Arroyo Grande, viewed from 

the north (A) and south (B). The large sand dune in the distance in A

(at the downstream end of this area) is migrating to the north, toward 

the arroyo network, but its sand is trapped by a wide drainage (not 

visible) between the dune and the arroyo, which prevents much of 

this sand from reaching the terrace into which the arroyo has incised. 

Note the thick zone of riparian vegetation.
23

The orientation of this dune’s slip face indicates that it is migrating in an upstream direction, such that sand is being transported 

into the wide bed of the tributary that lies between this large 

dune and the terraced area into which the main arroyo has 

cut. This large tributary likely intercepts most of the sand that 

migrates into it from the large dune, preventing the dune from 

acting as a significant source of aeolian sand for the Arroyo 

Grande terrace.

It is probable that flow of the Colorado River in the Arroyo 

Grande area has been affected multiple times in the past by 

debris flows on the debris fan at Granite Park, ~2 km downstream of the study region. At least four episodes of debris flow 

activity are recognizable in the geomorphology at Granite Park 

(Hereford and others, 2000), the most recent of those occurring 

in 1994. Debris flows at Granite Park would be expected to 

cause temporary damming of the river behind it, ponding water 

on the terraced areas at Arroyo Grande and inundating land on 

river left that is included in the study area.

Archaeological Significance

Four archaeological site complexes are present in the delta 

and terrace regions of the Arroyo Grande area. An additional 

three sites are located at higher elevation, near the basalt cliffs 

that define the lower canyon wall on river left. The largest of 

the sites is the area immediately affected by the large arroyo 

network, NPS site number G:03:064 (Fairley and others, 1994). 

This site, one of the largest and most significant in the western 

Grand Canyon, contains 16 known features exposed on the 

land surface with several additional hearths, charcoal lenses, 

and one metate exposed within the drainage network itself. 

The arroyo system that cuts through this large site is as much 

as 5 m deep in places and has contributed to significant erosion of these cultural features. NPS identifies this site as being 

in immediate danger of severe loss due to arroyo incision of 

the terrace on which the features are located (National Park 

Service, 2004). Features that are exposed on the surface today 

date primarily from the protohistoric and early historic era (Pai 

and Paiute occupation), whereas older dates (PI–III Formative) have been identified for features that are exposed at lower 

stratigraphic levels (as much as 3 m below the modern surface) 

within the arroyo walls (Fairley and others, 1994). 

Numerous artifacts that have been documented on the land 

surface indicate the cultural importance of this area. Several of 

the roasting features at this site are greater than six meters in 

diameter, implying extensive and repeated use over time. The 

artifact assemblage indicates that the occupants of this area 

participated in an extensive trade network. Imported artifacts 

include obsidian, Hopi pottery, and marine shell material. 

Arroyo Grande is a very important area in the historic 

archaeological record because there are several ethnographic 

accounts from the Paiute and Hualapai of how the area was 

used; known uses include religious practice and ceremonies, trade, complex social interaction including information 

exchange, and crossing of the river. Tribes continue today to 

view this area as a very significant part of their past, present, 

and future.

Stratigraphic and Geomorphic Analyses

Detailed stratigraphic logs for the Arroyo Grande study 

area are described in appendix 3 (figs. 50 through 65). With 

Figure 23. Aerial photograph of the Arroyo Grande area (taken in 

1965; Grand Canyon Monitoring and Research Center). The dashed 

line shows the approximate perimeter of the main arroyo. River discharge is between 680 and 792 m3

/s.

Figure 24. Photograph taken from within a wide drainage at the 

downstream end of the Arroyo Grande area, on river left (facing 

toward the river). The large sand dune is migrating northwest (upcanyon; toward the right of the photograph), but interception of sand by 

this drainage prevents much of it from reaching the terrace into which 

the deep arroyo network is cut.

Arroyo Grande Area
24 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

the exception of section 5, all stratigraphic descriptions from 

Arroyo Grande were obtained within the main arroyo network. 

Section locations were distributed such that representative 

samples of the along-shore and across-shore stratigraphy 

would be included. Section 5 was described from a wall of 

the secondary arroyo network that lies downstream of the 

major arroyo. No artificial trenches or pits were dug for this 

work; all stratigraphic information was obtained using the 

existing arroyo walls. After completion of the work, stratigraphic sections were disguised with loose sediment from 

the arroyo floor to conceal the work areas. Dispersed ash and 

charcoal are common in Arroyo Grande stratigraphy; this is 

believed to be the result of grass fires (discussed in more detail 

below). Unless specifically noted, mention of charcoal in the 

stratigraphic descriptions to follow does not refer to charcoal 

deposits of cultural significance (such as hearths or roasting 

pits). Geomorphic characteristics in the immediate area surrounding each of the features of site G:03:064 are discussed in 

appendix 3 (figs. 66 through 68). Stratigraphic data and geomorphic observations are summarized in the Arroyo Grande 

Site Summary section below.

Grain-Size Analyses

Samples were collected from selected stratigraphic units 

at Arroyo Grande to obtain grain-size data for several fluvial, 

aeolian, and slope-wash deposits. As discussed above in the 

section titled “Methods,” considerable variation in grain size 

and other sedimentary characteristics may occur between 

deposits generated by the same type of depositional event. 

However, these analyses provide a reference framework for 

comparing characteristics of different depositional environments relative to each other within each section, and for 

evaluating sedimentary dynamics within specific flood events. 

Grain-size data from Arroyo Grande samples are shown in figures 25 through 29. Figure 30 shows grain-size trends within 

four individual flood units from which multiple samples were 

taken at different depths within the flood deposit.

Results of grain-size analyses for samples taken from section 1 (a and b) at Arroyo Grande are shown in figure 25. The 

lowermost sample from the section 1b stratigraphy, sample 

AG Strat 1b #2, shows the very fine grained nature of the basal 

part of this flood deposit, which contains more than 74 percent silt and clay. The deposit coarsens upward (see also fig. 

30), similar to flood deposits described by Rubin and others 

(1998), which can indicate sediment-supply limitation during 

the flood that produced the deposit. The aeolian unit (sample 

AG Strat 1a #1) that overlies the flood sampled in section 1b 

is distinctly coarser than any of the samples from the fluvial 

deposit (fig. 25). In cases where aeolian deposits are produced 

by aeolian reworking of fluvial sediment, it is expected that 

the aeolian deposits will, in general, be coarser than the fluvial 

source sediment because of winnowing and removal of fines 

by wind. Sample AG Strat 1b #6, which was considered in the 

field to be most likely from the upper part of the flood deposit 

but possibly part of the overlying aeolian unit, has a grain-size 

distribution more consistent with the upper part of the flood 

layer than the overlying aeolian sand (fig. 25).

Data from two flood deposits within section 5 are shown 

in figure 26. It is apparent from this plot that unit H contains 

distinctly coarser sediment than unit C. Unit H coarsens 

upward (fig. 30), suggesting that sediment supply was limited 

during the flood that deposited this unit (Rubin and others, 

1998). Unit C, in contrast, fines upward, indicating that sediment-supply limitation was less significant or absent during 

this flood; this is the same flood unit in which rapid sediment 

deposition is inferred from vertically climbing eddy-pulsation 

ripples (figs. 55, 56). 

Grain-size data from seven stratigraphically sequential 

samples within section 6 are shown in figure 27. Samples AG 

Strat 6 #1 and AG Strat 6 #7 were collected from subaerial 

strata that underlie and overlie a flood deposit from which 

the intermediate five samples were obtained. The lower of 

the two subaerial deposits (AG Strat 6 #1) is coarser than the 

upper (AG Strat 6 #7), and both of them are coarser than any 

of the sampled flood sediment between them. These subaerial 

deposits include locally derived slope-wash sediments as well 

as charcoal and ash material. Charcoal and ash particles were 

present in small enough amounts that their presence is not 

believed to have significantly affected analyses of these samples on the Coulter LS 100Q laser particle-size analyzer. The 

fluvial deposit (AG Strat 6 #2 through #6; fig. 30) first coars- Cumulative percent finer

Grain size (mm)

0

20

40

60

80

100

0.001 0.01 0.1 1

Strat1a #1

Strat1b #2

Strat1b #3

Strat1b #4

Strat1b #5

Strat1b #6

Figure 25. Grain-size data from sections 1a (sample #1) and 

1b (#2 through #6), Arroyo Grande. See appendix 3 for sample 

descriptions.
25

0

20

40

60

80

100

0.001 0.01 0.1 1

AGStrat5C#1

AGStrat5C#2

AGStrat5C#3

AGStrat5C#4

AGStrat5H#5

AGStrat5H#6

AGStrat5H#7

AGStrat5H#8

Grain size (mm)

Cumulative percent finer

0

20

40

60

80

100

0.001 0.01 0.1 1

AGStrat8#1

AGStrat8#2

AGStrat8#3

Grain size ( mm )

Cumulative percent finer

Figure 26. Grain-size data from four samples taken from each 

of two floods within section 5 at Arroyo Grande (unit C and unit 

H). See appendix 3 for sample descriptions.

0

20

40

60

80

100

0.001 0.01 0.1 1

AGStrat10#1

AGStrat10#2

AGStrat10#3

AGStrat10#4

AGStrat10#5

Grain size (mm)

Cumulative percent finer

Figure 28. Grain-size data for samples from section 8, Arroyo 

Grande. See appendix 3 for sample descriptions.

Figure 29. Grain-size data for samples from section 10, Arroyo 

Grande. See appendix 3 for sample descriptions. 

Arroyo Grande Area

0

20

40

60

80

100

0.001 0.01 0.1 1

AGStrat6#1

AGStrat6#2

AGStrat6#3

AGStrat6#4

AGStrat6#5

AGStrat6#6

AGStrat6#7

Grain size (mm)

Cumulative percent finer

Figure 27. Grain-size data from subunit G3 within section 6, Arroyo Grande. See appendix 3 for sample descriptions.
26 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

ens upward and then fines again near its top. Such a grain-size 

progression is consistent with a deposit that mimics suspendedsediment characteristics of the flood hydrograph in the absence 

of significant sediment-supply limitation.

Three samples from section 8 show the different grain-size 

distributions that characterize various parts of a flood/landsurface ‘couplet’ deposit within this section (fig. 28). Sample 

AG Strat 8 #1, from the basal part of a fluvial deposit, contains 

nearly 94 percent silt and clay. Sample AG Strat 8 #2 was taken 

higher up in the same couplet from sediment that was interpreted in the field as either the upper part of the fluvial layer 

or part of the subaerial unit that overlies the fluvial part of this 

couplet. This sample is coarser than the base of the deposit, 

containing 37 percent silt and clay. Because this sample was 

also found to be slightly coarser than sediment that was clearly 

part of the subaerial portion of this couplet (sample AG Strat 8 

#3, which contained 46 percent silt and clay; fig. 28), sample 

AG Strat 8 #2 can be more reasonably interpreted as subaerial 

material (having undergone winnowing and possibly incorporation of locally derived sediment) rather than as the upper part 

of an intact fluvial layer. 

Results of grain-size analyses on five samples collected 

from section 10 are shown in figure 29. Samples AG Strat 10 

#1, 2, and 3 were collected from sediment in this section that 

was clearly fluvial in origin, containing well-defined fluvial 

sedimentary structures. Sample AG Strat 10 #1, a white silt 

that forms the base of that flood deposit, is much finer than the 

other samples from this section, containing 72 percent silt and 

clay. The uppermost two samples from section 10 were considered in the field to be of uncertain depositional origin, though 

an aeolian interpretation was favored on the basis of sedimentary structures. It is apparent from figure 29 that these two samples, AG Strat 10 #4 and #5, are coarser than the samples of 

fluvial sediment at this site, which would be consistent with an 

aeolian origin for these two. These two samples have grain-size 

distributions comparable to samples of known aeolian affinity, 

though in the finer grain sizes they plot within the region of 

overlap between fluvial and aeolian grain-size fields. On the 

basis of both field observations and grain-size data, an aeolian 

origin is most likely for these two samples.

Arroyo Grande Site Summary

In the stratigraphic sections examined at Arroyo Grande, 

it is apparent that Colorado River flood deposits dominate 

the stratigraphic record of the area in which the features of 

archaeological site G:03:064 are located. The number and total 

thickness of fluvial deposits increase toward the river; the best 

preserved record of flood deposition occurs here in sections 5 

and 6, closest to the river (see, for example, fig. 31). All profiles show some evidence of subaerial reworking and incorporation of locally derived (slope-wash) sediment between flood 

events (compare Benito and others, 2003); poorly sorted sediment with abundant lithic grains typically characterize interflood sedimentation and may occur as channel-fill lenses. The 

frequent occurrence of fluvial deposits overlain by these subaerial sedimentary packages led to the description, in the field, 

of ‘flood couplets’ that each contain a lower fluvial and upper 

subaerial member (see figs. 58, 59). The fluvial portion of flood 

couplets commonly, but not always, was seen to begin with a 

light-colored, very fine grained silt and clay layer. Within four 

flood units from which multiple samples were taken at different 

depths, two were found to coarsen upward (section 1b, and unit 

H of section 5), one was found to fine upward (unit C of section 5) and one had no distinct trend, first coarsening and then 

fining upward (subunit G3 of section 6). 

A stage-discharge relationship similar to those shown in 

figures 11 and 19 was not developed for the Arroyo Grande 

area because of limited access to driftwood on the densely 

vegetated lower terraces on river left. Two driftwood logs on 

river right that are believed to be associated with the 1957 flood 

(they are, in any case, not likely to be older than 1921, given 

their relatively good condition) yielded elevations of 428.5 

and 428.7 m. The highest fluvial deposits observed at Arroyo 

Section 6 (Unit G3)

Section 5 (Unit C)

Section 5 (Unit H)

Section 1

0

0.2

0.4

0.6

0.8

1

10 100

d50 (� m)

Normalized height within flood deposit

0

0.2

0.4

0.6

0.8

1

0 20 40 60 80 100

Silt + Clay content (percent)

Normalized height within flood deposit

�

�

Figure 30. Grain-size trends upsection for four flood units sampled at 

Arroyo Grande. A, Median grain size vs. normalized height within the 

flood deposit. B, Combined silt and clay content vs. normalized height. 

Two of the floods, unit H of section 5 and the flood within section 1b, 

coarsen upward. Unit C of section 5 fines upward. The flood sampled 

within section 6 (subunit G3) first coarsens upward then fines upward. 

Samples from coarser subaerial strata immediately above and below 

section 6 (subunit G3) are also shown (at normalized heights 1 and 0 

on the line for this sample, respectively). See appendix 3 for sample 

descriptions.
27

Grande are within 2–3 m of this and therefore would not have 

required a flood discharge substantially larger than that of the 

1921 flood (4,810 m3s, 170,000 ft3/s). 

The influence of locally derived sediment was seen to 

increase with distance away from the river (fig. 31), making up 

the great majority of section 11 and forming a substantial component of the stratigraphy at sections 7, 8, and 9. Channel-fill 

deposits, such as those observed in sections 4 and 11, indicate 

multiple episodes of gully formation and subsequent filling. 

Aeolian sediment constitutes a relatively minor volume 

in the sections studied; deposits that were clearly aeolian in 

nature (from the presence of sedimentary structures) occurred 

at sections 1, 2, and 10; where present, these aeolian dune 

deposits exhibited fairly deep roots, extending as much as 2 m 

beneath the land surface. The most likely sediment source for 

the small coppice dunes, now deflated, that are visible on the 

land surface today (for example, fig. 66) is the reworking of the 

extensive fluvial deposits that created the terrace morphology 

in this area. 

The presence of abundant charcoal and ash material in 

many of the stratigraphic horizons that represent subaerial 

exposure is most probably attributable to frequent grass fires 

in the area. It is likely that these were deliberately set by the 

inhabitants of the area, although lightning strikes could also 

have caused occasional fires. The Hualapai and Paiute Tribes, 

which have ancestral ties to the Arroyo Grande region, both 

carry cultural traditions that include the deliberate setting of 

grass fires. Loretta Jackson, Tribal Historic Preservation Officer 

for the Hualapai, has related that Hualapai tribal elders know 

of grass fires commonly set to initiate seed germination (L. 

Jackson, oral commun. with L. M. Leap, 2004). Ila Bullets, of 

the Kaibab Band of Paiutes, reports that grass fires were commonly initiated to facilitate seed collection and crop management and to prevent other, larger fires from starting. The setting 

of grass fires was practiced at certain prescribed times of year, 

and continues today (I. Bullets, oral commun. with L. M. Leap, 

2004). Other possible reasons for deliberately set fires include 

hunting and escape/warfare (see, for example, Powell, 1878; 

Beaglehole, 1936; Drucker, 1941; Kelly and Fowler, 1986; 

Boyd, 1999).

The features that form site G:03:064, all of which occur on 

or just beneath the land surface, are built primarily on fluvial 

sediment, though several (features 1, 2, 4, 5, and 6) are located 

in the area where small coppice dunes, now deflated, occur (fig. 

66). Several cultural hearth features, which are not included 

as features in G:03:064, are visible in the walls of the arroyo, 

having been built on fluvial and slope-wash sediment and subsequently buried by both fluvial and locally derived material. 

By far the greatest threat to cultural features at Arroyo Grande, 

both the surface features that make up G:03:064 and the older 

subsurface hearths, is arroyo incision. The steep arroyo walls 

consist of relatively poorly consolidated fluvial and aeolian 

sediment throughout most of the arroyo network; back-wasting 

of the walls continues to undercut features such as that shown 

in figure 67. Toward the distal ends of the arroyo arms, incision is taking place into more resistant, coarser grained sediment (the slope-wash and talus deposits near the cliff wall), 

which limits the rate at which incision can occur. All but one 

of the features associated with G:03:064 are located on the 

fluvial/aeolian terrace surface, however, and not in the slopewash-dominated area far from the river, resulting in the imminent threat to these features from arroyo incision. The smaller, 

secondary arroyo network that lies downstream of the primary 

arroyo (in the area where section 5 was measured) has begun to 

incise into the uppermost fluvial terrace to its north. Incision of 

the upper terrace by the secondary arroyo is expected to accelerate after the ~1.5-m-deep nickpoint that is currently located 

Figure 31. Proportions of fluvial, aeolian, and locally derived sediment, and sediment for which a depositional environment could not 

be confidently identified, in stratigraphic sections at Arroyo Grande 

(aeolian and fluvial sediment indicated only where confirmed by 

sedimentary structures). Stratigraphic section numbers are shown 

below the stacked-column graph for each; upper elevation (in m) of 

each section is shown above the column. The vertical axis indicates 

elevation referenced to the NAD83(1999) datum for the stratigraphic 

sections (shown actual height). Elevations of colored bars within each 

section indicate proportion of that sediment type only, and do not 

represent actual stratigraphic position of individual horizons within 

that section. Relative proportion of fluvial sediment can be seen to 

decrease with distance from the river, as locally derived and aeolian 

sediment are more volumetrically significant at distances far from the 

river and at higher elevation.

1

2

3

5

6

7

8

9

10

11

4

428

429

430

431

432

27 29 35 36 45 55 59 60 63 72 95

 Fluvial sediment

Aeolian sediment

Locally derived sediment

Sediment of unknown origin

427

Elevation (m)

Distance from river (m)

430.403

430.530

430.654

429.317

430.799

431.435

430.887

431.062

430.899

431.397

432.034

Arroyo Grande Area
28 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

at the site of section 5 retreats further and meets the upper terrace. The proximity of this nickpoint to the upper terrace at this 

time (2 m) implies that perhaps two or three intense rain storms 

could allow the nickpoint to move into the upper terrace area.

Although no samples were collected from Arroyo Grande 

for radiocarbon dating, numerous stratigraphic horizons in the 

arroyo walls contain sufficient ash material for radiocarbon 

dating purposes. Any future efforts to conduct radiocarbon 

dating work on charcoal (either cultural or noncultural ash, 

such as the apparent grass-fire horizons represented in several 

stratigraphic sections) could provide valuable age data that 

could be used to refine the flood history of this area, as well as 

providing dates for various stages of cultural occupation. 

Discussion and Conclusions 

These investigations of Holocene stratigraphy in 

archaeologically significant areas have been designed as a 

cooperative effort by geologists and archaeologists to collect data to help guide managers on future efforts to preserve 

archaeological sites in Grand Canyon. Developing a strategy 

for preservation of such cultural resources requires a detailed 

understanding of sedimentary and geomorphic processes with 

multiple time scales considered. In the shorter term, field 

evaluation of geomorphic processes (such as gully incision 

and dune migration) is essential to the success of erosion-mitigation efforts at archaeological sites (for example, checkdam 

construction, slope stabilization). The time scales on which 

these processes and mitigation methods are relevant in a particular location may vary from less than an hour (the duration 

of a rainfall event that can carve a new gully or further incise 

an existing one) to decades or more (the time scale on which 

the migration distance of large aeolian dunes becomes important). To develop a far-sighted, longer term approach to in situ 

preservation of sites throughout the river corridor, it is necessary to document in detail the geomorphic and stratigraphic 

context in which these sites were built and in which they were 

preserved. Such information is essential to the formulation 

of management decisions, including those that involve dam 

operation, with the aim of maintaining and restoring sedimentary deposits in which sites are located. The time scale 

on which these research topics remain relevant ranges from 

days (during which the effect of flow fluctuations, including 

a beach/habitat-building flow [BHBF], may be measured) to 

many years or decades, as anthropogenic impact on the river 

corridor continues.

In her recent comprehensive assessment of culturalresource issues in the river corridor, and of the research 

approach that is needed to best address the preservation of 

archaeological sites, Fairley (2003) has posed a series of 

research questions that encompass complex multi-dimensional aspects of geomorphic process and history, paleoclimate and ecology, and cultural interaction with the Grand 

Canyon landscape. Four of those research questions are particularly relevant to the present study: 

(1) What do the sedimentary structures and grain sizes 

in Holocene deposits reveal about the relative dominance of 

aeolian versus fluvial processes in prehistory, and how might 

these processes have influenced the preservation or erosion of 

archaeological sites in the past?

(2) What types of geomorphic processes were active on 

the landscape before settlement, and what processes created 

the landscape on which sites were subsequently located?

(3) How have postdepositional processes altered or 

removed portions of the archaeological record at sites?

(4) What do postdepositional sediments [those that postdate site formation] reveal about landscape changes since the 

time of occupation?

Stratigraphic and geomorphic data such as those collected 

during this investigation are essential to formulating answers 

to these and other research questions, which seek to better 

define the context in which prehistoric people lived in Grand 

Canyon. Understanding the complex interaction between the 

people who lived along the river corridor and their physical 

environment has great significance for enhancing archaeological studies of this area. The same types of data in turn form the 

basis by which management decisions must be made that have 

the potential to preserve or restore archaeologically significant 

sediment deposits. 

During the course of this study, the importance of answering the four research questions listed above on a site-by-site 

basis became particularly apparent. The relative dominance 

of fluvial, aeolian, and slope-wash sedimentation can differ 

widely between study sites and can vary substantially between 

stratigraphic sections within any one study site. It is difficult, 

and not necessarily informative, to form regionally generalized conclusions about the geomorphic processes that created 

the landscape before and after settlement, or to generalize the 

effects of postoccupation processes on archaeological site 

preservation, without actual detailed field study at a site in 

question. For example, the answers to these questions attained 

by our work at Arroyo Grande (dominance of fluvial and 

slope-wash deposition in various areas of site G:03:064, with 

arroyo incision having removed portions of features) cannot 

be assumed to affect other sites in western Grand Canyon, or 

even sites directly across the river, in exactly the same manner. 

Observations Related to Aeolian Sediment

Because this research team was originally assembled 

for the specific purpose of investigating the role of aeolian 

sediment in the preservation of archaeological sites, we offer 

conclusions on that topic based on information obtained 

during this stratigraphic study and from earlier reconnaissance 

work done in 2003. As an initial reconnaissance effort for this 

project, personnel from the USGS, NPS, Hopi and Hualapai Tribes, GCMRC, and the Western Area Power Authority 

(WAPA) participated in a river trip from May 4 to 19, 2003. 

NPS archeologists J. R. Balsom, L. M. Leap, and J. L. Dierker 

directed visitation of 38 archeological sites at which earlier 
29

monitoring work by NPS had indicated that aeolian sediment 

may have been significant, either as the material underlying the site or forming a protective cover. At each site, the 

sedimentologists made initial assessments of the depositional 

environments (aeolian vs. fluvial) on which the sites were 

located. Although a few of these sites are underlain by sediment deposited by slope-wash processes or by side-canyon 

floods, most sites were built on (or buried by) sand that was 

transported by wind from Colorado River deposits. Following 

the May 2003 reconnaissance trip, the list of study sites was 

narrowed to identify areas of highest priority that would then 

be examined within our budget and time constraints. The Palisades, Lower Comanche, and Arroyo Grande investigations 

described in this report were three of those areas selected for 

further study. 

Observations made during the May 2003 reconnaissance 

trip included the following:

1. A preliminary estimate is that roughly 90 percent of 

the sediment beneath or above these particular archaeological 

sites is Colorado River sand that was transported from fluvial 

deposits at the river margins by wind. A few sites were built on 

or buried by sediment deposited directly by large floods of the 

Colorado River. (These sites were selected for the purposes of 

this reconnaissance trip and are not typical of all sites.)

2. Aeolian transport appears to be limited largely by 

factors such as sediment supply and vegetation, rather than by 

wind. Reduction in the size of sand bars, increased vegetative 

cover, and increased submergence of bars can be expected to 

reduce aeolian transport of sand to the nearby dune fields that 

contain archaeological sites. Reduced aeolian transport can be 

expected to facilitate the growth of existing arroyos and establishment of new arroyos. At many of the sites visited in May 

2003, damage to archaeological sites by arroyo development 

and incision is apparent. Data and photographs collected by 

NPS archaeologists at repeated visits to many sites over a time 

frame of more than a decade support this observation. 

3. Some archaeological sites are threatened by small 

drainages that might be repaired by modest increases in aeolian sand deposition. Examples include sites visited at Lower 

Tanner and Palisades, where construction of brush check dams 

by archeological teams has slowed the incision of some gully 

systems; an increase in transport of aeolian sand to such areas 

is anticipated to further stabilize such archaeological features. 

Other sites (such as Arroyo Grande) are threatened by very 

large drainages that could only be filled by increases in aeolian 

transport larger than might reasonably be expected in the short 

term. 

4. The side walls of some drainages showed sedimentary structures recording prior gully erosion and prior filling by aeolian sand, but these structures showed filling of 

paleogullies that were smaller than the modern active arroyo 

channels.

Beginning in November 2003, six months after this reconnaissance river trip, several members of the research group 

(Draut, Rubin, Dierker, Fairley) installed weather stations at 

six locations along the river corridor that collect a continuous 

record of wind conditions and precipitation in the vicinity of 

selected archaeological sites. The purpose of these stations is 

to document wind speed/direction and measure aeolian sediment-transport rates using anemometers and sand traps. The 

work plan for these instrument stations calls for their continued operation until December 2005, after which time they are 

scheduled to be removed. Data collected at these weather stations will be discussed in a separate report.

Because the relative significance of various sedimentary 

and geomorphic processes can differ widely between sites, 

specific causes of archaeological-site erosion, and specific 

measures needed to rectify erosion, must be determined on a 

site-by-site basis. We propose a series of questions that must 

be answered to determine the extent to which aeolian sediment 

deposition affects the preservation potential of archaeological 

sites. The first two questions in this series are:

(1) What is the depositional context of sediment on which 

the site is built? 

(2) What is the depositional context of sediment that has 

buried/protected the site?

These two questions require detailed field analyses of the 

stratigraphic and geomorphic context of each site, using the 

methods described in this report for the work at Palisades, 

Lower Comanche, and Arroyo Grande. The results described 

in this report are intended to answer these research questions 

for the three locations considered. To move toward developing 

a plan for mitigation of archaeological-site degradation, additional questions follow. If the answer to question 2 is aeolian 

sediment, one asks:

(3) Is there evidence for loss of aeolian sediment that previously covered the site?

Establishing whether or not aeolian-sand cover has been 

lost from a site provides a means by which to gauge the risk 

or degree of site degradation. If no loss of aeolian sediment is 

suspected at a particular site, that site may not be at immediate 

risk of artifact loss; in such a case, it may not be necessary to 

continue with the remaining questions that identify the degree 

of sensitivity to dam operations that could have contributed to 

degradation. A loss of aeolian sediment at a site could be most 

accurately documented using repeated high-resolution survey 

mapping to quantify aeolian deflation and/or gully incision. If 

such methods are not available, loss of aeolian sediment can 

be qualitatively inferred using repeated ground-based photography and geomorphic and sedimentary evidence of deflation. 

Evidence for deflation can include pedestal development, lag 

deposits on the land surface, and exhumation of plant roots. 

Deflation of aeolian sand in an area that had formerly experienced aeolian deposition can result from continued wind action 

Discussion and Conclusions 
30 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

accompanied by reduced sediment supply in the source area 

from which the sand is derived. Therefore, if it can be established that degradation of the archaeological site is related to a 

loss of aeolian sediment cover, the next question that arises is:

(4) What is the source of the aeolian sediment that has 

buried the site?

Positive identification of the source area for aeolian sediment can best be made by documenting wind conditions at 

this location over a representative period of time. In this river 

corridor, the most prominent sources of new sediment from 

which the wind can mobilize sand are sand bars created by fluvial deposition of sediment (see, for example, Schmidt, 1990). 

It is also possible for large aeolian dune fields (which have 

developed by accumulation of windborne sand from fluvial 

deposits) to serve as source areas for other, secondary aeolian 

deposits located farther from the river. Records of wind speed 

and direction can be used to generate vector sums that demonstrate the potential for aeolian transport of sediment over time; 

this is the primary objective of data collection at the weather 

stations now in operation at six locations along the river corridor (mentioned above). Because wind conditions may vary 

widely from season to season, and even from year to year, the 

longer the time interval from which data are used, the more 

accurate net sediment-transport calculations and predictions 

will be. Once a source area has been identified on the basis of 

local wind conditions, it is necessary to ask:

(5) Has there been a demonstrated reduction in the source 

area from which this aeolian sand is derived?

Aerial photography and repeated high-resolution mapping can be used to identify historical changes in the amount 

of sand available in the source area for aeolian transport. A 

demonstrated reduction in the open-sand area at the source 

indicates that less sand would therefore be available for transport toward and redeposition at the archaeological site in question. The definition of “open sand” (sand that is available to be 

transported by wind) is somewhat loose, because multiple factors affect the entrainment of sand by wind to varying degrees. 

Aeolian sand transport is limited by the presence of vegetation (Olson, 1958; Bressolier and Thomas, 1977; Ash and 

Wasson, 1983; Wasson and Nanninga, 1986; Buckley, 1987; 

Bauer and others, 1996), interstitial moisture in the sediment 

(Sarre, 1988, 1989; McKenna Neuman and Nickling, 1989; 

Namikas and Sherman, 1995; Wiggs and others, 2004), trapping of sand by gullies (Bauer and others, 1996), nonerodible 

surface objects such as rocks (for example, Gillette and Stockton, 1989), and cryptogamic soil crusts (Leys and Eldridge, 

1998; Belknap, 2001; Goossens, 2004). A reduced potential 

for aeolian entrainment of sand from a given source area could 

therefore be manifested in the form of increased vegetation on 

the source-area sand bar, an increase in the moisture content 

of the exposed sand area (caused either by flow fluctuations or 

precipitation), increased exposure of rocks that obstruct wind, 

or development of cryptogamic soil crust on the source-area 

sediment. If a loss of open sand in the identified source area is 

inferred, one needs to ask:

(6) Could renewed deposition of aeolian sand have a substantial restorative effect on this site?

Sites at which renewed aeolian deposition would “substantially” improve the preservation of cultural features would 

be those at which the greatest threat of degradation is from 

deflation of sediment cover by wind, from incision by gullies 

small enough to be healed by wind-blown sand (those on the 

order of <1 m wide, judging by occasional exposures of filled 

gullies observed on the land surface and in stratigraphic sections by this research group; see also Thompson and Potochnik, 2000). Aeolian sedimentation is not expected to protect 

(in terms of its ability to prevent complete loss of the site) 

archaeological features that are more threatened by incision of 

a major gully, arroyo, or side-canyon channel than they are by 

the loss of aeolian sediment. 

In the case of sites that have been compromised by migration of aeolian dunes, leading to the destabilization of surfaces 

on which the sites were formed (for example, fig. 48), renewed 

deposition of aeolian sand during dune migration could rebury 

artifacts but would not lead to preservation of the site in its 

original, intact state if artifacts have migrated downslope. If 

the answer to question 6 is ‘yes,’ a final additional question 

that begins to involve larger-scale management decisions 

should be asked:

(7) How could this be accomplished?

This final question is obviously broad, but given that 

increased deposition of aeolian sediment at a particular site 

would (in our line of reasoning) be linked to an increase in 

sand available for aeolian transport from the source area, the 

question could be rephrased to ask, how could the source area 

for aeolian sediment at this site be enhanced? Because riverlevel sand bars constitute the largest source of new sediment 

that could be mobilized by wind, an increase in the area of 

open sand available on river-level sand bars would enhance 

the potential for aeolian sand transport and redeposition in 

the vicinity of the archaeological site considered. In Grand 

Canyon, management options include restoration of open-sand 

area on river-level sand bars throughout the river corridor 

using BHBF flows (Bureau of Reclamation, 1995). Behavior 

of a particular sand bar that is of interest as an aeolian-sediment source during a BHBF flow may be gauged using aerial 

photographs or mapping conducted before and after the 1996 

BHBF. However, future BHBF flows may induce local aggradation or degradation patterns that are not identical to those of 

the 1996 flow, if sediment concentration and flow operation 

are different from the 1996 controlled flood experiment. 

Additional management considerations related to the 

observations of this study are discussed below, as they involve 

processes not limited to aeolian sedimentation.
31

Potential Implications for Management 

Objectives

Observations from the three study areas discussed here lead 

to several implications for management action that could potentially help to restore the condition of some archaeological sites. 

First, based on observations relevant to question 1 above, large 

areas of fluvial deposits, such as those at Palisades and Arroyo 

Grande, form the most substantial substrate on which archaeological sites are located (see Hereford and others, 1996). The 

river discharge level represented by high-elevation flood deposits such as the well-defined fluvial terraces at Palisades and 

Arroyo Grande is substantially above the 1,270 m3/s (45,000 

ft3/s) 1996 BHBF flow and of the 2004 flood experiment (1,160 

m3/s; 41,000 ft3/s). In order to effect large-scale restoration of 

the fluvial terraces in these two (and other) areas, a flow level 

comparable to or exceeding the 1921 flood stage (4,810 m3/s, 

or 170,000 ft3/s) would likely be needed. In order to produce 

the desired deposition instead of erosion, a flood of this magnitude would also require very high sediment concentrations in 

the flow (see modeling work of Wiele and Franseen, 1999). An 

experimental flow of this magnitude is not currently within the 

management considerations of the Glen Canyon Dam Adaptive 

Management Program, and it is likely that sediment concentrations would be too low in a flow with such high discharge to 

have a substantial restorative effect. Were a sediment-rich, highdischarge flood to occur, a large volume of new high-elevation 

fluvial deposits could then act as a source for aeolian sediment, 

which could help preserve and protect archaeological sites in 

areas dominated by aeolian sediments (such as the large dune 

field at Lower Comanche or smaller coppice dune accumulations that overlie fluvial terrace sediment at Palisades and 

Arroyo Grande). Generating a major new fluvial deposit as a 

source for aeolian sand is anticipated to produce a much greater 

benefit to the condition of aeolian deposits than creating smaller 

sources of aeolian sand by simpler methods (for example, 

removing vegetation from selected channel-margin sand bars).

As discussed by previous researchers, Glen Canyon Dam 

operations are considered to have a potentially large effect on 

the condition of fluvial deposits (see, for example, Hereford and 

others, 1993a). By extension (Thompson and Potochnik, 2000), 

the operation of the dam has the potential to affect the condition 

of aeolian deposits as well. In contrast to fluvial and aeolian 

sediment sources, deposition of locally derived sediment has the 

least potential to be affected by Glen Canyon Dam operations. 

Local sediment delivery occurs at the sites studied in the form 

of slope-wash events, playa deposition, and distal debris-flow 

runoff. All of these processes occur irrespective of processes 

controlled by dam operations. It is possible, however, that the 

deposits produced by local sedimentation events may vary in 

their extent and location from the predam condition if, as Hereford and others (1993) and Thompson and Potochnik (2000) 

proposed, the base level onto which local sediment is delivered 

has been altered by dam operations. If the configuration of 

aeolian dunes in a dune field (such as the one at Palisades) has 

changed, or if a new gully has breached the dune field, local 

slope-wash events may produce additional gully incision that 

drains to the river rather than collecting in ponded areas to produce the playa deposits commonly seen at Palisades. The loss of 

locally derived sediment would have the greatest effect farthest 

from the river, where the proportion of local sediment is highest 

(see discussion of Palisades and Arroyo Grande stratigraphic 

sections, above). 

With respect to the archaeological sites investigated during 

this work, locally derived sediment was not found to play a 

volumetrically significant role in site preservation relative to 

the protective cover provided by thicker fluvial and aeolian 

deposits. However, this research group has observed exposures 

in other areas of the river corridor in which aeolian and locally 

derived (slope-wash and debris-flow) sediments interbed, such 

that a thin cover of coarse, poorly sorted local sediment forms 

a resistant cap that protects more easily erodible aeolian sediment beneath (Hereford and others, 1996). Examples include 

exposures at the mouths of small tributaries at river mile 60.7 

and at Monument Creek, river mile 93.9. By extrapolation, the 

potential reduction in local sediment delivery that could follow 

a reconfiguration of local drainage patterns might lead eventually to reduced preservation of aeolian sediment. 

Future Research Directions

It is apparent from the results presented here that sedimentary and geomorphic processes that affect archaeological 

site stability and preservation can vary widely even within a 

relatively small area. At Lower Comanche, some cultural features face erosion because of arroyo incision, while others have 

become (or have the potential to become) unstable because 

of dune migration processes. Treatment procedures designed 

to mitigate erosion at archaeological sites such as these, and 

others, depend greatly on site-specific parameters. Mitigation 

measures that increase the preservation potential of one site may 

be ineffective at others. To this end, the formulation of successful mitigation strategies must depend on detailed cooperative 

field investigations involving both archaeologists and geologists. 

Such a joint research approach could be used to further 

elaborate on the sequence of events that has led to the repeated 

fluvial-subaerial ‘flood couplet’ stratigraphy described at Arroyo 

Grande. Radiocarbon dating of charcoal and ash material in 

many of the stratigraphic horizons that represent subaerial exposure, both in cultural features (hearths) and in dispersed occurrences that likely represent grass fires, could be used to establish 

a chronology for flood events in the western canyon. The potential for detailed dating work in these deposits warrants further 

investigation, if this can be accomplished without unacceptable 

risk to the integrity of cultural material and if the collection of 

carbon samples from this particular area would be acceptable 

to the Hualapai Nation. The frequency of flood events, and of 

burning incidents during interflood times, has the potential to 

produce a high-resolution record of flood occurrence and to eluDiscussion and Conclusions 
32 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

cidate the timing of human occupation at this site (if grass fires 

were indeed deliberately set), issues discussed by Fairley (2003) 

as relevant to understanding human interaction with the prehistoric Grand Canyon landscape.

As Fairley (2003) has stated, cultural resource management in Grand Canyon National Park could benefit from more 

detailed quantification of erosion of archaeologically significant 

sediment deposits. Resolution of dam-induced erosional effects, 

a key component of both geomorphic and cultural-resource 

investigations, could be greatly facilitated if quantitative records 

of erosion rates were available from the predam time, but such 

data are not available, nor are erosion rates systematically quantified on a regional scale during geomorphic or archaeological 

monitoring work today. Erosion rates and processes could be 

definitively established through the more widespread use of 

repeated high-resolution survey mapping (which has been done 

at a number of locations by NPS and other research groups, 

as exemplified by Yeatts, 1996), the use of long-term camera 

placement, and the continued use of instrument stations to measure precipitation, wind magnitude and intensity, and aeolian 

sediment-transport rates. The Thompson and Potochnik (2000) 

restorative base-level hypothesis, and their predictive model for 

terrace erosion, could be tested systematically in representative 

river- and terrace-based drainage systems using similar methods. The strengthening or refutation of the restorative base-level 

hypothesis through quantitative testing of this model could 

generate significant progress in formulation of management 

strategies for the river corridor, given the implications of this 

model for relating geomorphic processes and archaeological site 

preservation to dam operations. 

Acknowledgements

This project has been sponsored by the U.S. Geological 

Survey and Bureau of Reclamation through the Grand Canyon 

Monitoring and Research Center (GCMRC). Permission to 

conduct the research was granted by Grand Canyon National 

Park under Research and Collecting Permit GRCA-2003-SCI0101. We are grateful to Loretta Jackson, Hualapai Tribal Historic Preservation Officer, for authorizing the work to occur at 

Arroyo Grande. We thank boatmen Brian Dierker, Steve Jones, 

and Kirk Burnett for their valuable logistical support during 

river trips in May 2003 and May 2004, and Carol Fritzinger 

of GCMRC for coordinating those trips. Jan Balsom, Chief of 

Cultural Resources at Grand Canyon National Park, has been 

instrumental in the development and planning of this research. 

Michelle Rubin provided field assistance during the 2003 and 

2004 river trips. Mike Dai and Tom Porter of the Arizona Water 

Resources District assisted with surveying operations and logistics at Arroyo Grande. Nancy Andrews of NPS contributed to 

the data preparation for monitoring work at archaeological sites. 

Erin Todd analyzed a sample of the Lower Comanche white 

layer under a petrographic microscope. This work has benefited 

from insightful discussion and comments by Ted Melis, Mary 

Barger, Loretta Jackson, and Richard Hereford. We thank Curt 

Storlazzi and Jon Warrick for their technical reviews of this 

manuscript, and Peter Stauffer for thorough editorial reviews 

and revision.

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37

Stratigraphic Sections

Palisades, Section 1

Stratigraphic heights in this exposure were originally measured 

in cm above a datum at the west end of the outcrop (on the 

arroyo floor). They have been converted here to cm below 

the ground surface, with the original measurements from 

R. E. Hunter’s notes listed in parentheses. A stratigraphic 

diagram for section 1 is shown in figure 32.

0 – 6 cm (116 – 124 cm), Unit A: Ground surface sediment. 

6 – 14 cm (110 – 120 cm), Unit B: This unit consists of 

poorly exposed sandy silt, sloping toward the arroyo. 

14 – 25 cm (99 – 110 cm), Unit C: This very fine silty sand 

unit contains silt laminae.

25 – 29 cm (95 – 99 cm), Unit D: Unit D is a reddish silt.

29 – 40 cm (85 – 95 cm), Unit E: This horizon is a very fine 

silty sand with fluvial ripple bedding.

40 – 53 cm (71 – 84 cm), Unit F: Planar lamination in this 

fine sand indicates probable aeolian deposition. One sample 

was collected from this layer for grain size analysis: Pal 

Strat 1, Unit F. 

53 – 64 cm (60 – 71 cm), Unit G: This unit consists of very 

fine to medium sand, with fluvial ripple bedding apparent. 

Eddy pulsation ripples (Rubin and McDonald, 1995) occur 

at 54 cm below the ground surface (70 cm original recorded 

depth). At 56 cm below the surface (68 cm originally 

recorded depth) a gray muddy silt occurs that is 3 mm 

thick. One sample was collected from unit G for grain size 

analysis: Pal Strat 1, Unit G.

64 – 76 cm (48 – 60 cm), Unit H: This horizon consists of 

fine sand with planar laminations and is interpreted as an 

aeolian deposit.

76 – 84 cm (40 – 48 cm), Unit I: This is a fine- to mediumgrained sand with occasional small pebbles and fluvial 

ripple bedding. The uppermost part of unit I contains a 1-

cm-thick gray silt layer with ripple structures.

84 – 105 cm (19 – 40 cm), Unit J: Unit J is a very fine to 

medium sand, with occasional small pebbles and fluvial 

ripple structures. A 3-mm-thick reddish silt is present at the 

top of this horizon.

105 – 124 cm (0 – 19 cm), Unit K: This lowermost unit 

contains fine to medium sand with planar lamination and 

cross-lamination in sets as thick as 10 cm. At the base of the 

section, the tops of scattered cobbles are exposed.

Section 1 summary.—This low-elevation exposure in the wall 

of the main arroyo trunk indicates multiple episodes of 

fluvial deposition and aeolian reworking of that sediment. 

Occasional pebbles incorporated into the upper parts 

of fluvial deposits are interpreted to reflect reworking 

processes at the land surface. 

Palisades, Section 2

A stratigraphic diagram for section 2 is shown in figure 33. 

Measurements are in cm below the ground surface.

0 – 9 cm (Unit A): This uppermost unit is an aeolian sand 

with soft, single-grain dry consistency, very friable when 

moist. Its lower contact is abrupt and smooth.

9 – 13 cm (Unit B): Unit B sediment is soft to slightly hard 

when dry, very friable when moist. Its lower boundary is 

abrupt and wavy. Fluvial climbing ripples are visible (see 

discussion of stage-discharge relationship for the Palisades 

area in main text). This unit consists of Colorado River 

flood sand. 

13 – 36 cm (Unit C): This unit is tan colored with a slightly 

reddish color in the lower 10 cm. Soft, single-grain in the 

upper part when dry, slightly hard in lower portion when 

dry; unit C is friable throughout when moist. There are 

thin, indistinct bedding planes visible; these dip mainly in 

Appendix 1: Descriptions of Stratigraphic Sections and Geomorphology 

at Archaeological Site Locations, Palisades Area

K – fluvial deposit

H – aeolian deposit

G – fluvial deposit (fluvial and wave ripples).

 Thin silt near top

F – aeolian deposit

E – fluvial deposit with ripples

D – reddish silt

C – laminated silt

B – poorly exposed sandy silt

A – surface sediment

50

100

0

Depth (cm)

J – reworked fluvial deposit w/ thin silt at top

I – reworked fluvial deposit w/ thin silt at top

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 32. Stratigraphic diagram for section 1 at Palisades.
38 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

the onshore direction and slightly downstream. Unit C is 

interpreted as aeolian sand.

36 – 55 cm (Unit D): Unit D is a reddish fine sand, with 

fluvial climbing ripples faintly visible. Sediment is hard 

to very hard when dry, friable when moist, and weakly 

cemented. Salt weathering is visible in the upper part of 

this horizon. Ripple migration direction has an offshore 

component; this is interpreted as a Colorado River fluvial 

deposit.

55 – 58 cm (Unit E): This is a very light colored fine sandy 

silt, deposited in water. Lenses of cross-laminated sand are 

present.

58 – 70 cm (Unit F): Unit F is poorly sorted with granules and 

coarse sand in its lower part, grading to fine sand with silt 

interbeds in the upper part. This horizon is tan colored, and 

appears to be the product of local deposition (slope-wash 

deposit).

70 – 81 cm (Unit G): This very poorly sorted, matrixsupported horizon contains dominantly fine sand but with 

clasts up to granule sizes; occasional small pebbles are 

present. Sediment is red and hard to very hard when dry, 

friable when moist. Sediment is locally derived; unit G 

may represent local distal debris-flow sedimentation. This 

unit may correspond to unit F of section 3 and unit E of 

section 4.

81 – 82 cm (Unit H): This horizon consists of muddy silty 

clay deposits. A 1-cm-thick lens of paper-thin laminae 

is present. Unit H was interpreted in the field as playa 

sediment. 

82 – 87 cm (Unit I): This is a very fine sand, horizontally 

bedded with red silty clay interbeds. Unit I is interpreted as 

aeolian deposition with episodic brief playa inundation.

87 – 91 cm (Unit J): Unit J is a fine sand, thinly bedded and 

lenticular.

91 – 94+ cm (Unit K, to base of section): This is a reddishcolored very fine sand and silt, with no sedimentary 

structures visible.

Section 2 summary.—This stratigraphic section consists of 

aeolian, fluvial, possible playa, locally derived slope-wash 

and distal debris-fan deposits. Aeolian sediment makes up 

the greatest thickness within this section (~39 percent), 

with fluvial and slope-wash/distal debris-fan sediment 

contributing ~28 percent and ~25 percent respectively. 

Multiple episodes of aeolian deposition followed by 

deposition of locally-derived sediment are evident from 

interbedding of those deposits (for example, units H and I). 

Aeolian deposition (with subsequent deflation and arroyo 

cutting) has been active most recently at this location. The 

most likely source for aeolian sediment at this location is 

reworked fluvial sediment. Two flood deposits are preserved 

in this section (units B and D), with a third waterlain deposit 

(unit E) possibly also representing a Colorado River flood.

Palisades, Section 3

A stratigraphic diagram for section 3 is shown in figure 34. 

Measurements are in cm below the ground surface.

0 – 30 cm (Unit A): This is a clean, fine to medium sand with 

well defined thin bedding, an aeolian deposit. There are 

several very thin, irregular red silt beds that may represent 

playa-wetting events. The lower contact is very irregular; 

disruption of the contact may be related to formation of salt 

crust.

30 – 39 cm (Unit B): Unit B is a fine to medium sand of 

uncertain depositional origin, alternating with thin silty 

clay beds. These may be locally reworked fluvial sands, 

interrupted by what appears to be occasional playa 

inundation. The upper contact of unit B is very irregular. 

Occasional vertical, silty-clay-filled desiccation structures 

originate from the upper contact.

39 – 46 cm (Unit C): This horizon consists primarily of fine 

sand, with occasional clay laminae that are thought to 

represent playa wetting. Occasional vertical, silty clay-filled 

desiccation structures originate from the upper contact of 

unit C.

46 – 73 cm (Unit D): Unit D is primarily a fine sand; its 

K – red silt/very fine sand

J – thinly bedded sand

I – aeolian with playa interbeds

H – playa sediment

G – distal debris flow

F – slope-wash deposit

D – fluvial deposit

C – aeolian deposit

B – fluvial deposit

A – aeolian deposit

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

20

0

Depth (cm)

40

60

80

E – waterlain sandy silt

Figure 33. Stratigraphic diagram for section 2 at Palisades.
39

redness and clay content increase up-section. Very thin 

beds of silty clay are present at a depth of 54 cm that likely 

represent playa sedimentation. The lower contact of this 

unit is abrupt, erosional, and very prominent. Unit D is 

interpreted as an aeolian deposit.

73 – 95 cm (Unit E): This is a cemented red sand that initially 

appears massive but, after drying, shows very faint bedding 

and cross lamination. A thin, white bed of silty fine sand 

occurs at approximately 85 cm. This depositional unit is 

truncated in the landward direction. There is a charcoal 

concentration at approximately 76–77 cm that contains 

diffuse ash material (not a cultural hearth feature). The red 

color and the presence of charcoal indicate that this unit is 

most likely the result of locally derived sediment deposition 

(possibly a playa layer).

95 – 108 cm (Unit F): The sediment in this horizon fines 

upward from coarse sand and gravel to a medium sand. 

Minor charcoal is present. This horizon is likely equivalent 

to unit E of section 4, which occurs at a comparable depth 

below the land surface at that site. This unit may also 

correlate with unit G of section 2. Unit F is most probably a 

distal debris-flow deposit.

108 – 124 cm (Unit G): This is a fine sand interbedded with 

thin, red, silty sands. Desiccation fractures originate in 

at least five levels within this unit. Lenticular bodies of 

cross-laminae are present. Thicker sandy beds appear to be 

aeolian, with small rivulets crossing the playa area.

124 – 156+ cm (Unit H, to base of section): This is a fine 

sand, in which no sedimentary structures are evident. Its 

origin is uncertain. One sample was collected from this 

horizon: Pal Strat 3, Unit H (145 cm depth).

Section 3 summary.—This stratigraphic section is located 

farther from the river than any other section studied at 

Palisades. The stratigraphy in section 3 contains numerous 

deposits of locally derived sediment, in the form of multiple 

thin playa beds and coarser distal debris-flow deposits. 

Playa sedimentation is represented by a volumetrically 

minor part of this section but has occurred frequently, 

judging from the interbedding of playa units with aeolian 

and possible fluvial strata. There is no unambiguous 

evidence for interpreting a Colorado River flood deposit 

in this section on the basis of sedimentary structures; flood 

events may be represented here by thin sand beds (distal 

flood deposits) that interbed with the playa horizons (units 

B, C, G, and possibly H).

Palisades, Section 4

Units A, B, and the upper part of unit C in section 4 are shown 

in figure 35. A stratigraphic diagram for section 4 is shown 

in figure 36. Measurements are in cm below the ground 

surface.

0 – 35 cm (Unit A): Unit A contains two very red, mud-rich 

beds (clay loam) separated by clean, fine-grained sand. The 

uppermost 10–12 cm of unit A are vesicular. This uppermost 

horizon in this section consists of playa deposits.

35 – 76 cm (Unit B): Fluvial climbing ripples occur in the 

upper 7 cm of unit B (35–42 cm depth below the land 

surface). Between 37 and 39 cm, the ripple migration 

direction is upstream, indicating flow in an eddy. At 67 cm, 

the ripple migration direction is dominantly offshore, with 

a downstream component. This unit is a Colorado River 

alluvial deposit. Three sediment samples were collected 

from this unit: Pal Strat 4, Unit B #1 (75 cm depth); Pal 

Strat 4, Unit B #2 (67 cm depth); Pal Strat 4, Unit B #3 (48 

cm depth).

76 – 86 cm (Unit C): Unit C consists of dark brown muds, 

interpreted as possible nearshore deposition.

86 – 92 cm (Unit D): This unit is a clean, light-colored fine 

sand; its depositional environment is ambiguous.

92 – 107 cm (Unit E): Unit E is a bright red, poorly sorted 

horizon of clay to very coarse sand. This may correspond 

to red gravelly sand distal debris-flow deposits observed 

elsewhere (for example, unit F of section 3, and possibly 

also unit G of section 2). One sample was collected from 

unit E: Pal Strat 4, Unit E (99 cm depth).

107 – 114+ cm (Unit F, to base of section): This basal unit 

H – structureless fine sand

G – aeolian/playa interbeds

F – distal debris flow

D – aeolian deposit

C – playa deposit

B – interbedded playa and

possible fluvial deposit

A – aeolian/playa deposit

Depth (cm)

50

100

0

150

E – playa/subaerial deposit

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 34. Stratigraphic diagram for section 3 at Palisades.

Appendix 1: Stratigraphic Sections and Geomorphology, Palisades Area
40 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

F – fluvial deposit

D – fine sand

C – dark brown mud

B – fluvial deposit

A – interbedded playa silt, fine sand

Depth (cm)

E – distal debris flow

50

100

0

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 35. Photograph of part of section 4 measured at Palisades. Unit A represents 

multiple episodes of playa deposition. Unit B is a Colorado River flood deposit (note 

fluvial climbing ripples). Unit C contains dark brown muds that may represent nearshore 

deposition.

Figure 36. Stratigraphic diagram for section 4 at Palisades. 
41

is a very fine tan-colored sand or sandy silt. Horizontal 

laminations and cross-laminations are present, with fluvial 

climbing ripples. This is a Colorado River fluvial deposit. 

One sample was collected from this unit: Pal Strat 4, Unit F 

(114 cm depth).

Section 4 summary.—Section 4 contains two Colorado River 

flood deposits with well-defined fluvial climbing ripples 

(units B and F). Two additional floods may be present in 

this deposit (units C and D); alternatively, units C and D 

may represent different phases of deposition during one 

flood event. Locally derived sediment also contributes 

significantly to this profile, with a distal debris-flow deposit 

(unit F) and a 35-cm-thick upper horizon (unit A) that 

represents multiple episodes of playa sedimentation.

Palisades, Section 5

Section 5 was a test pit 0.8 m deep, located on the edge of 

the playa area. A detailed stratigraphic log was not made 

for this pit. It was noted that the sediment in section 5 

was very different in appearance from that of section 4, 

approximately 70 m away. In contrast to the red-colored, 

playa-dominated sediment in the upper part of section 4, 

section 5 contained poorly consolidated, pale beige-colored 

sediment. No sedimentary structures were visible. This 

deposit was interpreted as a remnant of a fluvial terrace 

that is now surrounded by playa deposits; Hereford (1993) 

mapped a sediment deposit ~20 m from the section 5 

location as a remnant of the ‘ap’ fluvial terrace.

Stratigraphy and Geomorphology at Archaeological Site Locations

All archaeological sites discussed in this report are referred to 

by the site number assigned by NPS.

C:13:033—This site is constructed on debris-fan sediment 

originating from the unnamed tributary that debouches 

into the Colorado River on river left 400 m north of the 

Palisades Creek mouth. One small area of aeolian sand 

cover (maximum 10 cm thick, covering ~2 m2) is present 

immediately north of the room structure, apparently in 

the lee of the northern wall of the structure. With the 

exception of this small patch of sand, the remainder of the 

area consists of gravel, cobbles, and boulders related to the 

leveed debris fan immediately upslope and to the north of 

the site. The site is not buried by debris-flow sediment and 

apparently postdates the most recent debris-flow deposition 

in its immediate vicinity. 

C:13:098—This is the historic-era site that contains the 

Tanner-McCormick Mine and associated living structure 

and artifacts. A test pit was not attempted at this site. On 

the basis of field observations and aerial photographs, this 

site is within the boundary of the Palisades Creek debris 

fan. The area where this site lies was mapped by Hereford 

(1993) as within the geomorphic unit designated as ‘dfi’ 

(debris fan of intermediate age).

C:13:099—This is the lowest elevation archaeological site 

identified at Palisades. Clasts at the distal margin of the 

Palisades Creek debris fan (reddish gravel-rich colluvium) 

reach almost to the edge of this site. Immediately up-arroyo 

of feature 1 (next to NPS Cross-section #3 across the 

arroyo) are very fine-scale fluvial ripples. The upper portion 

of this flood deposit has been modified by aeolian processes 

and now forms a small dune surface (figure 37) such that 

~ 5 cm of aeolian sediment overlies ~ 6 cm of undisturbed 

fluvial deposit, which in turn overlies red-orange sediment 

interpreted as distal-debris-flow sediment. The fluvial 

sediment appears to be just overlying potsherds eroding out 

of the arroyo wall. The original depth of the archaeological 

material is difficult to determine. Feature 1 is built on 

Colorado River flood sediment and is buried by aeolian 

sediment (well-defined aeolian climbing-ripple structures 

are visible). Feature 3 (partly missing because of erosion) 

is located in a gravel unit that represents distal debris-flow 

sediment from Palisades Creek. Feature 5 is built on red 

gravelly slope-wash sediment directly, but has an apparently 

younger dune partly covering it (it is built on slopewash sediment but buried by aeolian sand, which is now 

undergoing deflation). Sediment that partly buries feature 

5 is structureless medium-grained sand. It is likely that this 

site was reoccupied (and feature 5 constructed) after debrisflow sediment partly buried older occupation sites (such 

as feature 3). A driftwood line just below (riverward of) the 

Figure 37. Juxtaposition of aeolian and fluvial sediment at archaeological site C:13:99, Palisades. The arroyo wall exposed in the foreground has incised into several features of this site. A thin cover of 

light-colored aeolian sediment has buried artifacts at this location. 

The site was formed on top of fluvial sediment and distal colluvium 

from the Palisades Creek debris fan, both of which are exposed in 

the arroyo wall.

Appendix 1: Stratigraphic Sections and Geomorphology, Palisades Area
42 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

site, adjacent to feature 5, has an elevation consistent with 

that of the 1958 Colorado River flood (3,060 m3/s; 108,000 

ft3/s). Summary: several features are partly buried by aeolian 

sediment, but were built on material that originated from 

Colorado River floods and distal colluvium from the nearby 

debris fan. The site is affected by arroyo incision (artifacts 

exposed in the northernmost branches of the main arroyo 

system).

C:13:100—This site occupies a partially deflated coppice 

dune area in the region mapped as ‘ec’ (eolian, coppice) 

by Hereford (1993) and is within the limit of the driftwood 

line believed to represent the 1921 Colorado River flood 

(4,810 m3/s; 170,000 ft3/s). No clear sedimentary structures 

were apparent in most of the sand/silt material that has 

filled in structures at this site, but in the northernmost 

structure, parallel-bedded sediment indicates likely aeolian 

deposition. Aeolian reworking of flood-derived sediment is 

likely. Feature 4 (vertical walls of sandstone from the Dox 

Formation outlining room blocks) is exposed in a shallow 

arm of the main arroyo network. A driftwood log directly 

above this wall, exposed in the gully that is being filled by 

migrating dune sand, is at the same elevation as the 1921 

driftwood line surveyed during this work. Aeolian sediment 

is filling in the arroyo as a sand dune approximately 2 m high 

is actively migrating to the north (approximately up-canyon; 

figure 38). Feature 7 was identified eroding out of an incision 

into fluvial/aeolian sediment in the area mapped as ‘lmt’ by 

Hereford (1993) and is apparently both built on and buried by 

fluvial sediment that has undergone aeolian modification. 

C:13:101—This site is located on an apparent fluvial cobble 

bar just riverward of the former location of the Beamer 

Trail. Driftwood present at this site is at an elevation 

consistent with the 1957 Colorado River flood event (3,540 

m3/s; 125,000 ft3/s). Riverward of this site (and the old 

Beamer Trail) is a relatively flat area with thin sedimentary 

cover over the river cobbles that probably represents a 

flood surface. Immediately landward of this site and the 

old Beamer Trail are aeolian dunes that have apparently 

undergone deflation. Sedimentary structures in one dune 

that is adjacent to the former Beamer Trail indicate aeolian 

stratification dipping to the north. This dune sediment 

consists of well-sorted fine to medium sand, and the 

north-dipping aeolian climbing ripples indicate northward 

dune migration. New artifacts have been exposed as dune 

migration and deflation occur. The depositional environment 

of sediment on which the site was constructed cannot be 

confidently ascertained, although rounded fluvial cobbles 

appear to underlie many of the artifacts riverward of the old 

Beamer Trail. Artifacts are buried by little or no sediment; 

where overlying sediment is present, it occurs in the form of 

small coppice dunes.

C:13:272—Gravel, cobbles, and boulders related to the 

Palisades Creek debris fan appear to underlie features at 

this site, though sand and silt form an extensive cover 

of variable thickness over these larger clasts (figure 39). 

Figure 38. Sand dune migrating northward (upcanyon; toward the 

left in this east-facing photograph), filling a small gully at the location 

of archaeological site C:13:100, Palisades. A driftwood log (far left 

of photo), partially buried by aeolian sand on which vegetation has 

grown, lies at the same elevation as a driftwood line identified as the 

1921 flood stage.

Figure 39. Thin sedimentary cover at archaeological site C:13:272, at 

Palisades. In the foreground are cobbles associated with a habitation 

structure at this site. Mesquite in the background has grown on sediment that partly covers the site. Sedimentary structures are absent in 

this sediment; its color and consistency suggest a fluvial origin rather 

than locally derived material from the Dox Formation (Stevenson 

and Beus, 1982). Aeolian reworking is evident in this deposit; a partly 

buried driftwood log is exposed at the northern end of this site in the 

same sedimentary unit. The sedimentary cover is inferred to be a 

fluvial deposit that has been reworked by wind into coppice dunes on 

which the mesquite has grown. This is within the ‘lmt’ (lower mesquite 

terrace) unit mapped by Hereford (1993). 
43

Features are apparently constructed on the distal debrisflow sediment, though possibly on the finer sand that covers 

the boulders and cobbles and provides a more habitable 

surface. The majority of the area covered by this site is 

within the ‘lmt’ unit mapped by Hereford (1993). Sand 

and silt have filled in several room-block features at this 

site. Sand and silt deposits in this area probably originated 

from Colorado River fluvial deposition during floods but 

have been modified by aeolian activity, evident from the 

modern dune morphology. No sedimentary structures are 

visible in the cryptogamic, bioturbated sediment that covers 

features of this site, but the surficial geomorphology is 

indicative of small (up to 1 m high) aeolian dunes that have 

undergone deflation, colonization by grasses and mesquite, 

and cryptogamic crust development. The color and 

consistency of the sediment cover suggests a fluvial origin, 

but the degree of sorting and the relatively coarse grain size 

(dominantly fine sand, with little silt present) are suggestive 

of aeolian reworking. This site lies immediately riverward 

of a driftwood line believed to represent the landward extent 

of the 1957 Colorado River flood (3,540 m3/s; 125,000 

ft3/s). Logs from this driftwood line are partly buried 

by cryptogamic surficial sediment apparently related to 

migration of aeolian sand dunes. 

C:13:334—This site is exposed on the surface of the playa 

deposits at the landward side of the Palisades area, although 

the artifacts appear to have eroded out of a remnant of a 

sediment deposit at slightly higher elevation than the current 

playa surface (mapped as ‘ap’ terrace by Hereford, 1993). 

There is little to no aeolian sediment cover apparent at the 

artifact location.

C:13:336—An exposure of sediment in a gully wall adjacent 

to this site revealed fluvial ripple structures; the elevation 

at this location is between the 1957 (3,540 m3/s; 125,000 

ft3/s) and 1884 (5,940 m3/s; 210,000 ft3/s) driftwood 

limits (with logs at the inferred 1884 flood level partly 

buried immediately landward of the site; 1921 driftwood is 

not clearly present at this location). Aeolian modification 

of this flood sediment is apparent, with coppice dunes 

on the surface (consistent with the ‘ec’ map unit (eolian, 

coppice) in this area by Hereford, 1993). The deflated 

morphology and cryptogamic crust on these dunes indicate 

relative inactivity at this time. The nature of the sediment 

on which the site was originally constructed is not clear, 

as the site consists largely of artifacts scattered on the 

ground surface. Burial by fluvial sediment (with discharge 

at the 1921 levels and greater) is inferred, as is subsequent 

aeolian modification of flood-deposited sediment. A minor 

contribution by fine-grained, pinkish interdune/playa 

sediment is apparent in the sediment cover also.

C:13:355—This site is located just south of an arroyo network 

between the Palisades Creek debris fan and the debris fan 

of the unnamed tributary 400 m north of Palisades Creek. 

Sediment in the arroyo walls is largely structureless and 

affected by significant bioturbation, but locally it contains 

fluvial climbing ripples that indicate deposition during 

Colorado River flood events. The high silt content and light 

color of the majority of the structureless areas of arroyowall sediment indicates derivation from the Colorado 

River, but without sedimentary structures this cannot be 

confirmed for much of the deposit. Slope-wash gravel and 

distal debris-flow sediment is also apparent in the arroyo 

walls, in greater proportions at the eastern (landward) end 

of the arroyo. The mouth of this arroyo enters the Colorado 

River in the large eddy on river left where a large sand bar is 

commonly present. Access to the arroyo mouth is restricted 

by very heavy vegetation. Two of the four features of site 

C:13:355 (features 1 and 4) were constructed on distal 

debris-flow sediment from Palisades Creek, with very minor 

modification on the surface by likely aeolian sediment (10–

12 cm thick). This thin aeolian sediment deposit at the site 

shows no sedimentary structures and is heavily cryptogamic, 

indicating very little active aeolian transport at this time. 

Feature 2 is similarly built on colluvial gravels and cobbles, 

near Feature 1. One additional feature of this site (feature 3) 

is located at lower elevation and is now affected by incision 

of the arroyo network mentioned above. Feature 3 is eroding 

out of an arroyo wall that contains thinly bedded fluvial 

ripples in cross-section. Radiocarbon samples collected 

from feature 3 by R. Hereford during excavation of this site 

yielded dates of A.D. 1030–1240, 1010–1210, and 1450–

1640 (Hereford, 1993). At this location, the stratigraphy of 

the arroyo walls also shows slope-wash and possible debrisflow gravels in addition to the fluvial sediment. Feature 3 

appears to have been built on and buried by fluvial sediment, 

in an area affected by slope-wash and debris-flow deposition. 

Appendix 2: Descriptions of 

Stratigraphic Sections and 

Geomorphology at Archaeological 

Site Locations, Lower Comanche Area

Stratigraphic Sections

Lower Comanche, Section 1

Section 1 is located beside an archaeological site that contains 

a roasting mound eroding into the arroyo drainage. The face 

of this stratigraphic exposure was not cleaned off or studied 

in detail, to avoid damaging the feature. From existing 

exposure, it is apparent that section 1 contains slumping 

aeolian sediment that overlies poorly sorted sand and 

gravel channel-fill sediment dominated by sandstone clasts 

from the Dox Formation. The inferred aeolian sediment is 

approximately laterally continuous with the aeolian deposits 

described in section 2.

Appendix 2: Stratigraphic Sections and Geomorphology, Palisades Area
44 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

Lower Comanche, Section 2

The photograph in figure 40 shows the arroyo wall where 

section 2 was described. Figure 41 shows the stratigraphic 

log for section 2. Measurements are in cm below the ground 

surface.

Surface: The surface has a crust 2 mm thick with cryptogams. 

It is very thinly bedded, with both horizontal and inclined 

beds. Surface material is fine sand, slightly hard, and is 

interpreted as slope-wash sediment and tributary alluvium, 

with a minor aeolian component.

0 – 15 cm (Unit A): Unit A contains primarily fine sand, with 

silt. No ripple structures are visible; bedding is dominantly 

horizontal, though some is inclined, and varies from 

indistinct to clear. The lower contact is abrupt and flat. 

This layer is interpreted as reworked slope-wash sediment 

and tributary alluvium, possibly with a minor aeolian 

component. 

15 – 51 cm (Unit B): Bedding in unit B varies from clear to 

indistinct. Sediment is very thinly bedded sand and silt, with 

bedding horizontal to subhorizontal. No cross-stratification 

is present. Three substrata exist within this horizon:

15 – 28 cm (Subunit B1): Fine sand, with indistinct 

bedding.

 28 – 40 cm (Subunit B2): This stratum has numerous silt 

laminae; bedding is subhorizontal. Silt is very soft and friable, with numerous 1-mm-wide rootlet pores filled with 

calcium carbonate. 

40 – 51 cm (Subunit B3): Very slightly harder than B2, 

this substratum has indistinct bedding. It consists of fine 

sand with a minor component of medium sand. The lower 

boundary is abrupt and flat.

51 – 78 cm (Unit C): Unit C contains distinctly laminated and 

indistinctly bedded sediment. Individual beds vary in color 

between tan and reddish. In indistinctly bedded strata, the 

texture is fine sand with a minor medium-sand component. 

There are occasional to many 1-mm-wide carbonate-filled 

rootlet pores. Indistinctly bedded horizons are redder and 

contain small, fine-sand-size lithic fragments from the Dox 

Formation. These are less well sorted. Distinctly laminated 

beds are light in color with numerous paper-thin laminae. 

They are relatively well sorted and contain numerous 

carbonate-filled rootlet pores. These appear to be tributary 

alluvial deposits. Laminae are subhorizontal, subparallel, 

and locally cross-stratified, but no climbing ripples are 

observed. Occasional charcoal fragments occur that are as 

much as 3 mm in diameter. Laterally, unit C grades into 

channel facies, filled with channel-fill gravels derived 

from the Dox Formation. The lower contact is abrupt, 

and erosional. One sample was collected from unit C for 

grain-size analysis: Com Strat 2, Unit C (sample depth not 

recorded).

78 – 79 cm (Unit D): This is the white layer observed 

throughout the Lower Comanche study area. In this locality, 

this stratum does not display the three phases observed 

elsewhere, but rather consists of a thin single bed that 

Figure 40. Stratigraphic section 2 at Lower Comanche, an exposure 

in the arroyo wall. The white layer is visible at a depth of 78 to 79 cm 

below the surface (arrow).

50

100

0

Depth (cm)

H – channel fill

G – channel fill, with gravel lenses

F – channel fill

E – channel fill, with gravel lenses

D – white layer

A – aeolian/slope-wash sand and silt

C – aeolian/slope-wash sand and silt

B1

B2

B3

B – thinly bedded sand and silt

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 41. Stratigraphic diagram for section 2 at Lower Comanche.
45

pinches out laterally into gravelly alluvium. This surface 

locally has 20–30 cm of topographic variation in this 

exposure. The lower boundary appears to be conformable, 

and there is no evidence of erosional activity (truncation 

of laminae). The upper surface is, however, eroded, which 

probably accounts for the absence of the upper two phases 

of this white-layer unit. Its thickness varies between 0.5 and 

1.5 cm locally.

79 – 83 cm (Unit E): This is one of several westward 

(riverward)-migrating channels representing paleoarroyo 

features in this locality. Thinly bedded silt laminae are 

present, primarily horizontal, though inclined at the edge of 

the channel. Silty fine sand occurs where this stratigraphic 

section was measured, but this interbeds laterally with 

channel gravels locally derived from the Dox Formation. 

The lower contact of unit E is abrupt, representing one stage 

in channel filling, but does not appear to be significantly 

erosive. This unit, which occurs as a horizon as much as 50 

cm thick in a nearby channel-fill exposure, is the edge of 

channel-fill deposits.

83 – 87 cm (Unit F): The facies within unit F strongly 

resembles unit E above (channel-fill material), with the 

exception that cross-stratification is observed in the channelfill deposits. This stratum also grades locally into channel 

deposits derived from the Dox Formation and locally 

reaches 50 cm in thickness. One sample was collected from 

unit F for grain-size analysis: Com Strat 2, Unit F (sample 

depth not recorded).

87 – 132 cm (Unit G): The texture of unit G is primarily fine 

sand but with occasional small pebble-gravel lenses. The 

lowermost deposits of this stratum (which are generally 

not observed) are gravel. Sediments at the top and bottom 

of unit G appear to be slightly cemented and are redder 

in color than the lower deposits of this unit. This horizon 

represents channel fill and in places is significantly thinner 

than at the recorded section location.

132 – 135+ cm (Unit H, to base of section): This lowermost 

stratum contains very fine sandy channel-margin deposits. 

These are thinly bedded, with laminae in some places and 

indistinct bedding in others. These sediments are interpreted 

as tributary alluvium and appear to have been modified 

by wetting and drying related to ground water, producing 

contorted liesegang-like oxidation bands in some areas. This 

would imply a relatively high water table, which is difficult 

to explain in this particular location.

Section 2 summary.—This profile represents various facies 

of tributary alluvium deposited in and adjacent to a series 

of eastward (shoreward)-migrating channels, which may 

interbed with sediment originally derived from fluvial 

and/or aeolian deposits. The white-layer horizon was 

deposited at a time when the deepest channels had largely 

filled but a gravel-bottomed channel still persisted adjacent 

to the bedrock valley wall. Additional tributary alluvium 

was deposited above the white layer, although associated 

channels are not as apparent or as deep as those from earlier 

times (at least until the modern arroyo-cutting episode). 

Sediments contained within this alluvial stack are unusually 

fine, and, with the exception of gravel-bottomed channels, 

contain very limited amounts of bedrock-derived sediment. 

The source of alluvial sediments is believed to be reworked 

aeolian deposits from upstream and valley-slope areas. 

The relationship of these sediments to modern coppice 

accumulations is not clear.

Lower Comanche, Section 3

The pit from which section 3 was described is shown in figure 

42. Figure 43 shows the stratigraphic profile for section 3. 

Measurements are in cm below the ground surface.

Surface: This profile is a pit dug into a flat land surface with 

sparse vegetation. The surface shows evidence of recently 

ponded water (abundant mudcracks) over a large area, 

and crust formation 2–2.5 mm thick. Minor mud curls are 

present; cryptogamic development is present but weak, and 

short grasses (cheat grass) have colonized the area.

0 – 5 cm (Unit A): Uppermost sediment in this section 

contains soft, very friable (to single grains) coarse, weakly 

subangular, blocky fine sandy silt. Sand content is estimated 

to be approximately 25–30 percent, with the remainder silt. 

Considerable bioturbation, primarily by rootlet growth, 

has obscured most sedimentary structures. Minor crosslamination is visible, but this unit is generally massive. The 

lower contact is clear to sharp, and wavy. An aeolian origin 

is inferred; the unit appears to be a loose aeolian cover on 

the surface.

5 – 20 cm (Unit B): Unit B is a silty fine sand that contains 

subhorizontal, paper-thin laminae throughout that may be 

related to wetting episodes. Sand content is estimated to be 

~75 percent, with the remainder silt. Sediment is friable, 

and is slightly more consolidated than unit A. No crossbedding is visible. The lower contact is somewhat arbitrary, 

and has been placed at 20 cm, in a zone where silt laminae 

become more prominent and frequent. These laminae may 

be remnants of a wetted surface, weathering out to appear 

more consolidated than the sediment above or below. Unit 

B is interpreted as a likely aeolian deposit on the basis 

of its texture and lamination. This may represent aeolian 

deposition with periodic wetting.

20 – 40 cm (Unit C): Lamination in unit C shows some 

disturbance attributed to bioturbation in the upper 3 cm 

of this horizon. There is a broad channel in unit C with a 

maximum depth of 8 cm that has lamination in the channelfill sediment (cross-lamination is not present). Scattered 

pebbles and granules occur in the channel-fill material, 

which may represent a deflation or lag deposit. Silt laminae 

are present at the base of the channel fill, but they become 

indistinct toward its edges. Sediment in the channel is 

dominantly soft, fine sand and silt with occasional pebbles 

and granules. Bedding both inside and outside the channel 

Appendix 2: Stratigraphic Sections and Geomorphology, Lower Comanche Area
46 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

fill is very thin to laminated, and approximately horizontal. 

There is a poorly defined central zone within unit C (outside 

and below the channel) ~7 cm thick that is slightly more 

consolidated and massive than the sediment above and 

below. This central consolidated sediment has indistinct 

minor lamination. Unit C contains ~25 percent fine sand, 

with the remainder silt (and rare pebbles). The unit coarsens 

downward, with the lowermost third of unit C significantly 

coarser than the sediment above it (fine to medium sand). 

Indistinct very thin subhorizontal bedding is apparent in 

this lower portion of unit C; silt laminae are fewer than in 

the upper part of the unit. The lower fine and medium sand 

is very soft and friable (to single grains). The unit’s lower 

boundary is defined at several prominent silt laminae that 

are better consolidated than the sediment above or below.

40 – 46 cm (Unit D): Bedding in unit D is much more distinct 

than in overlying unit C, with numerous subhorizontal 

paper-thin silt laminae. This horizon is very silty; no crosslamination is apparent. Some of the laminae in the lowest 

2 cm of unit D consist of material from the white-layer 

horizon. Laminae increase in frequency downsection just 

above the white layer. The boundary with the white layer 

(unit E below) is somewhat abrupt but gradational. Unit D 

appears to be aeolian, but deposited when standing or slowmoving water was present (judging from well-defined silt 

lamination and texture). 

46 – 48 cm (Unit E): Unit E in this section is the white layer 

observed elsewhere in the Lower Comanche area. Section 3 

is the northernmost location where it has been documented 

during this study. The top part of the horizon here is very 

well laminated. Its base is sharp in terms of composition, 

but deposition within this unit appears gradational. The 

three phases identified in this white layer in the more 

southern stratigraphic sections at Lower Comanche are 

also present in section 3 (a central, slightly darker, sandier 

layer is present within it). The lowest phase of the white 

layer contains small micropores from rootlet channels (scale 

1–3 mm), with vegetation occupying the channels. Its total 

thickness is 1.5 to 2.0 cm. The sediment is friable, very 

fine and powdery when crumbled; grain size is visually 

estimated to be fine silt. Small fragments of mica are visible 

within it.

48 – 63 (77) cm (Unit F): This sediment immediately below 

the white layer is silty fine sand. The upper 8 cm of unit F 

is similar to unit D, with frequent paper-thin silt laminae 

that decrease in frequency downsection. In places, some of 

those silt laminae appear to be composed of silt similar to 

that of the white layer (unit E). The contact between units 

E and F does not appear to be burrowed; the silt laminae 

in uppermost unit F may possibly be related to an early 

stage in the deposition of the white layer. The lamination 

immediately below the white layer is interpreted to reflect 

frequently wetted sediment. The rest of unit F (below the 8 

cm laminated upper region) appears much more massive; 

lamination disappears, and the sediment becomes slightly 

more consolidated and fines downward into fine sandy silt. 

The lower contact of unit F is inclined, as a small mound 

is apparent in the contact between units F and G in the 

southern corner of the section 3 pit. Where unit F overlies 

this mound, that unit is thinnest (15 cm thick); its maximum 

thickness (at the northern corner of the pit, away from the 

mound) is 29 cm. 

63 – 77 cm (Unit G): This fine to medium sand unit is present 

only in the southern corner of the pit, as a mound up to 14 

Figure 42. Section 3 at Lower Comanche. The white layer is visible, 

spanning a depth of 46 to 48 cm below the land surface.

50

0

Depth (cm)

H – slope-wash and aeolian sediment

G – sand mound

F – fine sand, silt with wetting

evidence

E – silt; white layer

D – probable aeolian, with evidence of wetting

C – probable aeolian; contains filled channel

B – probable aeolian sediment

A – probable aeolian sediment;

 evidence of repeated wetting

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 43. Stratigraphic diagram for section 3 at Lower Comanche.
47

cm thick. The mound that defines unit G is significantly 

coarser than overlying unit F. The contact between units F 

and G at this mound surface is abrupt and inclined. Unit G 

is massive but contains at least one inclined bedding plane 

(dipping off the mound, subparallel to its upper surface). 

Unit G fines downward. Its lower contact with unit H 

is abrupt and inclined toward the north (dipping off the 

mound). 

77 – 87+ cm (Unit H, to base of exposure): Unit H is a 

poorly sorted fine to very coarse sand that contains some 

clasts as large as granule size. Numerous lithic clasts are 

present; sandstone clasts from the Dox Formation can be 

identified, among other lithologies. Some stratification is 

present, but it is poorly defined. This unit is inferred to be a 

combination of aeolian and slope-wash sediment. 

Section 3 summary.—This profile represents episodes of 

aeolian and slope-wash sediment deposition and reworking, 

affected by repeated wetting events. The flat land surface 

into which this pit was dug is still an area that collects 

standing water, as evidenced by abundant mudcracks over 

a large area on the modern land surface. This location 

represents the northernmost site where the white-layer 

horizon has been observed.

Lower Comanche, Section 4

The stratigraphic diagram for section 4 is shown in figure 44. 

Measurements are in cm below the ground surface.

Surface: Section 4 is located ~10 m shoreward of section 6. 

Section 4 is exposed in a wall of the arroyo; fluvial deposits 

in the upper part of section 6 can be observed to pinch out 

in a shoreward direction before reaching the location of 

section 4.

0 – 13 cm (Unit A): The uppermost horizon consists of buffcolored sand and silt, with no discernible sedimentary 

structures. 

13 – 55 cm (Unit B): Unit B is a silty sand, with faint parallel 

bedding. Its color is slightly pinker than that of units A and 

C, above and below it, respectively.

55 – 93 cm (Unit C): Unit C is a buff-colored sandy silt with 

some parallel bedding structures. 

93 – 94 cm (Unit D): Unit D is the white layer observed in 

other areas at Lower Comanche, of unknown origin.

94 – 151 cm (Unit E): Unit E is a buff-colored silty fine sand, 

with some parallel bedding. 

151 – 197 cm (Unit F): This horizon contains coarse sand and 

gravel, derived locally from the Dox Formation, of which 

abundant lithic grains are present. Unit F contains a channel 

structure ~1.5 m wide, which has been filled with gravel 

from the Dox Formation. At the location of section 4 the 

channel base occurs at 186 cm; from 186 cm to 197 cm, the 

base of unit F, sediment consists of coarse sand and gravel 

dominated by lithic fragments from the Dox Formation.

197 – 213 cm (Unit G): This horizon consists of parallelbedded coarse sand and gravel, dominantly derived from the 

Dox Formation.

213 – 238 cm (Unit H): Unit H contains buff-colored sand 

and silt, with little apparent sedimentary structure.

238 – 255+ cm (Unit I, to base of section): This lowermost 

unit contains dominantly silt and sand, but with interspersed 

lithic fragments from the Dox Formation as much as 1 cm 

across. 

Section 4 summary.—Section 4 is interpreted to contain a 

combination of aeolian deposits (silt and sand) with locally 

derived sheet-wash deposits (gravel and coarse sand grains 

from the Dox Formation). The absence of diagnostic 

sedimentary structures in much of the silt/sand stratigraphy 

indicates that an aeolian origin cannot be confirmed; an 

alternate explanation for the parallel-bedded silt and sand 

units may be sheet wash (upper plane bed) redeposition 

of this finer sediment as a result of local precipitation and 

runoff events. The channel-fill sediment in unit F contains 

gravel (from the Dox Formation) as well as buff-colored silt 

and sand material.

Lower Comanche, Section 5

The stratigraphic diagram for section 5 is shown in Figure 45. 

Measurements are in cm below the ground surface.

Surface: This section is located at low elevation, closer to the 

river than any other section studied at Lower Comanche. 

Surface sedimentary characteristics are obscured by very 

heavy vegetation cover dominated by mesquite trees. 

0 – 48 cm (Unit A): The uppermost sediment in Section 5 is 

silty sand with well defined parallel bedding. This unit is 

interpreted to be most likely of aeolian origin; sedimentary 

structures are probably aeolian climbing ripple forms.

48 – 58 cm (Unit B): Unit B consists of medium to coarse 

sand, with a large proportion of lithic clasts derived from 

the Dox Formation. The upper and lower contacts of this 

unit dip landward (east). 

58 – 98 cm (Unit C): This unit contains pinkish fine sand, 

with beds of buff-colored very fine silty sand or sandy 

silt. Bedding is horizontal, with stratification disrupted or 

bioturbated. This unit could be either of fluvial or aeolian 

origin; disturbance of sedimentary structures makes the 

interpretation ambiguous.

98 – 101 cm (Unit D): Well-defined fluvial climbing ripples 

are present in this silty very fine sand horizon, interpreted as 

a Colorado River flood deposit. Troughs of the ripples are 

filled with very fine sediment (mud/silt/clay). 

101 – 122+ cm (Unit E, to base of exposure, which ends ~25 

cm below the arroyo floor): This lowermost unit of section 

5 consists of very poorly sorted sand that contains sandstone 

clasts from the Dox Formation. The general grain size is fine 

sand, but the clasts from the Dox Formation are as large as 

coarse sand. Subhorizontal stratification is visible. Deposition 

Appendix 2: Stratigraphic Sections and Geomorphology, Lower Comanche Area
48 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

was likely dominated by slope-wash sedimentation, possibly 

with aeolian sediment incorporated also. 

Section 5 summary.—Section 5 is the closest to the river of 

the measured profiles. One Colorado River flood deposit 

is clearly apparent (unit D); bioturbation and other surface 

modification of bedding render ~30 percent of the thickness 

in this section ambiguous (unit C). The rest of the section 

represents sedimentary processes operating at the land 

surface: aeolian deposition and local runoff are apparent in 

units B and D.

Lower Comanche, Section 6

The stratigraphic diagram for section 6 is shown in figure 46. 

Measurements are in cm below the ground surface.

Surface: The land surface above section 6 is cryptogamic, 

deflated, and locally heavily vegetated (the section is 

located under mesquite cover). All units in this profile are 

heavily bioturbated, with small rootlet pores and large roots 

disturbing stratification in every horizon.

0 – 42 cm (Unit A): Unit A is a bioturbated very fine sand 

with organic laminae. Lamination is approximately 

conformable with the land surface. This is a probable 

aeolian deposit. A concentration of granule-size charcoal 

fragments is visible that defines the lower contact of this 

horizon.

42 – 56 cm (Unit B): This is a bioturbated very fine sandy 

silt to silty very fine sand. Stratification is present, with 

some beds as much as ~2 cm thick. Some beds have a 

pinkish color, while others are more gray. Some beds dip 

riverward (west), while others are subhorizontal. Finer scale 

lamination is locally apparent that appears parallel to larger 

scale beds that dip riverward; these are possible Colorado 

River fluvial ripples, though the stratification and potential 

climbing ripple structures are faint and only locally visible. 

The lower contact of unit B is corrugated on a mm scale, 

with unit B displaying a slightly pinker color at the contact 

while unit C is slightly grayer. Some of the beds within unit 

B are also corrugated on a mm scale (most likely reflecting 

bioturbation). One sediment sample was collected from this 

horizon: Com Strat 6, Unit B.

56 – 80 cm (Unit C): Unit C is a silty fine sand that contains 

fine-scale laminations that dip generally landward (east). 

50

100

0

Depth (cm)

E – aeolian and locally derived

 slope-wash sediment

D – fluvial deposit, rippled

C – disturbed aeolian or fluvial;

 ambiguous

B – locally derived sediment

A – aeolian deposit

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 45. Stratigraphic diagram for section 5 at Lower Comanche.

50

100

0

Depth (cm)

150

200

250 I – sand, silt, and Dox lithic grains

H – structureless silt and sand

G – locally derived sand and gravel

F – Dox sand/gravel,

channel fill

E – silty fine sand

D – white layer

C – fine sandy silt

B – silty sand

A – structureless silt and sand

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 44. Stratigraphic diagram for section 4 at Lower Comanche.
49

Some of the stratification is parallel to the set boundary, 

while some dips landward (as if in an upstream migration 

direction). The lower part of unit C contains less clear 

stratification, though approximately parallel bedding is 

visible. The lower boundary of unit C is a thick organic 

concentration. The upper part of unit C is interpreted as 

Colorado River fluvial deposit. The lower half of this 

unit may be either fluvial or aeolian; stratification is too 

disturbed to distinguish diagnostic sedimentary structures.

80 – 81 (90) cm (Unit D): This horizon is a heavily organicrich layer of variable thickness (ranging from <1 to 10 cm) 

that represents a buried land surface. The large mesquite tree 

immediately above section 6 has formed roots in this layer 

where it outcrops ~3 m riverward of the measured profile. 

90 – 103 cm (Unit E): This is a very fine sandy silt. The lower 

half of unit E contains well-defined parallel lamination; 

in the upper half, lamination is much less distinct and 

is disrupted. The lower, well-laminated half pinches out 

landward of the exposure and may represent a former river 

channel pinching out laterally against the riverbank. In the 

offshore direction, this unit can be traced laterally for >6 m 

from the profiled area, where well-defined fluvial climbing 

ripples become apparent. Unit E is a fluvial deposit, though 

the upper half may reflect reworking of flood sediment at 

the land surface (degrading fluvial sedimentary structures). 

The lower contact of unit E, defined by a thin organic 

concentration, climbs upward to the landward side of the 

outcrop, consistent with the pinching out of a river channel 

against its bank. Unit E has thinned to almost no thickness 

within 1 m landward of the measured section, and thickens 

riverward. One sediment sample was collected from unit E: 

Com Strat 6 Unit E.

103 – 121 cm (Unit F): This unit, which is separated 

from unit E above by a thin organic-rich boundary, is 

better consolidated than unit E. Sediment is a sandy silt 

with small root holes in it. Bedding is subparallel and 

discontinuous; some stratification is wavy, approximately 

horizontal, and disrupted. The contact (organic layer) 

between units E and F is wavy, with 3–4 cm of topography 

(a disrupted surface that probably represents reworking of 

unit F sediment at the land surface). Depressions in this 

contact between units E and F are filled with sediment of 

unit F type. Stratification in unit F includes lamination 

that dips landward but not necessarily at the scale of 

subaqueous ripple structures. This could be aeolian 

material, though the upper 14 cm of unit F appear to locally 

contain faint fluvial ripple structures. Some lamination is 

approximately horizontal, while some dips landward very 

subtly. Faint ripple forms in the upper part of this unit 

appear to migrate riverward and slightly out of the outcrop 

plane (upstream) in one small area. The lower contact of 

unit F is defined by a slight increase in consolidation. One 

sediment sample was collected from this unit: Com Strat 6 

Unit F.

121 – 143 cm (Unit G): The upper boundary of unit G is 

marked by a subtle downward increase in consolidation; 

its lower contact is defined by the appearance of distinct 

subparallel laminae. The upper 5 cm of this horizon contain 

disrupted beds. Subparallel lamination is present that 

dips landward; three sets of such bedding are visible. The 

depositional environment of unit G is not clear. 

143 – 145 cm (Unit H): A 2-cm-thick silt bed defines unit 

H, which contains wavy, subparallel lamination that dips 

landward at a low angle. This may represent the distal 

edge of a Colorado River flood deposit, on the basis of its 

similarity to other flood-related silt beds observed at Arroyo 

Grande. The lower boundary of this layer is the white layer 

(unit I).

145 – 146 cm (Unit I): Unit I is the white-layer bed observed 

in other profiles at Lower Comanche. In this section it is 

0.5 to 1.0 cm thick, with a mottled, burrowed base that is 

indistinct. The top is also gradational, but less so than the 

base.

146 – 153 cm (Unit J): This is a massive, poorly stratified 

silty sand that has been modified by burrowing activity 

and apparent disturbance at the land surface. Its original 

depositional environment is assumed to be either aeolian 

or fluvial but cannot be confidently determined, given the 

extensive disturbance of sedimentary structures. The top of 

unit J is gradational, leading into the overlying white-layer 

horizon.

153 – 161 cm (Unit K): Unit K is a fine to medium sand 

with sand-size lithic fragments of sandstone from the Dox 

M – probable aeolian sediment; land surface

 with 4-cm-thick channel

L – probable aeolian sediment with charcoal

K – slope-wash sediment

J – disturbed aeolian or fluvial; ambiguous

I – silt; white layer

H – possible fluvial deposit

G – depositional setting ambiguous

F – possible fluvial deposit (upper part)

 but structures ambiguous

D – organic layer; buried land surface

C – upper part fluvial; lower part fluvial or aeolian

B – possible fluvial deposit

 (structures ambiguous)

A – aeolian deposit

50

100

0

Depth (cm) 150

E – fluvial deposit pinching out against bank

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Organic

horizon

Figure 46. Stratigraphic diagram for section 6 at Lower Comanche.

Appendix 2: Stratigraphic Sections and Geomorphology, Lower Comanche Area
50 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

Formation. Parallel lamination is present; a slope-wash 

or arroyo-type (local runoff) depositional environment is 

inferred.

161 – 168 cm (Unit L): This locally charcoal-rich horizon 

contains horizontal paper-thin laminae, some of which are 

wavy. Some bedding appears to be aeolian stratification 

that dips landward. Unit L is interpreted as likely 

aeolian sediment that has undergone modification and 

incorporation of charcoal material at the land surface. 

Dominant grain size is silty very fine sand.

168 – 188+ cm (Unit M, to base of outcrop at arroyo 

floor): This horizon contains dominantly silt, with some 

very fine sand. Very thin stratification is locally apparent 

and is suggestive of an aeolian deposit. The upper part 

of this unit contains some subhorizontal paper-thin wavy 

laminae. Some beds are pinkish, while others are more 

gray. A 4-cm-thick pebbly sand lens (medium to fine 

sand) occurs within unit M, which is filled by sandstone 

fragments from the Dox Formation that may be as large 

as >5 mm across. This sand lens likely represents a small 

rivulet channel incised into a largely aeolian deposit at the 

land surface. Many calcified white rootlets are present.

Section 6 summary.—This section, the second-closest to the 

river (after section 5) contains clear evidence (in the form 

of sedimentary structures) for at least one Colorado River 

flood deposit (unit E), and possibly as many as seven 

flood units if units with ambiguous sedimentary structures 

are in fact fluvial in origin (units B, C, E, the upper part of 

F, and possibly G, H, and J). Aeolian deposition is inferred 

for at least three of the horizons in this section (units A, L, 

and M, with possible aeolian sediment also occurring in 

units F, G, and J). Many of the units in this profile show 

evidence of having been exposed at the land surface, 

causing reworking, incorporation of locally derived 

sediment of the Dox Formation into fluvial or aeolian 

sediment, and disturbance of original stratification in these 

horizons.

Stratigraphy and Geomorphology at Archaeological Site Locations

C:13:273—This site contains several roasting features, one 

of which is one of the largest in eastern Grand Canyon. 

Arroyo incision to the north and east of the site will 

eventually threaten to undercut the larger roasting feature, 

while another has already been largely eroded by arroyo 

growth (fig. 47). Sedimentary exposure in the arroyo wall 

directly beneath the site indicates that the feature is built 

on alternating fine sand/silt and locally derived coarse 

sandstone slope-wash deposits from the Dox Formation. 

The site is at the modern land surface elevation, with a 

thin to absent aeolian sediment cover. Vegetation and 

cryptogamic crust development affect the modern surface. 

Excavation was completed in 1995 at the downstream side 

of C:13:273 and in 1997 at feature 5 of this site, which 

was located 70–130 cm below the surface and spanned a 

width of 40–85 cm (Leap, 1995; Matthews, 1997; Smith, 

1997; Yeatts, 1998). Two radiocarbon analyses, collected 

Figure 47. One of the roasting features, feature 3 (containing firecracked rock), at archaeological site C:13:273, Lower Comanche. The 

feature is affected by gully incision, as one branch of the main arroyo 

network has undercut much of this feature, causing artifacts to move 

downslope into the gully.

Figure 48. Roasting feature at archaeological site C:13:333 (within 

dashed line). This site, built on aeolian sand at high elevation in the 

large dune field at Lower Comanche, is affected by dune migration, as 

the dune on which it is built shows geomorphic evidence of northward migration (upcanyon, to the right of the photograph). Artifacts at 

this site have undergone some downslope movement (toward the left 

of the photograph) as sediment has been eroded by wind. It appears 

that the rocks of the roasting feature have armored the site to some 

degree; the rock mound shown faces south, downstream (into the 

direction of strongest winds at this site), apparently offering some 

protection to the sediment around it.
51

from the hearth of Feature 5, yielded dates of A.D. 620–

775 and 575–770 (Yeatts, 1998). This site is affected by 

visitor use of the Beamer hiking trail, though this is not 

perceived to have caused major impact (Leap, 1995).

C:13:274—This site is built on a thin accumulation of 

aeolian coppice-dune sediment that is now cryptogamic 

and deflated, near a large dead mesquite complex. The site 

is affected by minor gullying and deflation. The coppice 

dune is at most 1 m thick at its thickest part, and overlies 

distal debris fan sediment and reworked relict river 

cobbles derived from the older (believed to be Pleistocene) 

deposits at higher elevation to the east. The site is exposed 

at the modern land surface, apparently as a result of some 

aeolian deflation. 

C:13:333—Artifacts at this site are exposed by northward 

dune migration; the site is built on aeolian sediment 

and is now located within an elevated interdune area 

(not a true playa surface). Very minor cryptogamic crust 

development is apparent on the land surface, with minor 

deflation evident around local vegetation. The site appears 

to be located on or near its original surface elevation, but 

it is moving gradually downslope in response to dune 

migration. It appears that the fire-cracked rock associated 

with the roasting feature at this site may actually be 

armoring the site from more rapid deflation (fig. 48); some 

scour is evident around the base of the rock mound, and 

a small amount of sand forms a sand shadow in the lee 

(north) of this roasting feature. 

C:13:335—Cultural material at this location is exposed 

by deflation in the center of a large high dune (a small 

low area located at high elevation in the dune). This 

dune appears very active in general, although minor 

cryptogamic crust development has occurred in this 

deflated low area. Immediately to the south (downstream) 

of this site is a large active dune, across a small saddle 

from this site location. The slip face of that large active 

dune indicates migration to the north, toward the site. 

This site, considered relatively stable at present by NPS 

(National Park Service, 2004), is built on and buried 

by aeolian sediment, but it is being exposed by gradual 

deflation. 

C:13:337—This site, containing exposure of fire-cracked 

rock, is the northernmost site in the main Comanche dune 

field (fig. 49). The site was constructed in a small playa/

interdune area affected by dune migration and associated 

aeolian sediment transport. Recent ponding of water in 

this interdune area is evident; mudcracked sediment was 

visible in the vicinity of the site in May 2004. This site 

is presently considered by NPS to be relatively stable 

(National Park Service, 2004). The site is not currently 

buried by sediment, but is located in an area where aeolian 

sediment deposition associated with dune migration would 

be expected to eventually cover and re-expose these 

artifacts over time. 

C:13:373—This site is located at high elevation in the dune 

field and is eroding down the riverward side of a steep 

dune. It was originally built on aeolian sediment and is 

buried by a minor cover of aeolian material, though the 

site and its sediment cover have moved downslope as a 

result of deflation associated with dune migration. 

Appendix 3: Descriptions of 

Stratigraphic Sections and 

Geomorphology at Archaeological 

Site Locations, Arroyo Grande Area

Stratigraphic Sections

Arroyo Grande, Section 1

Section 1 was recorded in two parts, 1a and 1b, which are 

separated laterally by ~4 m (stratigraphic diagrams are 

shown in fig. 50). At both substations, the survey datum was 

placed at the base of aeolian sands. At both 1a and 1b, the 

upper 2.0–2.1 m of the section (including the uppermost 

~1.5 m, in which stratigraphic depths were not logged in 

detail) is aeolian sand. The sand is fine grained, not silty, 

and is characterized except in the upper part by subcritically 

climbing translatent strata (see Hunter, 1977a,b) that are 1–

Figure 49. Archaeological site C:13:337, consisting of fire-cracked 

rock concentrated under the tree. This site is built on an interdune/

playa area within the large dune field at Lower Comanche. This 

area is affected by aeolian sediment transport and dune migration 

processes; the site is presently considered relatively stable (National 

Park Service, 2004). 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area
52 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

3 mm thick and dip at angles of less than 10o. The upper 0.5 

m of the section has obscure or no visible stratification, and 

the upper structureless part grades down into the stratified 

part. The section below the aeolian sand differs in sections 

1a and 1b. Depths are recorded as depth below the base of 

aeolian sand. This aeolian sand is considered to be unit A in 

both sections 1a and 1b.

Section 1a

0 – 14 cm (Unit B): Unit B, which is separated from aeolian 

sand of unit A above by a sharp contact, contains silt or very 

fine silty sand. Sediment is laminated, with fluvial climbing 

ripples migrating toward 330o. The lower contact of unit B 

is gradational. This is interpreted as a Colorado River flood 

deposit. 

14 – 17 cm (Unit C): This is a clayey silt, whitish and 

consolidated, interpreted to be the basal unit of the 

overlying flood deposit. The base of this horizon is sharp. 

17 – 20+ cm (Unit D, to base of exposure): This unit contains 

silt or silty sand, with no apparent sedimentary structures.

Section 1b

0 – 8 cm (Unit B): This unit contains fine to very fine sand 

with planar lamination, truncated at an angle of less than 5o 

by contact with overlying aeolian sand. The base of unit B 

is gradational. The depositional environment is ambiguous.

8 – 45 cm (Unit C): Unit C consists of very fine sand, grading 

down into silt toward its base, with fluvial (Colorado 

River) climbing ripples that migrated toward the northwest 

(upstream). The base of this unit is gradational.

45 – 49 cm (Unit D): This unit is a clayey silt, whitish and 

consolidated, which probably forms the base of the flood 

deposit in unit C above. The base of this horizon is sharp. 

49 – 52+ cm (Unit E, to base of exposure): The lowermost 

unit exposed in section 1b is a coarse silt or very fine sand. 

It is micaceous, suggesting incorporation of locally derived 

sediment.

Six samples were collected from section 1: AG Strat 1 #1 

(example of aeolian sand with good ‘pinstriped’ climbingripple structure, sampled near Section 1a); AG Strat 1b #2 

(sampled silt from base of flood unit 0–1 cm above base 

of outcrop, slightly below arroyo floor); AG Strat 1b #3 

(sampled flood sediment 10–11 cm above base of outcrop); 

AG Strat 1b #4 (sampled flood sediment 20–23 cm above 

base of outcrop, in area that contains fluvial climbing 

ripples); AG Strat 1b #5 (sampled flood sediment 30–31 cm 

above base of outcrop); AG Strat 1b #6 (sampled 40–41 cm 

0

20

+40

Depth (cm)

Arroyo Grande, Section 1a D – structureless

silty sand

B – fluvial deposit

A – aeolian sand

50

+50

Arroyo Grande, Section 1b

E – locally derived sediment

D – basal unit of flood deposit

C – fluvial deposit

A – aeolian sand

0

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

C – basal unit of

flood deposit

 sand

B – fluvial or aeolian deposit, 

 ambiguous planar 

 laminations 

Figure 50. Stratigraphic diagrams for sections 1a and 1b, Arroyo Grande.
53

above base of outcrop; may be top of flood unit, or possibly 

in aeolian sand above the flood).

Section 1 summary.—This section (both parts 1a and 1b) 

is interpreted to include at least one, and possibly two, 

Colorado River flood deposits. Sediment with aeolian 

depositional structures makes up ~1 m of the section 

above the flood unit(s), indicating reworking of fluvial 

sediment into aeolian dunes. Above the well-defined 

aeolian sedimentary structures, another 30–60 cm of 

sediment thickness is poorly exposed or contains few 

diagnostic sedimentary structures. Considering the 

gradational upward transition from aeolian to structureless 

sediment, this uppermost structureless material may be 

bioturbated aeolian sediment.

Arroyo Grande, Section 2

Section 2 was logged in two parts, 2a and 2b, which are 

separated laterally by ~5 m (fig. 51). Section 2a is the 

upper portion of this profile, while 2b is the lower part 

(exposure was not accessible below 2a or above 2b); there 

is some overlap in stratigraphic depth between 2a and 2b. 

Depths are listed in reference to a survey datum within 

the section used for this exposure. An additional ~1 m of 

poorly exposed and unexposed sediment is present above 

the highest levels described in section 2a.

Surface: Deflated, cryptogamic soil cover with grasses 

and mesquite tree above profiled area. Slumped sand, 

silt, and gravel obscure the exposure of stratigraphy 

immediately below the surface at both sections 2a and 2b. 

Measurements are in cm above (+) and below an arbitrary 

datum in the section.

Section 2a

+50 – +38 cm (Unit A): The uppermost exposed sediment 

in unit A of section 2a is a fine to very fine sand. This 

material contains scalloped cross-bedding that youngs in a 

direction downstream with respect to the Colorado River. 

This unit is interpreted as aeolian dune sand.

+38 – +1 cm (Unit B): The upper part of unit B contains 

fine to very fine sand (the whole unit appears to coarsen 

upsection). Clear fluvial climbing ripples are present, with 

some horizontal stratification. The upper part of the unit 

is mostly well-stratified; some unstratified areas appear 

bioturbated. Ripple migration direction is upstream toward 

270° in one ripple set and toward 290° on another ripple 

0

10

+40 Depth (cm)

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Arroyo Grande, Section 2a

F – land-surface layer

E – lower part of fluvial couplet?

D – upper part of fluvial couplet?

C – fluvial deposit

B – fluvial deposit

A – aeolian deposit

50

100

+40

Arroyo Grande, Section 2b

G – fluvial deposit

F – land surface layer

E – couplet: thin

fluvial deposit

D – couplet: fluvial deposit

C – couplet: fluvial deposit B – fluvial deposit

A – fluvial deposit

Depth (cm)

Figure 51. Stratigraphic diagrams for sections 2a and 2b, Arroyo Grande.

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area
54 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

set. The lower portion of this inferred flood couplet is a 

light-colored fine silt bed (1–2 cm thick) at the base of 

unit B. Unit B is interpreted to be a couplet representing a 

Colorado River flood deposit.

+1 – 0 cm (Unit C): The base of unit C is the survey datum 

used to define stratigraphic depth in section 2 (a toothpick 

was used to mark this in the field). The upper part of the 

couplet is a brownish silt that is slightly coarser than the 

lower part. The base of unit C is a powdery, clean, whitish 

clayey silt; the unit has a bioturbated upper contact. Unit C 

represents an inferred fluvial deposit.

0 – 2 cm (Unit D): This unit is coarser than unit E below and 

consists of a brownish very fine sandy silt. The sediment 

is very well sorted and has been bioturbated. Unit D may 

represent the upper part of a flood couplet (the lower 

portion being unit E).

2 – 10 cm (Unit E): This is a light-colored bed, finer than 

unit F below. Paper-thin laminae are present in powdery, 

clean silt, although bioturbation obscures some of the 

stratification. Its upper contact is gradational, with a 1-cmthick pinkish band through the silt. Unit E is interpreted as 

a Colorado River flood layer and may be the lower part of 

a fluvial deposit that forms a couplet with overlying unit 

D.

10 – 20 cm (Unit F): Unit F is a brownish silty fine 

sand, with occasional disseminated charcoal fragments 

(of coarse sand size). There is crude, discontinuous 

horizontal stratification present, but this bedding is too 

poorly displayed to see real original bedding structures 

(possibly bioturbated). The upper contact is sharp. Unit F 

is interpreted as sediment that has undergone reworking 

and incorporation of charcoal while exposed at the land 

surface.

Section 2b

+44 – +1 cm (Unit A, to top of exposure where slumped, 

crusted sediment is present): Unit A contains well-defined 

fluvial climbing ripples. Ripple migration direction is 

upstream; this flood deposit is the same as unit B of section 

2a. There is a clear trend toward coarsening upward in this 

layer. This is interpreted as a Colorado River flood deposit.

+1 – 0 cm (Unit B): This thin flood couplet with a white silt 

base corresponds to unit C of section 2a. One sediment 

sample was collected from this layer: AG Strat 2b #1 

(sample in two parts): sample of silt (inferred Colorado 

River deposit) from unit B. 

0 – 8 cm (Unit C): The upper portion of this layer is gray to 

buff silty fine sand with a small amount of charcoal in it. 

The lower portion is a light-colored silt with paper-thin 

laminae. The upper contact of unit C is abrupt. This flood 

couplet may be the lateral continuation of a flood couplet 

defined by units D and E in section 2a. Unit C is interpreted 

as a fluvial couplet.

8 – 12 cm (Unit D): The lower part of unit D is a silt with 

paper-thin laminae, which grades upward into poorly 

stratified grayish silt with coarser charcoal pieces that 

suggest some reworking at the land surface. The upper part 

is only locally present, with a faint boundary between the 

upper and lower parts of the deposit. This is interpreted as a 

fluvial couplet.

12 – 13 cm (Unit E): The resistant lower silt portion of unit 

E (white, clean silt) is <1 cm thick. The silt is bioturbated, 

with several mm above it of coarser, disturbed sediment, 

similar to unit F below. Unit E is likely the very thin distal 

edge of a flood deposit couplet.

13 – 20 cm (Unit F): Unit F consists of a brown/gray silty 

fine to very fine sand with crude horizontal stratification and 

a small charcoal/ash concentration ~1 cm thick centered 

at a depth of 20 cm. The charcoal lens is bioturbated and 

spans just over 35 cm laterally. The upper contact of unit 

F is abrupt. The lower contact is inclined downstream and 

irregular; this unit thickens downstream, with the depth 

of the lower contact ranging from 20 to 35 cm. Unit F in 

section 2b is likely the same land surface that is described 

as unit F in section 2a, though section 2b contains evidence 

for more smaller floods between the reworking of sediment 

at the land surface in the two units F and the deposition of 

the next clear correlative layer between these two profiles 

(the large flood deposit referred to as unit B in section 2a 

and unit A in section 2b). Unit F is interpreted as sediment 

that has undergone reworking and modification at the land 

surface between Colorado River flood events.

20 – 105 cm (Unit G, to base of section at the arroyo floor):

The base of this layer is below the floor of the arroyo, 

so the complete thickness is not known. Well-defined 

fluvial climbing ripple structures are present in this fine to 

very fine sand, consistently migrating upstream. Ripples 

migrated toward 330o, indicating flow in an eddy during 

the flood event. Unit G is bioturbated and contains white, 

calcified rootlets; sediment is pinker than unit F above it. 

On the upstream side of the exposure, dune or bar foresets 

(crossbedding) are apparent that dip 28o toward 320o. 

Ripples were migrating toward 330o; this therefore suggests 

that the ripples were migrating over the crest of a bar form. 

The upper contact is defined by a color change from pink 

to brown/gray; pinker sediment likely reflects incorporation 

of locally derived sediment from slope-wash deposition. 

The upper contact is irregular, as described above, and is 

bioturbated. Unit G is a Colorado River flood deposit.

Section 2 (a and b) summary.—These two sections reflect 

multiple Colorado River flood events, with reworking of 

sediment at the land surface implied by the unit F in both 

sections. After deposition of unit F, several small fluvial 

deposits were emplaced, followed by a thick flood deposit. 

The exposure at section 2a demonstrates deposition of 

aeolian sand above that thick flood unit (likely aeolian 

reworking of sediment derived from flood deposits). 
55

Arroyo Grande, Section 3

A detailed stratigraphic log was not made for section 3. A 

stratigraphic diagram made based on a field sketch for 

section 3 is shown in figure 52.

Section 3 summary.—This profile appears to consist entirely 

of multiple sets of superposed flood couplets, despite the 

facts that (1) at least part of the profile was in a low channel 

at the time of deposition and (2) nearby profiles show 

multiple layers of slope-wash/channel gravels.

Arroyo Grande, Section 4

A photograph of the section 4 location is shown in figure 53; 

figure 54 shows the stratigraphic diagram for this section. 

Measurements are in cm below the land surface. On either 

side (upstream and downstream) of this measured section, 

facies are very variable laterally; the location of this profile 

in a zone of repeated channel incision and filling has 

resulted in little direct lateral continuity of stratigraphic 

horizons.

Surface: Deflated, grass-covered sediment with cryptogamic 

crust development. This section was logged immediately 

below a large mesquite tree with roots extending down 

over the arroyo wall. Metal pegs in this arroyo wall mark 

locations where samples were collected for radiocarbon 

dating (sample RC5) by Hereford and others (2000). 

0 – 18 cm (Unit A): Surface material, with no apparent 

sedimentary structures. Sediment is heavily bioturbated. 

Grain size is silty fine sand. Unit A has no distinct lower 

contact. Material is possibly reworked aeolian sediment, 

but sedimentary structures have been almost entirely 

obscured by root development. 

18 – 25 cm (Unit B): This unit contains a burned zone 

(burned in place, as is evident from a reddish oxidation 

layer at the base of the ashy soil area). The ashy soil is 

contained in silty very fine sand that is massive, heavily 

burrowed, and bioturbated by roots. The lower contact of 

unit B is indistinct.

25 – 40 cm (Unit C): The lowermost 3 cm of unit C are pale 

beige/gray silt, heavily bioturbated such that sedimentary 

structures are absent. The boundary between this silt and 

100

0

Depth (cm)

200

300

Possible aeolian, grading

down-section into fluvial

sands (based on topographic expression). Exposure inaccessible.

Flood couplet; lower part silt, grading upward into fine sand.

Flood couplet?

One or more floods

Flood couplet?

Flood couplet?

Flood couplet?

Flood couplet?

Flood couplet?

Flood couplet?

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 52. Field sketch of stratigraphic section at section 3, Arroyo 

Grande.

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area

Figure 53. Location of section 4 at Arroyo Grande. The profile height 

(at location of measuring tape) is approximately 3 m. This stratigraphic section is in a zone of repeated channel incision and filling (note 

beds that dip steeply to the right of the photo [north, shoreward]); 

there is therefore little lateral continuity of stratigraphic units at this 

site.
56 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

the overlying coarser upper part of the flood couplet is 

smooth. The lower boundary (between units C and D) is 

bioturbated. This is interpreted as a flood deposit that forms 

a sedimentary couplet.

40 – 78 cm (Unit D): The upper part of unit D is a 

structureless silty fine sand. Numerous carbonate-filled 

rootlet nodules are present, as are many open rootlet pores. 

Sediment is heavily bioturbated. The lower contact is sharp 

and smooth, with an apparent increase in calcium carbonate 

content directly on the lower contact. Unit D is interpreted 

as a likely fluvial deposit on the basis of stratigraphic 

association and couplet structure; the lowermost 6 cm of 

this unit is a whitish very clean silt, more resistant than the 

upper portion.

78 – 115 cm (Unit E): Unit E can be considered in four parts:

78 – 87 cm (Subunit E1): Structureless fine sand, possibly 

an aeolian deposit that may represent reworking by wind 

of the upper part of a fluvial deposit. Alternatively, this 

could represent local alluvial material, as it grades laterally 

into a gravel lens that fills a channel 0.5 m wide that lies 

immediately upstream of section 4.

87 – 90 cm (Subunit E2): This horizon consists of a 

burned zone with ashy soil (burned in place) with very 

little charcoal. The upper and lower contacts are strongly 

bioturbated, and are clear but wavy.

91 – 100 cm (Subunit E3): This layer is a silty fine sand 

that may represent a flood deposit, but lacks sedimentary 

structures. Subunits E2 and E3 together may represent a 

flood couplet.

 100 – 115 cm (Subunit E4): Structureless clean silt with 

abundant calcium carbonate. The lower boundary is very 

abrupt and smooth.

115 – 117 cm (Unit F): Unit F is another burned zone 

(burned in place) with ashy soil. The lower contact is very 

bioturbated but clear.

117 – 125 cm (Unit G): This unit is a whitish silt layer. The 

lower boundary is sharp and smooth.

125 – 135 cm (Unit H): The bottom of unit H contains fine 

sand, which grades into coarse sand and granules at the 

top. Laterally, this unit grades into a poorly sorted gravel 

bed that contains clasts as big as pebble sizes. The lower 

boundary of the upper part of unit H is difficult to see but 

is defined by an increase in silt content. The unit contains 

no clear sedimentary structures, but there is an increase in 

carbonate-filled root pores in the lowest 5 cm of unit H. The 

lower contact is indistinct and smooth. The lower silty 5 cm 

pinch out riverward. This couplet is interpreted as a fluvial 

deposit that has experienced subaerial reworking at its top.

135 – 148 cm (Unit I): Unit I represents another sediment 

couplet (fluvial base, subaerially reworked top). The coarser 

upper part of the couplet extends from 135 to 139 cm below 

the modern land surface, and is dominated by silty fine sand 

with very faint, thin bedding. The transition from coarser 

upper sediment to lower, finer sediment within this couplet 

is fairly abrupt. The lower part of the couplet (from 139 to 

148 cm) is a very clean silt. This lower sediment is locally 

laminated, and contains isolated small pieces of charcoal 

but no evidence of an actual burned zone (the charcoal 

was not burned in place). Many rootlet pores and burrows 

are present. This unit is inclined more steeply than other 

bedded areas (the lower, silt portion of the couplet dips 

at 15o toward 262o). This is the same unit that yielded a 

radiocarbon date range of 40 B.C. to A.D. 330 (sample 

RC5 of Hereford and others, 2000). The lower boundary is 

bioturbated but abrupt.

148 – 155 cm (Unit J): The upper part of this ‘couplet’ unit 

(148 to 151 cm) is a silty fine sand. This contains charcoal 

pieces and is burrowed with silt-filled rootlet channels. 

Parallel laminae (horizontal) are present. The boundary 

between upper and lower portions of this couplet is indistinct. 

The lower part of the couplet (from 151 to 155 cm) is a silt 

that gradually disappears in the downstream direction within 

this outcrop, grading into the unit from which the radiocarbon 

sample RC5 was obtained by Hereford and others (2000). 

Unit J thickens upstream in this outcrop. Its lower boundary 

is strongly bioturbated and indistinct. This is interpreted as 

another couplet most likely representing a fluvial deposit with 

a subaerially reworked top.

155 – 205 cm (Unit K): Locally, as much as 25 cm of 

50

100

0

Depth (cm) 150

200

L – (schematic) alternating fine (flood?) 

 beds and locally derived sand/gravel

K – Apparent couplet; likely fluvial 

 deposit

J – Couplet: inferred fluvial deposit

I – Couplet: inferred fluvial deposit

H – fluvial deposit, reworked at top

G – Silt, possible fluvial deposit

F – Burned zone, ashy soil

E4 – Possible flood deposit

E3 – Possible fluvial deposit

E2 – Burned zone, ashy soil

E1 – Possible aeolian sand

D – Couplet: inferred fluvial deposit

C – Couplet: inferred fluvial deposit

B – Burned zone, ashy soil

A – Surface sediment; likely

aeolian reworking

Sand

Silt

Gravel

Charcoal/ash

250

E

Figure 54. Stratigraphic diagram for section 4, Arroyo Grande.
57

topography occurs on the lower boundary of unit K. 

Downstream of this unit are two coarse gravel lenses 

(locally present only). This unit appears to be in part a local 

channel fill. There are many silty interbeds, but sediment 

generally grades from very fine sand at the base to fine 

sand at the top (coarsening upward). If unit K is a flood 

deposit, it may represent more than one flood event. The 

silty lower part of this apparent couplet is very variable and 

discontinuous. Where present, the lower sediment is a clean 

silt; this is discontinuous and locally lies directly above 

much coarser material of unit L. This is a possible couplet, 

but with the division between upper and lower portions of 

the couplet indistinct.

205 – 280+ cm (Unit L, to base of outcrop): The lowermost 

unit in section 4 contains alternating sequences of locally 

derived slope-wash material (lithic fragments) with silty 

sands as much as 10–12 cm thick. Beds of hard, cohesive 

clay are present in this sequence. Individual sandy beds 

vary between fine sands and pebbly gravels. Unit L may 

truncate an older Colorado River flood sequence at its base 

(inconclusive exposure). 

Section 4 summary.—This exposure appears to be on the 

edge of some kind of channel, with channel forms filled 

with gravel beds in lenses on either side (upstream and 

downstream) of the vertical cut where section 4 was 

described. This section is in the main trunk (center) of the 

primary arroyo network at Arroyo Grande; the trunk of this 

large arroyo system has likely been at this approximate 

location (with some lateral migration) for an extended 

period of time. The area represented by section 4 may not 

have been a true “arroyo” but instead a local broad channel 

carrying locally derived sediment as well as being the 

return channel area for Colorado River floods. 

Arroyo Grande, Section 5

Photographs from one horizon within section 5 (unit C) are 

shown in figures 55 and 56. The stratigraphic diagram for 

section 5 is shown in figure 57. Measurements are in cm 

below the land surface.

Surface: Deflated, grassy cryptogamic soil cover with 

pebbles of granite and amphibolite gneiss from talus slope 

behind and above. A pedestaled soil crust ~1 mm thick is 

present. Section 5 is cut into arroyo of lower terrace, just 

riverward of boundary between upper and lower terraces. 

Approximately 2 m landward of section 5 is a nickpoint 

where arroyo incision into lower terrace is about to join 

with upper terrace incision (the nickpoint is now ~3 m 

riverward of where it will meet the upper terrace drainage). 

0–19 cm (Unit A): This unit is surface sediment with no 

microstructure; it is very bioturbated with root development. 

Clasts up to pebble size of granite and gneiss are present, 

derived from the talus above. Within that 19 cm is (from 

15–17 cm below the surface) a heavy organic concentration. 

The lower contact of unit A is indistinct and gradational into 

better defined stratification until the microstructure in unit B 

becomes apparent.

19–38 cm (Unit B): Unit B consists of parallel stratification 

with beds defined by thin (less than 3 mm) coarse sand 

lenses with some organic concentration horizons as much 

as 1–2 cm thick. Several areas with subaqueous ripples are 

defined in coarse sand; rivulet development is inferred down 

a distal alluvial fan surface with slope-wash sediment being 

reworked. Finer-grained areas have a pinkish color. Unit B 

reflects apparent surface sedimentation and reworking on 

the distal edge of an alluvial fan.

38–65 cm (Unit C): This unit is bioturbated with white 

calcified rootlets, but contains very well defined fluvial 

climbing-ripple structure throughout. Cuts at different 

angles show that ripple migration direction was upstream 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area

Figure 55. Excellent exposure of fluvial climbing ripples in unit C of 

section 5, Arroyo Grande. The current direction during deposition 

was from left to right (upstream), indicating flow in an eddy. The steep 

angle of climb indicates rapid deposition; increasing climb angle 

upsection indicates increasing deposition rate in the later stages of 

the flood. Grain-size analyses show that this unit fines upward slightly. 

Unit C is slightly thicker in this exposure than the 15 cm measured at 

section 5 (this unit thickens riverward).
58 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

(climb toward 290°), apparently representing flow in an 

eddy, during the flood. Photographs (figs. 55, 56) show 

that ripples at the base of this unit base climb subcritically 

(lateral migration of ~6 cm in 1 cm of vertical aggradation). 

Ripple climb changes upward to almost vertical 

aggradation. Wavelength of the ripple set increases from 7 

to 10 cm at top of unit. Appears to fine upward; base grain 

size is fine sand while the top of the unit is very fine silty 

sand. This unit thins very slightly toward the shoreward end 

of outcrop (25 vs. 27 cm thick). Interpreted as a Colorado 

River flood deposit, unit C has a sharp lower contact with 

a color change from fine gray Colorado River sediment to 

brown-pink coarser material below. Four grain-size samples 

were collected from this unit: AG Strat 5 #1( Unit C base, 

0–1 cm above base of unit); AG Strat 5 #2 (Unit C 8–10 cm 

above base of unit); AG Strat 5 #3 (Unit C 16–18 cm above 

base of unit); AG Strat 5 #4 (Unit C 24–26 cm above base 

of unit).

67–75 cm (Unit D): Unit D consists of slope-wash sediment 

representing a land surface, with modifications by aeolian 

processes; this apparently represents a distal alluvial-fan 

environment. Sediment has a pink/brownish color, coarser 

than unit C above. Parallel bedding is defined by clasts as 

large as granule sizes that consist of K-spar-rich granite and 

amphibolite gneiss fragments. Bioturbated sediment, with 

white rootlets. Grain size ranges from granule down to fine 

and medium sand in parallel beds. Unit D is bounded by 

sharp upper and lower contacts.

75–79 cm (Unit E): This 2–4-cm-thick unit is a gray, finegrained subaqueous flood deposit, likely a Colorado River 

flood. Unit E contains well-defined subaqueous climbing 

ripples (small scale). Grain size is fine to very fine sand, 

with no evidence of lithic clasts as would be expected if 

it were a subaqueous deposit from an arroyo floor. Ripple 

migration direction is toward 254o, upstream (indicating 

an eddy). The unit thickens from 2 to 4 cm riverward in 

outcrop, over a distance of ~1.5 m. The lower contact is on 

an organic concentration a few mm thick (unit F), making 

the contact slightly irregular. 

79–85 cm (Unit F): This unit is 2–6 cm thick (thickens toward 

the shoreward end of the outcrop, where overlying unit E is 

thinner). It includes an organic horizon ~0.5 cm thick and 

is bioturbated with rootlets throughout. Unit F has a more 

pink/tan color than flood units (E and G) above and below. 

Find sand dominates, with no strong concentrations of lithic 

clasts but with some parallel bedding (not very distinct). 

The lower contact is sharp. This is an apparent land surface 

with aeolian modification, similar to units B and D above.

85–91 cm (Unit G): Unit G contains gray, fine to very fine 

sand with very well preserved subaqueous climbingripple structures. Ripples migrate toward 260o (upstream, 

indicating an eddy). The unit does not appear to coarsen 

upward. The lower contact is angular, separating two 

flood deposits (G and H) in vertical sequence. This unit is 

interpreted as another Colorado River flood deposit.

91–106 cm (Unit H): This unit is 12–15 cm thick. The base 

consists of fine sand, with a pinkish gray color. The top 

is fine to medium sand, pinker and coarser than unit G 

above. The base is organic rich, but contains better defined 

subaqueous climbing ripple development upward. This is 

interpreted to represent a separate flood deposit from unit 

G, with no other apparent land surface or unconformity 

between this and the overlying flood unit (G). Units G and 

Figure 56. Close-up view of one ripple crest from the unit C flood 

deposit of section 5, Arroyo Grande. The nearly vertical angle of climb 

implies rapid deposition. The ‘zig-zag’ upsection oscillation of the 

ripple crest in individual laminae is interpreted to be a result of eddy 

pulsation (Rubin and MacDonald, 1995) during deposition.

J – fluvial deposit

I – organic concentration; land surface

H – fluvial deposit

G – fluvial deposit

F – land surface with aeolian modification

E – fluvial deposit

D – slope-wash sediment

C – fluvial deposit

B – slope-wash sediment

A – land surface; slope-wash

and aeolian material

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

50

100

0

Depth (cm)

Figure 57. Stratigraphic diagram for section 5, Arroyo Grande.
59

H may have been two peaks of same flood event, or two 

separate floods that occurred in rapid succession without 

reworking by aeolian, slope-wash, or other processes on 

the land surface between flood events. Because unit H is 

truncated by overlying unit G, which contains sediment of 

different grain size and color, it likely represents a separate 

event. Unit H appeared in the field to coarsen upward. Four 

samples were collected for grain size analysis: AG Strat 5 

#5 (Unit H 1–2 cm above base of unit); AG Strat 5 #6 (Unit 

H 5–6 cm above base of unit); AG Strat 5 #7 (Unit H 9–10 

cm above base of unit); AG Strat 5 #8 (Unit H 13–14 cm 

above base of unit).

106–107 cm (Unit I): This unit consists of a concentration 

of organic matter as much as 1 cm thick, interpreted as a 

reworked land surface containing vegetation development 

that formed between Colorado River flood events.

107–135+ cm (Unit J, to base of outcrop): Contains pinkish 

sediment with well-developed bedding, but not climbing 

ripples. Grain size is fine to very fine sand, and the deposit 

does not appear to coarsen upward. Unit J is mostly parallelbedded with swales, suggesting possible subaqueous dunes. 

One small area of this unit, approximately 3 m laterally 

from the main logged section 5, contained subaqueous 

ripple forms. Unit J is interpreted as a Colorado River 

flood unit, possibly with its pinker sediment contributed by 

unusually high discharge from a tributary during the flood 

that deposited it. 

Section 5 summary.—This section is located in a secondary 

arroyo system incised into the lower terrace area at Arroyo 

Grande, riverward and downstream of the stratigraphic 

sections described in the main arroyo network. Depositional 

environments in this section alternate between Colorado 

River flood deposition and modification of this flood 

sediment by reworking at the land surface, including the 

addition of sediment derived from local slope-wash. A 

minor component of aeolian sediment is inferred in unit A. 

Arroyo Grande, Section 6

Photographs in figures 58 and 59 show detail of several 

stratigraphic horizons in section 6. A stratigraphic diagram 

for section 6 is shown in figure 60. Stratigraphic depths are 

reported in cm below the land surface. The survey datum 

for this section is 2.0 m below the terrace surface. 

0 – 80 cm (Unit A): Inaccessible area. At the base of this unit 

is a thin whitish layer interpreted to be the base of a flood 

deposit.

80 – 150 cm (Unit B): Unit B is a slightly sandy silt, with 

very little original stratification apparent. The unit is 

bioturbated, and charcoal is present at two horizons (depths 

not recorded). 

150 – 168 cm (Unit C): This is a silt that ranges in size from 

fine to coarse and slightly sandy, with paper-thin lamination. 

Fluvial climbing ripples are present; this is interpreted as a 

Colorado River flood deposit. This unit coarsens upward, 

and more bioturbation is apparent in its upper portion. 

168 – 198 cm (Unit D): Fluvial climbing ripples are present 

throughout this horizon. No distinct trend in grain size is 

apparent. Unit D is interpreted as another Colorado River 

flood layer.

198 – 228 cm (Unit E): This is a coarse silt, with fluvial 

climbing ripples indicating another flood deposit. 

Bioturbation increases upsection within this horizon. The 

lower contact is sharp, with a basal white, very fine grained 

clayey silt less than 1 cm thick.

228 – 231 cm (Unit F): This is a silt that contains fluvial 

climbing ripples and little bioturbation. At its base is 2–3 

mm of white, very fine-grained clayey silt that has a sharp 

base and a gradational top.

231 – 258 cm (Unit G): Unit G can be broken into three 

subunits, each of which is interpreted as a fluvial deposit 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area

Figure 58. Uninterpreted (A) and interpreted (B) views of sedimentary 

couplets in unit G, section 6 at Arroyo Grande. Each stratigraphic 

package shown is interpreted as a flood deposit that has subsequently been reworked at the land surface. Fluvial climbing ripples are 

present in the fluvial parts of each couplet. The subaerial portion of 

each couplet shown contains locally derived slope-wash sediment as 

well as charcoal and ash staining. The common presence of charcoal 

and ash in subaerial parts of couplets may reflect grass fires started 

by the prehistoric occupants of this area. Bioturbation has mixed 

subaerial sediment downward into the lower fluvial deposits.
60 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

that shows evidence of reworking at the land surface (figure 

58):

231 – 242 cm (Subunit G1): The upper part of this 

subunit is ~1 cm thick, containing charcoal-rich silt or fine 

sand with sharp contacts. The middle portion of subunit 

G1 contains silt with fluvial climbing ripples, and has a 

gradational base. The lower part of this subunit contains ~1 

cm of white, very fine-grained clayey silt. 

242 – 250 cm (Subunit G2): Upper part is ~1 cm thick, 

containing charcoal-rich silt or fine sand with sharp upper 

and lower contacts. The middle portion of this subunit 

contains silt with fluvial climbing ripples; its base is 

gradational. The lower part of subunit G2 contains ~1 cm of 

white, very fine-grained clayey silt.

250 – 258 cm (Subunit G3): The upper part of subunit G3 

is ~1 cm thick, containing charcoal-rich silt or fine sand with 

sharp contacts. The middle portion of this subunit contains 

silt with fluvial climbing ripples, and has a gradational base. 

The lower part of this subunit contains ~1 cm of white, 

very fine-grained clayey silt. Seven samples were collected 

from subunit G3 and the inferred land-surface sediment that 

immediately underlies (unit H) and overlies (subunit G2) 

that flood deposit (the sampled bed extends from 251 to 261 

cm below the land surface): AG Strat 6 #1 (260 – 261 cm 

below surface; interpreted as land-surface material (contains 

charcoal) immediately below the base of the next overlying 

flood deposit; bioturbation has mixed this material with the 

silty basal sediment of the overlying flood unit); AG Strat 6 

#2 (258.5 – 259.5 cm depth; white silty basal flood sediment); 

AG Strat 6 #3 (256 – 258 cm depth; within same flood couplet 

as #2; paper-thin wavy laminae in silty sand); AG Strat 6 #4 

(255 – 256 cm depth; wavy laminated sediment farther up 

in same flood couplet); AG Strat 6 #5 (252 – 253 cm depth; 

farther up in same flood unit); AG Strat 6 #6 (251 – 252 cm 

depth; uppermost laminated part of this flood deposit; affected 

by burrowing); AG Strat 6 #7 (250 – 251 cm depth; interpreted 

as land surface; charcoal-rich fine sand, burrowed from both 

above and below).

258 – 302 cm (Unit H): The upper part (1–2 cm) of unit H 

contains fine sand with charcoal and ash incorporated; 

bioturbation has mixed the silty base of subunit G3 into the 

coarser sediment of unit H. Fine sand grades down into silt 

Figure 60. Stratigraphic diagram for section 6, Arroyo Grande.

C – fluvial deposit

B – reworked surface sediment

A – poorly

exposed,

possible

aeolian

H – couplet with reworked land surface sediment near

top, fluvial near base

G3 – couplet of fluvial deposit with reworked land surface

G2 – couplet of fluvial deposit with reworked land surface

G1 – couplet of fluvial deposit with reworked land surface F – fluvial deposit, clayey silt at base

E – fluvial deposit, clayey silt at base

D – fluvial deposit

I – six couplets of fluvial deposit with reworked land surface

50

100

0

Depth (cm) 150

200

250

350

300

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Clayey silt

Figure 59. Uninterpreted (A) and interpreted (B) views of sedimentary 

couplets in the upper part of unit I, section 6 at Arroyo Grande. Each 

subunit is interpreted as a flood deposit that has subsequently been 

reworked at the land surface. Fluvial climbing ripples are present in 

the fluvial parts of each couplet. The blue dashed line in subunit I1 

is believed to represent a small scour-and-fill sequence within that 

flood deposit. The subaerial portion of each couplet shown contains 

locally derived coarse sand and gravel as well as charcoal and ash. 

Bioturbation has mixed subaerial sediment downward into the lower 

fluvial deposits; several larger burrows are indicated, but bioturbation 

is evident on a smaller scale as well.
61

or very fine sand with silt laminae several mm thick. Some 

bioturbation is apparent, especially in the upper half of this 

unit. Fluvial climbing ripples are present, especially in the 

lower half. The lower contact of unit H is gradational. The 

lower 1 cm contains white, very fine-grained clayey silt.

302 – 379+ cm (Unit I, to base of exposure): Unit I contains 

six subunits. Each subunit is interpreted as a Colorado 

River flood deposit that has been subsequently reworked 

at the land surface (fig. 59). Several of the subunits contain 

postflood charcoal-rich concentrations at the top. Four of 

the flood layers are overlain by a few mm to as much as 

1.5 cm of unstratified sand, coarser than any sand within 

the flood layer. Each of the flood layers has a thin (several 

mm to 1 cm) white, very fine grained clayey silt at its base, 

which may have a sharp base and gradational top. The bulk 

of each flood layer is silt or very fine sandy silt, in part with 

paper-thin lamination and fluvial climbing ripples. Some 

bioturbation occurs throughout. Depth ranges of individual 

subunits are: 302 – 323 cm (Subunit I1); 323 – 335 cm 

(Subunit I2); 335 – 341 cm (Subunit I3); 341 – 345 cm 

(Subunit I4); 345 – 349 cm (Subunit I5); 349 – 379 cm 

(Subunit I6).

Section 6 summary.—This section records the deposition of as 

many as 15 individual deposits from Colorado River flood 

events, separated by reworking at the land surface between 

floods. The typical flood sequence in this exposure contains 

a lowermost very fine clayey silt deposit, up to 1 cm thick. 

The bases of these fine silt layers are typically sharp, though 

subsequent bioturbation has modified their lower contacts 

in many cases. The fine basal clayey silt is overlain (often 

gradationally) by a coarser silt to fine sand deposit that, 

in this outcrop, commonly contains well preserved fluvial 

climbing ripple structures. The uppermost part of each 

sequence is typically bioturbated and, in the case of at least 

six of the flood events apparent in section 6, contains a 

concentration of charcoal and ash that indicates exposure of 

this material at the land surface.

Arroyo Grande, Section 7

The stratigraphic diagram for section 7 is shown in figure 61. 

Measurements are in cm below the ground surface. The land 

surface above section 7 is not accessible because its location 

is on a thin ridge separating two arms of the arroyo.

0 – 18 cm (Unit A): This uppermost sediment consists of 

alternating fine sand, silt, and small-pebble gravels. It is 

interpreted as dominantly slope-wash material. Several silt/

fine sand beds are present, approximately 5 mm thick, and 

appear to be crusts formed on former surfaces.

18 – 54 cm (Unit B): This horizon is laminated and appears 

to be waterlain, alternating fine sand and silt beds with 

laminae. There is distinct cross-bedding present, which is 

inferred to be of local origin rather than due to a Colorado 

River flood.

54 – 58 cm (Unit C): Unit C is a very fine sandy silt, strongly 

bioturbated and lensatic, that disappears to the north 

(shoreward). It may represent a Colorado River flood, but 

this interpretation is not certain. No depositional structures 

are visible.

58 – 67 cm (Unit D): This layer is a massive very fine sand, 

which may represent a Colorado River flood deposit.

67 – 67.5 cm (Unit E): This is a thin, discontinuous fine sandy 

silt lens, which may be the lower part of a flood deposit 

represented by unit D.

67.5 – 89 cm (Unit F): Unit F is a clean deposit of coarse 

sand and pebble-gravel, cross-bedded and lensatic; this 

comprises locally derived channel-fill deposits.

89 – 94 cm (Unit G): Unit G is a clean silt, laminated in its 

upper 2 to 3 cm, which is thought to represent a Colorado 

River flood deposit. 

94 – 99 cm (Unit H): This is a clean, structureless fine sand 

overlying a thin, discontinuous, obscure silt bed. These, 

together with the overlying silt of unit G, represent a flood 

sequence (‘triplet’) that dips to the south (riverward) into 

a small, channel-like depression. Units G and H appear to 

represent Colorado River fluvial deposition.

99 – 127 cm (Unit I): Unit I consists of highly variable sands 

(fine to coarse sand); textural variations are localized and 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area

Figure 61. Stratigraphic diagram for section 7, Arroyo Grande.

50

100

0

Depth (cm)150

200

250

Q

P

O

N – distal edge of fluvial deposit?

M – subaerial sands

L – two flood couplets

K – slope-wash deposit

J – fluvial deposit

I – slope-wash deposit

H

G

F – slope-wash deposit

E

D

C – possible fluvial deposit

B – slope-wash deposit

A – slope-wash deposit

300

fluvial deposit

fluvial deposit

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples
62 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

lensatic. Sediment in this horizon varies from clean to very 

poorly sorted and was apparently deposited by locally 

derived running water. The lower contact appears to be 

erosional. 

127 – 146 cm (Unit J): This is a fine sandy silt, possibly with 

finer interbeds in the upper part. Numerous carbonate-filled 

rootlet pores are present. Sediment appears to be very thinly 

bedded or laminated, but the structure is obscure. The lower 

4 cm of this horizon is bioturbated, clean, laminated silt. 

This silt-and-sand couplet appears to represent a single 

Colorado River flood event.

146 – 170 cm (Unit K): Unit K is a poorly sorted fine sandy 

gravel in its upper section, with cleaner fine sand in the 

middle and clean, coarse sandy pebble gravel in its lower 

part. Contacts within this stratum are gradational. Unit K 

was deposited by running water of local derivation.

170 – 185 cm (Unit L): This is a set of four beds: sand-siltsand-silt (in order downsection). The two sand layers are 

silty fine sands, separated by clean silt beds. The silt beds 

are laminated, especially in the upper and lower parts, 

whereas the sands appear structureless. This set of beds 

most likely represents two Colorado River flood deposits.

185 – 249 cm (Unit M): This horizon contains bedded sands 

(silty fine sand including poorly defined lenses of mediumcoarse sand with occasional small pebbles). Sediment is 

very micaceous. Internal contacts are gradual, which might 

suggest that these beds were deposited in one event, but 

a small patchy area of charcoal at approximately 240 cm 

suggests otherwise.

249 – 250 cm (Unit N): This is a very thin bed of very pure, 

whitish silt. This horizon is broken by occasional insect 

burrows, but remains surprisingly intact for so thin a bed. 

It cannot be traced for more than a few meters in any 

direction. Upper and lower contacts are abrupt and smooth. 

This may represent the distal edge of a Colorado River flood 

deposit.

250 – 280 cm (Unit O): This horizon consists of very clean 

fine sand. Bedding is very obscure, but slightly siltier layers 

appear to dip riverward. A few burrows filled with ashy 

charcoal are present in the upper 10 cm of this unit. The 

lower contact is abrupt and wavy. This may represent the 

upper part of a Colorado River flood couplet.

280 – 299 cm (Unit P): Unit P is a very clean silt, laminated 

in its upper and lower parts, that dips steeply westward 

(upstream with respect to the river). This may represent the 

lower part of a Colorado River flood couplet (the upper part 

being unit O above).

299 – 340+ cm (Unit Q): This lowermost stratum is a very 

clean, laminated fine sandy silt, with well-defined climbing 

ripples in its upper 8–10 cm (locally). This grades into the 

silt above (unit P) and is interpreted as a Colorado River 

fluvial deposit.

Section 7 summary.—Section 7 appears to be somewhat 

transitional between the zone dominated by slope-wash 

sediment (similar to section 11) and the flood-dominated 

profiles (for example, sections 3, 4, and 6). The profile 

contains some sediments at the base that are clearly 

deposited by Colorado River floods, along with a number 

of gravelly beds that are clearly of slope-wash origin. 

In between are sand/silt couplets that appear similar in 

composition and sequence to beds that have been identified 

as flood sediments (inferred from the presence of fluvial 

sedimentary structures) in other stratigraphic profiles at 

Arroyo Grande, but this profile contains few distinctive 

bedding structures. Section 7 contains evidence for as many 

as eight Colorado River floods (possibly unit C, units D/E, 

units G/H, unit J, two in unit L, unit N, and units O/P/Q). If 

these units are flood sediments, they were likely deposited 

on a surface that had a broad channel reaching from the 

terrace surface to the river.

Arroyo Grande, Section 8

The stratigraphic diagram for Section 8 is shown in figure 62. 

Measurements are in cm below the land surface.

Surface: Deflated, grassy sediment with cryptogamic soil 

cover. 

Figure 62. Stratigraphic diagram for section 8, Arroyo Grande.

50

100

0

Depth (cm) 150

200

250

M – slope-wash sand

L – Couplet: inferred fluvial deposit

K – Probable couplet, fluvial deposit

J – three thin fluvial deposits

I – slope-wash sediment

H – thin fluvial deposit

G – slope-wash and aeolian sediment

F – couplet: inferred fluvial deposit with

subaerially reworked top

E – couplet: inferred fluvial deposit

D – lower part of flood couplet?

C – fine sand, reworked flood sediment?

B – slope-wash sediment and

 possible aeolian

A – slopewash and

aeolian

sediment

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples
63

0 – 49 cm (Unit A): Unit A is variously bedded and/or 

laminated. This sediment contains locally derived pebbles 

and granules (granite and amphibolite gneiss grains) in 

places, but is dominantly silt or fine sand. Isolated charcoal 

pieces are present, not burned in place. The origin of unit 

A appears to be slope-wash and aeolian sediment, with 

multiple episodes of soil-crust development between 

deposition events. 

49 – 125 cm (Unit B): Many localized lenses of locally 

derived coarse sand and granules are present, with some 

clasts up to pebble size. A burning episode is apparent, 

spanning 3 cm (from 64 to 67 cm below the land surface); 

this area was burned in place and is dark gray. Abundant 

disseminated charcoal is visible in this burned horizon. 

The charcoal layer is in the shape of a thin basin; its lateral 

extent is indeterminate. Below this burned area, unit B 

becomes coarser downward; from 95 to 125 cm below the 

land surface, the sediment is essentially gravel. Multiple 

episodes of slope-wash deposition are evident in this unit, 

but the greatest proportion of sediment in the upper part of 

unit B is still fine sand.

125 – 143 cm (Unit C): Unit C consists of fine sand that is 

thinly bedded. No clear sedimentary structure is apparent. 

This fine sand is relatively poorly sorted and includes a few 

coarse sand grains. This texture may indicate local sediment 

derivation, but it may also represent reworking of the upper 

part of a fluvial deposit (with underlying unit D forming 

the lower part of a fluvial couplet). The upper part of unit C 

contains very thin (<0.25 cm) silt laminae; this layer could 

represent a Colorado River flood event or local ponding. 

The lack of visible mudcracks suggests that a fluvial origin 

is more likely than local ponding. Units C and D most likely 

represent a fluvial couplet.

143 – 150 cm (Unit D): Unit D, the lower portion of the 

possible fluvial couplet formed by units C and D, is a white 

resistant silty layer. Its uppermost 2 cm consist of bedded, 

alternating silt and fine sand. Both the upper and lower 

contacts of this 2-cm-thick upper region are abrupt and 

distinct. The lower silt layer is very clean and laminated, 

implying likely Colorado River flood derivation. Sediment 

is bioturbated and contains carbonate-filled rootlet pores. 

Unit D is very distinctly laminated with paper-thin laminae 

throughout its thickness.

150 – 157 cm (Unit E): The upper part consists of brownish 

silty fine sand. Sediment is structureless, with numerous 

bioturbation features. The contact between the upper sandy 

material and a lower silt portion is abrupt but bioturbated. 

The lower, silty unit contains silt that is less pure than that 

of lower unit D (best described as a fine to very fine sandy 

silt). This lower portion of unit E is very bioturbated and 

contains very faint paper-thin laminae. This unit is inferred 

to be another fluvial couplet deposit.

157 – 187 cm (Unit F): The upper portion of unit F contains 

a significant coarse-sand phase, with many granules and 

pieces of what appear to be mechanically deposited clasts 

of calcium carbonate. This carbonate material does not 

resemble rootlet filling, and may have been eroded from 

somewhere upslope and subsequently incorporated into this 

deposit. A burned zone 1 to 1.5 cm thick occurs within this 

upper sand portion (whether it was burned in place is not 

clear). Small pebbles are present at the same stratigraphic 

level as the charcoal. This upper sediment of unit F is 

interpreted to be the upper part of a flood couplet that 

has undergone significant reworking (and incorporation 

of locally derived material) at the land surface after 

initial deposition. The silty, lower phase of the couplet 

is laminated, clean, very pure silt. Its lower boundary is 

abrupt and bioturbated, with some cementation evident at 

its base. Unit F is interpreted as a fluvial couplet. Three 

sediment samples were collected: AG Strat 8 #1 (Unit F, in 

silt at base of unit; attempted to remove sand-filled burrows 

from sample); AG Strat 8 #2 (Unit F, 1 cm above basal 

silt in relatively clean sand); AG Strat 8 #3 (Unit F upper 

part, locally derived sediment with occasional charcoal; the 

sample contains clasts of consolidated silt).

187 – 200 cm (Unit G): This horizon consists of medium 

sand that includes clasts as large as granule size (poorly 

sorted). Mica flakes and local lithic fragments are abundant. 

The lower part of unit G contains horizontal bedding with 

fine sand beds (very thinly laminated). Shoreward of the 

described section is an ashy charcoal soil at the same level 

with pieces of fire-cracked rock that suggest a cultural 

hearth feature. Unit G is interpreted as locally derived 

sediment that has likely undergone aeolian modification. 

Small pieces of consolidated silt are apparent, which may 

be related to disturbance of a hardened silt layer (unit H) 

during original construction of the hearth feature. 

200 – 201 cm (Unit H): This is a thin white silt layer 

that contains very delicate lamination. Sediment is 

very bioturbated and may have been disturbed during 

construction of the hearth shoreward of section 8 (the 

hearth is cut into units G and H). This unit is most likely a 

Colorado River flood deposit, on the basis of its similarity 

to other interpreted flood silts in this and other sections.

201 – 204 cm (Unit I): Unit I contains medium to coarse 

sand, very poorly sorted. Sediment is locally laminated near 

the top but is generally structureless. This is interpreted as 

locally derived slope-wash material.

204 – 212 cm (Unit J): This horizon consists of three silt 

bands separated by sand. Each silt layer averages ~1 cm 

thick and is fairly disrupted and discontinuous but very 

pure where present. Sediment is very bioturbated, as are 

the boundaries of each silt band. The sandier units between 

silt beds are silty fine sand, with local horizontal parallel 

bedding. The uppermost sand bed in unit J contains flecks 

of charcoal and is slightly grayer (contains ashy soil). Unit J 

is interpreted as three thin flood deposits.

212 – 242 cm (Unit K): Unit K is dominantly a silty fine 

sand horizon, heavily bioturbated. Many insect burrows 

are present (it is a possible termite nest). Sediment appears 

very structureless. At a depth of 242 cm is a very thin (<0.5 

cm), discontinuous, very bioturbated silt layer. This silt bed, 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area
64 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

which is fairly pure but very disturbed and discontinuous, 

may be related to local wetting, but may alternatively 

represent the thin landward edge of a Colorado River flood. 

242 – 264 cm (Unit L): The upper part of unit L consists of 

a poorly sorted coarse to medium sand with a large biotite 

component, implying local derivation. This material grades 

downward into a much cleaner (better sorted) medium sand, 

which at its base becomes a fine sand. The lower boundary 

of the fine sand is abrupt and wavy. This unit is interpreted 

as a possible fluvial deposit that has undergone reworking at 

the land surface after deposition.

264 – 289+ cm (Unit M, to base of outcrop): The basal unit 

of section 8 contains a medium-grained sand with abundant 

biotite, implying local sediment derivation. This layer is 

structureless and has very uniform composition throughout 

its thickness.

Section 8 summary.—This section is interpreted to reflect 

alternating deposition of Colorado River flood sediment 

and locally derived slope-wash material. In the nearly 3 m 

thickness of this section, a maximum of 10 flood events 

may be identified. The smaller number of identifiable 

flood events in this section, compared with the 15 flood 

deposits in section 6 (which has a comparable thickness but 

is located 34 m riverward of section 8), is consistent with 

expectations that fewer flood events will have a discharge 

great enough to reach the more landward locations studied. 

Between floods, deposition of slope-wash and minor 

amounts of aeolian sediment are evident in section 8.

Arroyo Grande, Section 9

The stratigraphic diagram for section 9 is shown in figure 63. 

Measurements are in cm below the land surface.

Surface: At section 9, the land surface contains discontinuous 

cryptogamic crust. Vegetation locally includes grass, 

occasional mesquite, and creosote bushes. The surface is 

slightly deflated at this location, with 1–2 cm erosional 

pedestals.

0 – 13 cm (Unit A): Unit A contains poorly sorted silty fine 

sand. This horizon is bioturbated with rootlets, as are the 

other horizons in this profile.

13 – 14.5 cm (Unit B): This horizon is a thin, discontinuous, 

pale gray silt layer.

14.5 – 36 cm (Unit C): Unit C contains poorly sorted, 

primarily silty sands, with little obvious microstructure; it is 

interpreted as likely slope-wash sediment. 

36 – 38.5 cm (Unit D): This is a gravel lens containing lithic 

grains and clasts of K-spar-rich granite and amphibolite 

gneiss (material derived from local bedrock). This unit 

consists of slope-wash sediment.

38.5 – 43 cm (Unit E): Unit E contains massive, poorly sorted 

silty sands. 

43 – 45.5 cm (Unit F): This horizon contains poorly sorted 

silty sands, similar to unit E, but with ashy charcoal present.

45.5 – 57 cm (Unit G): Unit G contains massive, poorly 

sorted silty fine sand.

57 – 61 cm (Unit H): This layer is a gravel lens containing 

slope-wash sediment. It is identical in character, 

composition, and inferred origin to unit D.

61 – 139.5 cm (Unit I): In contrast to the sedimentary 

character of units A through G above, unit I is a relatively 

clean silty fine sand. Two subunits are present within 

unit I: its upper part is probably Colorado River flood 

sediments, parallel bedded, with possible aeolian 

reworking. The lower part of this stratum is clearly a 

Colorado River flood deposit, with parallel bedding and 

more distinct, fine-scale climbing ripples near the base of 

the stratum. The boundary between the upper and lower 

parts of unit I is indistinct, marked by increasing frequency 

of Colorado River climbing ripples downsection. 

139.5 – 141 cm (Unit J): This stratum is interpreted as 

another Colorado River flood deposit, with a silty lower 

section that grades upward into a silty fine sand. No 

distinct bedding structures were observed.

141 – 142.5 cm (Unit K): Unit K is a laterally continuous 

ashy charcoal lens, interpreted to represent a former 

subaerial land surface.

50

100

0

Depth (cm) 150 L – flood deposit

K – ashy charcoal lens

J – flood deposit

I – flood deposit

H – slope-wash deposit

G – poorly sorted sands

F – poorly sorted sands with charcoal

E – poorly sorted sands

D – slope-wash gravel

C – poorly sorted sands

B – discontinuous silt layer

A – poorly sorted sands

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 63. Stratigraphic diagram for section 9, Arroyo Grande.
65

142.5 – 164+ cm (Unit L, to base of section): This 

lowermost unit contains distinct fluvial climbing ripples, 

indicating a Colorado River flood deposit.

Section 9 summary.— This profile can be divided into two 

distinct sections dominated by two different depositional 

environments. The upper portion (units A through H) is 

predominantly locally derived slope-wash sediment, with 

interspersed poorly sorted silty sands that likely represent 

reworking and redeposition of fluvial or aeolian deposits 

by local runoff. The lower part of this profile (units I and 

below) contains multiple Colorado River flood deposits. 

There is evidence for at least two floods (units I and L), 

with unit J representing either the distal part of a third 

flood deposit or the lowermost silt layer at the base of the 

flood deposit in unit I. Between these two major flood 

deposits there is evidence for subaerial exposure and a fire 

(possibly, but not necessarily, indicating human occupation 

of the land surface), from the presence of the ashy charcoal 

in sediment that forms unit K.

Arroyo Grande, Section 10

The stratigraphic diagram and a field sketch for section 10 are 

shown in figure 64. Stratigraphic thicknesses in this profile 

were measured in terms of heights above the survey datum, 

which is located at the base of the uppermost Colorado 

River flood deposit. In this section, contacts between unit 

are inclined; a range of upper and lower depths is given for 

most units.

170 to 70 – 80 cm (Unit A): Unit A is land-surface sediment, 

with a poorly exposed upper part but becoming more 

stratified toward its base. The base of this material is 

gradational.

70 – 80 to 50 – 70 cm (Unit B): This is a fine-grained sand, 

with planar lamination. Unit B is interpreted as an aeolian 

deposit that contains subcritically climbing wind ripples.

50 – 70 to 30 cm (Unit C): This fine-grained sand contains 

planar lamination or ripple cross-lamination and includes 

a 15-cm-high bar or dune. This is most likely a Colorado 

River flood deposit, but may be aeolian in part.

30 to 0 – 4 cm (Unit D): Unit D consists of very fine sand 

that is silty in part, with fluvial climbing ripples. This 

grades down into 1–2 cm of white clayey silt. The unit is 

interpreted as a Colorado River flood deposit. The base of 

this unit at one location within the outcrop is the survey 

datum.

0 – 4 to -10 – -12 cm (Unit E): Unit E contains silt or silty 

very fine sand, with charcoal concentrations and at least 

one cobble at its base.

Section 10 summary.—This section resembles section 1 in 

that it includes both fluvial and aeolian sediment. The 

upper aeolian sediment is most likely reworked material 

from fluvial deposition. Unit C of this section contains 

sedimentary structures that are inconclusive; a fluvial 

origin for this unit is most probable but aeolian material 

cannot be ruled out. The basal unit, E, includes charcoal 

and coarser sediment that implies a land surface with 

incorporated locally derived material.

Five samples were collected from Section 10: AG Strat 10 

#1 (0–2 cm above base of outcrop; powdery white silt, 

consolidated, interpreted as base of a flood deposit); 

AG Strat 10 #2 (20–22 cm above base of outcrop; silty 

sediment within unit that contains fluvial climbing ripples); 

AG Strat 10 #3 (44–46 cm above base; same flood unit as 

sample #2, but on lee side of large bar/bedform); AG Strat 

10 #4 (57–59 cm above base; origin uncertain—possible 

aeolian deposit or possible subaqueous dune sediment); 

AG Strat 10 #5 (50–52 cm above base, but shoreward 

of measured section; sand forms swales with charcoal 

interspersed. Possibly on contact between fluvial and 

aeolian sediment, or possibly within aeolian unit. This 

sample is stratigraphically above sample #4).

Arroyo Grande, Section 11

The stratigraphic diagram for section 11 is shown in figure 

65. Measurements are in cm below the land surface.

Surface: Surface sediment cover consists of pebbly sand 

with grasses and cryptogamic crust. The surface has no 

indication of pedogenic modification.

0 – 10 cm (Unit A): Cryptogam-capped pedestals in this 

uppermost unit indicate that 1 to 1.5 cm has been removed 

from this surface very recently. Unit A contains a wide 

variety of sediment sizes; from 0 to 5 cm there are 

alternating thin beds of fine sand and very thin beds of 

laminated silty fine sand. From 5 to 10 cm there are very 

poorly sorted coarse sands containing locally derived lithic 

clasts, up to and including granule size. All materials in 

unit A were apparently deposited by slope-wash processes, 

involving flowing water of varying velocities. The upper 

part (0–5 cm) is indicative of slower moving water that 

deposited the laminated silt; some of the silt beds represent 

silt crust formation at times in the past. From 5 to 10 cm 

more rapidly flowing water is implied, and this lower 

part of unit A represents a form similar to a precursor of 

the modern arroyo. The lower contact of unit A is abrupt, 

marked by a prominent change in texture.

10 – 23 cm (Unit B): Unit B can be divided into several 

subhorizons:

10 – 13 cm (Subunit B1): This is a clean fine to medium 

sand, a possible aeolian deposit. Sediment is structureless, 

and the lower boundary is abrupt and bioturbated.

13 – 18 cm (Subunit B2): This subunit consists of 

alternating fine sand and silt. It is primarily fine sand, but 

contains silt beds of thickness 2 to 5 mm. This subunit is 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area
66 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

inferred to have formed under similar conditions to the 

upper part of unit A. It grades down into coarser sediment 

of subunit B3.

18 – 23 cm (Subunit B3): This horizon is a sand and 

gravel lens that is virtually identical in character to the 

lower part of Unit A. Its lower boundary is abrupt. Within 1 

m laterally, this same gravel lens contains grain sizes up to 

small pebbles. 

23 – 61 cm (Unit C): Superficially, this unit resembles flood 

couplets in other profiles, by consisting of an upper fine 

sandy bed underlain by a silty fine sand. In this case, the 

upper fine sand is thinly bedded and locally laminated, 

with laminae inclined toward the northwest (upstream). 

Although mostly fine sand, it is poorly sorted and includes 

some locally derived coarse sand and rare coarse sand 

and granule lenses (single grain thickness). This is clearly 

a water-deposited unit, but it was not deposited all at 

one time. It is most plausibly interpreted as slope-wash 

material, but alternatively may represent a fluvial deposit 

that has been heavily reworked with locally derived 

sediment having been incorporated into it. The silty lower 

part (5 cm thick, from 56 to 61 cm depth) is a silty fine 

sand to fine sandy silt. It is heavily bioturbated and appears 

to have been alternating thin silt beds and fine sand; it is 

too disturbed to say much else with confidence. The lower 

boundary of unit C is diffuse and bioturbated.

61 – 84 cm (Unit D): Unit D consists of sand deposited by 

flowing water (inferred to be slope wash). It is primarily 

a poorly sorted fine to medium sand, which is very 

micaceous with some locally derived coarse and and 

granule sized particles. Within 1 m on either side of the 

profiled area are gravelly channel-fill deposits at this depth. 

Figure 64. Stratigraphic diagram and field sketch of section 10, Arroyo Grande.

E – land surface material with charcoal, one cobble

D – fluvial deposit (fines down to silt, rippled)

C – possible fluvial deposit, possibly with aeolian

reworking. 15-cm-high bar/dune

B – aeolian deposit

A – poorly

exposed

sediment,

stratified

at base

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

0

20

40

60

80

100

120

160

140

0

20

40

60

80

100

120

140

160

Aeolian Colorado River

flood deposit

Base of flood layer

Datum

Sample locations

Dip° directions of ripple

cross-laminations

Ground Surface

X 1

2

3

4 5

Dip dir. 240° 

260

245

285

260

ENE (070°) WSW (250°)

215

220

225

040

245 265

020 295

Dip dir. 060° 

Structureless sediment
67

The gravels contain particles up to (locally) medium-sized 

cobbles. Bioturbation is limited, likely due to the large 

grain sizes present. Other than the gravel lens, unit D is 

structureless.

84 – 88 cm (Unit E): This is a silt which in some respects 

resembles flood silts seen in other profiles. It differs from 

other fluvial silt layers in being much less pure silt, with 

fine and medium sand grains present. The unit is heavily 

burrowed and can be traced laterally for only 3.5 m, 

implying that it may have filled a small channel. Some 

lamination is present in its upper part. Unit E grades 

into very coarse channel-fill gravels in the downstream 

direction (with respect to the Colorado River), and 

disappears to the west (upstream). This silt is underlain and 

overlain by higher energy deposits (gravels and coarser 

sand). The lower boundary is bioturbated but distinct.

88 – 192+ cm (Unit F, to base of section): This lowermost 

unit contains alternating lenses of high-energy waterlain 

deposits, which vary in texture from very poorly sorted 

sands to granules and medium-cobble gravels. These 

sediments are clearly locally derived and are a product of 

slope-wash and localized channel deposition. They appear 

to represent channel-fill gravels, forming lenses that vary 

from 10 to 40 cm in thickness. Unit F is interpreted to have 

been deposited in either surface tongues of coarse slopewash or in small channels.

Section 11 summary.—Stratigraphic profile 11 is a 

slopewash-dominated depositional sequence. Everything 

in the profile, except possibly the 84–88-cm silt (unit E), 

is slope-wash sediment. The relative coarseness of most 

sediment reflects proximity to runoff source. This profile is 

the most distant stratigraphic section from the river, and the 

closest to valley-wall slopes. and consists almost entirely 

of slope-wash material. The present surface is within the 

zone of pebbly surface sediment. There are indications of 

minor pedogenic alteration below the surface. A variety of 

sediment types is represented, ranging from pure silt to fistsize cobbles. In general, the sediments here are the coarsest 

that we have observed in any location in this study. The 

small hearth observed in place at approximately 40–50-

cm depth and east of the profile was burned in place, and 

a very thin granule/small-pebble line and small angular 

pieces of silt that underlie the hearth suggests that a small 

pit may have been dug in preparation for the hearth.

Stratigraphy and Geomorphology at Archaeological Site Locations

Of the four archaeological sites recorded in the Arroyo 

Grande area, observations of feature-specific localities were 

made only in reference to site G:03:064. Within this site, 

features are exposed on the modern land surface. Several of 

the additional features that are exposed within arroyo walls, 

below the surface, are described above as they relate to the 

measured stratigraphic sections.

The land surface around the archaeological features 

shows abundant evidence of recent deflation, cryptogamic 

crust formation, and pedestal development. Dune forms (now 

deflated) are visible near the upstream ends of the arroyo 

branches, above the locations of sections 1 and 2 described 

above (both of which contained sediment with aeolian sedimentary structures). Dunes in this area are shown in figure 66. 

The ‘roots’ of these dunes, where cultural features are located, 

may be fairly deep (for example, in Section 1, aeolian sediment extended from the surface to a depth of ~2 m); however, 

the primary terrace-constructing material has been fluvial 

sand, judging from the relative proportions of fluvial and aeolian sand present in the stratigraphic sections studied. 

Features 1, 2, and 3 are located in the area where coppice 

dunes are present on the surface; feature 1 has been bisected 

by an arm of the arroyo (fig. 67). At the location of feature 

3, topographic relief on the dunes reaches >2 m relative to 

the surrounding upper terrace surface. Features 4 and 5 are 

located in an interdune area that presently has an elevation 

above that of the surrounding upper terrace surface. The land 

Appendix 3: Stratigraphic Sections and Geomorphology, Arroyo Grande Area

50

100

0

Depth (cm)

150

F – slope-wash and local 

 channel-fill deposits

E – channel-fill silt; possible 

 fluvial deposit?

D – slope-wash sediment

C – slope-wash or reworked

 fluvial deposit?

B3 – slope-wash sediment

B2 – slope-wash sediment

B1 – possible aeolian sand

A – slope-wash sediment

Sand

Silt

Gravel

Cross-lamination

Fluvial climbing ripples

Charcoal/ash

Aeolian climbing ripples

Figure 65. Stratigraphic diagram for section 11, Arroyo 

Grande.
68 Sedimentology and Stratigraphy of the Palisades, Lower Comanche, and Arroyo Grande Areas of the Colorado River

immediately surrounding features 4 (which has been cut by 

the arroyo) and 5 has clearly been modified by wind but is 

now deflated and covered by cryptogamic crust, grasses, mesquite, and creosote bushes. Feature 6 is located on an area of 

the deflated upper terrace surface that is near, but not in, small 

coppice dunes and is beginning to be undercut by arroyo incision. Feature 7, on the upper terrace surface not immediately 

near coppice dunes, is not in imminent danger of being undercut by arroyo incision. Features 8 through 11 are all built near 

each other on the surface of the upper fluvial terrace, in an 

area where coppice-dune accumulation is not common. 

Features 12, 13, and 14 appear to have been built on 

small coppice-dune accumulations formed on the upper fluvial terrace. The main arroyo network has incised entirely 

around the perimeter of feature 12; the large mound of firecracked rock has effectively armored itself against gully incision while the sediment around it has been eroded away (fig. 

68). At least 10 cm of deposition appears to have occurred 

after construction of the feature, judging from a small exposure of probable aeolian sediment at the shoreward edge of 

the feature with an elevation above that of the roaster surface. 

Deflation has removed the rest of the sediment that had been 

deposited. Sediment around features 12 and 13 is largely 

structureless silty sand. Feature 13 has been affected by incision of one arm of the secondary arroyo network. Feature 14, 

a roasting feature consisting of fire-cracked rock and ashstained soil, is exposed in a small drainage in close proximity 

to features 12 and 13.

Feature 15 is the only recorded feature north of the 

arroyo network. This roasting feature consists of a mounded 

concentration of fire-cracked rock with a perimeter of dispersed fire-cracked rock. The depositional context of this area 

includes a significant contribution by locally derived slopewash sediment overlying the distal part of the main fluvial 

terrace. Arroyo incision has not yet affected this area of the 

fluvial terrace, though a nickpoint is close to the feature.

Figure 68. Feature 12 of archaeological site G:03:064 at Arroyo 

Grande. The large size of this protohistoric-historic era roasting feature, along with imported artifacts found at this site, indicates the social significance of this area. Like several other features at G:03:064, 

this feature has been affected by gully incision as the main arroyo 

network has grown. The rock concentration of this roasting feature 

has resisted erosion, armoring the structure while the terrace around 

it has eroded; a moat-like gully now surrounds the feature, the floor 

of which is ~0.5 m lower than the elevation of the adjacent terrace 

surface. At the back left side of the roaster, a small remnant of sediment is visible that indicates at least 10 cm of deposition that covered 

part of the structure after the site was built. The arroyo branch that 

affects this site has continued to undergo headward erosion behind 

the armored roasting feature.

Figure 67. Feature 1 of archaeological site G:03:064 at Arroyo Grande. 

This large prehistoric-historic era roasting feature has been eroded 

severely by gully incision in the main arroyo network, so that a crosssection of the roaster is now exposed in the arroyo wall. The inner 

concentration of fire-cracked rock (where food was covered and left 

to cook) is visible in the center, with two exposures of the ring-shaped 

pile of discarded fire-cracked rock on either side.

Figure 66. Dune forms built on the terrace at Arroyo Grande. These 

dunes are interpreted to have formed by aeolian reworking of fluvial 

sediment on this terrace. Dunes are now largely inactive, with vegetation and cryptogamic soil on the land surface. One branch of the main 

arroyo network is in the foreground (photograph faces northwest); the 

vertical arroyo wall in the foreground is approximately 1.5 m high.