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-"'44 ; pits. Sedimentary characteristics were described. and unit thicknesses were measured in detail. for most of lh~ sections disclIssed. At selected locations. sediment samples were col- lected for subsequent grain size analysis using a Coulter laser pi.lrticle-siu analyzer at the Grand Canyon Monitoring and Research Center laboratory in FlagstatT. Arizona. Geomor- phic observations. supplemented by shallow test-pits. were made in the general area of archaeological sites in order to evaluate sedimentary and geomorphic proct:sses directly relevant to each of the sites of interest. To standardize strati- graphic sections, all sections in this report are described beginning with the uppermost unit and progressing dO\'v'nsec- tion. Unless otherwise specified. measurements of strati- graphic depth refer to depth below the land surt:lce. Surveying equipment \\:as used to document the loca- tions of stratigraphic sections, as well as the thalweg location and headward ex lent of major arroyo branchf:s at Palisades and Arroyo Grande. At Palisades and Lower Comanche. the locations and elevations of logs and milled limber interpreted as drift\\'ood were also surveyed; these data are used to place flood events that occur in the stratigraphic record within the context of historic f100J discharge levels. Stratigraphic work, including sediment sampling. has been pem1itted at these \ociJtions under Nntiona) Park Ser- vice Research and Collecting Pem,it #GRCA-2003-SCI- 0101 issued by Grand Canyon National Park. Research that took place at Arroyo Grande, located in the Area of Coopera- lion (AOCI between Ihe Hualapai Nation and the National Park Service. was also authorized by the Hualapai Office of Cultural Resources. All stratigraphic investigations have been conducted in c\os~ collaboration bet\vcen geologists and archaeologists to ensure that culturally significant ar~as are treated with extreme care. Sediment pits and arroyo wall exposures that were logged and studied during this ".,'ork were located away from the immediate vicinity of archaeo- logical features. No cultural artifacts were exposed or col- lected during this work. All artificial pits were filled in at the completion of the field work to conceal the work arens. Identification of Depositional Environments Within a vertical stratigraphic exposure, small-scale sedimentary structures are used as the best diagnostic indi- cator of depositIOnal environment. Structures that are char- acteristic oftluvial and aeolian environments. respectively. can often be identified by the dimensions. scale. grain-size sorting, and spatial orientation of bedding. apparent in out- crop e:~posure. Diagnostic features of fluvial and aeolwn stratification have been described thoroughly else\\/here (for example. Walker, 1963; Stokes, 1908; Harms and olhers. 1975; Hunter, 1977a, b; McKee, 1979; Rubin and Hunter. 1982, 1987; Rubin, 19R7) and will nol be discussed in delail 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 flu- Methods 5 vial structures that represent upper plane-bed conditions). In sucn cases, depositional environment may sometimes be infcrred by observation of lateral gradation into other, more diagnostic stmctures. Numerous stratigraphic horizons studied in these and other areas of Grand Canyon do not contain well preserved sedimentary structures. In most cases, absence of sedimen- tary stmcttlres In fine-grained surficial sediment is iJ.ttribut- able to biologic dTecls. The presence of vegetation on the land surface interferes \vith the development of sedimentary structures as aeolian sediment is deposited: wind ripples forming on a sparsely v~getated surface have shorter crests and less regular trough elevation than on unvegctated sur- faces, 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. Bioturba- tion by trampling, animal hurrowing, and the growth of plant roots can disturb or destroy sedimentary structures after they have fonned. Particularly in cases where the rate of sedi- ment deposition is slow. bioturbation in both subaerial and subaqueous environments can quickly obscure sedimentary structures (for example, DOH and Bourgeois. 1982; Wheat- croft, 1990; Boudroau, 1994). Textural diff~rences (grain size analysis') may in some cases distinguish fluvial and aeolian material if sedimentary stnlctures are indistinct. However. the range of grain sizes observed to occur witnin 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 3eolian deposits in Grand Canyon. Textural analysis as a diagnostic (001 should therefore be used with caution. Ifsedimclltary structures are not distin..:t. reworking of a primary sedimen- tary deposit by wind may be inferred 011 the basis of the morphology of the dt.:posit (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 weath- ering of sediment grains) than primary fluvial or aeolian deposilS (for example, Benito and others, 2003). Surficial runotT events may result in the development of channel fea- IlIres; channels filled with locally derived sand and gravel were obser\led in a number of the stratigraphic sections described during this work. In some locations studied, nota- bly Arroyo Grande, subaerial reworking of sum cia I sedi- ment may be identified by the presence of charcoal and ash concentrations in the soil. Horizons that contain sLlch burned material are readily distinguish3ble from isolated cultural hearth features; the more widespread, difTuse charcoal occur- rences that are common in Arroyo Grande imply a regional burning event at the land surface, such as a grass fire.