holocene loess in illinois revealed by osl dating implications for stratigraphy and geoarchaeology of the midwest united states abstract most uplands in illinois are blanketed by late wisconsinan peoria loess and the conventional wisdom is that little or no dust accumulation occurred during the holocene 11 7 ka to present in a recent effort to investigate if illinois holocene loess was deposited and preserved we applied optically stimulated luminescence osl dating to seven locations where loess is thick these include four sites along the mississippi river in westernmost illinois and three loess bluffs south of the illinois river in central illinois our results suggest that holocene aged loess is preserved along the mississippi river in the state of illinois in contrast no holocene loess was found south of the illinois river in the central portion of the state in addition to its spatially limited distribution holocene loess thicknesses range only up to 1 m nevertheless loess stratigraphy and osl dating results provide the first holocene loess reported east of the missouri river our new finding also resolves the long time archeological puzzle that a number of artifacts of several thousand years old were buried by upland loess along the mississippi river

mississippi valley regional source of loess on the southern green bay lobe land surface wisconsin comment to the paper published by jacobs et al quaternary research 75 3 574 583 2011

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

a full glacial biota from southeastern iowa usa wisconsinan full glacial silts filling a swale exposed in conklin quarry johnson co iowa contain a large and diverse biota that includes pollen bryophytes vascular plant macrofossils small mammals molluscs and insects radiocarbon dates on wood from the top middle and bottom of the swale fill respectively were 16710 270 17 170 205 and 18090 190 yr bp the pollen diagram is dominated by picea spruce pinus pine and cyperaceae sedge and it records low pollen accumulation rates plant macrofossils include a number of tundra species along with picea and larix larch needles and small pieces of wood the insect fauna contains many species now confined to the forest tundra transition zone of northwestern yukon and alaska small mammals include the tundra indicators dicrostonyx collared lemming and probably microtus miurus singing vole together with boreal forest taxa the molluscs include extinct and relict species and show the widest range in present geographic distribution but rocky mountain and especially northern elements predominate in the swale fill all these lines of evidence lead to consistent palaeoclimato logical interpretation and palaeoecological reconstruction the dominant habitats represented by the biota and sedimentary environment collectively included open calcareous silty to sandy or gravelly upland sites minerotrophic fens wetlands pond or stream side clayey to sandy shores and shallow possibly ephemeral cold clear water ponds mean july temperatures were probably 11 to 13 c cooler than at present the biota indicates that a picea larix krummholz with extensive tundra openings was present in southeastern iowa between 18090 and 16710 yr bp

dynamics and sources of last glacial aeolian deposition in southwest france derived from dune patterns grain size gradients and geochemistry and reconstruction of efficient wind directions abstract dune pattern grain size gradients and geochemistry were used to investigate the sources and dynamics of aeolian deposition during the last glacial in southwest france the coversands form widespread fields of low amplitude ridges zibars whereas younger dryas parabolic dunes mainly concentrate in corridors and along rivers spatial modelling of grain size gradients combined with geochemical analysis points to a genetic relationship between coversands and loess the latter resulting primarily from dust produced by aeolian abrasion of the coversands the alluvium of the garonne river provided also significant amounts of dust at a more local scale the geochemical composition of loess shows much lower scattering than that of coversands due to stronger homogenisation during transport in the atmosphere overall sandy loess and loess deposits decrease in thickness away from the coversands dune orientation and grain size gradients suggest that the efficient winds blew respectively from the w to the nw during the glacial and the w sw during the younger dryas a comparison between the wind directions derived from the proxy data and those provided by palaeoclimatic simulations suggests a change of the main transport season ground surface conditions and their evolution throughout the year i e the length of the season with snow and frozen or moist topsoil and the seasonal distribution of wind speeds able to cause deflation are thought to have been the main factors that controlled the transport season in the study area

mars atmospheric circulation aspects from viking landers winds measured by the two viking landers have been filtered and then compared with predictions from the general circulation model and to orbiter observations of clouds and surface phenomena that indicate wind direction this was done to determine the degree to which filtered winds may represent aspects of the general circulation excellent agreement was found between wind direction data from lander 1 and the model predictions and orbiter observations for lander 2 agreement was generally good but there were periods of disagreement which indicate that the filtering did not remove other extraneous effects it is concluded that lander 1 gives a good representation of the general circulation at 22 5 deg n latitude but that lander 2 is suspect most wind data from lander 1 have yet to be analyzed it appears that when analyzed these lander 1 data covering 3 5 mars years can provide information about interannual variations in the general circulation at the lander latitude

late holocene eolian activity in the mineralogically mature nebraska sand hills abstract the age of sand dunes in the nebraska sand hills has been controversial with some investigators suggesting a full glacial age and others suggesting that they were last active in the late holocene new accelerator mass spectrometry radiocarbon ages of unaltered bison bones and organic rich sediments suggest that eolian sand deposition occurred at least twice in the past 300014c yr b p in three widely separated localities and as many as three times in the past 80014c yr at three other localities these late holocene episodes of eolian activity are probably the result of droughts more intense than the 1930s dust bowl period based on independent great plains climate records from lake sediments and tree rings however new geochemical data indicate that the nebraska sand hills are mineralogically mature eolian sands in nebraska have lower k feldspar and k2o rb and ba contents than most possible source sediments and lower k feldspar contents than dunes of similar age in colorado the most likely explanation for mineralogical maturity is reduction of sand sized k feldspar to silt sized particles via ballistic impacts due to strong winds over many cycles of eolian activity therefore dunes of the nebraska sand hills must have had a long history probably extending over more than one glacial interglacial cycle and the potential for reactivation is high with or without a future greenhouse warming

age of colluvium indicates accelerated late wisconsinan hillslope erosion in the upper mississippi valley colluvium on foot slopes in parts of north america and europe has been attributed to a major mass wasting episode during the last glacial period stratigraphic evidence and 14 c ages support this hypothesis for the northern part of the upper mississippi valley colluvium in this region grades laterally into or interfingers with fluvial sediment beneath the late wisconsinan savanna terrace colluvial foot slopes are truncated by fluvial surfaces postdating the incision that created the savanna terrace between 13 and 11 ka samples from within the colluvium have 14 c ages between 18 6 and 12 ka ages of 28 9 and 20 3 ka have been obtained from beneath colluvium and fluvial sediment inset into colluvial foot slopes has yielded ages between 12 5 and 9 8 ka widespread permafrost during the late wisconsinan glacial maximum may explain the onset of accelerated mass wasting that produced the colluvium although mass wasting apparently continued for some time during subsequent climatic warming the results described here imply major long term climatically driven fluctuations in sediment supply from hillslopes to the fluvial system in this region

loess mantle spatial variability and soil horizonation southern wisconsin usa abstract loess is an important quaternary sediment that records evidence of landscape instability in sediment source areas and landscape stability in accumulation areas in wisconsin usa loess mantles glaciogenic deposits dating to the last glaciation but the landscape distribution and effects of the loess mantle on soil formation have received very little attention this study investigates the influence of glacial landforms on the spatial pattern of loess thickness and how the presence or absence of a loess mantle overlying glaciogenic sediments has influenced soil profile morphology namely thickness and nature of horizonation loess thickness follows systematic trends along hillslopes with slope percentage explaining most loess thickness variation between summit and backslope positions in the presence of a loess mantle solum and bt horizon thickness is proportional to the thickness of the loess mantle in well drained profiles in the absence of a loess mantle landscape and sediment characteristics that control permeability and water throughflow appear to be the most important determinants of profile characteristics the loess mantle modulates soil formation above the lithologic discontinuity with sandy calcareous glacial sediments by controlling soil water behavior and having physical and chemical characteristics conducive to transformation to soil genetic horizons

mississippi valley regional source of loess on the southern green bay lobe land surface wisconsin response to comments by randall schaetzl page 574 583

pedological analysis as a key for reconstructing primary loess relief a case study from the magdalenian site in klementowice eastern poland abstract the subject of the study was a gentle and smooth loess slope featuring a magdalenian culture site located near the northern edge of the loess belt in eastern poland based on the probing of 130 soil profiles across a 10 10 m mesh within an area of 1 2 ha the land surface after the deposition of weichselian loess was reconstructed soil mapping indicated the patchwork character of the present soil cover which consists of luvisols of varying degrees of transformation resulting from agricultural use partly eroded and buried soils co occur in the upper part of the ridge flat non eroded soils in the middle part and eroded soils in the lower part in the archaeological site area with buried soils at the foot of the slope the thickness of the primary soil cover was reconstructed by means of geostatistical analyses this was used to reconstruct the primary undulating land relief characterised by numerous small convex forms small hillocks and hummocks and concave forms closed depressions and slope troughs these forms were levelled out by soil erosion in the course of agricultural use the planation of relief caused a decrease of the mean inclination by 25 and elevation difference by 13 the greatest changes occurred on the ridge flat where the denudation of small hillocks and the filling of closed depressions took place local inclinations decreased by up to fourfold and in some places the inversion of slope exposure occurred during the 210 years of agricultural use the eroded surfaces subsided by an average of 0 32 m and at most by 0 92 m which corresponds to an average of 1 5 mm and 4 3 mm per annum respectively less than 50 of the eroded soil material was deposited within the investigated slope most of the material was carried off beyond it the accumulated surface however is 2 5 times smaller than the eroded surface hence the accumulation indicators are slightly higher than the erosion indicators

the effect of surface curvature on the deposition of loess a physical model summary a physical model for the deposition of fine dusty material loess on curved surfaces is presented the model is tested in a sedimentation experiment with a symmetrical hill with length axis perpendicular to the flow direction in a wind tunnel it appears both from the model and the experiment that loess deposition will occur preferably on windward slopes and not on leeward ones in contrast to the widespread wind shadow concept below the deflation threshold loess deposition increases as wind speed increases above the deflation threshold an effective accumulation of the loess layer on the surface will occur within a well defined wind speed interval the loess layer on the surface will erode away only if wind speed has exceeded this interval for flat and dry surfaces free of roughness elements the upper limit of the interval is 7 m s free stream velocity for wet surfaces however it may be considerably higher 15 m s and more

complex response of a midcontinent north america drainage system to late wisconsinan sedimentation abstract the geomorphic evolution of mud creek basin in eastern iowa u s a serves to illustrate how geomorphic influences such as sediment supply valley gradient climate and vegetation are recorded in the alluvial stratigraphic record sediment supply to the fluvial system increased significantly during the late wisconsinan through a combination of periglacial erosion and loess accumulation subsequent evolution of the holocene alluvial stratigraphic record reflects long term routing of the late wisconsinan sediment through the drainage basin in a series of cut and fill cycles whose timing was influenced by hydrologic response to change in climate and vegetation when viewed in a regional context the alluvial stratigraphic record appears to reflect a long term complex response of the flu ial system to increased sediment supply during the late wisconsinan hydrologic and sediment supply changes accompanying the spread of euroamerican agriculture to the basin in the 1800s dramatically upset trends in sedimentation and channel behavior established during the holocene

morphology and paleoenvironmental significance of quaternary sand veins sand wedges and composite wedges tuktoyaktuk coastlands western arctic canada sand and sand ice fillings of quaternary thermal contraction cracks on summer and hadwen islands western arctic canada comprise sand veins sand wedges and composite wedges sand wedges in diamicton poor ice diamicton rich ice and ice rich sand generally have simple v shapes whereas those in ice poor sand vary from v shaped to irregular forms and may contain inclusions of host sand these morphological differences are explained in terms of the relative tensile strength of the wedge and host materials bundles of sand veins within sand wedges indicate discrete stages of wedge growth criteria previously proposed for identifying relict sand wedges are reevaluated 1 not all wedges are v shaped some are irregular forms with offshoot sand veins 2 a vertical or steeply dipping lamination is not apparent within all wedges some appear to have a massive fill suggesting that the sand source can be texturally and mineralogically very uniform 3 individual sand veins and groups of veins can be just as common within sandy host materials as the better known sand wedges composite sand ice wedges at crumbling point summer island commenced growth as composite wedges continued as sand wedges were modified by thermokarst and in some cases recommenced at a stratigraphically higher level as ice wedges the sand wedge and ice wedge stages reflect environmental change from cold arid and windy proglacial conditions during oxygen isotope stage 2 to warmer and wetter interglacial conditions during oi stage 1

evolution of eureka flat a dust producing engine of the palouse loess usa sedimentologic stratigraphic and pedologic evidence demonstrates that eureka flat ef a narrow 80 km long deflation plain in south central washington usa is the source for the thick and widespread loess of the palouse region located in the southern columbia plateau ef has been a depocenter for abundant clay to sand sized sediment derived from repeated glacial outburst floods that swept through the area during the pleistocene prevailing sw winds that were funneled across ef remobilized flood sediment into sand dunes sand sheets and loess an eolian sand sheet buried beneath 1 8 m of loess at the downwind end of ef attests to bioclimatically driven changes in the style of eolian deposition the depositional transition from sand sheet to loess is characterized by a change from wind rippled strata to structureless bimodal sand and silt with insect burrows and rhizoliths to unimodal sandy loess soil moisture and density of dust trapping vegetation control where sand dunes and loess accumulate up to 4 5 m of post lgm loess mantles the landscape adjacent to and downwind from ef some of the thickest loess deposits owe their origins to the sequestering of eolian sand particles by topographic traps such as the touchet river and its tributary valleys the removal of saltating sand from the eolian system promotes thick accumulation of loess downwind of topographic traps patterns of loess accumulation across the columbia plateau are identified on isopach maps of major pre and post lgm loess units and show that ef has been the major engine for the production of atmospheric dust and loess during both times

aeolian sediments on the north eastern tibetan plateau abstract aeolian sediments on the tibetan plateau are an important archive of palaeoclimatic information this study presents a detailed analysis of sediments from the donggi cona catchment on the north eastern part of the tibetan plateau long and short distance sediment transport leads to a complex pattern of aeolian sediment deposition that depends on climatic changes as well as on the availability of sediments based on the largest dataset of osl datings 51 from a single catchment on the tibetan plateau so far different periods of increased sediment transport have been reconstructed increased aeolian deposition in this high elevation environment started in the early holocene with the accumulation of sands from around 10 5 to 7 ka loess sediments have been preserved from a period from 10 5 to 7 5 ka both archives are related to the strengthening of the asian summer monsoons characterized by wetter and warmer climate this change in climate supported the trapping of aeolian sediments under full monsoon conditions from around 9 ka onwards fluvial processes resulted in erosion of the aeolian archives and the formation of colluvial sediments until 6 ka a dry and cooler climate resulted in the reactivation of dune sands from 3 ka to present possibly in combination with stronger human influence aeolian sediments on the tibetan plateau therefore indicate two different climatic modes during the early holocene wetter conditions were favourable to retain aeolian sediments the reactivation of sediment in the late holocene due to small scale disturbances in the vegetation cover points to a cooler and drier climate

a new model of topographic effects on the distribution of loess a model of topographic influence on the regional transport and accumulation of loess in sparsely vegetated landscapes is proposed based on the critical role played by saltating eolian sand in the entrainment and long distance transport of finer grained suspended dust in this model long term accumulation of loess occurs mainly downwind of topographic obstacles that limit eolian transport of sand and associated re entrainment of dust regional and local patterns of distribution of late wisconsinan loess in parts of the upper mississippi river basin are closely related to the distribution of topographic obstacles to sand transport these cases of topographic influence occur in a region where evidence exists for sparse tundra like vegetation during the deposition of loess in the late wisconsinan similar topographic effects may not be evident further south in the missouri and mississippi river basins because vegetation density was generally sufficient to prevent the movement of eolian sand out of source valleys thus topographic obstacles and dense vegetation may have similar effects on loess distribution under different environmental conditions

eolian sand and loess deposits indicate west northwest paleowinds during the late pleistocene in western wisconsin usa our study adds to the quaternary history of eolian systems and deposits in western wisconsin usa primarily within the lower chippewa river valley thickness and textural patterns of loess deposits in the region indicate transport by west northwesterly and westerly winds loess is thickest and coarsest on the southeastern flanks of large bedrock ridges and uplands similar in some ways to shadow dunes in many areas sand was transported up and onto the western flanks of bedrock ridges as sand ramps presumably as loess was deposited in their lee long linear dunes common on the sandy lowlands of the chippewa valley also trend to the east southeast small depressional blowouts are widespread here as well and often lie immediately upwind of small parabolic dunes finally in areas where sediment was being exposed by erosion along cutbanks of the chippewa river sand appears to have been transported up and onto the terrace treads forming cliff top dunes luminescence data indicate that this activity has continued throughout the latest pleistocene and into the mid holocene together these landforms and sediments paint a picture of a locally destabilized landscape with widespread eolian activity throughout much of the postglacial period

dust transport and the question of desert loess formation although contemporary dust storms are frequent in arid and semi arid areas desert loess deposits are poorly developed much of the world s loess occurs in mid latitude areas which experienced glaciation during the pleistocene ocean core evidence indicates that dust transport from sub tropical deserts increased during cold stages of the pleistocene but loess formed only on certain desert margins for reasons which have not been fully explained this paper re examines the mechanisms of dust transport and deposition and the circumstances leading to the accumulation of thick loess typical loess is composed mainly of medium silt grains which are transported in short term suspension a few metres above the ground significant thicknesses of loess form only when dust is trapped within a limited area often relatively close to the source dust particles finer than 20 m are transported mainly in long term suspension over a greater height range and may be widely dispersed the availability of silt and the frequency magnitude and direction of dust transporting winds are important factors governing the potential for loess formation but the existence of a suitable dust trap is particularly important traps may be formed by topographic obstacles areas of moist ground or vegetated surfaces vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid latitudes during the pleistocene dust blown during glacial periods from certain deserts notably in sinai soviet central asia and china accumulated as loess in neighbouring semi arid regions on the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps during most of the holocene net dust deposition rates in all desert marginal areas have been too low for significant loess accumulation this is mainly due to a reduction in silt availability and a tendency towards landscape stability reported dust storm frequencies during the past 50 years over estimate the longer term holocene dust flux due to the effects of human activities much modern dust owes its origin to erosion of cultivated soils in semi arid areas and is finer than typical loess

origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry muhs d r bettis iii e a aleinikoff j n mcgeehin j p beann j skipp g marshall b d roberts h m johnson w c benton r 2008 origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry gsa bulletin 120 11 12 1378 1407

transport direction of wisconsinan loess in southeastern minnesota a study of wisconsinan loess in part of southeastern minnesota confirms earlier suggestions that much of the loess in this region was not derived flora the floodplain of the mississippi river two wisconsinan loess units the peoria loess and roxana silt occur in the study area peoria loess 1 8 m thick fines systematically eastward from an abrupt western border toward the mississippi there are no apparent grain size trends away from other adjacent rivers peoria loess thickness generally decreases eastward but is highly variable probably because of differential erosion potential sources for this unit are pre wisconsinan sediments on the iowan erosion surface immediately west of the border of thick loess and wisconsinan glacial sources tens or hundreds of kilometers to the west the underlying roxana silt up to 1 m thick occurs only near the mississippi and fines away from that river the roxana silt deposit could reflect aggradation of the mississippi floodplain because of glacial activity upstream and does not have clear implications for upland vegetation near the study area by contrast transport of the peoria loess from distant glacial sources or wind erosion of upland surfaces closer to the loess deposit both imply extensive very sparsely vegetated surfaces west of the study area when the peoria loess accumulated

from dust to dust quaternary wind erosion of the mu us desert and loess plateau china the ordos basin of china encompasses the mu us desert in the northwest and the chinese loess plateau to the south and east the boundary between the mostly internally drained mu us desert and fluvially incised loess plateau is an erosional escarpment up to 400 m in relief composed of quaternary loess linear ridges with lengths of 10 2 10 3 m are formed in cretaceous quaternary strata throughout the basin ridge orientations are generally parallel to near surface wind vectors in the ordos basin during modern winter and spring dust storms our observations suggest that the loess plateau previously extended farther to the north and west of its modern windward escarpment margin and has been partially reworked by eolian processes the linear topography mu us desert internal drainage and escarpment retreat are all attributed to wind erosion the aerial extent of which expanded southeastward in china in response to quaternary amplification of northern hemisphere glaciation

geomorphological and palaeoclimate dynamics recorded by the formation of aeolian archives on the tibetan plateau abstract aeolian sediments are an important archive for palaeoclimate reconstructions on the tibetan plateau the accumulation of aeolian sediments and the formation of palaeosols are used as a proxy for dry and wet phases respectively during the last decade the number of osl optically stimulated luminescence ages of aeolian sediments has been rapidly increasing this study summarises the results from more than 300 individual osl ages despite the widespread occurrence of aeolian sediments on the tibetan plateau nearly all osl samples are located on the northeastern and the southern part in most regions the strongest accumulation occurred during the late glacial and in the early holocene this coincides with the strengthening of the asian summer monsoon on the tibetan plateau and wetter climate conditions after the relatively dry glacial times the development of a sufficient vegetation cover acting as a sediment trap seems to be an important requirement for the preservation of aeolian archives short dry periods like the younger dryas led to a more enhanced aeolian accumulation without destruction of previously formed archives in some regions the formation of palaeosols occurred mainly during the wet early and mid holocene but the number of ages is relatively small in contrast a second phase of aeolian accumulation during the late holocene was related to the weakening of the asian summer monsoon and drier climate conditions this indicates that the reaction of aeolian accumulation on the tibetan plateau depends on the previous climate state a comparison between the osl ages from aeolian sands and loess deposits yields a different timing of deposition of these two sediment types while the sand deposits in different basins resemble wet and dry phases loess deposits show no common signal between the analysed areas this indicates an influence of local factors such as varying source areas and erosive processes on the slopes in general the aeolian archives in the high mountain environment are governed by numerous factors besides the climatic influence

spatial temporal differentiation of eolian sediments in the yarlung tsangpo catchment tibetan plateau and response to global climate change since the last glaciation abstract eolian sediments an important paleoenvironmental archive are widespread in the yarlung tsangpo river ytr valley of the southern tibetan plateau tp but their chronology and spatial temporal differentiation are poorly understood and it is not clear whether eolian accumulation is controlled by global climate changes or local paleoenvironment or both in this study we applied quartz osl dating to 30 eolian sediment samples sandy loess and eolian sand from eight profiles in the ytr catchment our new dates were combined with 72 previously published eolian sediment ages osl tl and 14c for the ytr catchment to analyze the response of eolian accumulation to paleoenvironmental changes the overall dataset has eolian accumulation spanning the period from the last interglacial through to the little ice age ranging from 84 6 8 7 ka bp possibly as old as 118 11 ka bp to 0 4 0 1 ka bp with most occurring since the late glacial period 15 ka bp which suggests that the preservation of eolian deposits is controlled by geologic recirculation probability density function distributions pdfs of eolian ages from different parts of the ytr catchment show different age clusters suggesting that factors controlling eolian accumulation vary across the catchment so that spatial temporal disparities are inherent in the system to investigate the effect of regional and global paleoclimate since the last glacial maximum lgm on eolian deposition processes in the ytr we compared the pdf of ages from the combined dataset with a range of paleoclimate proxies the pdf shows significant fluctuations since the lgm including the younger dryas cold event there is no consistent eolian sediment response to changes of the 30 n summer insolation asian summer monsoon and westerlies phases of strong eolian sediment accumulation in the ytr basin do not show a simple correspondence with the classical global climate curve suggesting that eolian processes in the alpine valley environment may be modified by local responses to these changes

late pleistocene loess like deposits in the coastal area of south china abstract a layer of yellow silt with thickness from tens of centimeters to a few meters is widely distributed on low hills and terraces in the coastal area of south china the yellow silt has different sedimentological characteristics from other common types of quaternary sediments and the bedrock weathering material in the area five representative sections were studied using scanning electron microscopy grain size analysis diffuse reflection spectroscopy geochemical analysis and optically stimulated luminescence osl dating to reveal the sedimentary characteristics deposition environment origin and age of the yellow silt subrounded subangular quartz grains showed typical aeolian impact pits such as dish like and crescent pits and chemical precipitation and corrosion on grain surfaces the grain size composition grain size parameters and unimodal frequency distribution curves of the yellow silt were similar in all sections examined and consistent with those of north and central china loess the iron mineral content of the yellow silt was dominated by hematite indicating a similar sedimentary environment to that of typical chinese loess the major oxide composition of the yellow silt with respect to normalized upper continental crust ucc distribution patterns was close to that of the ucc and similar to north china loess and other aeolian deposits in china however the data suggested that the yellow silt in the coastal area of south china had suffered strong weathering after deposition the osl ages of the yellow silt indicated a deposition age of late pleistocene the results of the various analytical methods all showed an origin correlation and regular transition of loess deposits from northwest to southeast china hence it was concluded that the yellow silt widely distributed in the coastal area of south china is a loess like deposit formed during the late pleistocene it probably mostly originated from northwest inland china and constitutes the south extension of other loess in china

the chronological position of the lohne soil in the nussloch loess section re evaluation for a european loess marker horizon abstract the loess section of nussloch in southwestern germany is a key profile for the reconstruction of the terrestrial palaeo environment of central europe at the time of the last glacial and interglacial recently the significance of the site for palaeo environmental and geoarchaeological research increased additionally with the discovery of unique palaeolithic cultural remains from anatomically and culturally modern humans that invaded southwestern germany during the weichselian wurmian pleniglacial period at nussloch a chrono stratigraphical key position is taken by a middle pleniglacial cambisol remain which under the designation lohne soil serves as an important pedostratigraphic marker horizon for the greater area repeatedly greenland interstadial gis 8 was suggested as a likely period of soil formation for the lohne soil this interpretation is yet not justified on the basis of published chronometric data critical assessment of the data points to a later period of soil formation likely during gis7 to gis5 this conclusion is supported by a new set of radiocarbon ages for the nussloch site which are presented here for the first time consequences of a revised chronology for correlations of pleniglacial cryosols below and above the lohne soil with greenland interstadials are discussed the implications are important for european loess research as the nussloch section serves as a reference base for loess sections throughout europe

plant and insect remains from the wisconsinan interstadial stadial transition at wedron north central illinois abstract organic material exposed within a small swale fill in pit 6 of the wedron silica sand co near wedron in lasalle county illinois includes well preserved pollen plant macrofossils and insect remains this material occurs in slackwater sediment in the lower part of the peddicord formation which was deposited as existing valleys were dammed by fluvial aggradation during the initial late wisconsinan advance of laurentide ice into the wedron area wood from the organic horizon has a radiocarbon age of 21 460 470 yr b p isgs 1486 the pollen spectrum is dominated by picea pinus and cyperaceae plant macrofossils comprise a mix of boreal forest taxa including picea larix laricina and the moss campylium stellatum subarctic species including betula glandulosa empetrum nigrum and selaginella selaginoides along with the predominantly arctic vaccinium uliginosum var alpinum dryas integrifolia and rhododendron lapponicum the insect fauna contains the western montane ground beetle opisthius richardsoni several arctic subarctic ground beetles including diacheila polita helophorus sibiricus and pterostichus cryobius caribou and a diverse assemblage of insects that today inhabit the boreal forest we interpret the biotic record to record a phase in the transition from closed boreal forest to open tundra as climatic conditions deteriorated in advance of continental glaciation

loess studies in central united states evolution of concepts abstract few words in the realm of earth science have caused more debate than loess it is a common term that was first used as a name of a silt deposit before it was defined in a scientific sense because this loose deposit is easily distinguished from other more coherent deposits it was recognized as a matter of practical concern and later became the object of much scientific scrutiny loess was first recognized along the rhine valley in germany in the 1830s and was first noted in the united states in 1846 along the lower mississippi river where it later became the center of attention the use of the name eventually spread around the world but its use has not been consistently applied over the years some interpretations and stratigraphy correlations have been validated but others have been hotly contested on conceptual grounds and semantic issues the concept of loess evolved into a complex issue as loess and loess like deposits were discovered in different parts of the us the evolution of concepts in the central us developed in four indefinite stages the eras of 1 discovery and development of hypotheses 2 conditional acceptance of the eolian origin of loess 3 bandwagon popularity of loess research and 4 analytical inquiry on the nature of loess toward the end of the first era around 1900 the popular opinion on the meaning of the term loess shifted from a lithological sense of loose silt to a lithogenetic sense of eolian silt however the dual use of the term fostered a lingering skepticism during the second era that ended in 1944 with an explosion of interest that lasted for more than a decade in 1944 r j russell proposed and h n fisk defended a new non eolian property based concept of loess the eolian advocates reacted with surprise and enthusiasm each side used constrained arguments to show their view of the problem but did not examine the fundamental problem which was not in the proofs of their hypothesis but in the definition of the term between 1944 and about 1950 the debates about loess reached a maximum level of complexity the main semantic problem was submersed in peripheral arguments about physical properties and genetic interpretations the scholarly treatment of the subject by fisk and russell stimulated quality responses from a diversity of earth scientists interested in academic and applied studies particularly geo history pedology soil mechanics and stratigraphy the long lasting popularity of loess studies during the bandwagon era lasted to about 1970 by that time the analytical and technical interests had attracted the mainstream into the fourth era with a focus beyond the old arguments although fisk and russell found themselves defending an unpopular theory they stimulated a scientific interest in the late quaternary history of the mississippi valley that may never be exceeded

the nature origin and accumulation of loess abstract loess can be defined simply as a terrestrial clastic sediment composed predominantly of silt size particles which is formed essentially by the accumulation of wind blown dust there are three fundamental requirements for its formation 1 a sustained source of dust 2 adequate wind energy to transport the dust and 3 a suitable accumulation site most loess has been modified to some degree by local reworking bioturbation and syn depositional weathering but a process of loessification is not necessary for a dust deposit to qualify as loess the terms primary loess and secondary loess have previously been used to described wind deposited loess and redeposited loess respectively but it is more accurate to describe material reworked and redeposited by non aeolian processes as loess derived colluvium or loess derived alluvium during the quaternary loess formed in three main situations 1 mid continental shield areas beyond the limits of ice sheets periglacial loess 2 on the margins of high mountain ranges perimontane loess and 3 on the semi arid margins of some lowland deserts peridesert loess peridesert loess is relatively poorly developed partly due to the limited efficiency of silt generating mechanisms in low land deserts compared with glaciation but also important is the tendency in arid areas for dust to be dispersed over a wide area and to be rapidly reworked after deposition particularly in areas which experience frequent climatic changes the remarkably thick loess of china and central asia results from an unusual combination of conditions which has persisted for much of the last 2 3 million years namely rapid uplift of the tibetan plateau and surrounding mountain ranges high rates of sediment production and supply to adjacent basins a strong northwesterly and westerly wind regime and the existence of effective dust traps downwind of the source regions

aeolian deposition of dust over hills the effect of dust grain size on the deposition pattern wind tunnel experiments were conducted to investigate the effects of topography on the grain size characteristics of aeolian dust deposits experiments were performed on three isolated hills having various size and aspect ratios the longitudinal profile of the median grain diameter was investigated for each hill the longitudinal dust deposition profile was also studied for nine grain size classes of between 10 and 104 m as were wind and dust concentration profiles in the atmosphere upwind of over and downwind of a hill the wind tunnel experiments show that the grain size characteristics of aeolian dust deposits are affected by topography most apparent is the occurrence of a zone of reduced grain size on the leeside of hills which extends from just upwind of the summit to a distance of several times the height of the hill slightly coarser than normal dust is deposited on the concave windward hill slope and in a zone downwind of the area of reduced grain size but the increase in grain size in these zones remains very small although the normalized dust deposition profile for a hill does not vary substantially as a function of grain size systematic trends are observed the most important tendencies are 1 a progressive extension in the downwind direction of a zone of decreased dust deposition on the leeside of a hill the coarser the grains the further downwind the zone of reduced deposition extends 2 a progressive increase in dust deposition immediately upwind of a hill the finer the grains the higher the deposition value upwind of a hill becomes both tendencies are explained by the difference in inertia of the grains which is controlled by grain size copyright 2005 john wiley sons ltd

holocene palaeosols and aeolian activities in the umimmalissuaq valley west greenland aeolian sand sheets and active dunefields preserve an ancient holocene land surface represented by palaeosols that occur around the present ice margin in the kangerlussuaq area west greenland to determine the relation between holocene aeolian activities and periods of soil formation both substantially dependent on the deglaciation history palaeosols aeolian sand sheets and dunefields were analysed using field data grain size analyses optically stimulated luminescence dating and ams 14c data in an area of about 15 km2 of the umimmalissuaq valley palaeosols are developed close to the ice margin 60 wt are comparable with aeolian sand sheets currently formed at greater distances 4 5 km from the present ice margin we propose a transport distance for fine aeolian sediments in which the palaeosols are formed of at least 4 km from inboard of the present ice margin soil for

topographic and climatic influences on accelerated loess accumulation since the last glacial maximum in the palouse pacific northwest usa abstract topographic and climatic influences have controlled thick loess accumulation at the southern margin of the palouse loess in northern oregon juniper and cold springs canyons located on the upwind flank of the horse heaven hills are oriented perpendicular to prevailing southwesterly winds these canyons are topographic traps that separate eolian sand on the upwind side from thick accumulations nearly 8 m of latest pleistocene to holocene l1 loess on the downwind side silt and sand rich glacial outburst flood sediment in the umatilla basin is the source of eolian sand and loess for the region sediment from this basin also contributes to loess accumulations across much of the columbia plateau to the northeast downwind of cold springs canyon mt st helens set s and glacier peak tephras bracket 4 m of loess demonstrating that approximately 2500 g m 2 yr 1 of loess accumulated between about 15 400 13 100 cal yr b p mass accumulation rates decreased to approximately 250 g m 2 yr 1 from 13 100 cal yr b p to the present tephrochronology suggests that the bulk of near source palouse loess accumulated in one punctuated interval in the latest pleistocene characterized by a dry and windy climate

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

loess of the upper mississippi valley driftless area abstract loess of the driftless area includes four distinct late quaternary lithostratigraphic units the wyalusing new loveland roxana and peoria formations erosion has removed parts or all of the pre late wisconsian loess at many sites these formations consist largely of loess and retransported loess that typically occurs on uplands terraces and valley margins in the region the oldest widespread formation probably 125 000 yr b p is here formally named the wyalusing formation the three younger formations correlate with the loveland probably 125 000 yr b p roxana 55 000 25 000 yr b p and peoria 25 000 12 000 yr b p loesses recognized elsewhere in the midcontinental united states the soil stratigraphic morphology of the wyalusing loveland and roxana peoria couplets appears to represent two distinct loess sedimentation sequences related to major expansion and contraction of the laurentide ice sheet during oxygen isotope stages 6 and 3 2 respectively the occurrence frequency of loess units is inversely related to age illustrating the erosional nature of the driftless area landscape the occurrence of four loess units at some sites in driftless area stratigraphic sections corroborates loess stratigraphy along the length of the mississippi valley where typically not more than four or five loess units are found and they represent only the late and middle pleistocene

age erosion constraints on an early pleistocene paleosol in yukon canada with profiles of 10be and 26al evidence for a significant loess cover effect on cosmogenic nuclide production rates abstract wounded moose type paleosols developed on remnant deposits of late pliocene to early pleistocene pre reid cordilleran ice sheet cis glaciations in central yukon canada it is an important regional soil geomorphic marker at the boundary between early cis advances and the non glaciated regions of yukon and alaska yet at present its age is poorly constrained between the reid 0 2 ma and earliest 2 84 ma cis advances here we apply depth profiles of in situ produced cosmogenic 26al and 10be to obtain both a minimum exposure age 1 12 0 44 0 36 ma 2 and maximum erosion rate 1 1 0 9 0 5 m myr 1 for the wounded moose paleosol our results show that this soil formed under exceptionally stable conditions max erosion rate similar to polar bedrock erosion rates and that it pre dates the emergence of the 100 ka eccentricity climate cycle contrasting our results from single and joint nuclide depth profile models reveals a significant discrepancy between calculated and effective 10be and 26al production rates 40 65 of expected values we interpret this discrepancy as the result of intermittent loess cover with a time averaged depth between 60 and 110 cm which significantly reduced apparent exposure ages obtained from the single nuclide model the observation of such a significant loess cover effect on cosmogenic nuclide production has implications for exposure dating in glacial and periglacial environments a multi nuclide sampling strategy is required to quantify this effect

last glacial loess in the conterminous usa the conterminous united states contains an extensive and generally well studied record of last glacial loess the loess occurs in diverse physiographic provinces and under a wide range of climatic and ecological conditions both glacial and non glacial loess sources are present and many properties of the loess vary systematically with distance from loess sources united states midcontinent last glacial loess is probably the thickest in the world and our calculated mass accumulation rates mars are as high as 17 500 g m 2 yr at the bignell hill locality in nebraska and many near source localities have mars greater than 1500 g m 2 yr these mars are high relative to rates calculated in other loess provinces around the world recent models of last glacial dust sources fail to predict the extent and magnitude of dust flux from the mid continent of the united states a better understanding of linkages between climate ice sheet behaviour routing of glacial meltwater land surface processes beyond the ice margin and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region r 2003 elsevier ltd all rights reserved

the role of climate and local regolith landscape processes in determining the pedological characteristics of aeolian dust deposits across south eastern australia abstract fine grained aeolian sediments are an important component of many loessic soil landscape systems across south eastern australia these loessic soils are commonly related to the deposition of parna a red clayey calcareous material proposed to have been transported predominantly as silt sized pellets and companion silt grains however it is apparent that loessic soils of south eastern australia do not necessarily exhibit or retain the morphological features regarded as typical of parna this may be due to variations in post depositional pedogenic processes operating over long periods of time it is important to understand this inconsistency in loessic soil expression because the physico chemical and structural attributes of these profiles may significantly affect catchment wide processes such as salinisation and erosion in this paper the pedological features of a range of previously identified loessic profiles occurring across south eastern australia are examined to assess the relative importance of climate and local landscape processes on profile characteristics most of the sampling sites occur in mid to upper slope positions while the annual average rainfall of the sites ranges from 440 mm to 650 mm the deepest profiles in the driest locations tend to yield whole coloured slightly alkaline slightly saline and sodic subsoils with moderate cation exchange capacities profiles in the wettest locations are generally characterised by smaller cation exchange capacities are acidic non saline and non sodic and feature prominent mottling and sesquioxide nodules measurements and predictions of the structural attributes of the profiles indicate that all topsoils are quite stable to wetting whereas subsoils exhibit variable stabilities depending on physico chemical properties these results appear to demonstrate a broad climosequence effect on loessic soil attributes the climosequence effect was compared to the changes in loessic profile characteristics down a case study toposequence in southern new south wales the toposequence indicates that local landscape features such as slope and drainage have also been important in determining the properties of these materials as has re working by colluvial processes we conclude that to explain adequately the current characteristics of profiles containing fine grained aeolian sediments particularly their salinisation potential and stability to erosion it is critical to consider both the past and present climate and the local soil forming processes

last glacial aeolian landforms and deposits in the rhone valley se france spatial distribution and grain size characterization abstract in the rhone valley a north south oriented cenozoic rift in southeast france thick pleistocene loess deposits have been recognized since the beginning of the last century these loess records which are disconnected from the north european loess belt nelb are of significant interest to document the evolution of perimediterranean landscapes and environments during the last glacial to overcome the poor precision of available aeolian distribution maps aeolian deposits were mapped using the topsoil textural database provided by the land use and cover area frame statistical survey project lucas the grain size distribution of aeolian sand and loess was first determined using 116 samples taken from surveyed outcrops then the areas showing a similar grain size composition were extracted from the lucas rasters the resulting map reproduces the conventional maps correctly but suggests a more significant extension of loess in better agreement with the known distribution of outcrops the map shows that the distinctive morphology of the valley dominantly controls the distribution of aeolian deposits the deflation related landforms i e yardangs closed depressions pans and desert pavements are widespread south of narrowings of the rhone valley between latitudes 44 n and 45 n they indicate palaeowinds blowing from the north northwest aeolian sand loessic sand sandy loess and loess deposits successively spread on both sides of the rhone river the loess is characterized by a coarse texture main mode around 60 m strong local thickness 5 m limited extension and abundant bioturbation this preservation results from the persistence of a shrub vegetal cover during the coldest and driest phases of the last glacial that allowed for trapping the saltating and suspended particles close to the alluvial sources

chronologic evidence for multiple periods of loess deposition during the late pleistocene in the missouri and mississippi river valley united states implications for the activity of the laurentide ice sheet abstract the loess stratigraphy of the mid continental u s is an important proxy record for the activity of the laurentide ice sheet in north america one of the most outstanding problems is deciphering the age of loess deposits in this area during the late pleistocene radiocarbon dating of snails and thermoluminescence dating of the fine silt fraction 4 11 m from loess at the loveland loess type section loveland iowa and a recent excavation at the pleasant grove school section madison county illinois provide new chronologic control on loess deposition in the mississippi missouri river valley chronology indicates that the loveland loess is illinoian in age 135 20 ka but is not correlative with the teneriffe silt which is dated to 77 8 ka concordant radiocarbon and thermoluminescence age estimates demonstrate that the roxana silt and a correlative loess in iowa the pisgah formation is probably 40 30 ka old these age estimates in conjunction with previous results indicate that there were four periods of loess deposition during the last 150 ka at 25 12 ka 45 30 ka 85 70 ka and at ca 135 20 ka this chronology of loess deposition supports the presence of both a late illinoian and early wisconsinan loess and associated soils thus there may be more than one soil in the loess stratigraphy of the mid continental u s with morphologies similar to the sangamon soil the last three periods of loess deposition may be correlative with periods of elevated dust concentrations recorded in the dye 3 ice core from southern greenland this is particularly significant because both areas possibly had the same source for eolian particles reconstructions of atmospheric circulation for glacial periods show a southerly deflected jet stream that could have transported dust from the mid continental usa to southern greenland lastly the inferred record of loess deposition is parallel to a chronology for deglaciation of the laurentide ice sheet deciphered from chronologic and stratigraphic studies of raised glacial and marine sediments in the hudson bay lowlands canada these chronologies indicate that the laurentide ice sheet was quite dynamic during the late pleistocene advancing and retreating across north america at least four times during the last 150 ka

mississippi valley regional source of loess on the southern green bay lobe land surface wisconsin loess has been recognized on the glacial land surface of the green bay lobe for over 100 yr but no systematic explanation of the source of the loess has been advanced intriguingly the loess on the green bay lobe land surface is thicker than predicted by regional thinning trends from the mississippi valley and is geographically separated from much loess of southwest wisconsin by a sandy region devoid of loess mapping based on soil survey interpretation indicates that the loess occurs above an escarpment marking the eastern end of the sandy loess free region particle size fining trends demonstrate that the loess was transported by northwesterly winds clay mineralogy of the green bay lobe loess is distinctly different than glaciogenic sediments and matches loess of the mississippi valley indicating a regional source and long distance transport of the loess we propose the loess was transported from the regional source along a surface of transport produced by migration of eolian sand through low relief landscapes including the glacial lake wisconsin basin eolian sand migration caused repeated entrainment of dust leading to east southeastward transport the loess accumulated above an escarpment that limited sand mobility and re entrainment of loess beyond this topographic barrier

up in the refrigerator geomorphic response to periglacial environments in the upper mississippi river basin usa abstract james c knox was best known for his work on holocene and historical changes in fluvial systems but he also had a long standing interest in the effects of late pleistocene periglacial environments on landscape evolution in parts of the upper mississippi river basin that were just outside the laurentide ice sheet margin or as knox put it up in the refrigerator knox and others in the quaternary community of the midwestern u s often suggested that hillslope erosion was accelerated under periglacial conditions so that glacial periods have had a dominant effect on the landscape we see today this paper reviews the evidence and reasoning supporting that view in a study area of the upper mississippi basin bordered on three sides by ice margins of the last glaciation including the wisconsin driftless area and adjacent landscapes sparse but compelling paleoecological data and relict ice or sand wedge polygons provide clear evidence for a cold climate and widespread permafrost around the peak of the last glaciation in highly dissected relatively high relief parts of the study area the loess and soil stratigraphy on ridgetops and the colluvial mantles on steeper slopes are best explained by highly effective hillslope erosion including solifluction during and just after the last glacial maximum knox used the post depositional truncation of a loess unit to quantify contrasting late pleistocene and holocene sediment yields from a small driftless area watershed while the late pleistocene yield indicates accelerated erosion it is still lower than modern sediment yields in many tectonically active or semiarid landscapes and it may reflect deposition of highly erodible loess as well as effects of periglacial conditions the views of knox and other midwestern geomorphologists on landscape evolution through glacial interglacial cycles were highly influenced by the work of robert v ruhe ruhe proposed that an episode of widespread erosion during and just after the last glacial maximum can explain enigmatic aspects of quaternary stratigraphy and the soil landscape on the iowan erosion surface a very low relief landscape of the study area ruhe s key evidence is still valid though it needs to be separated from an implausible model of landscape evolution interpretation of the iowan erosion surface and other low relief landscapes just outside the ice margin also requires recognition of the profound effect of eolian processes on those landscapes under periglacial conditions many new insights on landscape evolution in the study area could result from wider application of cosmogenic nuclide based methods to assess glacial interglacial changes in basinwide rates of erosion and residence time of soils just as important a need exists for much more field based characterization of hillslope fluvial and eolian sediments for comparison with those of modern permafrost regions and past periglacial environments in europe

roxana silt of the upper mississippi valley lithology source and paleoenvironment significant lithological differences exist between the middle wisconsinan roxana silt loess of the upper mississippi valley the valley north of 41 5 and that of the type area near st louis missouri primary differences are that the upper mississippi valley roxana silt is much thinner finer textured and more weathered than that of the type area extrapolation of radiocarbon dates brackets sedimentation of upper mississippi valley roxana silt between 55 and 27 ka which is in close agreement with dates from the type area radiocarbon dates and paleobotanical data indicate that the upper mississippi valley roxana silt accumulated at a maximum long term average sedimentation rate of about 7 cm 1 000 yr within a boreal forest environment whereas the type area roxana silt accumulated at a maximum long term average sedimentation rate of about 60 cm 1 000 yr within a prairie like environment multiple regression analysis shows that more than 75 of the variance in thickness and texture between the upper mississippi valley roxana silt and that of the type area is explained by parameters of source valley width source valley azimuth and distance of the depositional site from the source valley this indicates that although the lithologic characteristics and sedimentation rates between the two regions are quite different they are consistent with a unified loess sedimentation sys tem linked to the ancient mississippi river valley a proglacial source for the roxana silt and a middle wisconsinan advance of the lauren tide ice sheet is a topic of scientific debate because recent findings show a lack of roxana correlative till deposits in the midwest alternative hypotheses include a glaciolacustrine source a regional slope erosion source and a cold climate desert dust source tests of these hypotheses support a proglacial origin for the roxana silt multiple regression analysis of loess thickness and texture along with paleobotanical evidence indicates that the roxana silt was blown from the ancient mississippi valley rather than from a cold climate desert source or from flood plains linked to a glaciolacustrine source geochemical and mineralogical composition of the upper mississippi valley roxana silt compared to out wash derived peoria loess indicates that detritus from regional hillslope erosion is not a unique component of the roxana silt combined data indicate that the midcontinental roxana silt was derived as loess blown from flood plains of middle wisconsinan proglacial rivers between 55 and 27 ka