proximal cretaceous tertiary boundary impact deposits in the caribbean trace element isotopic and mineralogic studies indicate that the proposed impact at the cretaceous tertiary k t boundary occurred in an ocean basin although a minor component of continental material is required the size and abundance of shocked minerals and the restricted geographic occurrence of the ejecta layer and impact wave deposits suggest an impact between the americas coarse boundary sediments at sites 151 and 153 in the colombian basin and 5 to 450 meter thick boundary sediments in cuba may be deposits of a giant wave produced by a nearby oceanic impact on the southern peninsula of haiti a approximately 50 centimeter thick ejecta layer occurs at the k t boundary this ejecta layer is approximately 25 times as thick as that at any known k t site and suggests an impact site within approximately 1000 kilometers seismic reflection profiles suggest that a buried approximately 300 km diameter candidate structure occurs in the colombian basin

weak bedrock allows north south elongation of channels in semi arid landscapes abstract differences in the lengths of pole and equator facing slopes are observed in a variety of landscapes these differences are generally attributed to relative variations in the intensity of mass transport processes on slopes receiving different magnitudes of solar radiation by measuring anomalies in the planform characteristics of drainage networks we demonstrate that in the most asymmetric landscapes this asymmetry primarily arises from the equator ward alignment of low order valley networks valley network asymmetry is more severe in rocks expected to offer little resistance to erosion than in more resistant rocks when controlling for climate this suggests that aspect driven differences in surface processes that drive differences in landscape evolution are also sensitive to underlying rock type

stiffness of soil geosynthetic composite under small displacements ii experimental evaluation abstractwhile most soil geosynthetic interaction models have focused on the characterization of failure conditions little emphasis has been placed on models and parameters suitable for characteriz

eco geomorphic implications of hillslope aspect inferences from analysis of landscape morphology in central new mexico 1 we investigate the influence of hillslope aspect on landscape morphology in central new mexico where differences in soils vegetation and landforms are observed between mesic north facing and xeric south facing slopes slope area and curvature area relations derived from a digital elevation model dem are used to characterize the opposing hillslope morphologies in all geologies and elevation ranges studied topographic data reveal significantly steeper slopes in north facing aspects and shallower slopes in south facing aspects north facing slope curvatures are also greater than south facing curvatures using a conceptual slope area model we suggest that for a given drainage area steeper north facing slopes imply lower soil erodibility we argue that this interpretation consistent with recent views of ecosystem control on semiarid erosion rates shows the influence hillslope aspect on topography and its associated vegetation communities observed valley asymmetry in the region reinforces this concept and suggests a long term legacy of aspect modulated ecogeomorphic processes

rock strength along a fluvial transect of the colorado plateau quantifying a fundamental control on geomorphology a b s t r a c t bedrock strength is a key parameter that influences slope stability landscape erosion and fluvial incision yet it is often ignored or indirectly constrained in studies of landscape evolution as with the k erodibility parameter in stream power models empirical datasets of rock strength suited to address geomorphic questions are rare in part because of the difficulty in measuring those rocks at earth s surface that are heterolithic weak or poorly exposed here we present a large dataset of measured bedrock strength organized by rock units exposed along the length of the trunk green colorado river through the iconic colorado plateau of the western u s measurements include field compressive tests fracture spacing and selby rock mass strength at 168 localities as well as 672 individual tensile strength tests in the laboratory tensile strength results are compared to geomorphic metrics of unit stream power river gradient and channel and valley bottom width through the arid colorado plateau where the influence of bedrock is intuitive but unquantified our dataset reveals logical trends between tensile and compressive strength as well as between strength rock type and age in bedrock reaches of the fluvial transect there is a positive rank correlation and a strong power law correlation between reach averaged rock strength and unit stream power as well as a linear relation between tensile strength and river gradient expected relations between fracture spacing and topography are masked partly by the massive yet weak sandstones in the dataset to constrain values for weak rock types such as shale we utilize the inverse power law scaling between tensile strength and valley bottom width to estimate their effective tensile strength results suggest that tensile strength varies to at least an order of magnitude smaller values than evident with directly testable rocks in this landscape and values for erodibility k in numerical simulations may be informed by this dataset in terms of landscape evolution these results support the finding that equilibrium adjustment to bedrock strength not differential uplift or transient incision is the first order control on large scale fluvial geomorphology in the colorado plateau this has broad implications for the interpretation of topography in terms of tectonic drivers

indian plate motion and shape constraints on the geometry of the himalayan orogen abstract sea floor palaeomagnetic data that reflect variations in rate and vector of indian plate movement and rotation suggest that initial collision between india and asia occurred at about 50 55 ma ago as the pre collisional indian plate was diamond shaped with the northern margin comprised of two oblique boundaries collision was earliest where these boundaries meet or in what is now the northwest himalaya oblique convergence along each of these two boundaries would generate rotation of thrust sheets as they climb on to the indian plate the oroclinal shape of the main himalayan chain to the east of the northwest himalayan syntaxes reflects a combination of the effects of oblique convergence post collisional anticlockwise rotation of the indian plate and the pinning of the main thrusts at their northwestern terminations by crust thickened during the earliest collisional stage the indian plate rotation enhances a strike slip component of movement along the western oblique margin with the transpressively sinistral chaman fault zone now acting as a continental escape structure

chicxulub crater a possible cretaceous tertiary boundary impact crater on the yucatan peninsula mexico we suggest that a buried 180 km diameter circular structure on the yucatan peninsula mexico is an impact crater its size and shape are revealed by magnetic and gravity field anomalies as well as by oil wells drilled inside and near the structure the stratigraphy of the crater includes a sequence of andesitic igneous rocks and glass interbedded with and overlain by breccias that contain evidence of shock metamorphism the andesitic rocks have chemical and isotopic compositions similar to those of tektites found in cretaceous tertiary k t ejecta a 90 m thick k t boundary breccia also containing evidence of shock metamorphism is present 50 km outside the crater s edge this breccia probably represents the crater s ejecta blanket the age of the crater is not precisely known but a k t boundary age is indicated because the crater is in a thick carbonate sequence shock produced co2 from the impact may have caused a severe greenhouse warming

spherules from the late cretaceous phosphorite of the fatehgarh formation barmer basin india abstract a first report of discovery of spherules glassy balls highly magnetic fine dust and microbracciated matrix in the fatehgarh formation of barmer basin rajasthan india is being presented in this paper the fatehgarh formation is a mixed siliciclastic carbonate and phosphorite formation of cretaceous age in the barmer basin that comprises sediments of middle jurassic to lower eocene age the phosphorite zone in the fatehgarh formation is 8 metre thick zone that comprises phosphatic sandstone bone bed bedded phosphorite and phosphatic and non phosphatic gastropod beds the spherules occur in a thin phosphatic clay mud and silt band of bone bed which also yielded a very rich and diverse microvertebrate assemblage with a dominant late cretaceous maastrichtian form of igdabatis along with forms comprising of semionodontid lapisosteum and enchodontid the end cretaceous is marked for a mass extinction of numerous species including dinosaurs an extraterrestrial impact is interpreted as the reason for this mass extinction whether these spherules are related to the volcanic source or k t boundary impact ejecta found at caribbean and gulf of mexico region needs detailed chemical and age characterization for which study is in progress

how long is a hillslope hillslope length is a fundamental attribute of landscapes intrinsically linked to drainage density landslide hazard biogeochemical cycling and hillslope sediment transport existing methods to estimate catchment average hillslope lengths include inversion of drainage density or identification of a break in slope area scaling where the hillslope domain transitions into the fluvial domain here we implement a technique which models flow from point sources on hilltops across pixels in a digital elevation model dem based on flow directions calculated using pixel aspect until reaching the channel network defined using recently developed channel extraction algorithms through comparisons between these measurement techniques we show that estimating hillslope length from plots of topographic slope versus drainage area or by inverting measures of drainage density systematically underestimates hillslope length in addition hillslope lengths estimated by slope area scaling breaks show large variations between catchments of similar morphology and area we then use hillslope length relief structure of landscapes to explore nature of sediment flux operating on a landscape distinct topographic forms are predicted for end member sediment flux laws which constrain sediment transport on hillslopes as being linearly or nonlinearly dependent on hillslope gradient because our method extracts hillslope profiles originating from every ridgetop pixel in a dem we show that the resulting population of hillslope length relief measurements can be used to differentiate between linear and nonlinear sediment transport laws in soil mantled landscapes we find that across a broad range of sites across the continental united states topography is consistent with a sediment flux law in which transport is nonlinearly proportional to topographic gradient

thresholds of strath genesis deduced from landscape response to stream piracy by pancho rico creek in the coast ranges of central california recent compelling arguments regarding the overwhelming influence of drainage basin lithology on strath formation montgomery 2004 require reevaluation of the role other factors play in this process pancho rico creek prc is a bedrock eroding stream in the tectonically active central coast ranges of california late quaternary time stream piracy of the upper san lorenzo creek catchment by prc caused instantaneous at graded timescales catchment area expansion from 99 5 156 6 km 2 and a significant change in the lithologic composition of prc channel bedload this naturally occurring experiment in the southern gabilan range provides insight regarding the influences of catchment area and channel bedload composition on formation of fluvial straths in the context of newly recognized controls on bedrock channel erosion sklar and dietrich 2001 stock and others 2005 distinct compositional differences in pre and post capture prc alluvium facilitates deducing from alluvial stratigraphy pre and post capture fluvial processes the pre capture drainage basin of prc known as pancho rico valley is entirely underlain by predominantly fine grained friable clastic marine sediments of the pancho rico formation pre capture prc alluvium consists only of pancho rico formation clasts but post capture prc alluvium also includes relatively competent clasts of franciscan complex rocks pre and post capture straths formed in pancho rico valley only along reaches of prc where drainage area was is greater than ca 60 km 2 this indicates that a minimum catchment area is necessary for prc to carve straths spatial and temporal patterns of strath formation in pancho rico valley support the hypothesis that distribution of straths within a drainage basin underlain by mechanically weak rock is largely dependent on catchment area two hypotheses are proposed to explain differences in the character of pre and post capture straths as well as the processes leading to their formation and or preservation pre capture straths are paired major straths which indicate that lateral incision prevailed over vertical incision during a given interval of graded time for example bull 1990 post capture straths are unpaired minor straths which indicate simultaneous vertical and lateral incision during a given interval of graded time for example bull 1990 one hypothesis proposes that prc was able to simultaneously over graded timescales lower and widen its channel in post capture time because the presence of relatively competent bedload enhanced abrasion and accelerated channel denudation relatively incompetent bedload precluded simultaneous channel lowering and widening in pre capture time an alternative favored hypothesis is that post capture minor straths are present along prc because they are buried under franciscan complex derived gravelly alluvium which is more resistant to weathering than pancho rico formation derived gravelly alluvium thin deposits typical of minor erosional straths and composed of readily weathered pancho rico formation bedload were insufficient to protect straths from weathering and erosion in pre capture time pre capture straths survived only when buried by substantial amounts of alluvium as may have occurred during an episode of strath formation and alluviation caused by climate change

terrestrial chondrules glass spherules and accretionary lapilli from the suevite ries crater germany abstract until recently no terrestrial analogues of meteoritic and lunar chondrules were known only rare glass spherules from the lonar crater india and black magnetic spherules from various localities have been recorded the impact breccia suevite of the no rdlinger ries crater germany contains both chondrules and glass spherules and in addition accretionary lapilli all of which are found imbedded within the fine grained matrix of the suevite the chondrules display many of the textural features characteristic of meteoritic and lunar chondrules lithic chondrules and fluid drop chondrules are present the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition on the whole fluid drop chondrules lithic chondrules and glass spherules are rare in the suevite with fluid drop chondrules prevailing detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules accretionary lapilli also represent material formed as a result of impact

stiffness of soil geosynthetic composite under small displacements i model development abstractwhile significant emphasis has been placed on the quantification of soil geosynthetic properties under failure conditions studies of properties that are suitable for characterizing this in

aspect related microclimatic influences on slope forms and processes northeastern arizona 1 climate is a major control on geomorphology yet the effects of aspect related differences in microclimate have been little studied we examined several 60 100 m deep canyons in semiarid northeastern arizona where rock type and structure are essentially constant but where field data and a high resolution digital elevation model reveal consistent morphologic and microclimatic differences between asymmetric north and south facing sideslopes cliffs account for 29 of the vertical relief of south facing slopes but only 2 5 of north facing slopes excluding cliffs south facing slopes are 1 3 steeper than north facing slopes and have significantly less weathered bedrock we monitored air surface and subsurface temperatures and soil moisture at 0 5 h intervals at four locations over 1 year south facing slopes were 1 4 5 6 c warmer and soil moisture tension at 10 cm depth averages at least 78 kpa lower drier than on north facing slopes the dominant rock type in the study area morrison formation sandstone weathers primarily by clay hydration these sandstones form disaggregated mantles where weathering exceeds erosion but also maintain steep slopes and cliffs where little weathered south facing slopes were too dry during most of the instrumented year for significant clay expansion whereas the north facing bedrock slope was moist all year cliff growth thus occurs preferentially on warmer and drier slopes where weathering is reduced small north facing cliffs typically 3 m could have formed during the holocene but cliffs up to 70 m high on southerly aspects require more time to form and likely persisted or expanded under cooler and wetter late pleistocene climates

steering of westerly storms over western north america at the last glacial maximum the last glacial maximum hydroclimate over western north america differed from the modern climate a proxy model comparison suggests that the glacial storm track was squeezed and steered by atmospheric high pressure systems

spheroids at the cretaceous tertiary boundary are altered impact droplets of basaltic composition sand size spheroids of k feldspar in the cretaceous tertiary c t boundary clay at caravaca southern spain were interpreted by smit and klaver as having solidified from a melt resulting from the impact of a large extraterrestrial body sand size spheroids of k feldspar glauconite and magnetite quartz have been found in the c t boundary clay in northern italy and spheroids of k feldspar and pyrite were found in the boundary clay at deep sea drilling project site 465a in the central pacific these spheroids have textures similar to those of rapidly crystallized feldspar and mafic silicates they are interpreted as diagenetically altered microcrystalline spherules of basaltic composition produced by the impact of a large asteroid in an ocean basin at the end of the cretaceous they are analogous to the glassy microtektites produced by impacts on more siliceous target rocks 21 references 4 figures

improved understanding of geogrid response to pullout loading insights from three dimensional finite element analysis soil reinforcement has rapidly become one of the most common soil improvement techniques used in geotechnical engineering understanding the behavior of a geogrid under pullout loading is essential

ecohydrologic role of solar radiation on landscape evolution solar radiation has a clear signature on the spatial organization of ecohydrologic fluxes vegetation patterns and dynamics and landscape morphology in semiarid ecosystems existing landscape evolution models lems do not explicitly consider spatially explicit solar radiation as model forcing here we improve an existing lem to represent coupled processes of energy water and sediment balance for semiarid fluvial catchments to ground model predictions a study site is selected in central new mexico where hillslope aspect has a marked influence on vegetation patterns and landscape morphology model predictions are corroborated using limited field observations in central nm and other locations with similar conditions we design a set of comparative lem simulations to investigate the role of spatially explicit solar radiation on landscape ecohydro geomorphic development under different uplift scenarios aspect control and network control are identified as the two main drivers of soil moisture and vegetation organization on the landscape landscape scale and long term implications of these short term ecohdrologic patterns emerged in modeled landscapes as north facing slopes nfs get steeper by continuing uplift they support erosion resistant denser vegetation cover which leads to further slope steepening until erosion and uplift attains a dynamic equilibrium conversely on south facing slopes sfs as slopes grow with uplift increased solar radiation exposure with slope supports sparser biomass and shallower slopes at the landscape scale these differential erosion processes lead to asymmetric development of catchment forms consistent with regional observations understanding of ecohydrogeomorphic evolution will improve to assess the impacts of past and future climates on landscape response and morphology

evidence for deposition of 10 million tonnes of impact spherules across four continents 12 800 y ago airbursts impacts by a fragmented comet or asteroid have been proposed at the younger dryas onset 12 80 0 15 ka based on identification of an assemblage of impact related proxies including microspherules nanodiamonds and iridium distributed across four continents at the younger dryas boundary ydb spherule peaks have been independently confirmed in eight studies but unconfirmed in two others resulting in continued dispute about their occurrence distribution and origin to further address this dispute and better identify ydb spherules we present results from one of the largest spherule investigations ever undertaken regarding spherule geochemistry morphologies origins and processes of formation we investigated 18 sites across north america europe and the middle east performing nearly 700 analyses on spherules using energy dispersive x ray spectroscopy for geochemical analyses and scanning electron microscopy for surface microstructural characterization twelve locations rank among the world s premier end pleistocene archaeological sites where the ydb marks a hiatus in human occupation or major changes in site use our results are consistent with melting of sediments to temperatures 2 200 c by the thermal radiation and air shocks produced by passage of an extraterrestrial object through the atmosphere they are inconsistent with volcanic cosmic anthropogenic lightning or authigenic sources we also produced spherules from wood in the laboratory at 1 730 c indicating that impact related incineration of biomass may have contributed to spherule production at 12 8 ka an estimated 10 million tonnes of spherules were distributed across 50 million square kilometers similar to well known impact strewnfields and consistent with a major cosmic impact event

aspect dependent variations in regolith creep revealed by meteoric 10be although variations in insolation and emergent feedbacks among soil moisture vegetation and soil cohesion are commonly invoked to explain topographic asymmetry that depends on aspect few studies have directly quantified the efficiency of regolith transport along hillslopes of opposing aspect we utilize meteoric 10 be concentrations in regolith n 74 to determine mass flux along equatorial facing and polar facing hillslopes in three forested upland watersheds in and adjacent to the susquehanna shale hills critical zone observatory in central pennsylvania usa in combination with regolith depth measurements and high resolution topography these fluxes allow us to evaluate transport rate laws and the efficiency of regolith creep concentrations of meteoric 10 be in regolith along six separate transects imply that regolith flux is similar along all hillslopes despite differences in topographic gradient and regolith thickness comparison of flux with regolith depth and topographic gradient reveals that transport depends on regolith depth and that regolith creep is twice as efficient along low gradient south facing slopes with thin regolith as compared to steep north facing slopes mantled with thicker regolith we suggest that the observed topographic asymmetry in these watersheds has evolved over geologic time as a result of differences in the frequency of freeze thaw events between hillslopes of opposing aspect

hillslope asymmetry maps reveal widespread multi scale organization 1 hillslope asymmetry is the condition in which oppositely facing hillslopes within an area have differing average slope angles and indicates aspect related variability in hillslope evolution as such the presence orientation and magnitude of asymmetry may be a useful diagnostic for understanding process dominance we present a new method for quantifying and mapping the spatial distribution of hillslope asymmetry across large areas resulting maps for the american cordillera of the western hemisphere and the western united states reveal that hillslope asymmetry is widespread with distinct trends at continental to drainage scales spatial patterns of asymmetry correlate with latitude along the american cordillera mountain range orientation for many ranges in the western united states and elevation in the idaho batholith of the northern rocky mountains spatial organization suggests that non stochastic process driven controls cause these patterns the hillslope asymmetry metric objectively captures previously documented extents and frequencies of valley asymmetry for the gabilan mesa of the central california coast range broad scale maps of hillslope asymmetry are of interest to a wide range of disciplines as spatial patterns may reflect the influence of tectonics atmospheric circulation topoclimate geomorphology hydrology soils and ecology on landscape evolution these maps identify trends and regions of hillslope asymmetry allow possible drivers to be spatially constrained and facilitate the extrapolation of site specific results to broader regions

the root of branching river networks models and field measurements together show that the branching patterns of fine scale river networks are the result of coupled instabilities in the erosional processes that drive valley incision the mechanisms that create the familiar patterns seen in branching river networks are poorly understood here the authors use a simple numerical model together with field measurements to establish that the branching patterns of fine scale river networks are the result of coupled instabilities in the erosion processes involved in valley widening and channel incision this means that river networks are not a consequence of random topology but of erosional mechanics and the branching patterns can provide information about the geological and environmental conditions during their formation branching river networks are one of the most widespread and recognizable features of earth s landscapes and have also been discovered elsewhere in the solar system1 2 but the mechanisms that create these patterns and control their spatial scales are poorly understood theories based on probability3 4 5 or optimality3 6 7 8 have proven useful9 but do not explain how river networks develop over time through erosion and sediment transport here we show that branching at the uppermost reaches of river networks is rooted in two coupled instabilities first valleys widen at the expense of their smaller neighbours and second side slopes of the widening valleys become susceptible to channel incision each instability occurs at a critical ratio of the characteristic timescales for soil transport and channel incision measurements from two field sites demonstrate that our theory correctly predicts the size of the smallest valleys with tributaries we also show that the dominant control on the scale of landscape dissection in these sites is the strength of channel incision which correlates with aridity and rock weakness rather than the strength of soil transport these results imply that the fine scale structure of branching river networks is an organized signature of erosional mechanics not a consequence of random topology

symmetry randomness and process in the structure of branched channel networks the branched structure of channel networks has a primary impact on the spatial distribution of elevation water and life across earth s surface from the hillslope to the continental scale and is also observed on other planets however the link between this dendritic multiscale structure and the erosional processes that sculpt it has remained elusive for more than six decades in fact many topologic measures fail to distinguish natural networks from those generated by random walks here we show that a fundamental multiscale topologic symmetry is ingrained into the structure of these networks and reflects the equal elevation drop spanned by flows that split at the drainage divide and meet again downslope we demonstrate that this symmetry distinguishes random walk networks from natural ones captures the temporal evolution of these networks and divulges information about the processes that shape them

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

strain distribution within geosynthetic reinforced slopes geosynthetic reinforced slopes are conventionally designed using methods based on limit equilibrium in order to estimate the factor of safety against internal stability using these methods the distribution of the reinforcement peak tensile forces with height must be assumed a linear distribution of reinforcement peak tension with height with zero tension at the crest and maximum peak tension at the toe of the structure has often been assumed although this assumption may be appropriate for the design of vertical geosynthetic reinforced walls little evidence has been collected so far justifying this distribution for the design of geosynthetic reinforced slopes a combination of centrifuge testing and digital image analysis is undertaken in order to obtain the strain distribution within geosynthetic reinforced slopes under prefailure conditions specifically digital image analysis techniques are used to determine the displacement distribution along reinforcement layers in reduced scale models subjected to increasing g levels a sigmoid function was useful to fit raw displacement data and estimate the strain distribution along reinforcement layers analysis of reinforcement strain results shows that the location of the reinforcement maximum peak strain does not occur near the toe of the structure but was located approximately at midheight of the reinforced slopes at the point along the critical failure surface directly below the crest of the slope the pattern of reinforcement peak strain with height obtained for prefailure conditions is similar to that obtained for failure conditions the estimated factor of safety is found to be a good indicator of the magnitude of the reinforcement maximum peak strain for geosynthetic reinforced slopes built with different configurations

soil development over mud rich rocks produces landscape scale erosional instabilities in the northern gabilan mesa california in soil mantled landscapes the length of hillslopes is set by the relative efficiency of hillslope transport processes and incision by channelized flows in cases where rapid channel incision exposes less weathered more resistant rocks incision should slow creating a damping feedback that prevents runaway channel expansion we present observations from the northern gabilan mesa central coast ranges california which is underlain by mud rich rocks that provide less resistance to erosion than the soils produced from them this strength inversion arises because unconfined mud rich units of the pancho rico formation undergo a process called slaking in which pervasive fractures spaced centimeters apart develop when subjected to wet dry cycles in contrast consecutive wetting and drying cycles have produced a cohesive regolith with higher tensile strength than the slaked bedrock exposure of the relatively weak bedrock may create a positive feedback where the acceleration of erosion within the bedrock undermines the cohesive regolith and allows channels to expand headward at the expense of hillslopes geomorphic mapping reveals numerous channelized features that have incised deeply into bedrock and crosscutting relationships and topographic analyses indicate that these features have consumed regions of the landscape previously occupied by hillslopes however these features are only present in south flowing drainages where soil is thinnest and therefore bedrock is most likely to be exposed by increases in erosion rate preferential headward expansion of these south flowing channels has established a profound topographic asymmetry where south flowing tributaries occupy 75 of drainage basin width these results suggest that processes operating at the scale of the critical zone which may be unique to certain rock types conspire to produce an instability that shapes the morphology of drainage basins

40ar 39ar age of cretaceous tertiary boundary tektites from haiti 40ar 39ar dating of tektites discovered recently in cretaceous tertiary k t boundary marine sedimentary rocks on haiti indicates that the k t boundary and impact event are coeval at 64 5 0 1 million years ago sanidine from a bentonite that lies directly above the k t boundary in continental coal bearing sedimentary rocks of montana was also dated and has a 40ar 39ar age of 64 6 0 2 million years ago which is indistinguishable statistically from the age of the tektites