On the formation of sand ramps: A case study from the Mojave Desert

Sand ramps are dune-scale sedimentary accumulations found at mountain fronts and consist of a combination of aeolian sands and the deposits of other geomorphological processes associated with hillslope and fluvial activity. Their complexity and their construction by wind, water and mass movement means that sand ramps potentially hold a very rich store of palaeoenvironmental information. However, before this potential can be realised a full understanding of their formation is necessary. This paper aims to provide a better understanding of the principal factors influencing the development of sand ramps. It reviews the stratigraphic, chronometric and sedimentological evidence relating to the past development of sand ramps, focussing particularly on Soldier Mountain sand ramp in the Mojave Desert, as well as using observations of the modern movement of slope material to elucidate the formation of stone horizons within sand ramps. Findings show that sand ramps cannot easily be interpreted in terms of a simple model of fluctuating palaeoenvironmental phases from aeolian dominated to soil/fluvial dominated episodes. They accumulate quickly (perhaps in < 5 ka), probably in a single phase before becoming relict. Based on the evidence from Soldier Mountain, they appear strongly controlled by a ‘window of opportunity’ when sediment supply is plentiful and cease to develop when this sediment supply diminishes and/or the accommodation space is filled up. Contemporary observations of stone movement both on rock and sandy sloping surfaces in the Mojave region indicate movement rates in the order of 0.6 and 11 mm yr− 1, which is insufficiently fast to explain how stone horizons could have been moved across and been incorporated into sand ramps on multiple occasions. Stone horizons found within the aeolian sediments lack evidence for soil development and are interpreted as very short-term events in which small streams moved and splayed discontinuous stone horizons across the sand ramp surface before aeolian deposition resumed. Surface stone horizons may form by creep from mountain slope sources across sand ramps but require enhanced speed compared to measured rates of runoff creep. We propose the mechanism of fluvio-aeolian creep. Our study suggests that current models of alternating aeolian and colluvial deposition within sand ramps, their palaeoenvironmental significance and indeed how sand ramps are distinguished from other dune forms require amendment

A comparison of conventional and 137 Cs-based estimates of soil erosion rates on arable and grassland across lowland England and Wales

Soils deliver a range of ecosystem services and underpin conventional global food production which must increase to feed the projected growth in human population. Although soil erosion by water and subsequent sediment delivery to rivers are natural processes, anthropogenic pressures, including modern farming practices and management, have accelerated soil erosion rates on both arable and grassland. A range of approaches can be used to assess and document soil erosion rates and, in the case of the UK, these mainly comprise the 137Cs-based approach, conventional surveys using volumetric measurements, integration of information on suspended sediment flux, fine sediment source apportionment and landscape sediment retention and traditional bounded hydrological monitoring at edge-of-field using experimental platforms. We compare the erosion rates for arable and grassland in lowland England assessed by these different techniques. Rates assessed by volumetric measurements are similar to those generated by integrating information on suspended sediment flux, sources and landscape retention, but are much less than those estimated by the 137Cs-based approach; of the order of one magnitude less for arable land. The 137Cs approach assumes an initial distribution of 137Cs uniformly spread across the landscape and relates the sampled distribution to erosion, but other (transport) processes are also involved and their representation in the calibration procedures remains problematic. We suggest that the 137Cs technique needs to be validated more rigorously and conversion models re-calibrated. As things stand, rates of erosion based on the distribution of 137Cs may well overstate the severity of the problem in lowland Britain and, therefore, are not a reliable indicator of water erosion rates

Badland and gully erosion in the Karoo, South Africa

To the European eye, the Karoo is an ancient landscape untouched by major climate change or glaciation and evolving through erosional processes since deposition of Jurassic rocks, uplift, and the break up of Gondwana about 180 million years ago (McCarthy and Rubidge 2005). This break up marked the end of ~300 million years of sedimentation, largely under arid conditions, and a ~2 million year episode of violent volcanic eruptions and the outpouring of basaltic lava covering virtually the whole of southern Africa. Since this time, the interior of South Africa has been dominated by erosion (McCarthy and Rubidge 2005). The evidence of relatively recent landscape degradation is ubiquitous and has been noted in reports, diaries, and articles for over 100 years (Hoffman and Ashwell 2001; Hoffman et al. 1999; Beinart 2003). In many respects, the situation is similar to semiarid landscapes in the Midwestern United States and in Australia. Gully systems and, in some cases, badlands have been assigned to the influence of European farming systems and in particular to the introduction of large numbers of domesticated sheep and cattle, thus destabilizing hillslopes and impacting rivers (Patton and Schumm 1975; Fanning 1999

Badland and gully erosion in the Karoo, South Africa

To the European eye, the Karoo is an ancient landscape untouched by major climate change or glaciation and evolving through erosional processes since deposition of Jurassic rocks, uplift, and the break up of Gondwana about 180 million years ago (McCarthy and Rubidge 2005). This break up marked the end of ~300 million years of sedimentation, largely under arid conditions, and a ~2 million year episode of violent volcanic eruptions and the outpouring of basaltic lava covering virtually the whole of southern Africa. Since this time, the interior of South Africa has been dominated by erosion (McCarthy and Rubidge 2005). The evidence of relatively recent landscape degradation is ubiquitous and has been noted in reports, diaries, and articles for over 100 years (Hoffman and Ashwell 2001; Hoffman et al. 1999; Beinart 2003). In many respects, the situation is similar to semiarid landscapes in the Midwestern United States and in Australia. Gully systems and, in some cases, badlands have been assigned to the influence of European farming systems and in particular to the introduction of large numbers of domesticated sheep and cattle, thus destabilizing hillslopes and impacting rivers (Patton and Schumm 1975; Fanning 1999