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 stone layers resulting from 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. Our new work based on the evidence from the Soldier Mountain sand ramp in the Mojave desert, that their formation appears 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 indicate movement rates 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. Our study suggests that sand ramps cannot easily be interpreted in terms of a simple model of fluctuating palaeoenvironmental phases from Aeolian dominated to soil/fluvial/colluvial dominated episodes. As a result their palaeoenvironmental significance and indeed how sand ramps are distinguished from other dune forms require amendment