Backwasting rock sheets of the Navajo Sandstone, Utah, USA

On many Navajo Sandstone outcrops in southern Utah, steep, decimetre-high scarps (downslope-facing margins of rock sheets) interrupt smooth slopes covered by crustose lichens. Scarps form where sheeting joints and water emerge at the land surface. Weathering scarp faces causes upslope backwasting sheets leaves behind smooth, unweathered surfaces that become stabilized We hypothesize subsurface, low-angle, open fractures porous permeable rocks are barriers to downward flux through vadose zone. Runoff lateral flow uppermost deliver scarps, inducing backwasting. Monitoring temperature water/ice content within two surficial revealed that, winter 2020–2021, snowmelt entered lichen-covered persisted for 64 days underwent 46 freeze/thaw cycles. Water gained from summer rains remained a maximum 11 days. Flaking dangling lichen crusts overhang indicate is now active. conclude cycles (rather than salt precipitation) drive our study sites, all which located more 1400 m above sea level climate relatively cool, snowfalls frequent. Backwasting would likely be rapid extensive under cooler, wetter (periglacial) climate. Although broad, gently sloping, dominate surface, they not downwasting; weathering instead restricted near-vertical, lichen-free, scarps.