Aspect‐related microclimatic influences on slope forms and processes, northeastern Arizona

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.