Morphometric analysis of alluvial terraces and terrace remnants of increasing age demonstrated geomorphic evolution through time, with a decrease in extent of original planar terrace tread morphology and an increase in frequency of steeper slopes and convexo-concave land elements. The number of loess sheets and the thickness of loess increased across the three youngest terraces. The next oldest (ca. 150 ka) terrace remnant had the greatest maximum number of loess sheets (3) and loess thickness (8 m) but the loess mantle was highly variable. A detailed loess stratigraphic analysis and the morphometric analysis place this terrace in a transition between dominantly planar, uniformly loess-mantled landforms and loess-free ridge and valley terrain exemplified by the oldest terrace remnant. Variations in thickness and preservation of loess sheets demonstrated spatially and temporally variable erosion during loess accumulation.
To test our hypothesis of loess persistence we calculated critical steady-state hillslope curvatures from a soil transport model, calibrated for the study area, above which the uppermost loess sheet (L1, max. thickness 1.8 m) should be completely eroded. We compared loess distribution mapped in the field to values of slope curvature calculated from topographic surveys and found two of three critical curvature values had acceptable predictive ability. Where predictions failed this is probably due to transient responses related to active steam incision. Because all critical curvature values predicted presence of loess on the oldest terrace remnant where there is none we conclude that important factors other than morphometric ones are important in determining loess distribution in loess landscapes in active tectonic regions. These may include internal changes to regolith affecting erodibility, or vegetation or topographic interaction with wind patterns affecting loess trapping.