SOIL GEOMORPHOLOGY OF THE NORTH FLANK OF THE SAN BERNARDINO MOUNTAINS, CALIFORNIA: A SUMMARY OF INTERACTIONS AND FEEDBACKS BETWEEN SOIL DEVELOPMENT AND LANDSCAPE EVOLUTION AT VARIOUS TEMPORAL AND SPATIAL SCALES

Typically soil geomorphology studies do not evaluate the combined influences of multiple soil forming factors on soil landscape evolution. The alluvial fan piedmont of the north flank of the San Bernardino Mountains is an excellent study area in which to evaluate soil landscape evolution as a function of climate, tectonics and rock type. Approximately 40 soils were examined on alluvial fans that range in age from Middle Pleistocene to latest Holocene and that range in composition from primarily carbonate to primarily granite rock types. Field and laboratory investigations of these soils revealed that soil spatial variability is largely controlled by sediment composition. Carbonate-dominated alluvial fans are characterized by soils with strong petrocalcic horizon development. Granite-dominated alluvial fans are ultimately characterized by soils with argillic horizons and little accumulation of pedogenic carbonate. Geomorphic and geologic mapping, long profile measurements of alluvial fan surfaces and ephemeral streams, and infiltration measurements of fan surfaces revealed that sediment-composition-related soil development significantly influences geomorphic processes and the overall landscape evolution of the San Bernardino piedmont. When tectonic folding deforms rock-like petrocalcic soils, prominent anticlinal ridges form and episodically dam and/or reroute drainages along the mountain front. These ridges strongly influence local base-level and consequently the spatial and stratigraphic distribution of sediments in limestone-dominated portions of the piedmont. Argillic horizons in granite-derived deposits produce significant runoff and are easily eroded and often reworked into younger deposits. Folds forming in these deposits are thus topographically less pronounced. In addition, drainage basins that are underlain by preweathered granite bedrock are sensitive to short-term climate change. Holocene sediments thus dominate the stratigraphy of alluvial fans adjacent to these basins. Overall soil and landscape variability along the north flank of the San Bernardino Mountains strongly reflects interactions and feedbacks at different temporal and spatial scales between climate, tectonics, geomorphology and soil development itself.