SOIL WEATHERING ON TOPOGRAPHICALLY PAIRED SITES, CLEARWATER NATIONAL FOREST, IDAHO

This study evaluates weathering and the influence of slope on soil development in the Clearwater Mountains, Northern Rocky Mountains, Idaho. The surrounding landscape consists of a series of ridges with occasional rounded summits and steep, V-shaped valleys. Secondary ridges extend nearly perpendicular to the main divides, sloping steeply toward streams on the valley floor. Bedrock is predominantly high-grade schist mantled by volcanic glass-enriched loess. Soils form in residuum, and colluvium admixed with loess on some slopes exceeding 17º. Soil development was evaluated using field, laboratory,and petrographic methods for paired sites at summit, backslope, and footslope positions. Soils developed in the glass-enriched loess and underlying residual and colluvial metamorphic material are 2 to 3 meters thick on summit and footslope positions and 1 to 1.5 meters thick on even the steepest slopes. Accordingly, pedogenic development in steep, mountainous terrain here is greater than what has been reported for similar landscapes. Comparisons of particle-size distribution, extractable Fe, fabric analysis and cutan development were particularly useful when characterizing summit and backslope positions and in differentiating summit from backslope sites. For example,summit soils showed prominent accumulation zones of silts and clays and of citrate-extractable oxides, whereas these constituents were distributed more homogenously in backslope soils. Petrographic analysis indicated retention of rock fabric in lower (residual) portions of summit profiles, while fabrics of backslope soils showed a pronounced disorder indicative of the colluvial nature of the material. Soil properties were not as readily characteristic of the footslopes in this study, because the sampled footslope sites differed from each other considerably with respect to loess thickness and degree of ash-colluvium mixing. On these steep slopes, we hypothesize that relatively small differences in slope length or slope-length segments contribute disproportionately to soil-landscape relationships.