WSP10425 - Laserfiche WebLink

Home Browse Search

OOnJ5 Soils Soils in the planning unit are influenced by the type of geologic "parent" materials as well as the differences in climate, vegetation, and topography. The igenous volcanic materials develop as fine to coarse textured, medium to well drained soils. They usually are slightly acid to neutral in reaction. As a rule. these soils have a low mass movement potential-meaning they are not likely to slip or slide downslope. This potential rapidly increases, however, on slopes exceeding 40 percent and on north and northwest facing slopes. The erosion hazard potential-erodibility-for these igneous soils is generally moderate in comparison to other soils found in the unit. Factors such as ground cover, slope and intensity, and amount of precipitation must be considered to determine the actual amounts of soil erosion. The erosion potential also increases on slopes over 40 percent and in areas where the depth of soil material is shallow. Basalt flows in the San Miguel area have produced soils of fine textures with very high cobble and stone content. These soils are on flat slopes although poor vegetative cover contributes to high soil losses through erosion, Soils developing on gneiss materials are similar in character to those developing on volcanic material. The soils developed from sedimentary sandstones and shales in the western portions of the unit generally have higher mass movement potential and higher erosion potential. Shallow soils developed from shale parent material have the highest erosion potential within the unit. Soils formed from the more recent glacial deposits are generally deep, gravelly to cobbly in texture, and well drained. Erosion and mass movement potential are moderate for most of these glacial soi Is, but also become high on slopes over 40 percent. Flood plain alluvial deposits result in stratified sand, silts, and clays. Mass movement potential in these soils is low due to the flat terrain, but erosion potential is rated high. These soils also have a high fertility rating and generally support good ground cover. This vegetation and the gentle slopes result in less actual erosion than on most of the other soils in the unit. Landslide materials derived from quartz latite and other igneous rocks form soils which are deep, well drained, and medium textured. Mass movement potential is generally high in these areas and slippage can be observed in many specific locations. Erosion potential for these soils is generally low. The geology fact sheet and map provide additional information about the soils and their locations.