| description:
|
These map units show the areal extents of significant volumes of earth, debris and rock fragments that have accumulated as a result of past debris flows and debris slides and, to a lesser extent, rock falls and rock slides. Debris flow deposits mainly occur in valleys and can grade upslope into debris slide, rock fall, and rock slide deposits nearer steep source areas. The mapped slope movement deposits are typically composite features that formed as a result of multiple slope movement processes and events of various ages from prehistoric to modern times. The bulk of the deposits are likely prehistoric in age, but their ages have yet to be verified by modern age-dating techniques. These deposits consist of heterogeneous mixtures of clay, silt, and sand particles with gravel- to boulder-sized rock clasts in various stages of weathering and decomposition. Areas mapped as slope movement deposits met two or more of the following criteria: 1) exhibited an elongate, lobate or fan shape, or other landform characteristic of a slope movement deposit, visible at a scale of 1:7,500 using the LiDAR DEM; 2) had an adequate upslope source area where past slope movements could have initiated; and/or, 3) were verified in the field to contain gravel- to boulder-sized clasts or other textures and depositional structures that characterize deposits produced by slope movements (i.e. matrix supported or imbricated clasts, or scour and fill structures). Many narrow stream valleys likely contain slope movement deposits that are not feasible to map at this scale. |