note the flat, high-elevation areas on the north and south side of the valley, which are formed from the deposition of glacial sediment. this implies that the north fork stillaguamish river has cut down, or incised, into this sedimentary deposit. which of the triggering mechanisms below is likely implicated in the failure of most or all of the stillaguamish landslides? group of answer choices slope steepening and undercutting by stream erosion increasing thickness of material on the top of the hillslope through sedimentation seismic shaking increased housing development in the area