The answer is the parent material. Parent material is what occurs on the surface before soil begins to form. This is found in new environments such as after a volcanic activity. Pioneers organisms of an ecosystem and abiotic elements of weathering such as water and temperatures begin to weather the rocks into smaller particles, Humus and organic matter from life accumulate with the minute rock particles to form soil over time. The topmost soil is the most developed and this is why soil has profiles towards the deep into the parent rock material, depending on the stage of weathering.
Flash flooding is most likely to occur when heavy rain falls on (1) deforested landscapes with clay soils.
Weathering is related to the breaking down and loosening of rock or soil into smaller pieces, but the weathered pieces remain in place. Erosion is related to the movement of weathered (and sometimes non-weathered) pieces away from the source.
