Figure 2 shows the general geology of the Puget Sound area. The glaciers deposited the soil layers that make up your slope. Your shoreline bank or bluff could be composed of one or more of these soil layers. Some isolated areas of Puget Sound shorelines consist predominantly of rock instead of soil. In the construction industry, glacial till is frequently referred to as hardpan because it is very dense, making it difficult to dig.
Hardpan is a mixture of clay, silt, sand, and gravel.
Common soil layer sequence found along coastal slopes. Sand and gravel soils usually contribute to the stability of a slope in the absence of water. However, water readily flows through sand and gravel. Water can reduce the stability of the slope when it accumulates above a soil layer that is not as permeable as the sand and gravel. Today's shoreline is a result of continuing geologic evolution since the time of the glaciers.