"Coastal areas are particularly susceptible to oil pollution. When a large spill drifts ashore, a fraction of the oil may become trapped in sediments and persist, in some cases, for years. This is in contrast to conditions in the open sea, where currents and diffusion usually rapidly reduce the concentration of oil. The immediate effects of heavy oiling of the shore zone can be evidenced by the death of plants and animals due to smothering and toxicity. In the longer term, the effects are more variable and subtle. Key factors influencing the fate of oil on the shore are the porosity of sediments and the waveerosion activity acting on them. In high energy environments, (such as rocky shores) the stranded oil may coat the rocks and gradually harden by weathering into a tough tarry "skin". The oil is gradually removed by wave erosion, although pools of oil are likely to collect in hollows among the rocks, protected by a skin of weathered oil, and may remain for a long time."
USCG Sector Mobile Digital Area Contingency Plan: http://ocean.floridamarine.org/acp/mobacp/