In winter the ocean storms claw into the base of these cliffs, undermining them. Along some stretches erosion is gradual, with little rivulets of sand running down the cliff face into the sea. At other places, especially where there are large deposits of clay, the process can be dramatic. Whole slabs of the marine scarp may shelve off at once, and a single stretch of cliff may lose 30 or 40 feet in one storm. In still other areas the beach appears to be accreting, with wide terraces of sand covered with beach grass building out from the cliff base. But overall the Outer Beach continues to erode, losing an average of three feet a year. On the barrier beaches the erosion process is somewhat different, though the overall effect is also retreat. Barrier beach dune lines are dynamic systems. That is, they retreat and maintain themselves by moving with, rather than resisting, the ocean’s power. Normally the foreslopes of the dunes flatten out during a severe storm, presenting a less steep face to the waves, which helps to dissipate their force. Meanwhile wind and occasional storm surges that break through the line carry sand into the estuary or marsh behind the dunes. These deposits are gradually colonized by beach grass and other beach plants, which begin the process of building up another dune line as the foredunes continue to erode. Evidence of this gradual retreat can be observed by the occasional emergence in front of the present dune lines of peat ledges, the remains of a salt marsh or freshwater bog that once lay behind a former line of dunes. In a healthy system and under normal conditions a barrier beach “rolls over on itself” in a smooth progression landward. On occasion, however, even this process is overwhelmed by an unusual manifestation of the sea’s power. One such manifestation was the Great Blizzard of February 6-7, 1978, called “The Storm of the Century.” Carrying 15-foot tides and hurricane-force winds, this storm rearranged many parts of the Cape’s shoreline. Monomoy Island, for example, was severed in two, and remains so today. But nowhere were the effects more dramatic and visible than at Eastham’s Coast Guard Beach. Storm surges breached the dune line, flattening 90 percent of the dunes themselves, carrying off most of the beach cottages, including Henry Beston’s Outermost House, and totally destroying the Seashore’s bathhouse and large parking lot. Provincetown has always been the Cape community that has had to contend most with change. During the 19th century the moving dunes of the Province Lands threatened to bury its houses and silt up its vital harbor. Subsequent erosion control and beach grass plantings have so far kept the dunes at bay, but a more implacable force may now be threatening the town.