COASTAL GEOLOGY OF CAPE COD NATIONAL SEASHORE

Cape Cod is the result of glacial deposition by the Laurentide ice sheet during the Late Wisconsinan. After the Laurentide ice sheet retreated from New England beginning around 18,000 years BP, Cape Cod emerged as a series of end moraines and outwash plains (Larson, 1982). Immediately following deglaciation, the shoreline of Cape Cod was an irregular hillock of unconsolidated sand and till covering bedrock to depths as great as 250 m (Oldale, 1992). Estimates of the maximum retreat of Cape Cod since waves began eroding its shoreline about 4000 BP have been approximately 4 km (Strahler, 1988). Consequently, Cape Cod National Seashore is extremely susceptible to natural weathering agents such as wind, waves, sea level fluctuations, storms, and tides (Figure 1). The shoreline of the outer cape is oriented such that the dominant east-northeast waves produce a bi-directional longshore transport system that transports outwash sediment eroded from cliffs to the north and south (Fisher, 1987). Sediment transported north is incorporated into the enlarging Provincetown spit system. Conversely, sediment transported to the south is supplied to the southern barrier spit system that extends from Coast Guard Beach to Monomoy Point. The portion of the seashore that extends from Ryder Beach to Great Island within Cape Cod Bay also experiences a net southerly longshore transport which has resulted in the growth of Great Island and the formation of Jeremy Point.