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therefore the most disturbed and least stable area in the 

 dune system. The foredune is naturally adapted to 

 rebuilding itself after a major disturbance. Because of the 

 dynamic nature of the foredune, it is not a continuous ridge 

 but has intermittent gaps caused by blowouts, slip faces, 

 and the development of new dunes (Larsen, 1969). Behind the 

 foredune is the interdune which varies in width from 300 

 feet (100 m) to 900 feet (300 m) , sloping gently upward to 

 the west. In general it has a flat Ammophila grassland 

 appearance, but in many areas there are blowouts and new 

 dunes developing. In addition, there are a number of 

 relatively deep, wet depressions which provide a suitable 

 habitat for the establishment of a variety of trees, shrubs, 

 and herbs. 



At the western boundary of the interdune and running 

 parallel to the beach is the backdune ridge. This is the 

 most dramatic feature in the dune system, generally ranging 

 from 25 feet (8.3 m) to 40 feet (13.3 m) tall while reaching 

 heights of over 50 feet (16 m) . The highest of these 

 backdunes is High Sandy, which can be seen between parking 

 lots 2 and 3. The backdune ridge is also discontinuous, 

 with gaps up to a half mile (0.8 km.) in some places. The 

 backdune ridge is relatively stable, being covered by a 

 variety of plants, but in a number of places there is an 

 active slip face which is slowly moving in a westerly 

 direction (Larsen, 1969). Due to its height and relative 

 stability the backdune ridge provides excellent protection 

 from sand and salt spray. Thus, behind the ridge small 

 "sunken" forests develop which are populated by a variety of 

 plants. Because of the destructive nature of the salt and 

 sand spray, which frequently occurs over the top of the 

 ridge, none of the trees are ever able to grow above it. In 

 relatively undisturbed areas behind the ridge, such as 

 behind High Sandy, trees up to 50 feet (16 m) have been able 

 to survive. In the Parker River National Wildlife Refuge 

 the road runs behind the stable backdune ridge. 



The drumlins at the southern end of Plum Island, 

 including Cross Farm Hill, Stage Island, Bar Head, and Grape 

 Island, are unique to this barrier beach dune system. The 

 drumlins are large mounds composed of clay, sand, gravel, 

 boulders, and other glacial debris formed by the Wisconsin 

 Ice Sheet (Sears, 1905). As will be discussed in a later 

 section, these drumlins provided a point of attachment for 

 the developing island. All the drumlins except Grape Island 

 are directly connected by sand (Fig. 2). The 30 acre (13 

 ha) island is connected to the sand dune system by salt 

 marsh. The till soils of the drumlins are very fertile and 

 provide the only suitable areas for cultivation on the 

 island. 



The island is a dynamic system which is ever changing 

 in appearance. A most dramatic example of this occurred 

 when the Merrimack River changed course. The river 

 originally flowed out to the ocean through what is now The 



