atmospheric conditions and influence of 

 associated land masses, and the waters 

 become cooler than nearby coastal waters. 



Buildup of seawater ice on New Eng- 

 land tidal flats, both north and south of 

 Cape Cod, commonly occurs in winter. The 

 appearance and extent of the ice is de- 

 pendent upon tidal fluctuation, location, 

 and severity of the winter. Because of 

 tidal action, the ice moves back and forth 

 across the flats resulting in appreciable 

 geomorphological effects upon the sediment 

 through accretion, erosion, and transport. 

 Boulders weighing several tons have been 

 transported considerable distances by ice 

 at Barnstable Harbor (Redfield 1972). Salt 

 marsh turf may also be transported onto 

 tidal flats by ice movement. Shortly after 

 breakup of the ice in early spring, ero- 

 sional scars in the sediment are evident. 

 Most of the scars are quickly removed by 

 tidal and wave action. Although ice 

 occurs regularly on New England tidal 

 flats, relatively little is known about 

 its effects on the biota. Ice scouring 

 can remove or displace infaunal and epi- 

 faunal organisms. Freezing of the sedi- 

 ments to a depth of 5 to 10 cm (2 to 4 

 inches) may also occur, although little is 

 known about what effect this has on the 

 organisms living in the sediment. During 

 periods of severe and prolonged ice build- 

 up on tidal flats, birds that use the 

 areas as feeding sites may have to forage 

 elsewhere. 



Storms that pass through New England 

 also affect the sedimentary features of 

 tidal flats. Both northern and southern 

 New England normally experience three to 

 five major storms each year, usually in 

 fall and winter. Winds in New England are 

 predominantly from the southwest but dur- 

 ing winter are likely to shift to the west 

 or northwest. Occasionally winds come 

 from the northeast and are typically asso- 

 ciated with the most severe storms (the 

 classic "nor 'easter"). Hurricanes occur 

 in New England - the last major storm hit 

 the coastline in 1954. 



Fog is common in the coastal zone 

 especially in northern New England. Fog 

 occurs at any time of the year although 

 dense fog is associated with the warmer, 

 summer months. The presence of fog on 



the tidal flats acts to insulate organisms 

 living on or in the sediments from desic- 

 cation and allows less hardy organisms to 

 survive in intertidal areas during periods 

 of intense solar heating. 



1.3 GEOLOGICAL, PHYSICAL, AND CHEMICAL 

 CHARACTERISTICS OF TIDAL FLATS 



On a geologic timescale, coastal ma- 

 rine environments of New England represent 

 systems that have continually changed. 

 Since the last Pleistocene glaciation epi- 

 sode, the coastline has slowly subsided 

 and sealevel has progressively risen. The 

 net effect is a slow migration of the sea 

 into the lowlands, altering coastal habi- 

 tats. Historical reconstructions of many 

 New England estuarine systems show the 

 transitional nature of tidal flat habi- 

 tats. Flats develop as depositional fea- 

 tures expanding at the expense of tidal 

 channels and eelgrass beds and they in 

 turn are invaded by the progression of 

 salt marsh vegetation (Redfield 1967). 



The formation of tidal flats and 

 their sedimentary characteristics are pri- 

 marily dependent upon the physical and 

 biological environment (e.g., tidal cur- 

 rents, wave action, and biologically- 

 induced sediment mixing), the nature and 

 source of available materials, and the 

 glacial history of New England. Vast 

 deposits of coarse-grained sediments left 

 by glacial activity are responsible for 

 the general restriction of sand flats to 

 Cape Cod and southward. Mud flats, more 

 commonly found in northern New England, 

 are derived from land-based sources, and 

 transported by river systems. Sediments 

 are also deposited on tidal flats by cur- 

 rents from offshore sources or through the 

 erosion of adjacent tidal flats or shore- 

 lines. 



Sediments of tidal flats can be 

 characterized in various ways. Geologists 

 prefer to use the bulk properties of the 

 sediment (e.g., median grain size, percent 

 silt-clay fraction). Sandy sediments are 

 those having less than 5% of their weight 

 composed of silt-clay-sized material 

 (particles less than 62 jjm in diameter), 

 while muddy-sands and sandy-muds consist 

 of 5% to 50% and 50% to 90% silt-clay. 



