respectively. Muds are sediments with 

 greater than 90% silt-clay fraction. Biol- 

 ogists, on the other hand, have attempted 

 to view sediments with a higher degree of 

 resolution. Sediments are described by 

 biologists according to their particulate 

 constituents: these consist of a complex 

 array of organic and inorganic forms, 

 varying in size, shape, and qualitative 

 nature (Table 1; Figure 4). Most of the 

 sediments found in New England tidal flats 

 are dominated by siliceous sands, clay 

 minerals, and organic-mineral aggregates 

 (detritus). The abundance and variety of 

 particle types vary spatially and verti- 

 cally within the sediment (Johnson 1974; 

 Whitlatch 1981). A larger variety of par- 

 ticle types is usually found in the upper 

 layers of the surface than in deeper lay- 

 ers. Muddy sediments have a greater pro- 

 portion of organic-mineral aggregates than 

 sandy sediments. 



Examination of the surface of tidal 

 flats reveals undulations and ripples 

 formed by waves and currents sweeping over 

 the flats. Large grains tend to accumulate 

 on the front of the ripples while smaller 

 grains tend to concentrate on the back 

 side of the ripple marks. Sand and mud 

 flats may or may not be dissected by chan- 

 nels. When they occur, the channels form 

 meandering depressions roughly perpendicu- 

 lar to the creeks that border the flats 

 and are more common on the lower portion 

 of the flat (Figure 1 ). 



Tidal action is responsible for sedi- 

 ment movement and control of sediment tex- 

 ture as currents continually resuspend and 

 transport sediments. In exposed areas 

 where there are high current velocities 

 and turbulence, sediments are generally 

 composed of coarse, unstable sands and 

 cobble. In more protected areas, reduced 



Figure 4. Viewed microscopically, tidal flat sediments are a complex array of organic 

 and inorganic particulate material. The large (0.2 mm) plant fragment from cordgrass, 

 Spartina alterniflora , is the source of much of the detritus entering many New England 

 tidal flat ecosystems. Photo by R.B. Whitlatch, University of Connecticut. 



