PART IV: TYPICAL BORROW SOURCES 



30. Geological research, the ICONS studies, and Corps project borrow 

 source investigations have shown that certain types of relict and modern 

 deposits, occurring in marginal marine and continental shelf environments, are 

 predominantly composed of sand- size sediment, potentially usable as beach fill 

 material, while others contain predominantly fine-grained silt and clay-size 

 material or glacial till unsuitable for beach fill. Many of these deposits 

 are associated with geomorphic features that can be identified on large-scale 

 bathymetric charts. Those associated with sandy sediments can be given empha- 

 sis in planning exploration programs. The geomorphic and lithologic character 

 of the more common types of deposits formed in marginal marine and shelf areas 

 are discussed below. 



31. Deposits predominantly consisting of silt and clay-size material, 

 usually with some sand, are widespread in marginal marine and continental 

 shelf environments. In marginal marine environments, they are usually modern, 

 active deposits, and are most commonly found in (a) back-barrier marsh, tidal 

 flat, and open lagoon areas; (b) protected bays and estuaries where wave 

 energy is low; and (c) some shore- face areas. On the continental shelves, 

 silt and clay deposits usually have little if any relief. Modern shelf depos- 

 its of silt and clay are mostly found in the deeper mid and outer shelf areas, 

 and where wave shadow effects are created by headlands and shoal fields . 

 Relict deposits of silt and clay may occur on any part of the shelf. Com- 

 monly, these deposits are remnants of back-barrier and estuarine sediments 

 that were formed during the Holocene transgression. Predominantly silt-clay 

 back-barrier and shelf deposits often contain substantial amounts of sand-size 

 material. Usually, this material is in the very fine to fine size range 

 (Wentworth scale). This size range, together with the silt and clay compo- 

 nents, renders such material generally unsuitable for beach fill. 



Tidal Inlet Shoals 



32. Inlets are considered to be any narrow opening connecting two 

 bodies of water. Tidal inlets are breaches in barrier islands that connect 

 the open body of water to the lagoon, marsh, and tidal creek complex in the 

 lee of the barrier. Inlets of this type are comparatively small and often 



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