66 



shallow water depths over the continental shelves, thus reducing the 

 force with which they strike the shore (fig. IV. 1.2). Where the Con- 

 tinental Shelf is wide, waves reach the shore with greatly decreased 

 power and tend to move existing sediments around rather than cutting 

 the shoreline to produce new ones. 



Along the Atlantic and gulf coasts of the continental United States 

 the Continental Shelf is generally about 50 to 100 miles wide and ter- 

 minates at depths ranging from 300 to 900 feet. Within this regime 

 four significant differences in conditions on the shelf are reflected in 

 the estuarine zone : 



(1) The Gulf of Maine forms an embayment between Cape 

 Cod and Nova Scotia, and the general configuration of deep 

 basins close to shore with broad banks seaward of them is unique 

 to this part of the coast (fig. IV.1.3). Wliile the shoal waters 

 on the shelf serve to protect the New England coast from the full 

 force of the ocean swells, the deep embayment near shore and 

 the narrow trough which connects it to the ocean cause the great 

 tide ranges and strong currents characteristic of the region. 

 These currents tend to reduce deposition of sediments close in- 

 shore, particularly along the Maine coast where the tide range 

 is greatest and the currents strongest. 



(2) Cape Hatteras is a region where the deposition of sedi- 

 ments on the wide shelf at the meeting place of two major ocean 

 currents has resulted in the building of a series of barrier islands 

 out over the shelf and the formation of a Avide shallow embay- 

 ment (Pamlico Sound) behind them (fig. IV.1.4). This sedimen- 

 tation process has reduced the width of the 'Continental Shelf 

 to less than 20 miles at this point and created the infamous 

 Diamond Shoals seaward of the barrier islands. 



(3) South Florida, from Miami to beyond the Florida Keys, 

 has virtually no Continental Shelf; this is probably related to 

 the passage of the Gulf Stream through the narrow channel 

 between the Bahama Islands, Cuba, and Florida (fig. IV.1.5). 

 These same islands, however, serve to protect the southern part 

 of Florida from heavy ocean swells, while the steady current 

 keeps sediments from depositing on the offshore coral forma- 

 tions of the Florida Keys and tends to spread coral growth 

 northward along the Florida Coast. 



(4) The Mississippi River, draining about 41 percent of the 

 continental United States, has built a delta entirely across the 

 Continental Shelf and now deposits most of its sediments on the 

 slope beyond (fig. IV. 1.6). The generally enclosed nature of the 

 embayment forming the Gulf of Mexico has permitted the for- 

 mation of this delta and its associated channels and marshlands, 

 as well as the combination of barrier island and coastal marsh- 

 land formation which makes up the majority of the Gulf of 

 Mexico estuarine systems. 



On the Pacific coast of the continental United States, the Conti- 

 nental Shelf is 2 to 20 miles wide and terminates at depths of 300 to 

 600 feet. Pouring over this narrow, steep shelf is the full force of 

 the Pacific Ocean swell; this makes for excellent surfing, but it also 

 leads to considerable erosion of the shoreline. Shoreline erosion by 

 wave action with the development of a beach and bluff configuration 

 is typical of this part of the coastline (fig. IV.1.2). Strong currents 



