marsh plants to form a cohesive deposit that may function for a time as beach 

 protection. 



4. Consolidated Material . 



Along some coasts, the principal littoral materials are consolidated 

 materials, such as rock, beach rock, and coral, rather than unconsolidated 

 sand. Such consolidated materials protect a coast and resist shoreline 

 changes. 



a. Rock . Exposed rock along a shore indicates that the rate at which 

 sand is supplied to the coast is less than the potential rate of sand 

 transport by waves and currents. Reaction of a rocky shore to wave attack is 

 determined by (1) the structure, degree of lithif ication, and ground-water 

 characteristics of the exposed rock and (2) by the severity of the wave 

 climate. Protection of eroding cliffs is a complex problem involving geology, 

 rock mechanics, and coastal engineering. Two examples of this problem are the 

 protection of the cliffs at Newport, BQiode Island (U.S. Army Corps of 

 Engineers, 1965) and at Gay Head, Martha's Vineyard, Massachusetts (U.S. Army 

 Engineer Division, New England, 1970) . 



Most rocky shorelines are remarkably stable, with individual rock masses 

 identified in photos taken 50 years apart ( Shepard and Grant, 1947). 



b. Beach Rock . A layer of f riable-to-uell-lithif ied rock often occurs at 

 or near the surface of beaches in tropical and subtropical climates. This 

 material consists of local beach sediment cemented with calcium carbonate, and 

 it is commonly known as beach rock. Beach rock is important to because it 

 provides added protection to the coast, greatly reducing the magnitude of 

 beach changes (Tanner, 1960) and because beach rock may affect construction 

 activities (Gonzales, 1970). 



According to Bricker (1971), beach rock is formed when saline waters 

 evaporate in beach sands, depositing calcium carbonate from solution. The 

 present active formation of beach rock is limited to tropical coasts, such as 

 the Florida Keys, but rock resembling beach rock is common at shallow depths 

 along the east coast of Florida and on some Louisiana beaches; related 

 deposits have been reported as far north as the Fraser River Delta in 

 Canada. Comprehensive discussions of the subject are given in Bricker (1971) 

 and Russell (1970). 



c. Organic Reefs . Organic reefs are wave-resistant structures reaching 

 to about mean sea level that have been formed by calcium carbonate-secreting 

 organisms. The most common reef-building organisms are hermatypic corals and 

 coralline algae. Reef-forming corals are usually restricted to areas having 

 winter temperatures above about 18° C (Shepard, 1963, p. 351), but coralline 

 algae have a wider range. On U.S. coastlines, active coral reefs are 

 restricted to southern Florida, Hawaii, Virgin Islands, and Puerto Rico. On 

 some of the Florida coast, reeflike structures are produced by sabellariid 

 worms (Kirtley, 1971). Organic reefs stabilize the shoreline and sometimes 

 affect navigation. 



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