TREADWELL 



33 



maximum height is about 15 feet, and their bases extend twenty or thirty feet below feea level. 

 Width of the largest cheniers, most of which are buried under marsh, is about two miles. Veg- 

 etation is chiefly slash pine, live oak, and palmetto, and underbrush is thick, but does not 

 approach the density found on natural levees. 



The cross-sectional appearance of a chenier is shown in Fig. 3. It is a sand wedge partly 

 buried under marsh and mudflat deposits, here shown resting on lake and bay deposits underlain 

 by compact Pleistocene material. 



Being thick lenses of sand, cheniers are excellent trafficable units. They support weight 

 better than levees, and because they are porous, are as easily traversed during wet weather as 

 during dry weather. 



Present sand beaches are confined to Breton and Chandeleur Islands, a chain of islands 

 about fifty miles long and one mile wide. The average height of the islands is about five feet, 

 although some sand dunes rise to fifteen feet. Beaches are found only on the seaward side of 

 the islands. The Sound side is black mangrove swamp. Beaches rest on buried swamp which 

 they override as they move inland, and beach thickness approximates elevation. The trafficable 

 value of beaches is similar to that of cheniers except that, because the beaches are not as thick, 

 they will not support as much weight. Along the inner margin of Chandeleur and Breton Sounds 

 are many small shell beaches, but they are too discontinuous to have any appreciable traffic- 

 ability value. 



One other marshland feature needs to be considered— the marsh itself. Marsh composes 

 over 95% of all land south of the Pleistocene terrace. Its elevation varies from about sea level 

 to one or two feet. Marsh vegetation consists of various grasses, rushes, and sedges, which 

 are normally higher and thicker in fresh water than in salt water marsh. Fresh water marsh 

 is confined to more inland areas, and is composed of highly organic clay, silt, and vegetation. 

 Silt is deposited on the outer marshes when storm tides sweep over them. 



The section in marsh areas is identified on the left margin of Fig. 2. Typically, there 

 are 20 to 25 feet of soft organic clay resting on river bar material. In many places this or- 

 ganic material is so soft that a pipe can be pushed into it by hand until the hard bar materials 

 are encountered. This is in sharp contrast to levee materials which are very compact and 

 "tough," 



-ft 



MARSH^ 



s> 



n 



NEW MARSH 



MUD fLAT OtPqSfX 



BEACH SANDS AND SILTS 



OLDER MARSH DE P0S'rT5~ 



LAKE AND BAY DEPOSITS 



OXIDIZED PLEISTOCENE DEPOSITS 



™ 



Figure 3. Cross section of a chenier. 



