32 



TREADWELL 



TRAFFICABLE UNITS 



Natural levees are the dominant topographic feature of the marshland and form the only 

 continuous high ground. The levees radiate in a branching pattern from a focal point at New 

 Orleans. Levee heights near New Orleans are about ten feet, but gradually diminish seaward 

 as the levees dip beneath marsh level. Levee widths reach one and one-quarter miles on upper 

 parts of the distributaries and gradually narrow toward downstream regions. Under natural 

 conditions vegetation is very dense, mainly live oak and various kinds of underbrush. Natural 

 levees are composed of silty clay and silt, and ordinarily have " high moisture content because 

 the water table is close to the surface. The upper portion is usually oxidized, and reducing 

 conditions prevail below. Dry levees are excellent trafficable units; however, after rain they 

 become sticky, and ordinary vehicles are easily bogged. The effective trafficable limits of 

 levees are always outlined either by trees or agricultural areas, whicK allow easy determina- 

 tion of this boundary on aerial photographs. 



An ideal cross section of the sedimentary sequency associated with natural levees is 

 presented in Fig. 2. The basal layer is marine sand antedating delta encroachment onto the 

 area. Overlying this are pro-delta clays which are fine, stream-carried sediments deposited 

 in advance of distributary mouths. River bar material rests on the pro- delta clays. Bar sedi- 

 ment is deposited around distributary mouths, as the stream velocity is checked by waters of 

 the Gulf of Mexico. Fine, silty clays and clays are deposited concomitantly with bar sediment 

 in interdistributary areas and lense into bar material. Above this are the natural levees. 

 Levee silts and clays lake the shape of a double wedge, thickest near the stream channel and 

 thinning into the flanking marsh or lake deposits. Channels of abandoned distributaries are 

 located between the natural levees. The basal portion of channels typically contains sands and 

 silts, and the upper portion is filled with fine clays and vegetation. A tidal channel frequently 

 occupies the remnant distributary channel. 



Stranded beach ridges , or cheniers , are secondary trafficable units in this area. In the 

 western part of the State they are better developed, and owing to the lack of natural levees, they 

 are the only means of land travel in the marsh. 



Cheniers of this eastern area are largely buried under marsh, but are continuous under 

 marsh between outcrop areas. These features nearly always form a linear pattern. Their 



■J'^^^M!^^:^ 







y 



(SILTY CLAY 

 AND SILT 



SLTY CLAY AND ] 

 _'~2^---,^EGETATI0N^ J, 



SILT AND CLAY ! 



SAND 



CLAY 



Figure 2. Ideal cross section of natural levee. 



