214 tuknbull et al. RIVER ALLUVIUM PROBLEMS [Ch. 12 



Natchez, Mississippi. The sedimentary sequence in both the valley 

 fill and terrace deposits is indicated in the coarse to fine gradation 

 of the materials in these sections. Loess is represented as restricted 

 chiefly to crests of terrace hills of the eastern valley wall. 



Meander-Belt Sediments 



When sea level reached what is essentially its present stand, the 

 Mississippi River ceased carrying coarse sediments into its valley and 

 achieved what has been called a "poised" state. This is a condition 

 in which the river has no pronounced tendency to do much valley 

 deepening or filling. However, the behavior of the river is such that 

 earlier sediments through which it flows become reworked to produce 

 meander-belt deposits having distinctive properties. 



In a poised river, meandering or the development of "bends" is a 

 trademark. And, as these loops in the river's course are continuously 

 enlarging and being abandoned while newer belts form, they result in 

 reworked sediments that are left as superficial scars covering much of 

 the flood plain. Resulting sediments of the following sorts are typical 

 in the Mississippi River Valley (Fisk, 1947). Figure 3 depicts the 

 various types of deposits. 



Point-bar deposits. As meander loops increase in size, the core of 

 land in their interior expands through the addition of a series of arcuate 

 ridges and intervening swales, which result from river deposition in a 

 strip-like manner on growing bars. Most of the sediment contributed 

 consists of sand and coarse silt. Topographically, these ridges may 

 average 6 to 10 feet in height and usually conform to the channel 

 curvature within which they were formed. As a result of frequent 

 downstream migrations of channels which accompany bar development, 

 these strips often truncate each other and otherwise complicate their 

 patterns. Such truncation of bar ridges and the relation of these 

 ridges to each other as individual accretions are shown in the point- 

 bar deposits pictured in Fig. 3. 



Swale fillings. The swales, or topographic lows between accretion 

 bars, often develop dense willow growths and trap fine sediments 

 during high water long after their bordering ridges have developed. 

 Thus these ridges become separated by intervening shallow strips of 

 silts and silty clays which contain much organic matter. Within the 

 point bar of Fig. 3 are swale strips situated between the bar ridges. 

 The swales can be recognized by their heavy vegetation. 



Natural levees. Natural levees are the higher parts of a river's 

 flood plain, and they flank the immediate sides of its channel. These 

 deposits are laid down in the form of thin layers resulting from deposi- 



