Ch. 12] ALLUV1ATI0N 211 



their individual elevations (Fisk, 1944) . The oldest of these terraces 

 has the highest topographic expression, whereas the remainder have 

 progressively lower elevations in order of decreasing age. This con- 

 dition has resulted from a progressive continental uplift which has 

 raised the deposits vertically, thus protecting portions of them from 

 removal by subsequent erosive action of the Mississippi River. Their 

 present areal expression is that of strips bordering the modern Al- 

 luvial Valley and elongated "islands," such as Crowley's Ridge, which 

 divide its northern portion. Southward, in southern Louisiana, these 

 terrace formations have been downwarped as a result of sinking in the 

 Gulf Coast geosyncline and plunge under Recent deposits of the 

 Mississippi deltaic mass. 



The areal distribution of these sedimentary units in relation to the 

 modern Mississippi River and to physiographic basins and ridges 

 within its valley is illustrated in Fig. 1. 



Each of the terrace deposits reflects a sequence of deposition which, 

 as a result of lesser modification by erosion, is ideally shown in the 

 Recent alluvial fill. During its last period of overly steepened gradi- 

 ent, the Mississippi River transported very coarse loads containing 

 much gravel, derived chiefly from adjacent Paleozoic upland areas. 

 However, with the slow decrease in gradient resulting from gradually 

 rising seas, much of this coarse load was deposited, thus forming the 

 present valley fill. Eventually, the Mississippi River's gradient slack- 

 ened to such a point that gravels were no longer being transported, 

 although alluviation was continuing with further advances in sea 

 level. Under these circumstances, the alluvial deposits were formed of 

 finer materials, mostly sands. 



Thus it can be seen that, on the whole, the Recent alluvium can be 

 divided sharply into two types: (1) the lower graveliferous section 

 and (2) an upper, essentially non-graveliferous section. 



In the graveliferous section, about 95 percent of all samples con- 

 tain some gravel; in the non-graveliferous section, less than 1 percent 

 of the samples show any gravel whatsoever. In general, the gravel- 

 iferous section contains gravels somewhat in excess of 25 percent by 

 weight, the remainder being coarse sand. On the other hand, some 

 parts of the graveliferous section may contain only fine- or medium- 

 grained sands with little or no gravel. In general, the cobbles, where 

 present, are larger at depth and smaller in the shallower layers. It is 

 common, north of the Louisiana-Arkansas boundary, to find occasional 

 cobbles 1 foot in diameter, but southward there is a rapid decrease in 

 size, so that in Louisiana it is uncommon to find specimens over 3 

 inches in diameter, and toward the coast gravel over 1 inch in diameter 



