Ch. 12] ALLUVIATION 213 



is rarely found. Also, going north anywhere beyond the Louisiana- 

 Arkansas line, the gravels lie close to the surface, often within 15 

 feet; however, coming south into Louisiana and southern Mississippi, 

 the cover thickness increases so that the gravels are 100 feet below 

 the surface in the vicinity of Houma, Louisiana. Thicknesses within 

 the graveliferous sections as a result of slope controls during deposi- 

 tion show somewhat less of a distinct north-south variation than is 

 found in the overlying finer deposits. The gravel section near Sikeston, 

 Missouri, is 180 feet in thickness. Near Memphis, Tennessee, the sec- 

 tion is on the order of 100 feet but increases again to approximately 

 150 feet in the latitude of Yazoo City, Mississippi. Near Natchez, 

 Mississippi, the value is about the same, but near New Orleans a 

 thickening occurs in the deep-lying gravels so that 200-foot units can 

 be measured. Also, there are important changes in the gravel sections 

 within local portions of the valley, the gravels being thicker toward 

 each tributary stream, where they assume a fan-shaped form. 



The non-graveliferous deposits are themselves divisible into two 

 phases: 



(1) A pervious phase composed mostly of clean, well-washed sands 

 exists in the lower part of the section. These thin out toward the north, 

 where the whole section diminishes, and also thin southward because 

 after alluviation became advanced sand was no longer carried in large 

 quantities by the river below the central part of the valley. 



(2) The upper, less pervious phase consists of a layer of silts and 

 clays which thin to the north but thicken southward and become 

 especially thick in southern Louisiana, where they are part of the 

 deltaic mass. These are the most typical materials of the modern 

 Mississippi River flood plain, and they are the most important in their 

 effects on modern river behavior. 



A special sort of sediment, the origin of which has been disputed in 

 the Lower Mississippi Valley, is the loess that caps almost all terrace 

 deposits along Crowley's Ridge and the eastern Alluvial Valley walls. 

 It extends from north of Cairo, Illinois, all the way southward to 

 Bayou Sara in Louisiana. The loess is a homogeneous, unstratified, 

 calcareous silt with some plasticity and has a tendency to vertical 

 cleavage. Its thickness on hill crests varies from as much as 65 feet 

 near the Alluvial Valley bluffs to about 10 feet or less on terrace 

 margins. There it merges into a similar soil material known as brown 

 loam. 



In Fig. 2 there is depicted a geological section through Recent and 

 Pleistocene deposits of the Mississippi Valley in the latitude of 



