Chapter 12 



SEDIMENTARY GEOLOGY OF THE ALLUVIAL VALLEY 

 OF THE LOWER MISSISSIPPI RIVER AND ITS IN- 

 FLUENCE ON FOUNDATION PROBLEMS 



W. J. Turnbull, E. L. Krinitzsky, and S. J. Johnson 



Respectively Engineer, Geologist, and Engineer 



Soils Division, Waterways Experiment Station 



Corps of Engineers, Department of the Army 



Vicksburg, Mississippi 



ALLUVIAL DEPOSITS 



Alluviation in the Lower Mississippi Valley 



The most important recent geological influence on Mississippi River 

 behavior has been the advance and retreat of continental glaciers. It 

 has been estimated, and relatively accurately because the terminal 

 moraine outlines are fairly well known, that 20 million square miles 

 of land surface were covered by ice at its maximum extent. The 

 estimates of thickness of this great ice cap are not in such common 

 agreement, but a figure of 1 mile for average maximum thickness is 

 often given. This would represent a volume of 20 million cubic miles 

 of landlocked water. Theoretically, such a vast volume when re- 

 moved from the ocean basins would be sufficient to lower sea level 

 somewhere in the neighborhood of 450 feet, even if allowance is made 

 for the present-day ice masses which have not yet been melted. With 

 sea level at such a considerably lower stand during times of glacia- 

 tion (Daly, 1929), the Mississippi River was flowing with an overly 

 steep gradient and thus eroded deeply into its valley. But, with the 

 rise of sea level during waning of glaciation and the consequent loss of 

 gradient along the Mississippi River's course, alluviation of its former 

 incised valley took place. 



Since continental glaciation has occurred during five distinct times, 

 there are an equal number of such alluvial deposits in the Lower 

 Mississippi Valley. These are in the form of the modern Alluvial Val- 

 ley fill plus four distinct terraces, which are recognized according to 



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