2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



obliterating the shore lines of the preceding epochs. Thus arose a 

 very gently sloping alluvial plain bordered on each side by more 

 plainly marked terraces. 



INTRODUCTION 



The name Mississippi Embayment is applied to a great extension 

 of Cretaceous and younger sediments in the Mississippi Valley and 

 adjacent areas below the vicinity of the mouth of the Ohio River. 

 This includes all of Louisiana, a large part of Alabama, Mississippi, 

 and Arkansas, and smaller areas in Missouri, Tennessee, Kentucky, 

 and Illinois. 



The Gulf of Mexico has repeatedly invaded the embayment. The 

 remarkable Upper Cretaceous shells at Coon Creek, Tenn., the Paleo- 

 cene oysters of the Clayton Limestone, the well-preserved late Eocene 

 shells at Jackson, Miss., and Montgomery, La., the Oligocene fauna 

 at Vicksburg — all give unmistakable evidence of some of those 

 invasions. 



Rivers have been flowing into the embayment since Mesozoic time. 

 Their muddy waters played a large part in filling it with sediments. 

 Such formations as the Porters Creek Clay (Paleocene) and the 

 Yazoo Clay (late Eocene) were doubtless built of mud brought down 

 by these rivers. 



Quaternary deposits cover a vast area in the Mississippi Embay- 

 ment. Borings through these sediments prove that before their deposi- 

 tion through-flowing rivers, including the Ohio, the Mississippi, the 

 Ouachita, the White and, the Red, had carved valleys into the older 

 formations of the Coastal Plain, and that these rivers flowed into the 

 Gulf of Mexico somewhere beyond the present coast. The contem- 

 porary shore line may have lain not far from the edge of the Continen- 

 tal Shelf (Fisk, 1944, p. 38). Sea level at that time probably lay 

 400 to 450 feet lower than now (Fisk, 1944, p. 68). 



At that early time the Mississippi River flowed west of Crowley 

 Ridge. The Ohio joined it below Natchez, and the Arkansas came 

 in below latitude 30°, near Franklin, La. Later, the Ohio, flowing 

 in a lower valley, captured the Mississippi above Vicksburg, and still 

 later diverted it through Thebes Gap (Fisk, 1944, fig. 45). 



These ancient valleys are now buried under a great accumulation 

 of gravel, sand, and silt upon which the rivers now meander with 

 greatly reduced gradients. Fisk (1944, p. 17) estimates that this 

 alluvium "has a volume of approximately 1,000 cubic miles and an 

 average thickness of 132 feet." Krinitzsky (1949, p. 13) increases 



