90 W. UPHAM — THE SUCCESSION OF PLEISTOCENE FORMATIONS. 



observed the Lafayette gravels in the Ohio valley in southern Indiana 

 about 150 miles from the Mississippi. McGee states that the Lafayette 

 beds attain their maximum thickness, which is 200 feet or more, in the 

 region about tlie mouth of the Mississippi, and that they vary thence to 

 a thin veneer, the thickness being proportional directly with the volume 

 of neighboring rivers and inversely with the extension inland. 



Previous to the maximum advance of the ice-sheet, the Mississippi 

 river and all its large tributaries eroded broad and deep valleys through 

 the Lafayette formation and underlying strata, cutting at New Orleans 

 to a depth of at least 760 feet below the present sea level. Along the 

 central valley, from Cairo to the Gulf, this erosion averages probably 

 200 feet in depth upon a belt 500 miles long, with a width of 50 to 100 

 miles, excepting isolated plateau remnants of the Lafayette and older 

 beds, of which the largest are Crowley's and Bloomfield ridges, in 

 Arkansas and Missouri. The land during the valley erosion was cer- 

 tainly 760 feet higher than now, but this I think to be only a small 

 fraction of its uplift. From the transportation of northern Archean 

 pebbles and cobbles of crystalline rocks to the Lafayette beds of the 

 lower Mississippi and of Petite Anse island, on the Gulf shore, in the 

 direct line of the axis of the Mississippi valley, Hilgard believes that 

 during the deposition of these beds the valley had a greater descent and 

 stronger currents of its river floods. He suggests that the increased 

 altitude of the interior of the continent needed to give these formerly 

 more powerful currents may have been 4,000 to 5,000 feet, being suffi- 

 cient, probably, to bring the cold climate and ice accumulation of the 

 Glacial period. 



Marine submergence of the low coastal and Mississippi valley areas 

 occupied by the Lafayette formation is supposed by McGee and Spencer 

 to have been requisite for the deposition of its sand and gravel beds, 

 but they see that immediately afterward the land was much higher than 

 now to permit the extensive and deep river erosion of that time. A 

 simpler view of the epeirogenic movements, closing the Tertiary era and 

 inaugurating the Quaternary, seems to me to be found in ascribing these 

 beds to deposition by flooded rivers descending from the Appalachian 

 Mountain region and from the Mississippi basin, spreading gravel, sand, 

 and loam over the coastal plain and along the great valley during the 

 early part of a time of continental elevation. The land had lain during 

 the long Tertiary periods at lower altitudes, and its surface was largely 

 enveloped by residual clays and by alluvial sand and gravel. With the 

 elevation of the continent, increased rainfall and snowfall and resulting 

 river floods swept away these superficial materials from the higher lands 

 and spread them on the coastal plain and along the Mississippi valley, 



