LOESS AND OTHER MODIFIED DRIFT DEPOSITS. 95 



Laterally and downward the loess in some places grades into the Port 

 Hudson clays, which are continuous along the bottom of the Mississippi 

 A'alley from the mouth of the Ohio to the Gulf, having a thickness of 

 more than 600 feet at New Orleans. Near their base both the loess and 

 Port Hudson beds, which are classed together by McGee as equivalent 

 with the Columbia formation of the Atlantic coastal plain, often enclose 

 sandy and gravelly layers, and they very commonly appear to be con- 

 formable along the great river, as observed by Hilgard, with the under- 

 lying beds of the Lafayette formation. Glacially ground rock flour is 

 the chief or sole ingredient of the Mississippi loess, and it constitutes a 

 part of the Port Hudson clays, while their greater part, however, consists 

 of the very fine silt of this river and its tributaries during a time of 

 depression of the land which followed its elevation and the»attendant 

 erosion of the Lafayette sand and gravel and of the Tertiary strata 

 beneath. The coarse basal beds in the Port Hudson and loess or Colum- 

 bia formation imply at its beginning a close relationship with the condi- 

 tions of deposition of the Lafayette, and they both seem referable to the 

 same great epeirogenic uplift. During the early part of this time of 

 elevation, before the rivers obtained too steep gradients, I think that the 

 Lafayette formation was deposited ; and later, while the depression from 

 this elevation was bringing the continent once more down to approxi- 

 mately its present level, as soon as the streams again flowed with such 

 slopes as to allow them to lay down sediments, they formed the Port 

 Hudson, loess, and Columbia deposits. 



Successively as the ice-sheet retreated, all the areas which it had cov- 

 ered, first at the south, then intermediate, and latest at the north, were 

 overspread with modified drift along the avenues of drainage. Besides 

 its deposition in the principal valleys, the glacial rivers flowing down 

 from the melting ice-sheet accumulated eskers, kames and gravel and 

 sand plains where now no streams exist. Among these records of the 

 drainage of the wasting ice-sheet, none perhaps are more remarkable 

 than the eskers of loess in northeastern Iowa, w^hich McGee describes 

 under the aboriginal name paha. He also finds in that district, as I had 

 similarly observed in the northwestern part of the same state, that the 

 margin of the ice as a high barrier formed a boundary of the loess when 

 it was being deposited. The west side of the Minnesota and Iowa ice- 

 lobe was forming the Altamont moraine contemporaneous with adjacent 

 river floods spreading loess across the Missouri valley, and I beheve that 

 the loess deposition was mainly continuous, accompanying the gradual 

 and widely extended but wavering departure of the ice-sheet from its 

 farthest boundary to this outermost of the conspicuous morainic belts. 



XIII— Bull. Geol. Soc. Am., Vol. 5, 1893. 



