360 Ohamberlin and Salisbury — Relationship of 



But in the central part of the basin of the Mississippi River 

 below the latitude of glaciation. the difference between the 

 deposits made by the waters originating in the melting ice, 

 and the subjacent strata, appears to be much less obvious than 

 in many other regions. In consequence, the recognition of 

 the Pleistocene formations along the course of the Mississippi 

 south of the limit of the general drift sheet, and therefore the 

 correlation of the northern and southern Pleistocene forma- 

 tions, has not been free from difficulties. 



The Loess. — The highlands bordering the Mississippi Valley 

 on either side, between the parallels of 37° 30' (the southern 

 limit of glacial drift), and 35° are overspread by loess. The 

 loess extends much below this latitude, but that is the lower 

 limit of the area especially under consideration. A belt of 

 loess similarly disposed occurs along the course of the Ohio, 

 for a considerable distance above its junction with the Missis- 

 sippi, though the loess along this stream is less constant in 

 character, and its facies often different from that of the Missis- 

 sippi River loess. Consonant with the general habit of this 

 formation, the loess is best developed on the highlands im- 

 mediately adjacent to the rivers. In such situations, the loess 

 possesses the loose, open texture which is one of its most diag- 

 nostic characteristics. As the formation is traced eastward or 

 westward from the Mississippi, or northward or southward 

 from the Ohio, these characteristics gradually disappear. The 

 open texture becomes less pronounced, and by almost imper- 

 ceptible degrees passes from that of a loose, light loam, 

 through that of a clayey loam, to that of a loamy clay. So far 

 does this gradation proceed, that the texture of the loam at 

 some distance from the streams, resembles much more closely 

 that of the residuary earth upon which it rests, than that of 

 the typical loess, on the immediate borders of the valley. 



A change in color accompanies the change in texture. The 

 huffish color which everywhere characterizes the river bluff 

 loess in its normal development, becomes deeper with increas- 

 ing distance from the streams, so that along the borders of the 

 loess, where the texture has come to simulate closely that of 

 the residuary earths, the color has become notably deeper than 

 on the bluffs immediately fronting the river, and the deepen- 

 ing color has been a constant approximation to the color of the 

 underlying residuary earth. 



Accompanying the change in texture and color, are changes 

 in the chemical character of the loess. Xear the rivers it 

 almost uniformly contains a considerable percentage of carbon- 

 ates. Here too, it very generally contains shells, and concre- 

 tions of calcium carbonate, the one a partial cause, and the 

 other at once an index and a result of its calcareous character. 



