DERIVATION OF COMMON AND MARL-LOESS 169 



rial was not of local origin, but \yas brought down by the rivers them- 

 selves. The ultimate source, as indicated by the composition, as well 

 as by partial direct tracing, was in the glacial ice of the lowan sheet to 

 the north. 



Derivation of the Loess of the Common Type 

 evidences pointing to aqueous accumulation of part of the loess 



General statement. — In the introduction it was suggested that the silts 

 included under the common type are possibly not all of the same 

 origin. While, considered as a whole, the common loess of southern 

 Indiana and Illinois appears to be very largely of eolian origin, there 

 are (certain portions lying outside of the limits of marl- loess which, 

 though exhibiting the non-calcareous composition, the clayey texture, 

 and the buff, brownish, or reddish colors of the typical eolian loess, 

 have, nevertheless, a number of peculiarities of composition and topog- 

 raphy which are suggestive of aqueous accumulation. These features 

 are considered in the following paragraphs. 



Large pebbles. — A considerable number of instances were noted in the 

 Boonville quadrangle of glacial pebbles apparently occurring in the loess 

 of the common type outside the glacial limits. They were of the weath- 

 ered type, such as occur in the characteristic Illinoian drift a little far- 

 ther north. The pebbles, which varied from 2 to 5 inches in diameter, 

 were obtained from the ordinary weathered roadside exposures; hence 

 it can not be stated with absolute certainty that they were found in situ^ 

 though the considerable number noted is suggestive that such was prob- 

 ably the case, especially as none of the number found bore any evidences 

 of human handling, and were of an entirely different character from the 

 fresh Wisconsin gravels sometimes used as road metal near the cities. 

 None were found above an altitude of 500 feet above tide, though this 

 is not so significant here as in the case of the marl-loess farther west, as 

 very little of the land rises above this level in the quadrangle mentioned. 



Loess-covered till plains. — Southwest of Petersburg there are extensive 

 plains standing at 500 feet above tide. The main mass of these Pleis- 

 tocene deposits, which sometimes reach 100 feet or more in thickness, is 

 composed of glacial drift, largely till, but the surface is everywhere cov- 

 ered by a mantle of brownish loess. The stream channels which cut 

 into the plains show that, although the loess surface is nearly flat, the 

 underlying till is marked, as indicated by the varying thickness of the 

 loess mantle, by a surface of gentle but distinct undulations. The filling 

 of the depressions and the upbuilding from the uneven surface of a level 

 plain is not in accordance with the mode of occurrence of the loess above 



XXIV— Bull. Geol. Soc. Am., Vol. 14. 1902 



