60 F. W. Sardeso?i — What is the Loess f 



but more level land farther from the great rivers, the loess was 

 thinner and was also more oxidized. Often it is all oxidized. 



From my own observations compared with published descrip- 

 tions, I feel safe in saying that, as a rule, the loess loam is what 

 has been called sedimentary loess. If it were such, why is it 

 not calcareous as are the known glacial silts ? And if the 

 " bluff " loess is derived from it, why is the former calcareous, 

 since the latter is not ? There is no explanation. 



The above criticism applies to the- loess as a rock type, but 

 when the Loess formation is considered another distinction 

 should be admitted, namely that there are two distinct loess 

 formations, as H. F. Bain describes in northwestern Iowa,* 

 which are comparable to the older and younger loess of the 

 Rhine Valley . Each may bear loam. 



The loess as a formation bears loam both upon and within. 

 Upon the loess, the loam represents the portion which has been 

 oxidized since the loess formation was deposited, while in the 

 same it indicates patches of vegetation that were contempo- 

 raneous with the deposition. The loess loam, in combination 

 with land and fresh water shells, forms the very strong argu- 

 ment in favor of the purely geolian origin of the loess. The 

 seolian hypothesis is the one which permits the several parts of 

 the loess to be both readily interpreted and easily recognized 

 in the field, and it has therefore a distinct advantage over the 

 semi- seolian hypothesis. It is further competent to explain 

 without inconsistency the dune sands on the river bluffs and 

 their graduation into the loess ; wind polished pebbles lying 

 between till and loess are recognized ; two kinds of stratified 

 silts existing with the loess or graduating into it are identified 

 as respectively glacial silt from which the loess seems to have 

 been largely derived, and modified or washed loess. Banded 

 structure in the loess,f which is not true stratification, indicates 

 the line of a moist zone where vegetation thrived and oxidized 

 the loess. Dry runs and very probably rodent burrows and 

 mounds can be easily recognized ; and the occurrence of land 

 and fresh water shells is not anomalous. Finally, when it is 

 observed that much loess accumulated before one glacial period 

 or ice advance and little before another, the same is consist- 

 ently attributed to climatic conditions, to which kind of cause 

 the glaciers themselves are referred. 



University of Minnesota, Minneapolis. 



* Iowa Geol. Sur., vol. viii, p. 240. 



f See for example, fig. 32, p. 235, vol. vii, Iowa Geol. Sur. 



