Thicki 



LOESS 645 



lickness. The loess of the Mississippi basin rarely is more 

 than a score or two feet thick, and this only along the main valleys; 

 but exceptionally its thickness approaches 100 feet. Thicknesses of 

 10 feet are much more common than greater ones. 



Accessories. The loess contains characteristic accessories of 

 two kinds, concretions and fossils. The concretions are of lime 

 carbonate and iron oxide. Many of the former are irregular, 

 and of such shapes as to have been called "petrified potatoes"; 

 but many of them have other shapes. The ferruginous con- 

 cretions take various forms, one of which is the "pipe stem," 

 perhaps formed about rootlets. The fossils are chiefly gastropods 

 (Fig. 534), almost wholly of land species, or of such as frequent 

 isolated ponds. The other fossils are bones and teeth of land 

 mammals. 



Origin. There has been much diversity of opinion as to the 

 origin of loess, the fundamental question being whether it is aqueous 

 or eolian. There is little doubt that the loess-like silts which occur 

 in the terraces of rivers are of fluvial origin; but some would not 

 regard them as loess. Some, indeed, would restrict the term to an 

 eolian product. 



There is a growing conviction that most of the loess on the 

 'uplands, in the United States at least, is eolian. The river flats 

 are supposed to have supplied much of the material of the loess, the 

 alluvial silt being whipped up by the winds and re-deposited on the 

 adjacent uplands. The rivers are thus made essential factors in its 

 distribution, though not the direct agents of deposition. This hy- 

 pothesis seems on the whole best to fit the phenomena of the larger 

 part of the upland loess of the Mississippi basin. The constituents 

 of the loess, which appear to have come from the glacial drift, were 

 derived largely from the deposits made by glacial waters, or from 

 later flood plain silts derived from the glacial formations; but it is 

 probable that some of the loess was derived from glacial drift direct- 

 ly, before it became clothed with vegetation. 1 



1 References. Loess is described in the geological reports of many of the 

 states of the central Mississippi basin. Other references are McGee, Eleventh 

 Ann. Kept., U. S. Geol. Surv.; Chamberlin and Salisbury, Sixth Ann. Rept., U. S. 

 Geol. Surv.; Shimek, Am. Geol., Vols. XXVIII and XXX, Bull. la. Lab. Nat. 

 Hist., Vols. I, II, and V, Proc. la. Acad. Sci., Vols. Ill, V, VI, and VII; Leverett, 

 Am. Geol., Vol. XXXIII, and Monog. XXXVIII; Calvin, Bull. Geol. Soc. Am., 

 Vol. X, p. 119; Ch;imlH-rlin, Jour. Geol., Vol. V, 1897; Davis, Explorations in 

 Turkestan, 1905; and Willis, Researches in China, Vol. I, Carnegie Institution. 



