Gregory — Progress in Interpretation of Land Forms. 123 



The Loess as a Glacial Deposit. 



A curious side-product of the study of giaciation in 

 North America is the controversy over the origin of loess. 

 The interest aroused is indicated by scores of papers in 

 American periodicals and State reports of the last quar- 

 ter of the 19th century — papers which bear the names of 

 prominent geologists. 



The " loess" in the valley of the Rhine had long been 

 known, but the subject assumed prominence by the pub- 

 lication in 1866 of Pumpelly 's Travels in China. 62 Wide- 

 spread deposits 200 to 1,000 feet thick were described as 

 very line-grained yellowish earth of distinctive structure 

 without stratification but penetrated by innumerable 

 tubes and containing land or fresh-water shells. Pum- 

 pelly considered these deposits lacustrine, a view which 

 found general acceptance though combated by Kingsmill 

 (1871), 63 who argued for marine deposition. Baron Von 

 Richthofen's classic on China, which appeared in 1877, 

 amplifies the observations of Pumpelly and marshals the 

 evidence to support the hypothesis that the loess is wind- 

 laid both on dry land and within ancient salt lakes. The 

 conclusions of Von Richthof en were adopted by Pumpelly 

 whose knowledge of the Chinese deposits, supplemented 

 by studies in Missouri, of which State he was director of 

 the Geological Survey in 1872-73, placed him in position 

 to form a correct judgment. He says : 64 



"'Recognizing from personal observation the full identity of 

 character of the loess of northern China, Europe and the Mis- 

 souri Valley, I am obliged to reject my own explanation of the 

 origin of the Chinese deposits, and to believe with Richthofen 

 that the true loess, wherever it occurs, is a sub-aerial deposit, 

 formed in a dry central region, and that it owes its structure to 

 the formative influence of a steppe vegetation. 



The one weak point of Richthofen's theory is in the evident 

 inadequacy of the current disintegration as a source of material. 

 When we consider the immense area covered by loess to depths 

 varying from 50 to 2,000 feet, and the fact that this is only the 

 very finest portion of the product of rock-destruction, and again 

 that the accumulation represents only a very short period of 

 time, geologically speaking, surely we must seek a more fertile 

 source of supply than is furnished by the current decomposition 

 of rock surface. 



It seems to me that there are two important sources : I. The 

 silt brought by rivers, many of them fed by the products of 

 glacial attrition flowing from the mountains into the central 



