CHAMPLAIN EPOCH. 559 



The cause of the flooded condition of the rivers mid lakes was partly 

 the depression of the land, by which the sea entered into the old glacial 

 beds, forming estuaries ; partly the smaller angle of slope of the rivers, 

 by reason of which the waters in their lower parts ran off less rapidly, 

 and therefore were more swollen, and therefore also deposited more 

 sediment ; and partly the greater abundance of the water-supply, from 

 the melting of the glaciers. The mud-supply also was then very great, 

 as shown by the immense deposit, and also by the cross-lamination (p. 

 174) so common in these deposits. 



Origin of the Loess. — Over large areas bordering the Mississippi 

 and its tributaries, and forming the conspicuous bluffs of these rivers, 

 there is found a peculiar deposit of very fine, even-grained, and usually 

 unstratified material, remarkable for forming by river-erosion perpen- 

 dicular walls — although soft enough to be easily spaded. It is usually 

 destitute of organic remains, but when these are found they consist 

 of fresh-water shells, and especially of land-shells. When fresh- water 

 shells are found, the material is usually obscurely stratified. Similar 

 bluff-materials are found bordering nearly all the European rivers, such 

 as the Rhine and Danube, and is there called Loess, and referred to the 

 Champlain epoch. 



A somewhat similar material, however, is found also spread almost 

 evenly over wide areas nearly everywhere, especially in arid regions, and 

 having no obvious connection with any rivers. Such is es}3ecially the 

 case in Northern China, where Richthofen finds it covering thousands 

 of square miles, and in places one thousand or more feet thick. Russell 

 also finds a somewhat similar unstratified deposit covering large areas 

 of the Basin region, and sometimes locally called " adobe." 



There has been much discussion about the origin of these deposits. 

 The Loess of the Mississippi and its tributaries, as also the European 

 rivers, was probably deposited in the flooded lakes and in the slack- 

 ened waters of the flooded rivers of the Champlain epoch. It is poor 

 in fossils, because the waters were ice-cold. It is unstratified, because 

 the waters were overloaded with the very finely triturated material left 

 by the retreating ice-sheet. 



The Loess of Northern China, Richthofen thinks, is an JEolian de- 

 posit — i. e., a deposit of wind-borne dust from the arid regions to the 

 northwest.* The unstratified superficial soil of the Basin region, Rus- 

 sell thinks, is due partly to wind-borne dust, but mainly to rain-wash 

 — i. e., to the semi-liquid, creamy mud washed down the bare slopes by 

 heavy rains ; for in these arid regions, although rain is rare, it falls in 

 torrents. f The unstratified soil, often called Loess, covering the hilly 



* American Journal of Science, vol. xiv, p. 487, 1877. 

 f Russell, Geological Magazine, vol. vi, p. 289, 1889. 



