TEE LOESS. 415 



not come so much within my own field of observation as 

 other glacial deposits have done. Still, in southern Indiana 

 and Illinois, and in western Iowa and eastern Nebraska, I have 

 been able to study many typical regions of this deposit ; and 

 the more attention I have given to the subject, the more I 

 have been led to magnify the agencies of the Ice period in 

 producing the results both positive and negative. I have 

 come, therefore, to set an increasingly high estimate upon 

 the suggestions of Mr. Upham, made after he had concluded 

 his survey of the terminal moraine in Minnesota, Iowa, and 

 eastern Dakota : 



When the ice-sheet extended to the Coteau du Missouri, 

 the Coteau des Prairies, and the Kettle-Moraine, the floods 

 formed by its summer meltings were carried southward by the 

 present avenues of drainage, the streams which occupied the 

 areas between its great lobes in order from west to east being 

 the Big Sioux, Mississippi, and Wisconsin Rivers. The vast 

 glaciers which were gathered up on the Rocky Mountains, and 

 the ice-fields which sloped downward to their termination at 

 the coteaus and the moraine north and east in Minnesota and 

 Wisconsin, supplied every summer immense floods laded 

 with silt, sand, and gravel, that had been contained in the 

 melting ice. Very extensive deposits of modified drift were 

 thus spread along the course of the swollen Missouri and Mis- 

 sissippi. The Orange sand and gravel, described by Professor 

 E. W. Hilgard and others in the lower Mississippi Valley, ap- 

 pear to have been deposited in this way, but during the earlier 

 Glacial epoch, when an ice-sheet reached in Dakota beyond 

 the Missouri River to a termination forty miles west and 

 twenty miles southwest of Bismarck, into northeastern Kansas, 

 half-way across the State of Missouri, and nearly to the Ohio 

 River. 



In the closing stages of this epoch, and during the time 

 succeeding, till the date of the terminal moraine of the coteaus, 

 and especially at the final retreat of the ice-sheet of this later 

 epoch, the deposition of the overlying, finely pulverized, are- 

 naceous and calcareous silt, called the Bluff formation, or loess, 

 took place. This covers considerable areas along the Mis- 



