LOESS AND THE IOWAN DRIFT. 357 



where they formed extensive bars, and whence they were 

 carried to the adjoining slopes, where vegetation had already 

 secured a foothold. 



That more or less extensive sand-dunes were also formed 

 before the deposition of the loess along this border, appears 

 in the fact that finer sands underlie this loess very commonly 

 along the greater part of the border. The fact that these sands 

 are so commonly present was emphasized by McGee for the 

 northeastern part of the area, and has been observed by all the 

 Iowa geologists who have worked in the Iowan border terri- 

 tory.* 



These sands suggest a sand-dune origin rather than water- 

 deposition, by the fineness of the sand and the absence of 

 pebbles, by the lamination characteristic of sand-dunes, by the 

 frequent alternation of very fine and somewhat coarser sands 

 often seen in sand-dunes, and by the vertical outlines of the 

 deposits which, at least in the border area north of Iowa City, 

 are like the vertical contours of ordinary sand-dunes. These 

 sand-dune conditions have been preserved in the same region 

 to some extent even to the present day, but after the disap- 

 pearance of the Iowan ice they were at their best, and evi- 

 dently formed the characteristic surface deposit just outside of 

 the Iowan border. As vegetation gained a footing upon the 

 dunes, soils were formed, vegetation increased, and the de- 

 position of loess was made possible. The sand-dunes along 

 the border of the drift, as well as the extensive bars in the 

 streams occupying the same border territory, furnished a large 

 amount of dust, and the immediate vicinity of the drift-border 

 received larger quantities of this material than the outlying 

 districts, and hence accumulated more loess. Borders of large 

 streams, especially those flowing in broad valleys, have the 

 same advantage and accumulate more loess than regions more 

 remote. 



As a matter of fact Leverett himself based his opinion con- 



*See especially Calvin's discussion in Iowa Geol. Sur. , vol. vii, pp. 



88-9, 1897. 



