RELATION OF DUNE SANDS TO TERRACE GRAVELS 359 



cially along tlie limit of the highest flood line. Further, the same struct- 

 ural differences are seen between the river sand and the dune sand on 

 the bank or terrace as are commonly seen between modified drift and 

 dune sands throughout this region. 



TIME OF FORMATION OF DUNE SANDS 



The time when these dune sands accumulated can not be exactly de- 

 termined, although whenever the revision of the Saint Anthony Falls 

 recession shall be undertaken their comparative age may be quite exactly 

 defined. No further estimate will now be attempted than this : The falls, 

 which have receded about 8 miles since Glacial time in this region, had 

 not begun the last fourth of their now accomplished work at a time 

 when the dune sands were comparativel}'' as now. They have doubtless 

 been drifting ever since the time of the glacial retreat from this area. 



Lag Gravels 



As will be seen from the foregoing statement, lag gravels * frequently 

 occur on the till and modified drift. The pebbles composing them are 

 derived from the drift and are distinguishable from their association on 

 the drift, and more often from their smoothly polished surfaces — espe- 

 cially uj)per surfaces, which are quite evidently planed by wind-driven 

 sand. 



Summary 



Loess is found as a thin veneer widely distributed throughout eastern 

 Minnesota. It lies on many of the drift hills and plains, but its accu- 

 mulation was seemingly too slow for the formation of '' bluff loess ; " 

 hence, it appears as " loess-loam." 



The contemporaneous dune sands are far more abundant than the 

 loess; they are widely distributed, typical exposures being described 

 from separated localities, as Minneapolis, Pine City, and the banks of 

 Kettle river. Deposits equally well marked and extensive are seen in 

 adjoining Wisconsin, 



. Lag gravels are noted in a few localities, as Minneapolis. These de- 

 posits are recognized by their position at the top of the unmodified and 

 modified till and their content of pebbles with polished upward lying 

 surfaces. 



These eolian deposits are always distinguishable from the drift material 



*J. A. Udden : The mechanical composition of wind deposits, Augustana Library Publications, 

 no, 1, 1898, p. 7. 



