MODE OF DEPOSITION OF THE LOESS. 301 



That this region had an unfavorable altitude for drainage in the pre- 

 ceding Sangamon interglacial stage, and probably stood much lower than 

 at present, seems evident from the shallowness of the valleys which were 

 opened on the surface of the Illinoian drift. The drainage conditions seem 

 to have become still more unfavorable during the silt deposition, so that 

 erosion was either suspended or became so weak that it could not keep 

 pace with deposition. This increased imjjerfection of drainage conditions 

 seems best explained by a depression of the land. This being granted, 

 there may be found but little occasion for dispute between the advocates 

 of the eolian and the aqueous hypotheses, it being only necessary to decide 

 whether or not depression stopped short of submergence. The pebbles 

 and occasional sandy beds found in parts of this silt may help in deciding 

 this question. 



The source of the material is a question of prime importance, but so 

 far as the region under discussion is concerned it is largely undetermined. 

 The part which covers the glacial drift seems from the examinations made 

 by Salisbury to have been dei'ived to some extent from glacial deposits, 

 there being minerals present which abound in these deposits and are not 

 present in neighboring rock formations; but the quartz which forms the 

 great body of the silt may easily have been derived from various rock 

 formations, near or remote, east, south, or west from the glaciated districts, 

 and also from the drift. 



It remains to consider the influence of organic agencies, the view having 

 been presented by Orton that the white clay of southwestern Ohio is merely 

 the fine material brought up by burrowing animals, earthworms, etc., from 

 the underlying till. This view was suggested before the relationship to the 

 loess had been determined and before the underlying Sangamon weathered 

 zone had been clearly recognized. While therefore the effectiveness of 

 such agencies to produce deposits of considerable bulk is not questioned, 

 the reference of this silt deposit to such agencies can scarcely be sustained. 

 The disturbance produced by these organic agencies has not been suffi- 

 ciently deep to greatly affect the buried Sangamon soil and weathered zone, 

 there being, as indicated above, a clearly marked line separating the weath- 

 ered surface of the Illinoian till sheet from the overlying silt. The inad- 

 equacy of organic agencies to account for the surface silt becomes still more 

 apparent when the great bulk of the silt farther west is considered, the 

 thickness of the loess in parts of the Mississippi Valley being over 100 feet. 



