IOWA ACADEMY OF SCIENCES. 51 



and undermining. It is not difficult to suppose that the appear- 

 ance at that stage may have been not very unlike what is now 

 seen in the ' ' bad lands ' ' of South Dakota, where rigid loams 

 over-lie firm .clays or rocks. 



There may have been successive local base levels, each hav- 

 ing its labyrinthine ravines, alluvial fans, and terraces. 



If, on the contrary, there was much rainfall, so as to keep 

 much of the loess plastic, there would be very low or no abrupt 

 banks, but a general slow mud flow more or less rapid down 

 the slopes. In such a case the early topography would have 

 been a succession of flat upland and sag-like valleys with sides 

 gently sloping or marked with landslides of greater or less 

 extent. 



In time the valleys would reach their lower base-level, the 

 loose deposits would become more perfectly drained, the breaks 

 would be gradually worn off by erosion though some stand as 

 shoulders on the hillsides to the present time, and the region 

 gradually put on its present aspect. 



Another circumstance in the process, though probably 

 exceptional, should not be omitted. A ravine may have become 

 dammed by a landslide in its lower course, and the portion 

 above may have been deeply filled with accumulations from 

 the sides and wash from above. Subsequently, the barrier 

 which may have itself disintegrated and been recemented, so 

 as to appear a part of the original banks, is cut through and the 

 ravine again works back over its original course. 



If vegetation had accumulated in the bottom, a pseudo-forest 

 bed may be thus formed. I examined a case of this sort in 

 Mills county, Iowa, where cedars of considerable size had been 

 buried sixty to eighty feet. The bottom of the ravine was yel- 

 low till, and the first thought was that it was an old land sur- 

 face under the loess, but other facts, particularly the roots of 

 one much higher up, showed that this growth was long subse- 

 quent to the original deposition of that formation. 



It should be remembered that this theory is intended to 

 have special application to the widest and probably oldest 

 loess deposit. Some of the lower and more conspicuous are 

 evidently of much later date. They are simply heavy silt 

 deposits capping high terraces of deposition along the princi- 

 pal streams. 



With this incomplete presentation I leave the theory for 

 your criticism and hope that portions of it, at least, may be 

 found of assistance in further investigation. 



