302 C. R. Keyes — Kolian Origin of Loess. 



somewhat in excess of what it would be for the whole stream. 

 One-tenth instead of one-fourth of an inch might be perhaps 

 more nearly correct for the mean yearly deposition. St. Joseph, 

 Kansas City, Glasgow and St. Charles should have unusually 

 heavy deposits of loess. 



There is another factor to be taken into consideration in 

 estimating loess deposition. Loess is not governed by the 

 ordinary laws of erosion. While the deposits are subject to 

 degradation and the action of running water, neither of these 

 agencies is as destructive as in the case of most other soft 

 materials. Loess is porous, and absorbs as a sponge most of 

 the rain-waters falling upon it. Only the severest freshets 

 erode it appreciably. Its capacity for resisting weathering and 

 erosion are well shown by the perpendicular sides of road cut- 

 tings made in it, where the marks of the pick and shovel 

 remain visible for several years. 



Loess districts appear to be areas of exceptional fertility. 

 Plant life flourishes luxuriantly even when in adjoining tracts 

 not covered by the deposit only a scant vegetation is supported. 

 The peculiar porosity of the loess gathers in the maximum 

 amount of water, holds it, and gives it out again gradually, 

 during the dry season. The belt is one of unusual dampness 

 and there is within its limits always an abundance of moisture 

 for plant life. 



The plant roots penetrate the loess to great depths, and 

 this is perhaps the main cause of the marked vertical cleavage 

 developed in many of the deposits. The roots, instead of 

 spreading out a few inches beneath the surface as in most soils, 

 in loess appear to penetrate straight downward much farther 

 than is usually the case. In decaying, the exterior corky layer 

 of the rootlets lasts much longer than the other parts. As the 

 interior disappears the outer tube finally collapses, leaving a 

 flat band or ribbon-like film that long resists further decay and 

 finally only the insoluble mineral particles. If it can be 

 shown that the cleavage is eminent from bottom to top of some 

 deposits and only at the top of others, a criterion might be 

 established for distinguishing between wind and water deposits. 



The dense vegetable growth well protects the loess from the 

 destructive effects of wind aud water. When once deposited 

 the silt particles are only with great difficulty disturbed. Silt 

 dust blown up from the valley strikes the thick vegetation and 

 is acted upon in the same way that it is in water when the 

 current is checked. The particles come to rest around the 

 roots and gradually build up the ground. .Each year's vege- 

 tation is on a little higher level than the last. 



A characteristic feature of some loess deposits is the small, 

 cylindrical, concretionary masses that are commonly called loess 



