IOWA ACADEMY OF SCIENCES. 49 



■ which we frequently find half way up a steep slope, epecially 

 at the end of a spur running out toward a bottom land, a verti- 

 cal cliff 10, 15, 25 or 50 feet in height. Also the irregularly 

 terraced appearance which steep bluifs often show. From 

 •excavations in such bluffs I have noted that these are caused 

 by a succession of nearly parallel faults running with strike of 

 the slope which extend vertically through the dry, rigid por- 

 tion of the loess to the moist lower layers, or to the drift clays 

 below. They might be compared to crevasses in a glacial 

 rapid. 



This same property appears often in the sides of canyon-like 

 ravines. In fact, the cutting back of a ravine is first due to 

 the concentration of water in a depression in the lower surface 

 of the loess, which escapes as a spring. This washes away 

 the buttressing or enclosing material, the plastic loess escapes, 

 the superincumbent mass settles down, becomes in turn mois- 

 tened, plastic and washes away, and the process is repeated 

 backward, following the v6in of water. The sides also recede 

 until their base has risen above the plastic effect of the 

 water. But wet weather may further widen the canyon or 

 cause it to throw out branches. The vertical cleavage and 

 inherent rigidity of the dry loam cause it to stand indefinitely, 

 while the underlying drift clay is being eroded. In a dry cli- 

 mate this stage might continue long, as it occurs to-day in sim- 

 ilar formations in central Nebraska and Dakota. 



To understand still better the origin of the wide vertical 

 range of the loess, we need also to study somewhat the under- 

 lying formations. The most prevalent underlying formation of 

 the loess in the Misssouri valley is bowlder clay or till. This, 

 though quite impervious, is usually traversed by oblique seams 

 which cross each other, dividing the mass into polygonal blocks. 

 These seams are followed more or less by infiltrating water. 



There is without doubt more or less motion along these 

 seams, in fact they are not unlikely due to strains produced by 

 gravity upon the mass, or else by contractions caused by 

 consolidation or drying. Moreover, the upper portions of 

 this till are often quite easily rendered plastic by standing 

 water. 



Again, below the bowlder clay, which varies much in thick- 

 ness and is sometimes subdivided, there is usually a layer of 

 sand several feet in thickness. This, if exposed by the forma- 

 tion of ravines becomes a very unstable foundation and the 



i [la. Acad. Sci , Vol v.] LA.prll 28, 1898 ] 



