PLEISTOCENE GEOLOGY. 261 



grains derived from the Denver beds and by its resistance to the passage 

 of bowlders such as constitute the glacio-natant drift in the loess proper. 



The maximum observed thickness of this formation is 35 feet. It lias 

 been noted in isolated patches from Overland Part to the Seventh Street 

 Bridge, the largest being that at the mouth of < rreen Mountain (\ >ry ) < Ireek. 

 It- greatesl width, al righl angles to the course of the stream, is about 

 one-fourth of a mile. 



The exposed surfaces of the fluvial loess an- frequently stained with 

 calcareous material, ami pulverulent calcareous concretions abound in the silt. 



Minute mollusks of the genera Pupa, Planorbis, Succinea, Physa, and 

 Limnea are common fossils in the fluvial loess. Numerous bones of the 

 ancient horse, bison, and elephant, together with those of small rodents, 

 have also been found in it. 



THE LOESS. 



The eastern portion of Colorado below the present level of 5,800 feet, 

 wherever not denuded by recent erosion, is covered by a mantle of fine, 

 porous, uonindurated material that possesses the physical characteristics of 

 a typical loess. It has the cuboidal fracture and remarkable homogeneity 

 that produces vertical faces of erosion, and bears no evident relation to the 

 varying composition of the floor upon which it rests. Near the foothills its 

 color is dark-brown; at Denver it has faded to an ash-brown or reddish- 

 buff, while in the eastern portion of the State it is nearly devoid of coloring 

 matter. For about a yard below the surface' it has a somewhat darker stain 



from the infiltration of carbonaceous matter, resulting from the decomposi- 

 tion of the scants- vegetation of the plains. Except lor these accidental 

 variations of color there is nothing in the external appearance of the mate- 

 rial that would enable one to distinguish samples obtained from localities 

 hundreds of mile- apart. Material having similar characteristics extends 

 through Kansas and Nebraska to the Missouri Valley. Loess, us is well 

 known, is not a definite chemical compound, but a mechanical mixture of 

 very finely comminuted detritus, more or less decomposed, and generally, 

 though not necessarily, infiltrated with carbonates (and sometimes phos- 

 phates) of lime and magnesia. 



The loess of eastern Colorado shows in its physical characteristics 



