SIMILARITY OF PLAINS SOILS TO LOESS 707 



extensive mud flats and sand bars bordering the great stream. This 

 source is no doubt more than ample to supply all of the necessary mate- 

 rials for these loess deposits as thay appear today; yet it now appears 

 probable that a considerable proportion of the fine silts, if they may be 

 so called, actually comes from other places. Although at the present 

 moment quantitative determinations are not available, the volume of 

 wind-blown dusts derived from the dry upland plains to the west must 

 be very large. The latest considerations on this point suggest that not 

 only the contiguous country and the semi-arid belt, but the arid region 

 also, is a large contributor to the loess of the Mississippi Valley. 



The loess of the Missouri Eiver in Iowa and Missouri owes its charac- 

 teristic peculiarities to the fact that the wind-blown dusts were deposited 

 under conditions of moist climate. Farther up the stream it gradually 

 loses its typical aspects, assumes an ashen color, and finally becomes in- 

 distinguishable from the "bad-lands" marls. Tracing of the loess directly 

 westward from the Missouri Eiver in Kansas and Nebraska is difficult, 

 because none of the larger streams are notably degrading their channels. 



IDENTITY OF PLAINS DEPOSITS WITH ADOBE 



On the dry Mexican tableland, where the chief building materials are 

 sun-baked bricks, the latter are known as adobes, and the loamy soils 

 from which they are made is called adobe. In all physical respects except 

 the ashen color adobe is indistinguishable from the loess. Adobe is 

 essentially wind-blown dust accumulated on the surface of the desert. 

 Throughout the arid region of the United States its character remains 

 constant. In the Great Basin, in the Californian Gulf basin, on the 

 Colorado plateau, and on the Mexican tableland its features are the same, 

 and it is everywhere easily recognizable. Unlike the soils of moist lands, 

 adobe bears no relation to the rocks beneath. 



There is, therefore, considerable genetic significance in the association 

 of the Plains deposits and the adobe soils of the desert. The relation- 

 ships between the two are not so impressive to one coming directly from 

 a moist climate as they are in making the approach from the arid side. 

 The similarity in physical characteristics must be more than merely 

 coincidental. Once compared in the field, there is little hesitancy in 

 pronouncing the adobe soil of the arid regions the same as the ashen 

 loams of the Great Plains. 



DISTRIBUTION OF COARSER DETRITAL MATERIALS 



Since the Plains deposits are mainly loams throughout their great 

 thickness, the occurrence of sands and gravels in some places in abun- 



