86 PHYSICAL GEOGKAPHY. 



general stratification, or successive changes in the vertical 

 accumulation of the materials of the drift as originally 

 deposited, has been observed in Iowa. Modifications of it, 

 in some cases closely approaching true stratification, have 

 taken place, but this was evidently subsequent to its original 

 deposition, and will be presently noticed under the head 

 of Altered Drift. It may be remarked here, however, that 

 the eye may occasionally catch a trace of a horizontal 

 arrangement of large pebbles and small boulders in such 

 places, for example, as the face of a railroad excavation in 

 the drift upon the high prairies of the State. This apparent 

 modification probably has no direct connection with the 

 more distinctly stratified altered drift of the valleys, but the 

 subject will be again referred to further on. 



Neither the clay, sand, gravel, or boulders, are confined to 

 any particular part of the deposit vertically, but either or 

 all of them may occur at its base, middle or surface. 

 Generally, however, the whole surface of the drift, especially 

 that of the higher prairies, is covered with the finest 

 materials alone, the coarse materials and stiff clays being 

 usually covered from sight. This is particularly the case in 

 eastern and southern Iowa, where boulders upon the higher 

 surface are rarely seen, but in northern Iowa the higher 

 surfaces sometimes have boulders and gravel exposed quite 

 conspicuously upon them. 



The different materials of the drift have little more regu- 

 larity in their geographical distribution than in their vertical 

 arrangement, but that distribution, indefinite as some of its 

 outlines are, throws considerable light upon the origin of the 

 deposit. It may be said in general terms that in northern 

 and northwestern Iowa the drift contains proportionally 

 more boulders, pebbles, and sand, than elsewhere, and that 

 in other parts of the State it contains proportionally more 

 clay and other finely comminuted materials; yet in the 

 first named region there is enough of the clayey material, as 

 a general rule, to produce an excellent soil. Occasionally, 

 however, the elevated ridges and knolls there have a poor 



