SURFACE DEPOSITS. 101 



2. ALTERED DRIFT. 



It has already been shown that the drift as originally 

 deposited was a heterogeneous accumulation of various mate- 

 rials, without any arrangement of them into strata, or any 

 regular separation of them from each other. The greater 

 part of it remains to this day in apparently the same con- 

 dition in which it was left by the glaciers; but the waters 

 resulting from the latter as they receded by aggressive 

 melting along their southern borders, together with such as 

 constantly fell and still falls upon the surface, have produced 

 a certain degree of modification in a part of it. This modifi- 

 cation consists in the production of more or less horizontal 

 arrangement of its materials, or a more or less complete 

 separation of them from each other. 



The materials thus modified we designate as Altered Drift; 

 restricting the use of the term, so as to apply only to such 

 materials as have been re-arranged but not transported to 

 any considerable distance since their accumulation as drift. 

 This restriction seems necessary, because reference is here 

 had only to the Altered Drift as it appears in the valleys, and 

 where it is sometimes difficult to define the limit between it 

 and the unaltered drift on the one hand, and between it and 

 the alluvium on the other. The truth is, in the valleys these 

 surface deposits are all composed of or derived from the 

 same materials and differ from each other only in the degree 

 to which their materials have been disturbed, separated, or 

 re-arranged since their original accumulation.^ 



Some good examples of the Altered Drift may be seen 

 along the base of the bluffs which border the flood-plain 



*It will be observed that little reference is here made to the alteration of the drift 

 that must have taken place upon the surface immediately upon the close of the 

 Glacial epoch, and before the river valleys were excavated, There seems to be some 

 evidence that this modification was almost co-extensive with the deposit itself, and 

 that those cases of indistinct stratification upon the high prairies before mentioned, 

 are a part of the same wide-spread modification which the original deposit may be 

 supposed to have received. Prof. St. John reports observations in central and western 

 Iowa which led him to some such conclusion. Much further observation is necessary 

 before a complete knowledge of this subject can be obtained. 



