CRITERIA REQUISITE TO A GLACIAL AGE 77 



els as a source of really good evidence of maiis glacial age. It is 

 clear that these gravels are liable to present many misleading 

 cases, and this is a reason for special caution on the part of 

 geologists, while it is a sufficient ground for anthropologists to 

 withhold the acceptance of supposed evidence of this class until it 

 shall be conceded to be demonstrative by the most critical gla- 

 cialists. 



IV. FLOOD-PLAIN DEPOSITS. 



The preceding discussion relates to clear, well-stratified 

 gravel, sand, and silt deposits of the same composition as the 

 true glacio-fluvial formations and of the same general aspect and 

 part of the same complex train of assorted drift deposits. Over 

 these deposits of glacio-fluvial aspect there is usually spread a 

 layer of more or less structureless silt, with intermixed sand and 

 gravel, in which human relics are not infrequently found. There 

 is scarcely a presumption that these surface deposits are of gla- 

 cial age. They are generally the products of the river floods 

 formed during the stages when the river is sinking its channel 

 into the underlying gravel plain, a stage which is usually reached 

 some time after the glacial floods ceased their work of aggrada- 

 tion. Usually it is not until after the work of adjustment follow- 

 ing the close of the glacial stage has been completed, and proba- 

 ably in some cases only after a period of aggradation, due to the 

 special erosion of the drift succeeding the retreat of the ice, has 

 passed. 



All flood-plain surfaces of this class should be excluded from 

 ground likely to give any good evidence of contemporaneity with 

 the ice age. 



v. BLUFF-BORDER ACCUMULATIONS. 



There is a class of deposits closely correlated with river 

 bluffs that have played a part in the discussion of man's antiquity 

 in America. In these cases the bluffs have been cut from plains 

 of glacial or glacial-like gravels, and the deposits on the bor- 

 ders of the bluffs have on this account been referred to a glacial 

 age. There are at least two classes of these. The first is a 

 form of the preceding flood-plain surfacing. As is well known, 

 where a river overflows its bordering bluffs and banks, its largest. 



