772 T. C. CHAMBERLIN 



of the Missouri valley has been given in the preliminary con- 

 siderations. If neither of the strong bowldery terraces of so 

 late a stage of glaciation as the Wisconsin are represented at the 

 site of the burial, there is but scant ground to assume that the 

 earlier and much feebler and much more erosible glacio-fluvial 

 deposits of the Iowan are preserved. 



The natural conclusion is, therefore, that the little relic-bear- 

 ing deposit in the valley at Concannon's belongs either to the 

 same class as the silt terraces of Dakota, to which it bears a 

 measure of resemblance, or to some later stage. 



Specific views. — While, as before remarked, the case is per- 

 haps not a wholly declared one, and an unqualified identification 

 may not be entirely warranted, the range of tenable interpre- 

 tation seems to me to lie within narrow limits. 



I. The most conservative cmd the most probable view. — All the 

 essential facts known to me seem to be explicable on the follow- 

 ing lines which involve the minimum of action and of assump- 

 tion, and which appeal only to the natural order of things. The 

 first stage of essential action is assigned to a time when the chan- 

 nel of the Missouri river ran immediately past the mouth of the 

 tributary valley and was higher than now to such an extent as 

 to be in erosive adjustment with the tributary at the top of the 

 rather heavy limestone layer which lies just below the tunnel. 

 It has already been noted that where a strong stream like the 

 Missouri passes hard by the mouth of such a tributary, two 

 effective conditions of erosion are supplied. The tributary has a 

 low point of discharge and hence a high gradient, and its detri- 

 tus is immediately swept away by the great river. During this 

 stage the rock surface under the relic-bearing deposit was devel- 

 oped by the removal of the shales above, and the lower slopes 

 adjacent were measurably denuded because the conditions were 

 favorable to erosion and the shales were easily cut away. After 

 a stage of erosive adjustment of this kind, a change of relations 

 was brought about by the diversion of the channel of the Mis- 

 souri river to some other portion of the broad valley, attended 

 by the substitution of a flood plain at the mouth of the tribu- 

 tary. As the vertical range of water is now twenty feet or more, 



