CRITERIA REQUISITE TO A GLACIAL AGE 73 



normally an effective mode of action on the flood plains. And 

 so a portion of a fluvial plain that has never been occupied by 

 the river channel, but only by its upper floods, if indeed there 

 be such a portion, would not be greatly open to the suspicion 

 of having been reworked by scour-and-fiU. But every part of 

 the flood plain that is occupied at intervals by the stream's 

 channel is subject to scour-and-fill, during the whole period of 

 such periodic occupancy, and the contents of its deposits are 

 subject to all the uncertainties of origin already pointed out. 



Utiequal effect on aggrading afid degrading rivers. — Scour-and- 

 fill affects both aggrading and degrading rivers, but it does not 

 affect them equally. In aggrading rivers, especially those rap- 

 idly and declaredly building up their bottoms, as in the case of 

 typical glacial streams, the river is usually brolsen up into a 

 plexus of branches by the successive filling up of the channel 

 bottoms and the diversion of the streams. Deep scour-and-fill 

 is not a pronounced habit of such streams. Already overbur- 

 dened with detritus, they have not that reserve capacity for 

 taking up a new burden that clearer streams have, and their 

 division into numerous weaker branches takes away the cumu- 

 lative power which the combined and concentrated current of 

 the clearer degrading streams possesses. The action reaches its 

 greatest efficiency, it would seem, when the whole stream con- 

 centrates its full force in a limited portion of its channel, and 

 when it is free to take on temporarily a large additional burden 

 of detritus. The action is greatest at the turns of the streams 

 where the arrested or diverted momentum of the current devel- 

 ops a powerful rotary movement of the water. 



Breadth of action hi the great streams of tJie glacial a^-ea. — Now 

 this reworking of the fluvial deposits along the courses of the 

 Missouri, the Mississippi, the Ohio, and many of the other large 

 streams in the glacial area, occupies or has recently occupied 

 nearly or quite all the space between the bounding bluffs. Here 

 and there within protected recesses, or in other favored spots, there 

 may be a few exceptions, but in most of these cases even it may 

 be difficult to prove that the remnant deposits were not reworked 

 in postglacial times, though somewhat remote ones. The only 



