72 STUDIES FOR STUDENTS 



and filling. It is equally clear that any relics that may come 

 within the river's action, except such as float, are liable to be 

 buried to depths equal to the difference between the height of 

 the flood plain and the depth of the deep holes in the bottom of 

 the river. In the great rivers, this range not infrequently reaches 

 100 feet and more, and is very commonly three and fourscore 

 feet. In many cases the scour reaches the rock bottom, and 

 at such places the entire bottom deposit is undergoing rework- 

 ing, and modern relics may be introduced into any of its depths. 

 This phenomenon is most strikingly displayed in the Missouri 

 river, where the engineers tell us that transient scour often 

 reaches bed rock, and that the scooping out and filling up fol- 

 low one another with almost seasonal frequency. So pronounced 

 is this action that the impression has been given that the whole 

 bottom deposit is bei?ig shifted tiirn-by-turfi down stream. While this 

 may be an overstrained deduction, it is supported by a great 

 mass of very important facts, too much neglected. The erosion 

 of the rock bottom of most streams is accomplished by such 

 alternate shiftings of the loose bottom material. At any one 

 time, this loose material covers most of the bottom of most 

 degrading streams even, and the wear on the rock bottom is 

 accomplished first here and then there by the shifting of the 

 covering of loose material. In practically no streams at all, 

 except perhaps mountain cascades, is the whole bottom exposed 

 to erosion at the same time. In the drift-filled bottoms of the 

 great branches of the Mississippi system, it is wholly within 

 bounds to regard at least the upper 40 or 50 feet of the 

 deposit over which the river meanders as subject to scour-and- 

 fiU and to entertain the suspicion that the deeper portions 

 down to 100 feet or more may be similarly affected. To make 

 out a good case for the antiquity of relics buried in such a 

 deposit seems well-nigh impossible. 



Scour-and-fiU affects more or less every portion of a river 

 deposit that has formed a part of the channel bottom at any 

 time in its history. This action does not usually much affect 

 the flood plains while they re^nain such. There are exceptions 

 even here, and not very infrequent ones, but scour-and-fill is not 



