474 J. E. TODD THE CHA^^XEL OF THE MISSOURI RIVER 



There may be little trace of such a yalley. The surface which was 

 oiaciated in the Wisconsin stasre and the sheet of till left has been so 

 little eroded that the lines of drainage, possibly resulting from the con- 

 figuration of the preglacial surface, have not been distinctly outlined. 



GLACIAL BOULDERS NEAR BISMARCK 



2. The finding of boulders 125 feet below the surface at Bismarck is 

 also inconclusive. It is known that in time of flood streams scour out 

 their beds to an indefinite extent. This takes place especially in the na^r- 

 row portion of the course. There the velocity of the stream is quickened 

 and its eroding and transporting power are increased.* 



The depths to which rivers may scour in this way has been found to be 

 far bej'ond what most students heretofore have recognized. Surface 

 conditions may be considered as deepening such erosion. Fine material, 

 such as sand, may be picked up quickly, while heavy material may be 

 very little affected; and yet, when we think of the rounded character 

 of the pebbles within a short distance of their starting point, we may 

 be assured that they also are moved in time of flood to an indefinite 

 degree. It will be readily accepted that the coarser the material the less 

 distance it is transported and the more promptly it will be dropped. 



It is a question as to how deep such scouring may extend. It is very 

 important in the Missouri Eiver. It has been known by direct measure- 

 ment that the bottom of the river, which may be only 15 feet in depth at 

 ordinary stage, may be in a few hours deepened to 90 or even 100 feet 

 at some places. In the valley of the Missouri Eiver the depth of such 

 influence seems to have been 125 to 150 feet below the surface of the 

 bottom land. The depth, no doubt, increases with the volume of the 

 stream and the softness of bedrock below. When we remember the shaly 

 and clayey nature of bedrock at Bismarck and the tremendous floods 

 poured out from the melting glacier of the Wisconsin stage, a depth of 

 125 feet is not therefore enough to be very sure. 



A recent paper by Dr. E. B. Matthews discusses "The deeps of the 

 lower Susquehanna river.'-'^ It seems probable that those deeps are due 

 to the erosive as well as solvent action of the water of the river and may 

 be considered as formed analogous to the scour of streams, which we 

 liave been discussing. Some of those deeps were over 150 feet in depth. 

 The full significance of the boulders at Bismarck is simply that thev were 

 deposited at a stage of the river when the 30-foot terrace was formed, 

 which, from the configuration of the surface, seems to have been in the 



* Bulletin 158, U. S. Geol. Surv., p. 150. 

 5 Bull. Geol. Soc. Am., vol. 30. 



