250 PRINCIPLES OF STRATIGRAPHY 



Recent experiments by Sorby (66 : i8o) indicate tbat a current 

 of about 6 inches per second is sufficient to slowly drift along 

 granules of common sand having a diameter of about a hun- 

 dredth of an inch. No rippling of the sand was produced until the 

 velocity was somewhat greater. In the bed of the River Rhine at 

 Breisbach the following measurements were made by Suchier 

 (71 : JJ7 ; Penck-51 : 28^) to ascertain the velocity at which various 

 sediments begin to move, (i) through the influence of the cur- 

 rent alone, and (2) after the sediments on the bottom are stirred 

 up: 



A. With stream bed covered by fine sediment. 



Under action of current alone, no movement found, with 



bottom velocity at 0.694 m. per sec. 



After being stirred up, the movement of the sediment be- 

 gan for fragments of the size of beans, when bottom 

 velocity reached 0.897 rn. per sec. 



Fragments of the size of hazelnuts, when bottom velocity 



reached 0.923 m. per sec. 



Fragments of the size of walnuts, when bottom velocity 



reached i .062 m. per sec. 



Fragments of the size of a pigeon egg, when bottom 



velocity reached 1.123 m. per sec. 



B. With river bottom free from sediment, the smallest particles 



are moved when the current velocity reaches at 



bottom 1.180 m. per sec. 



^Pebbles of pea and hazelnut size move freely under a 



velocity of i .247 m. per sec. 



With noticeable noise at 1.300 m. per sec. 



Pebbles of walnut size are moved without stirring and 

 such of 250 gr. weight after stirring up, with current 



at 1 .476 m. per sec. 



Pebbles of 1,000 gr. wt. rolled at 1-589 m. per sec. 



C. General movements of pebbles: 



Up to the size of pigeon eggs, at 1.623 m. per sec. 



Up to the size of hens' eggs, at 1.7 1 7 m. per sec. 



(including such of 1,500 gr.) 



Pebbles of less than 2,500 gr. wt. are moved at 1.800 m. per sec. 



All pebbles moved at 2.063 m. per sec. 



It is evident that it requires a much greater velocity to start the 

 movement than is required to keep it up. This is shown by the fol- 

 lowing comparison : 



