THE WORK OF STREAMS 



131 



what as one might move sand grains on a table by dragging 

 the outstretched hand across them. Material is held in 

 suspension chiefly by minor up-moving currents. Since 

 rock material is on the average two and one half to three 

 times as heavy as the water it displaces, it tends, under 

 gravity, to sink to the bottom. In standing water it sinks 

 vertically. In a stream whose water is moving horizontally, 

 two forces act upon it. Gravity, of course, seeks to draw it 

 directly to the bottom (G, Fig. 122), while the current tends 

 to move it in the direction of its flow (C, 

 Fig. 122). The sediment accordingly fol- 

 lows a course (S, Fig. 122), which is a re- 

 sultant of the combined forces. It reaches 

 the bottom in the same time it would in 

 standing water of the same depth. In na- 

 ture, however, stream water rarely, if ever, 

 moves horizontally for any distance. Bowl- 

 ders and other irregularities on the bottom 

 deflect portions of the main current ob- 

 liquely upwards. Projections of the bank 

 likewise create subordinate currents, some 

 of which move upwards. Sediment settling 

 to the bottom along an oblique path may 

 encounter such up-going currents and be 

 lifted by them. Presently sinking again, it 

 may again be lifted or may reach the bot- 

 tom, perhaps to be presently carried up once more by other 

 upward currents. Material the size of sand, and larger, prob- 

 ably rarely makes extended trips in suspension ; instead, many 

 short trips are interrupted by periods when it rests upon the 

 bottom, or is dragged and rolled along it. On the other hand, 

 mud and silt are often carried long distances before settling. 

 Nevertheless, even fine material probably normally requires 

 thousands of years to make the trip from the sources of the 

 larger rivers to their mouths, for it is dropped on the way 

 many times for long periods, perhaps helping to form bars and 



forces acting upon 

 a particle at A in 

 the horizontal cur- 

 rent of a stream, 

 and the general 

 course which the 

 particle takes in 

 sinking to the bot- 

 tom. 



