36 PHYSICAL GEOLOGY 



the larger and heavier pieces first, the smaller and lighter 

 later. 



Offshore waters are frequently agitated down to the bottom 

 by winds and tides, the undertow (a from-shore movement of 

 the water which has come in with the waves), and by various 

 currents. The sediment on the bottom is rolled and dragged 

 about by these movements of the water, and is often shifted 

 long distances before reaching a final resting place. Normally 

 the bottom water moves most, close to shore where it is shal- 

 low, and frequently only coarse material, such as gravel, comes 

 to rest there, all sand and mud being swept away. Farther 

 out the quieter bottom water is able to move only mud par- 

 ticles, and drops any sand it may have had. Still farther from 

 shore the bottom waters become so quiet with increasing depth 

 that even the finest mud comes to rest upon the floor. Thus 



FIG. 15. Diagram showing the relations to one another and to the land, of 

 beds of gravel, sand, and mud. 



the stream-borne waste from the land tends to accumulate in 

 belts of gravel, sand, and mud, which merge gradually into one 

 another (Fig. 15). Since the depth of the water and the 

 strength of the waves and currents vary at points equally dis- 

 tant from the shore, different material is likely to be accumu- 

 lating at these different places ; traced alongshore, gravel 

 may give way to sand and sand to mud. Furthermore, the 

 agitation and depth of the water vary from time to time at a 

 given place, because of alternating storms and calms, high and 

 low tides, etc. Hence the character of sediments is subject to 

 changes vertically as well as horizontally, and alternate layers 



