26 trask. PRINCIPLES OF SEDIMENTATION [Ch. l 



the rate at which the delta sinks. If the rate of sinking is rapid, the 

 mouth of the delta becomes embayed and longshore currents cannot 

 effectively remove the debris until the delta builds forward to a point 

 where the longshore currents can sweep the shore. 



In the Mississippi delta in some places, masses of clay rise up 

 through the sediments to form mud lumps or mud boils (Shaw, 1913). 

 The mud is pushed up in much the same way as mud rises in front 

 of an embankment which fails because of overloading. Perhaps a 

 similar overloading due to natural causes takes place in deltas. At 

 any rate, displacement of masses of mud in this manner, if found in 

 ancient sediments, might well be interpreted as evidence of contem- 

 poraneous diastrophism when it really was only a readjustment of 

 unstable sediments due to imposed loads. 



Desert Areas 



In desert or semi-arid regions, rainfall is characterized by infre- 

 quent but intense storms of local extent. After such storms the water 

 commonly flows down the slope of the land in a sort of sheet, re- 

 sulting in the formation of gently sloping cones (Paige, 1912; Law- 

 son, 1915; Woodford, 1925; Bryan, 1936; Gilluly, 1937). The shape 

 of the cone seems to be a function of the average load and the average 

 velocity of water during time of flood. If irregularities protrude above 

 the level of the surface of equilibrium they are worn away; if irregu- 

 larities extend down below the surface of equilibrium they are filled up. 

 The upward ends of the fans abut against a steep mountain slope or 

 merge into a stream coming down through a valley in the mountains. 

 As the mountain surface is eroded backward or as the upward end of 

 the fan moves mountainward, the relationships of load to average dis- 

 charge and to profile of equilibrium change, with the result that de- 

 posits tend to accumulate in the lower parts of the fan and to be thin 

 on the upper parts. The thinly covered rock surfaces on the upper 

 parts of fans are called pediments. 



The deposits generally consist of two sizes of particles. The larger 

 group of particles consists of angular or subangular fragments of rock, 

 commonly ranging from 1 to 10 inches in diameter; and the smaller 

 group consists of particles of coarse sand or fine gravel size. The 

 larger particles ordinarily form less than 25 percent of the weight of 

 the rock, and the finer particles more than 75 percent. Apparently 

 the size of the smaller particles is a function of the transportive abil- 

 ity of the water. The size of the larger particles seemingly is gov- 

 erned by the capacity of the water to push them along over the smaller 

 particles, which act somewhat like roller bearings. 



