Ch. 1] WATER . 11 



if more than one process is operating. For example, if the locus of dep- 

 osition receives debris from two or more sources the material de- 

 posited will have one or more size distributions deposited upon another, 

 with the result that the size distribution of the sediments becomes 

 skewed or has two or more modes. Likewise, if an area is subject to 

 variations in average velocity, a similar result could arise. Also, as 

 discussed below, sediments deposited upon submerged ridges commonly 

 are disturbed or riled by moving currents, which cause the fine par- 

 ticles to be carried away and deposited elsewhere. The sediments on the 

 ridges are relatively well sorted, but those deposited in the lee (if the 

 word lee can be used with respect to moving water) are likely to be 

 skewed because they receive the normal supply of detritus for that 

 particular distance from land as well as debris winnowed away from 

 the nearby ridge. 



If the velocity of the water changes so that it moves at a different 

 velocity or in a different direction for any appreciable length of time, 

 or if the supply of debris changes in character, the resulting size distri- 

 bution of the sediments is different. In this way laminae or strata are 

 formed. The development of layers is one of the principal character- 

 istics of sediments (Andree, 1916; Barrell, 1917; Bucher, 1919; Antevs, 

 1922; Rubey, 1930; Weller, 1930; McKee, 1939; Payne, 1942; Ailing, 

 1945). 



Sediments ordinarily are deposited in nearly horizontal layers. 

 However, if the surface upon which they are laid down is uneven, the 

 individual layers tend to conform to that uneven slope. With con- 

 tinued deposition, the inequalities in slope become less, and eventually 

 the angle at which the layers are deposited corresponds to the equi- 

 librium position for the prevailing load and velocity of water. This 

 initial dip of the sediments is usually small, but in areas of variable 

 currents and on deltas it may be appreciable. In such places the dip 

 varies in direction and amount and gives rise to cross-bedding. Varia- 

 tions in velocity of water ordinarily are greater in shallow water than 

 in deep water; consequently cross-bedding is more likely to be indic- 

 ative of a shallow-water origin of sediments than of a deep-water 

 origin. Cross-bedding, however, can form in water of any depth, so 

 long as variable currents exist at or near the bottom. 



When streams enter the sea or lakes, the velocity is checked and 

 part of the transported *load is deposited. Unless longshore currents 

 transport the debris away, deltas form. Deltas are flat and gently 

 shelving on top near where the stream enters, but, at some point out- 

 ward from shore, the flat top of the delta gives way to a relatively 

 steep front, which is constantly built forward at essentially the same 



