18 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



into the soil. Their plentifulness or scarcity modifies the rates at 

 which environmental conditions change. 



Changes in environmental conditions in turn influence the environ- 

 mental factors themselves. Diastrophism, or deformation of the earth, 

 exerts a dominant influence upon environment, either directly by alter- 

 ing the shape of the land and the distribution of water, or indirectly 

 by modifying the climate and thus in turn the rate of erosion, trans- 

 portation, and deposition. The movement of water, particularly cur- 

 rents in the sea, is greatly influenced by diastrophism (Bailey, 1936; 

 Krynine, 1940). 



Oceanic Pkocesses 



The chief locus of deposition of sediments is the ocean. Any person 

 attempting to study deposition in the sea should be familiar with the 

 general principles of oceanography. For a comprehensive description 

 see The Oceans, by Sverdrup, Johnson, and Fleming (1942). 



The surface temperature of the ocean is high near the tropics and 

 low near the poles. The temperature of the water decreases with depth 

 in all latitudes, but below a depth of 5,000 feet it is remarkably con- 

 stant with respect to latitude, being everywhere less than 40° F. at 

 that depth. As the temperature of sea water decreases, its density in- 

 creases. Heavy water cannot long overlie light water; it must sink 

 to a level where it encounters water of its own density. Thus, once 

 cold and heavy water has sunk to a lower level in the sea, special con- 

 ditions must develop to cause it to rise to higher levels. Hence the 

 water in the ocean is essentially stratified with respect to density. 

 Water within individual layers is free to move within that layer. In 

 fact, in some places water at different depths is flowing in different 

 directions. Water moving at a given level may be deflected upward as 

 it passes over a ridge. The effect of submerged ridges on the turbu- 

 lence and the vertical motion of water is similar to the effect of moun- 

 tain gaps on wind. This disturbance of water over ridges results in 

 winnowing of the finer particles and causes the sediments on ridges to 

 be more coarse-grained than those upon adjoining slopes in deeper 

 water (Trask, 1932). No matter what the depth of water, the sedi- 

 ments on the Mid-Atlantic Ridge in more than 10,000 feet of water 

 are materially coarser than those on either side of the ridge (Bradley 

 etal, 1942). 



Evaporation of ocean water increases the density. If the density 

 increases, a column of water of unit height and cross section weighs 

 more than a column of lighter water of the same height and unit cross 



