THE COMPOSITION OF THE EARTH 



55 



currents change frequently, the inclined lamina? will slope in 

 different directions, and meet each other at various angles. 

 This structure is called cross-bedding or oblique lamination 

 (Fig. 35). It is especially characteristic of deposits made by 

 streams, and is found in many wind deposits (Fig. 83, p. 94). 

 It is developed also off ocean and lake shores, where the water 

 is shallow enough to be agitated frequently at the bottom. 

 Conglomerates and sandstones are cross-bedded more often 

 than other kinds of sedimentary rock (Why?). 



Ripple marks. The rhythmical movement of shallow 

 waters often develops on the bottom miniature ridges, com- 

 monly an inch or two from crest to crest. Such ridges are 

 known as ripple marks (Fig. 36). Often they may be ob- 



FIG. 36. Ripple marks upon a sandy beach, at low tide. (Greger.) 

 From which direction did the waves which formed the ripple marks come ? 



served on the sandy beds of clear and shallow streams. 

 Here the rudely parallel ridges extend crosswise of the current, 

 each having a relatively long and gentle slope upstream, 

 and a shorter and steeper slope on the downstream side. 

 Sand grains are rolled by the current up the gentle slope to the 

 crest, whence they fall down the steep slope into the trough. 

 By a continuation of this process, the ridges shift slowly 

 in the direction of the current. Ripple marks are produced, 

 too, along lake shores and seacoasts, particularly by undu- 

 latory movements of the undertow, out to depths of twenty 

 to thirty, or even more feet. (What things determine how 

 far from shore they may be formed ?) Ripple marks may be 

 preserved in the consolidated sediments, and are especially 



