56 



PHYSICAL GEOLOGY 



common in sandstones. Ripple marks are also formed in 

 sand by wind (Fig. 89, p. 97). 



Sun cracks. -When the water in roadside pools evaporates, 



the bottom mud 

 shrinks and cracks, 

 forming the familiar 

 mud cracks or sun 

 cracks (Fig. 37). If 

 the clay particles 

 were of uniform size, 

 and drying equal 

 everywhere, the 

 shrinkage cracks 

 would probably be 

 arranged in regular 

 FIG. 37. Mud cracks. (Fairbanks.) figures, after the man- 



ner of certain cooling lavas (p. 53). As these conditions 

 seldom hold, the cracking is usually irregular. Sun cracks 

 may form extensively in sediments that are exposed along 

 seashores during low tide, in dry interior basins on the smooth 

 mud floors of shallow and 

 temporary lakes (playas), 

 and about lake borders 

 and along stream courses 

 when the water is low. 

 If the sun-cracked surface 

 is exposed for a sufficient 

 time,it will harden enough 



V 



so that the cracks will not 



be washed out readily 



by the returning waters, 



which may fill them with 



other material and so 



preserve them permanently (Fig. 38) . (In which of the above 



situations are the chances for preservation best? Why?) 



Shales contain sun cracks more often than do other rocks. 



FIG. 38. Cast of sun cracks in sand- 

 stone. ,' natural size. (J. Geikie.) 



