FOOTPRINTS 



227 



FIG. 83. — Sun-cracks in sandstone. (Photograph by Pynchon.) 



Rain P7'ints are sometimes preserved in fine sand or mud in 

 the same fashion. The raindrops make little pits on the soft 

 surface, which are circular when the drops fall vertically, or oval 

 and with edge raised on one side, when the rain is driven obliquely 

 before the wind. A gentle deposition of fine sediment upon the 

 pitted surface will preserve the marks. 



Tracks of Anh?ials are not infrequently found in the strata, and 

 have been preserved in much the same way. Marine animals, such as 

 worms and crustaceans, leave tracks in fine sand, which are covered 

 so gradually and gently by fresh layers that the marks are not 

 disturbed. Tracks of land animals may be preserved when made 

 on the tidal flats of sheltered estuaries, where the surface is some- 

 what hardened by the exposure to sun and air. On the open sea- 

 beach such tracks would be soon obliterated by the waves. 



Marks of this latter class, sun- cracks, rain prints, and tracks of 

 land animals, show that the surface on which they occur was exposed 

 to the air, either periodically by the ebb and flow of the tides, or 

 at stages of low water in rivers and lakes. They are illustrations 

 of the way in which conditions, long passed away, have been 

 recorded for ages in the solid rocks. 



Concretions, or Nodules, are developed after the formation 

 of strata. They are balls or irregular lumps of material differing 



