40 New York State Museum 



slopes than those made by water. Wave marks are 

 formed on the beaches today as very low, narrow, wavy 

 ridges, the later ones cutting into those previously made 

 during recession of the tide. In rocks they indicate the 

 direction of the sea during deposition of the original sedi- 

 ments. Rill marks include the systems of channels formed 

 by the backwash from waves breaking on the beach and 

 by streams emptying over clayey and sandy flats. The 

 channels in the former case branch and rebranch going 

 up the beach, in the latter case going down the beach. 

 Channels are also scoured out by water around imbedded 

 obstructions, such as pebbles and shells, as it flows down 

 a sandy beach. Sun cracks or mud cracks are a network 

 of fissures formed in mud and clay deposits due to shrink- 

 ing and cracking under the drying effect of the sun's rays, 

 thus dividing the surface into polygonal areas. Material 

 is later deposited in these cracks, and they are very well 

 preserved in consolidated material of shallow water 

 origin, essentially continental in origin. They are some- 

 times formed on tidal flats or along low, exposed lake 

 shores, but they are most characteristic of river flood 

 plains and playa deposits and are best developed under 

 arid or semiarid conditions in a warm climate. Rain 

 prints may also be preserved in muds, clays and fine sands 

 and they also are particularly characteristic of continental 

 mud deposits. Animal tracks in the form of foot prints 

 or trails are to be found in rocks derived from fine- 

 grained sediments. Tracks of animals are found along 

 shores of lakes and the sea, but they are more apt to be 

 the tracks of aquatic animals. Terrestrial animals leave 

 footprints and trails in the fine clays and muds of river 

 flood plains or playa deposits and these are among the 

 best preserved tracks. Tracks made along a sea beach 



