48 New York State Museum 



prints of land animals, ripple marks, cross-bedding etc. 

 Lime rocks that show sun cracks arc of continental 

 origin, but they are rare. Sediments laid down in lakes 

 or the sea are well-stratified ; river deposits and de- 

 posits formed by glacial streams are poorly stratified. 

 Wind-blown dust shows little or no stratification, but 

 wind-blown sand is well stratified and shows cross- 

 bedding and ripple marks. Residual deposits, glacial 

 deposits and material transported only a short distance 

 are composed of angular fragments. Water-deposited 

 material is rounded to a greater or less degree depend- 

 ing upon the amount of transportation and wear and 

 rapidity of deposition. Eolian deposits of sand have 

 well-rounded grains. 



Climate and physiographic conditions at the time of 

 deposition may also be evident from structures in the 

 rock. Mud cracks, foot prints etc. have their best 

 preservation under semiarid conditions. Interbedded 

 gypsum, salt etc. indicate the same thing. Red color 

 in rock may be due to deposition under arid condi- 

 tions, but red color is also found in deposits of warm, 

 humid climates due to the dehydration of ferric hy- 

 drates. Glacial deposits, of course, indicate a cold 

 climate ; arkosic sandstones a relatively dry climate. 

 Desert conditions are indicated where thick beds of 

 eolian sandstones are found ; and where sands are 

 coated with a red oxide of iron they are of desert 

 origin. Coal beds indicate a marsh condition and a 

 warm or cool, moist climate. Where there is a large 

 percentage of kaolin in deposits the area which was 

 the source of the muds must have been undergoing 

 warm, moist conditions which favor decomposition. 

 In an arid or semiarid climate deposits have a large 



