Handbook of Paleontology 39 



chalks have been discussed above (page 18) . Marl is the 

 term used for deposits of a loose, earthy or friable char- 

 acter which consist chiefly of an intermingling of car- 

 bonate of lime or dolomite with clay in varying propor- 

 tions. Marls grade into clays and shales. The color is 

 usually gray but the presence of organic matter or oxide 

 of iron gives blue, black, yellow or green colors. There 

 are sandy marls which are full of quartz sands with other 

 minerals. Shell marls are full of shell fragments of vari- 

 ous kinds mixed with clay. They are whitish earthy de- 

 posits. Greenland marl (page 35) is the name inappro- 

 priately applied to greensands. 



Structures of Sedimentary Rocks 



The structures to be found in sedimentary rocks may 

 be divided into two groups. The first group includes the 

 original surface features and original structures and the 

 second later structures due to deformation. 



Original surface features and structures. Among 

 the surface features to be looked for are ripple marks, 

 wave marks, rill marks, sun cracks, rain prints and ani- 

 mal tracks. Ripple marks may be due to waves or to 

 currents of wind or water, the latter often known as 

 current marks, and are to be looked for mostly in coarse 

 and fine sandstones. Wave-made ripple marks are com- 

 mon on the beaches and are symmetrical, that is, have 

 equal slopes on each side of the crest line. Ripple marks 

 due to currents of water or wind are unsymmetrical, but 

 water current, like wave-made ripple marks, have the 

 coarser grains of sand in the troughs while the wind or 

 eolian ripple marks have the coarser grains on the crests. 

 Eolian ripple marks also may be formed on much steeper 



