40 PHYSICAL GEOLOGY 



great departure from horizontality indicates later disturb- 

 ance. 



Nonmarine fragmental rocks. While the ultimate goal 

 of running water and of the waste it carries is the sea, much 

 material is deposited in lakes, along valley bottoms, and in 

 other situations on the land. These sediments, like marine 

 beds, may become firm rock by cementation. Beds formed 

 in lakes that have since been destroyed usually betray their 

 origin by their form and attitude, and by the fossils which they 

 contain (p. 267). 



River deposits also have distinguishing characteristics, 

 some of which are suggested by Figure 185. Long, relatively 

 narrow strips of coarse material indicate former positions of 

 the shifting stream channel, while the broader layers of fine 

 material were spread upon the flood plain by the quieter 

 waters of the overflow. Cross-bedding (p. 54) and great 

 irregularity of stratification are among the most characteristic 

 features of stream deposits. Occasionally, they contain river 

 and land shells. River deposits will be considered hi greater 

 detail in Chapter V. 



Other sedimentary rocks. Certain special classes of sed- 

 imentary rocks, some of them very important, may best 

 receive attention in later connections. These include gypsum 

 and rock salt, precipitated from solution under special condi- 

 tions (p. 268), the iron ores (p. 323), and a few rocks formed 

 by organisms, or themselves organic, like coal (p. 379), to- 

 gether with deposits made by winds (p. 98) and by glaciers 

 (pp. 204, 212). 



Summary. The more important points concerning the 

 origin of sedimentary rocks are the following : (1) Loose sur- 

 face material is being formed constantly by the decay and 

 breaking up of solid rock. (2) Various agents which transport 

 material on the land, particularly rivers, are shifting this rock 

 waste to new situations, especially to the sea. (3) In the pro- 

 cess of transportation and deposition it is more or less per- 

 fectly sorted, and beds of gravel, sand, and mud result. 



