402 A. W. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



bedding is eminently characteristic of torrential stream deposits. 3 The 

 direction of inclination indicates the direction of the current, and since 

 in a fan this direction changes around the periphery such change must 

 also be looked for in the direction of the dip of the layers in various con- 

 temporaneous parts of the formation. Such a type of cross-bedding re- 

 sembles essentially a series of superposed deltas of small height, each 

 consisting of foreset and topset beds. The foreset beds become tangent to- 

 ward the bottom, as in ordinary deltas, merging into the underlying hori- 

 zontal beds, which in turn constitute the topset division of the next lower 

 series. The topset beds are generally somewhat coarser than the foreset 

 beds, and it not infrequently happens that the topsets of an earlier part 





Figure 1. — Diayram of compound oblique Cross-bed ding 

 The type most common in torrential deposits 



of a series are somewhat lower than those of the later part of the same 

 series. Generally a sharp contact is shown between topset and foreset 

 beds, the latter showing evidence of a certain amount of truncation before 

 the topset beds were laid down. In general the layers showing the oblique 

 bedding are not more than 3 or -i feet in vertical thickness, but the indi- 

 vidual oblique beds may be 6 feet or more in length. The topsets are 

 usually not more than half as thick as the oblique series. The angle of 

 inclination of the oblique series varies witli the grain of the material and 

 other factors, but generally falls around 30 degrees from the horizon- 

 tal. In some cases as many as six or eight or even more successive series 



3 W. II. Hobbs: The Guadix formation of Granada. Bull. Geol. Soc. Am., vol. 17, 

 pp. 285-294. 



