276 MATERIALS AND THEIR ARRANGEMENT 



When beds incline in a single direction, they form a monocline. 

 When they are arched up as in a fold, they form an anticline (Figs. 

 249 and 250). The anticline may depart from its simple form, as 



Fig. 254. Syncline, C. and O. canal, 3 miles west of Hancock, Md. Shale and 

 sandstone, near base of the Silurian. (Walcott, U. S. Geol. Surv.) 



shown in Fig. 253. The downfold corresponding to an anticline is 

 a syncline (Fig 254.) When beds assume the position shown in Fig. 

 251, the folds are isoclinal. When considerable tracts are bent so 

 as to form great arches or great troughs with many minor undula- 



Fig. 255. Generalized fan fold of the central massif of the Alps. (Heim.) 



