42 



EARTH FEATURES AND THEIR MEANING 



FIG. 21. Diagrams representing a, an 

 anticline ; 6, a syncline ; and c, a mono- 

 cline. 



latter have been protected under an arch of the more competent 

 layer (pi. 2 A). 



The arches and troughs of the folded strata. Every series 

 of folds is made up of alternating arches and troughs. The arches 

 of the strata the geologist calls anticlines or anticlinal folds, and 

 the troughs he calls synclines or synclinal folds (Fig. 21). When a 



stratum is merely dropped in a 

 bend to a lower level without 

 producing a complete arch or a 

 complete trough, this half fold 

 is termed a monocline. 



Any flexuring of the strata 

 implies a reduction of their 

 surface area, or, considering a 

 single section, a shortening. If the arches and troughs are low 

 and broad, the deformation of the strata is slight, the shorten- 

 ing is comparatively small, and the folds are described as open 

 (Fig. 22 6). If they be relatively both a t 



high and narrow, the deformation is 

 considerable, a larger amount of crustal 

 shortening has gone on, and the folds 

 are described as close (Fig. 22 c). This 

 closing up of the folds may continue 

 until their sides have practically the 

 same slope, in which case they are said 

 to be isoclinal (Fig. 22 d). 



The elements of folds. Folds must 

 always be thought of as having ex- 

 tension in each of the three dimensions 

 of space (Fig. 23), and not as properly 

 included within a single plane like the 

 cross sections which we so often use in 

 illustration. A fold may be conceived 

 of as divided into equal parts by a plane 

 which passes along the middle of either the arch or the trough, 

 and is called the axial plane. The line in which this plane inter- 

 sects the arch or the trough is the axis, which may be called the 

 crestline in an anticline, and the troughline in a syncline. 



In the case of many open folds the axis is practically hori- 



FIG. 22. A comparison of 

 folds to express increasing 

 degrees of crustal shortening 

 or progressive deformation 

 within the zone of flow : a, 

 stratum before folding; 6, 

 open folds; c, close folds; 

 d, isoclinal folds. 



