Geology Reference
In-Depth Information
the southwest, although there is no control as to exactly where the fold nose occurs.
The northwestern contours are all interpreted to lie on the limb of a single structure,
just as inferred by parallel contouring (Fig. 3.1b). The group of 600-ft contours in the
southeast remains a problem. A syncline seems possible.
It is possible to obtain dramatically different results by “highly” interpretive con-
touring of sparse data (Fig. 3.2b). The two maps in Fig. 3.2 are completely different,
although they are derived from exactly the same data. The difference between the two
maps reflects the different assumed regional trends. There is no basis for choosing
which map is better, given the available information. The simplest and most useful
additional information for selecting the best interpretation is the bedding attitude,
because the attitudes should indicate the trend direction (Sect. 3.6.1).
3.3.4
Smooth vs. Angular
Structure contours are usually drawn as smooth curves. This is appropriate for circu-
lar-arc and other smoothly curved fold styles. Many folds, however, are of the dip-
domain style for which the structure contours should be relatively straight between
Fig. 3.3.
Map and cross sections of dip-domain style folds in the Appalachian fold-thrust belt in Penn-
sylvania. (After Faill 1969)
