The nature of folded structures becomes particularly significant in landform studies where several contrasting lithologies are involved. Much depends on the degree of dip. In a simple series of folds, anticlines (upfolds) may be distinguished from synclines (downfolds). The folding may be symmetrical, or where the limbs of the fold differ in dip they may be asymmetrical. Degrees of increasing asymmetry can be recognized in overturned and recumbent folds. In some cases major anticlines and synclines may have minor folds superimposed on the flanks of the main structure, creating anticlinoriums and synclinoriums. In Britain, examples of these structures are provided by the Weald and Hampshire basins respectively.
The relationship between a series of folds and relief may be considered direct where anticlines and synclines form high and low relief respectively. The Jura Mountains in northern Switzerland are a well-known example. Quite frequently, however, there is an inversion of relief, in which the axial lines of the anticlines are deeply eroded, and the synclines form the high ground. One explanation of this cites the fracturing of the apex of the anticline as a zone of weakness, leading to rapid erosion of this zone. It seems that erosional rates may be sufficiently effective during slow rates of folding so that the synclines become the highest points in the area within a short time after folding commences.
The landform expression of individual limbs of folds depends primarily on two factors: the inclination of the strata, and the arrangement of varying lithologies. One significant effect of inclination is that the relative erodibility of a rock is expressed most accurately in the landscape when dips are vertical, and least accurately when dips are low. In the latter case, a resistant cap-rock may protect weaker underlying strata. Moderate dips accentuate the importance of gravity in erosional processes, particularly on sea cliffs and other similar situations where the lower end of a tilted block of strata is being eroded away, and the dips are seaward. The effect of varying lithologies in tilted strata is to produce examples of differential erosion, of which the alternations of scarp and vale in the scarp lands of southeast England are a well-known example.
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