Page 152 - Fundamentals of Geomorphology

SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 135

Hogback Homoclinal ridge Cuesta Plateau Butte
Dip Scarp
Anti-dip Strike Dip slope slope
stream stream stream
Mesa

Figure 5.16 Landforms associated with dipping and horizontal strata – cuesta, homoclinal or strike ridge, hogback,
butte, mesa, and plateau. The chief streams found in landscapes with dipping strata – strike streams, anti-dip streams, and
dip streams – are shown. Notice that a cuesta consists of a dip slope and a steeper escarpment of scarp slope. The black
band represents a hard rock formation that caps the butte, mesa, and plateau.

including the Congo basin, Sudan basin, and Karoo commonly develop along the strike. Strike streams
basin. gouge out strike valleys, which are separated by strike
ridges. Tributaries to the strike streams enter almost at
right angles. Those that run down the dip slope are dip
Folds, rivers, and drainage patterns
streams and those that run counter to the dip slope
Geomorphologists once described individual streams are anti-dip streams. The length of dip and anti-dip
according to their relationship with the initial sur- streams depends upon the angle of dip. Where dip is gen-
face upon which they developed. A consequent stream tle, dip streams are longer than anti-dip streams. Where
flowed down, and was a consequence of, the slope of the the dip is very steep, as in hogbacks, the dip streams and
presumed original land surface. Streams that developed anti-dip streams will be roughly the same length, but
subsequently along lines of weakness, such as soft strata often the drainage density is higher on the anti-dip slope
or faults running along the strike of the rocks, were sub- and the contours are more crenulated because the anti-
sequent streams. Subsequent streams carved out new dip streams take advantage of joints in the hard stratum
valleys and created new slopes drained by secondary while dip streams simply run over the surface.
consequent or resequent streams, which flowed in the Most stream networks are adapted to regional slope
same direction as the consequent stream, and obsequent and geological structures, picking out the main fractures
streams, which flowed in the opposite direction. This in the underlying rocks. The high degree of conformity
nomenclature is defunct, since it draws upon a presumed between stream networks and geological structure is evi-
time-sequence in the origin of different streams. In real- dentintheninechiefdrainagepatterns(Morisawa1985).
ity, the entire land area drains from the start, and it is A tenth category, irregular or complex drainage, which
patently not the case that some parts remain undrained displays no unambiguous pattern, could be added – as
until main drainage channels have evolved. Modern could an eleventh, deranged drainage, which forms
stream nomenclature rests upon structural control of on newly exposed land, such as that exposed beneath
drainage development (Figure 5.16). In regions where a a retreating ice sheet, where there is almost no struc-
sequence of strata of differing resistance is tilted, streams tural or bedrock control and drainage is characterized