Page 153 - Fundamentals of Geomorphology

136 STRUCTURE

by irregular stream courses with short tributaries, lakes, and secondary tributaries run parallel to the main
and swamps. Figure 5.17 shows the major types of streams. It is associated with alternating bands of
drainage pattern and their relationship to structural hard and soft dipping or folded beds or recently
controls: deposited and aligned glacial debris. Fold mountains
tend to have trellis drainage patterns. An example is
1 Dendritic drainage has a spreading, tree-like pat- the Appalachian Mountains, USA, where alternating
tern with an irregular branching of tributaries in weak and strong strata have been truncated by stream
many directions and at almost any angle. It occurs erosion.
mostly on horizontal and uniformly resistant strata 4 Radial drainage has streams flowing outwards in all
and unconsolidated sediments and on homogeneous directions from a central elevated tract. It is found
igneous rocks where there are no structural controls. on topographic domes, such as volcanic cones and
Pinnate drainage, which is associated with very steep other sorts of isolated conical hills. On a large scale,
slopes, is a special dendritic pattern wherein the trib- radial drainage networks form on rifted continental
utaries are more or less parallel and join the main margins over mantle plumes, which create litho-
stream at acute angles. spheric domes (Cox 1989; Kent 1991). A postulated
2 Parallel drainage displays regularly spaced and Deccan plume beneath India caused the growth of a
more or less parallel main streams with tributaries topographic dome, the eastern half of which is now
joining at acute angles. Parallel dip streams domi- gone (Figure 5.18a). Most of the rivers rise close
nate the pattern. It develops where strata are uni- to the west coast and drain eastwards into the Bay
formly resistant and the regional slope is marked, of Bengal, except those in the north, which drain
or where there is strong structural control exerted north-eastwards into the Ganges, and a few that
by a series of closely spaced faults, monoclines, or flow westwards or south-westwards (possibly along
isoclines. failed rift arms). Mantle plumes beneath southern
3 Trellis drainage has a dominant drainage direction Brazil and southern Africa would account for many
with a secondary direction parallel to it, so that pri- features of the drainage patterns in those regions
mary tributaries join main streams at right angles (Figure 5.18b–c).

a
b
c
d
( ) Dendritic ( ) Parallel () Trellis ( ) Radial () Centrifugal
e

h
i
() Centripetal ( ) Distributary ( ) Rectangular () Annular
f
g

Figure 5.17 Drainage patterns controlled by structure or slope.
Source: Mainly after Twidale and Campbell (1993, 342) and adapted from Twidale (2004, 173)