Minor forms developed on steep slopes - Landforms and Geology of Granite Terrains - page 194

(b)

(a)

Figure 8.18. Kluftkarren (a) at Devil's Marbles, Northern Territory, Australia; (b) at Paarlberg, near Cape

Town, South Africa; note flared basal wall in foreground, where regolith formerly lapped up
against the bedrock.

but run in fresh, massive rock for the most part, even though joints are present nearby (Fig. 8.19).
Some run diagonally across major joints. Also, on Wudinna Hill, some grooves have been eroded
preferentially in the porphyritic granite that comprises the greater part of the residual, rather than
in locally developed aplitic veins and lenses, though they have preferentially exploited the junc-
tion between the two rock types, and where this has occurred, the gutters are bordered by raised
rims of aplite. Thus, although there is a general structural control of distribution of grooves in
some areas, most are developed independently of structure in detail. Most workers have concluded
that grooves and gutters are due partly to chemical and partly to mechanical processes. Debate on
the origin of the channels has revolved around the relative significance of mechanical abrasion
and chemical weathering, particularly by moisture, and around the role of biota, especially such
plants as lichens.
Branner (1913) noted and investigated flutings, some of them 2 m deep, developed on granite
and syenite residuals in Brazil. He attributed them largely to the mechanical action of running
water. Others have also emphasised mechanical work. Klaer (1956), for example, considered both
mechanical and chemical processes, but because both bed and sidewalls of the channels are
smooth, emphasised abrasion by running water. Tschang (1961) suggested that the Pseudokarren
of Pulau Ubin are due to wash.
Others, such as Ule (1925), attribute grooveswholly to chemical weathering. Scholz (1947,
p. xlix) noted runnels on granitic inselbergs at Vredenburg and Witteklip, in the Western Cape
Province of South Africa, and thought them due to “ solvent action of downward trickling solutions
charged with cyclic salts and organic acids … ”. The latter, he thought, derived from soil-filled
rock basins developed in the upper slopes of the residuals. Support for such an origin involving the
dominance of the chemical action of water comes from the development of runnels on the faces of
blocks and boulders that generate insignificant volumes of runoff; from their occurrence on the
interior walls of hollows and on overhanging faces draining very small areas, where the water
adheres to the steep slopes by surface tension ( Figs 8.17c and f ) and from pitting found in the beds
of several of the channels ( Fig. 8.20).