Geology Reference
In-Depth Information
9.4.1 Terminology
The flattish, upper surfaces of bornhardts, and the surfaces of platforms or rock pediments are fre-
quently scored by channels cut in fresh granite (Figs 8.18, 8.19, 9.2a and 9.13) . These forms are
known by various names: Rille, Granitrille, Silikatrille, Karren, Pseudokarren, lapiƩs, lapiaz, can-
nelures, acanaladuras and so on. The channels of gentle slopes commonly merge downslope with
those of steeper inclines. Here, those on gentle gradients are referred to as runnels or gutters, from
their similarity to the roof drains of houses. As on steep slopes, joint-controlled forms are known as
Kluftkarren, slots or clefts (Figs 8.18 and 9.6b). Those that are fed principally by seepage from
patches of regolith or soil are called decantation runnels or gutters.
9.4.2 Description
Gutters several centimetres deep and wide are well-developed in granite in some river beds ( Fig.
9.14a) . They run parallel to the direction of flow. Scallops and potholes, the latter, due to the
grinding action of cobbles and boulders, are commonly associated with them (Fig. 9.14b) , and
others adopt the shape of a river channel ( Fig. 9.14c).
In addition, many flattish granite surfaces are drained by systems of comparatively narrow,
deep channels or gutters which link basins to form a rudimentary drainage system ( Fig. 9.2a) .
Most of these gutters are flat-floored and steep-sided. The channel sidewalls are commonly under-
cut, and some are bordered by raised, levee-like rims (see below). Many of their courses are clearly
guided by fractures ( Figs 9.6b and 9.13), and some of these are V-shaped in cross section.
Such structural influences are not as great, or as consistent, as might be supposed, however. Some
gutters, for example, run along joint traces for a few metres and then diverge downslope, follow-
ing the steepest decline and running across partings with either no discernible diversion or with
only minor dislocation. Some drain directly to the margins of the hill where the water runs over
steep slopes either in linear channels or in thin sheets spread over the rock surface (and adhering
to very steep and overhanging slopes by surface tension). Water draining to the margins of
Figure 9.13.
Gutters draining into fracture-controlled Kluftkarren with soil accumulation, Corrobinnie
platform, northern Eyre Peninsula, South Australia.
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