Democracia y representación
RECUADRO DE OPINIÓN 4
4.3.1 FORM
4.3.1.1 Bedrock Pediment Surface
Transverse profiles of the bedrock pediment surface derived from seismic determinations in association with augering and digging are shown in Appendix II. On GA the bedrock pediment surface lies very close to the subaerial pediment surface and in places is exposed
(e.g. on transects 5 and 6). The transported regolith is generally
thin, even near the footstream. The general slope is away from
the nick, but reversed slopes are common over short distances and
rare over long distances (however, see the lower 100 m along transect 4). The depth of the footstream channel averages 1 m and may even be cut
into the bedrock pediment surface (e.g. transect 7). Beyond the footstream
the bedrock surface is planar and lies close to the subaerial surface (e.g. on transects 7 and 8).
On GB the bedrock pediment surface is similar. Although
very irregular sections are found in detail (e.g. near the nick on transect 6), the bedrock surface generally lies close to the subaerial
surface. The bedrock surface is exposed in places (e.g. parts of
transects 7 and 8), and may even project above the general level of the subaerial surface (e.g. the isolated tors on transect 9 and a dike on transect 5).
On GC and GD the transverse profiles of the bedrock pediment surface are similar, with the exception that they have both been exposed
and lowered by erosion in nickline depressions. The bedrock pediment
surface projects above the general level of the subaerial surface on GD10 as a small tor, and is exposed along substantial lengths of GC5 and GC7. Transect GC3 is the only transect on granite which shows a generally
planar bedrock surface dipping at a slightly steeper angle than the
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corresponding subaerial surface throughout its length.
Longitudinal profiles of the bedrock pediment surface shown
in Appendix III display similar features. It should be noted that small
rises and falls on the subaerial pediment surface opposite small re-entrants and salients respectively (noted by Mabbutt 1966) are not found on the bedrock pediment surface (see the profiles for GA).
Trend surfaces of the bedrock pediment surface have high coefficients of determination. The third-degree surface for GA
(figure 4.48) indicates that near the nickline the slope is approximately
in a transverse direction. However, away from the nickline, the
longitudinal gradient (i.e. parallel to the direction of flow of the footstream) becomes more important. A similar trend is shown by the
subaerial pediment surface. The correlation of the two surfaces can
be assessed visually by means of the overlay on figure 4.48. The
close visual correspondence is confirmed by Mandelbaum’s d statistic
using computed values of the third-degree trend surfaces (<i=0.194). On GB the height of the bedrock pediment surface generally
decreases away from the nickline (figure 4.49). However, certain
transects (e.g. 6 and 7) are oblique to the general trend. There is a
close correspondence between the trend surfaces of the subaerial and bedrock pediment forms: Mancjelbaum’s <7=0.205.
On GC the height of the bedrock pediment surface decreases steadily away from the nick in a direction intermediate between
transverse and longitudinal to the nickline (figure 4.50). A similar
trend is found on the subaerial pediment surface: Mandelbaum's <7=0.146. On GD the general trend of the bedrock pediment surface is also similar to the general trend of the subaerial pediment surface. The third-degree surface of bedrock pediment height is very similar to the first-degree surface (coefficients of determination are 0.914 and 0.889 respectively): the general trend is SE-NW and the slope shows a slight exponential increase towards the nickline (figure 4.51).
Mandelbaum's <7=0.164 confirms that there is a strong correlation between the form of the bedrock and subaerial pediment surfaces.
4.3.1.2 Weathering Front
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The bedrock pediment surface is not formed by the weathering front, but is separated from it by a very variable thickness of saprolite as shown by the profiles in Appendix II. In most soil profiles the bedrock pediment surface is clearly defined and weathered rock passes abruptly
into transported regolith (4.2.2.3.5). Occasionally the bedrock
surface is formed by a corestone partially emerged from the surrounding saprolite (e.g. in the nickline depression at GC).
The depth of the weathering front could not be determined continuously along each transect, but from the available data the maximum and minimum depths in metres from the subaerial surface to
the weathering front are:
GA GB GC GD
maximum 22.95 20.00 18.65 18.24
minimum 0.00 0.00 3.21 0.00
(Depths of 0.00 correspond to dikes on GA and GB and to a small tor on GD10).
Isopachs of depth to the weathering front have lower coefficients of determination than the corresponding trend surfaces of the bedrock surface. On GA the trend bea^rs little relation to distance from the nickline (figure 4.52) and only explains 21% of the variation in
the data. The greatest depths are found near the nick on transects 2 and
9.
The trend surfaces for GB are not significant at 0.20 p. On GC the trend is not related to distance from the nickline (figure 4.53). This surface explains 62% of the variation in the data.
On GD there is no general pattern (figure 4.54) and this surface only explains 17% of the variation in the data.
Although joints are probably important in determining
the depth of weathering on different parts of the footslope, no joint pattern can be discerned from aerial photographs, transverse profiles,
or isopach surfaces. Other variations in the depth of the weathering
It should also be noted that some of the detailed irregularities of the weathering front shown in Appendix 1]_ may be based on misinterpreted data derived from the refraction of seismic waves by corestones in the saprolite. It is often difficult to identify such features from seismic records alone.
The form of transverse profiles of the weathering front in the vicinity of the nick deserves especial attention for the purposes of evaluation of the rival hypotheses of backwearing and downwearing which are considered later. Although pits dug at the nick indicated that the bedrock is weathered (4.2.3.2), the seismic records do not indicate a common form of the weathering front in this vicinity. On the one hand, certain transects reveal that there is localised
overdeepening at the nick relative to adjacent areas further away from the backing hillslope (e.g. transects 1, 3, 4, and 9 on GA; transects 2, 5, and 7 on GB; transect 2 (?) on G D ) . On the other hand some transects do not (e.g. transects 2, 5, 6, 7, 8, and 10 on GA: transects 3, 6, and 10 on GB; all transects except number 5 (for which there is no data) on GC; transect 1 on G D ) .
The relationship between the form of the weathering front
at the nick and the form of the weathering front on the backing hillslope could not be determined adequately. The interpretation of augering
on the backing hillslope was virtually impossible, since when obstructions were hit it was impossible £o determine whether the auger had reached
the weathering front or simply another corestone below which lay more
saprolite. Nevertheless there is no doubt that the rock between corestones on the backing hillslope immediately above the nickline is very weathered: in one case an auger driven into this material reached a depth of 2.1 m before striking hard rock.
4.3.2 PROCESS
Weathering and erosion of the bedrock pediment surface are not confined to places where it is exposed subaerially, but are also important where it is buried.
Excavations indicate that the bedrock pediment surface is
formed in saprolite with very rare unweathered corestones. Thin sections of saprolite taken from buried bedrock pediment surfaces show that
feldspars and micas are altered to clay minerals. On all cases the saprolite is damp, and on GD it is distinctly waterlogged even in the
dry season. Augering of the saprolite on each footslope indicates that the
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moisture content increases with depth (in the dry season at least). The
removal of weathering products in solution and suspension is achieved by
eluviation. Evidence is given by the formation of illuvial argillans in the
saprolite, especially along microfissures, and even in minute fractures in
polycrystalline quartz at GB and G C . Such eluviation could cause
both the bedrock and subaerial pediment surfaces to be lowered.
The seismic velocity of the saprolite varies considerably, but generally decreases from about 6000 fps to 3000 fps transversely across the bedrock surface, indicating an increase in the degree of weathering
away from the nickline. The velocity at the weathering front increases
to 8000 fps.
Seismic traverses along the nickline do not indicate a
greater degree of weathering in this vicinity. The average velocity
of the saprolite there is 5850 fps.
The mechanisms and role of mantle-controlled planation in the trimming of the irregularities of the bedrock pediment surface and the weathering front are considered in 4.5, since the evaluation of the importance of this process must be seen against the background of other aspects of pediment evolution which are discussed first in section 4.4 (e.g. whether backwearing or downwearing occurs).
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