THE GRID NETWORK

The boundaries of a case-study were determined as follows. Having selected a suitable area during general reconnaissance,

boundaries for a detailed study and interesting features were then

assessed with the aid of aerial photographs from the top of the residual or valley side overlooking the footslope. Radio contact was maintained with the field assistant on the footslope: he was directed to certain features and reported on them.

The grid network was based on ten transects, each beginning at the nickline and usually terminating at a footstream or reverse of slope (figure 3.2). The magnetic orientation of these transverse transects was determined from many locations on the top of the backing hillslope and from the end of the footslope. Prismatic compasses were used to sight along the line of maximum slope on the backing hillslope. If the readings did not vary by more than 3° the mean was taken, giving the magnetic orientation of transverse transects.

A baseline was established at right angles to the direction of transverse transects (AB on figure 3.2). Primary survey points were located along the baseline, and the distances between transects (either 40 m or 50 m depending on the width of the footslope in a longitudinal direction) were measured along it. Since the baseline was to be used for primary survey points, and since sampling points were more closely spaced near the nick (see below), the baseline was located sufficiently close to the nick for easy reading of the levelling staff. The exact location of the baseline depended in part on the vegetation: the clearing of vegetation to obtain a clear line of sight along it was . always kept to a minimum.

Two variations on this orientation of the grid network were necessary. First, if the nickline had a general change of direction

(i.e. in plan view), it was necessary to change the orientation of the transverse transects to maintain their collinearity with the direction of maximum slope on the backing hillslope (see for example figure 4.21). The orientation of the baseline was changed accordingly. Second, on the two footslopes surrounding residuals studied in detail (GB and BD) transects were radial from the very top of the residual. Each transect was 36° of arc from an adjacent transect. There was no baseline on

these case-studies.

Transverse transects were numbered from 1 to 10 inclusive, and intersected the baseline at points 1 alpha to 10 alpha respectively (figure 3.2). Each transverse transect began at the nick (termed sampling point x ) and was chosen by eye. It lay at the first point of substantial angular change of slope when moving upslope from the

footslope to the backing hillslope. On granite this was generally easy to select, whereas on basalt and-some sandstone cases it was often more

difficult, but was usually associated with a marked decrease in the size of gibbers.

Each transect ended at sampling point j which was at or very near (within 3 m) the end of the footslope. This point was also

selected by eye. It coincided either with a general reversal of slope or a stream channel, and often both. It was a point where surface processes other than wind operating areally on the footslope cease to operate due to either gravity (i.e. slope reversal) or to the start of linear erosion (i.e. a stream). This point was usually easy to define. In the case of a channel, sampling point j was located 3 m towards x to • permit gibber sampling on the footslope itself rather than from in the channel (see 3.2.2.2.3). This resulted in transects of various lengths.

There were three exceptions to this: on GA transects were extended beyond the end of the footslope and were of equal length; on SC transects terminated at a change of lithology; on SD transects

terminated at a clifflet. Reasons are given in the respective chapters. The spacing of sampling points along transverse transects raised certain problems on account of the various lengths of transects

(the longest was 620 m on SA and the shortest was 30 m on BD). An

equidistant spacing of sampling points was not adopted for three reasons. First, the longer transects (e.g. on SA) would have an excessive number of sampling points if sampled at a frequency suitable for the shorter transects on BD. Second, the area nearest the nick is considered to be of greater relevance in the study of pediment evolution than areas remote from it: equal spacing of sampling points would give equal attention to the remote areas. Third, and most important, equal

spacing of sampling points along transverse transects makes comparisons between transects very difficult indeed (be they on the same or on different footslopes). For example, a sampling point 100 m from x on transects of 120 m and 400 m represents points | and £ the length of the transects measured from the nick respectively. Clearly, in terms of topographic location they are not comparable.

Accordingly, a spacing of sampling points was adopted which satisfied these conditions:

(b) sampling points were more closely spaced near the nick. (c) each point on every transect had a directly comparable point

on every other transect.

The spacing of sampling points is shown in figure 3.3. Each sampling point between

point x

point j

point x

transect was 100 m in length, sampling points were located at 50 m, 25 m, 12.5 m, 6.25 m, etc. This process of halving the distance was made eight times. An additional station was located - of the distance downslope from

x.

x

j

GA2x,

4j.