Surface samples

Surface samples of the transported regolith were taken

directly below the 12.5 cm by 7.5 cm filing card to a depth of 7.5 cm. The average sample weight was 1800 gm. When the sampling point was on solid in situ rock, no sample was taken, and where the transported

*

regolith was less than 7.5 cm deep the sample was smaller. A note was also made if coarse material was concentrated on the surface at the sampling point (see 3.2.2.2.1).

It was not envisaged that laboratory time would be available to analyse samples from every sampling point on every transect, since this would have given 1320 samples for sediment-size analysis. (In addition it was estimated that 300 samples would be taken from pits.) Accordingly, only alternate transects were sampled in this way.

3.2.2.2.3 Gibbers

Gibber measurements were also made at each sampling point on alternate transects. A gibber was defined for this study as a rock or mineral with two axes greater than 12.7 mm (i.e. would not pass through a ^ inch sieve) which was lying freely on the subaerial surface

(David 1950, p. 25), and which required only slight pressure (relative

to its weight) to move it. In other words it was not buried or partly

buried by finer material and was easy to pick up.

Any gibbers lying totally beneath the filing card at the sampling point were first measured and identified before being added to

the surface sample, A gibber lying partly beneath the filing card was

only added to the surface sample if half or more of the gibber (in plan

view) lay beneath the card.

Because time was limited it was not possible to measure more

than 50 gibbers at each sampling point. Such a number is inadequate for

accurate shape determination. However, since no changes in gibber shape along a transect were apparent, and since no process operating

transversely across the footslope was thought capable of modifying

gibber shape significantly, this was not a serious limitation. Only the

density and size of gibbers appeared to change significantly, and a

sample of 50 was considered adequate to show these characteristics. The

additional work to count several hundred gibbers at a sampling point (if indeed such a number could be found) would have seriously reduced the time available for other work.

The area from which gibbers were taken around each sampling

point is shown diagrammatically in figure 3.4. The gibbers were taken

starting at the sampling point and moving outwards to a maximum distance of three metres or up to half way to the next sampling point, whichever

was the lesser distance. The sample was complete when 50 gibbers had

been taken or when the whole area had been stripped of gibbers,

whichever obtained first. At x the gibbers were sampled from an area in

the shape of a semi-circle.

The measurements made were those suggested by Cailleux (1945)

shown in figure 3.5. Since the three major axes were measured to

calculate size, the two additional measurements of shape were made

mineral was also identified. The maximum distance to the sampling point from which gibbers were sampled was noted and used later to calculate gibber density.

3.2.2.2.4 Pits

Pits were dug to obtain samples for size analysis and

impregnation, to examine profiles, and to verify seismic results. The number of pits dug on each case varied, but as a minimum included:

(a) pits to in situ rock at the nickline at lx, 3x, 5xy 7x, and

9x.

(b) pits at 5/, 5g, 5h, 5i, and 5j to give information on profile change in a transverse direction across the footslope.

(c) pits at Ig, 3gt 7g, and 9g to give information on profile development at several points not too remote from the nick. All pits were dug to in situ rock or about 2 m whichever occurred first. Moderate use was made of gelignite in some pits,

remembering that undisturbed oriented samples and other samples were to be taken. A pit-shaped area was generally loosened with gelignite, dug

out, and then a face was worked back to a point undisturbed by the explosion. No samples affected by the use of gelignite are known to have been taken. Samples of approximately 2000 gm were taken wherever macroscopic changes in colour or texture were evident. They were tested

in the field for colour and pH. Where no macroscopic changes were evident, samples were taken every 30 cm and similarly tested.

Representative oriented samples of regolith were taken from several pits for impregnation and microscopic work. In some pits additional auger samples were taken below 2 m and tested for pH and colour, and later used for laboratory work. Samples of weathered or unweathered in situ

rock were taken from exposures in pits, and observations were made

wherever possible on the bedrock/transported regolith interface, and the unweathered bedrock/saprolite interface.

3.2.2.3 Seismic Surveying

3.2.2.3.1 Introduction to the refraction method

The bedrock pediment surface was surveyed along the whole length of each transverse transect using the refraction method with a Huntec FS-3 seismograph. Two profiles longitudinally across the footslope were also

determined on each case. Seismic transects were usually sounded in both

directions, to give a total of over 70 km of seismic survey.

Refraction may be defined as the change in direction of

propagation of a seismic wave when it crosses the interface of two media with different acoustic properties at any angle other than a right angle. The physical principles of seismic refraction have been discussed many times (e.g. Huntec 1966, Hawkins 1961, Hales 1958) and need not be repeated here.

The suitability of the refraction method for studying

pediments was indicated by Weise (1972), but he did not publish any plotted

profiles. The method is particularly suited to continuous profiling of

boundaries between materials of markedly different acoustical properties (usually expressed as the velocity of a wave through the material in feet or metres per second (fps or mps)).

3.2.2.3.2 The critical distance (X )

c

Figure 3.6A shows a simplified version of a time-distance graph which would be produced by the FS-3 for the bedrock profile of

figure 3-6B. The velocity of the regolith is 2000 fps (Vx), and of the

bedrock is 15000 fps (V2). For simplicity the discontinuity is regular

and does not dip.

Hammerpoints are located every 10 ft away from the geophones

and we consider only the first wave to arrive. When the hammerpoint is

close to the geophones the first waves to arrive have travelled

horizontally at V x in the regolith. However, at a critical distance

(termed X ) two waves will arrive at the same time: one has travelled

o

horizontally at velocity Vi from A to D; the other has travelled ABCD,

and the time 'gainedT travelling at V 2 along BC has offset the time 'lost* along AB and CD at Vj . At hammerpoints located at more than the

critical distance from the geophones, the refracted waves will arrive first.