Hang- en puinmorfometrie van heuwels met 'n dolerietdekrots in die semi-ariede Suidelike Vrystaat

English: An investigation of the literature on slopes revealed that some researchers found a relationship between rock fragment size and slope angle on debris slopes whilst others find no such relationship at all. This project endeavored to empirically test whether such a relationship exists. The literature reveals confusion regarding the concept of debris slopes. It was found that the terms scree slope, debris slope and boulder- controlled slope were used interchangeably. Research findings of scree slopes were used for debris slopes and vice versa. The debris slopes identified for this project conform to the requirements as stated by Young (1972) and comprises a regolith covered slope occurring beneath a free face on which boulders display an upwards increasing tendency in size. Four hills in the southern Free State (semi-arid climate) were selected and on these ten profiles were surveyed. The literature reveals purposive sampling as opposed to random sampling of hills for study and this project is no exception. The choice of profiles were led by the requirements proposed by Young (1972) as well as including only hills with a debris slope of the Beaufort series overlain by a dolerite caprock. Control of the latter variable successfully allowed for isolating the origin of debris on the debris slope (as these comprised dolerite from the caprock). Research that did not control this variable ran into trouble isolating the exact origin of debris on the slopes which impacted on results and conclusions. The profiles were surveyed using an Abney level and the profile- meter of Le Roux (1980) which is 2m in length. Profiles were surveyed down the true slope (perpendicular to the contours) and a line sample of all debris 30 mm and larger on the profile were included for study. The debris sample is therefore representative of a particular measurement of slope. Differences amongst researchers exist regarding the downs lope boundary of a debris slope. This also impacts on research findings. In this project the downslope boundary was determined using the two variables of slope angle and percentage debris cover as opposed to only using slope angle (and this value was in many cases arbitrarily determined) as revealed in previous research. Thus for this study the debris slope comprises the part of slopes commencing at the foot of the free face with the lower boundary based on a low slope angle (≤ 5°) combined with a low percentage debris cover (≤ 2% > 30mm diameter). In previous research only one dimension of debris was measured as opposed to measurement of all three dimensions as in this project. A variety of indicators for debris size were calculated and correlated with slope angle and transformations of angle. These indicators were also correlated with distance from the bottom of the free face. Small, but statistically significant correlations were obtained. Comparison of correlations obtained for the different indicators indicated trends in the data which impacted on the research findings. Calculation of the indicators also allowed for controlling variation in calculating correlations. This demonstrated why many researchers were led to deduce a causal relationship between slope angle and debris size. Manipulation of data which included the isolation of outliers as well as variations in measuring length indicated increases in values obtained for correlations. The latter indicated the importance of the size of the measuring length in obtaining results. This revealed a possible reason why researchers using a measuring length of >2m found a causal relationship between slope angle and debris size. Measurement of all three dimensions of the debris allowed for an investigation into the shape of particles. This demonstrated the importance of particle shape on sorting as well as in the processes operating on slopes and hence the effect of particle shape on slope development. Slope angle frequency distributions were examined and it revealed that debris slopes in the 5° to 10° and 15° to 20° categories could be considered stable for the sampling space. Slopes above 350 are rare and therefore can be regarded as unstable by using ergodic reasoning. From the research it was concluded that debris slopes in this area are mainly structurally controlled but that slope angle and shape comprise a complex function of structure and process over time. With a relatively stable base level slope development is the result of parallel retreat.