Welsh Journals

by a factor of between 2.2 and 7 A-the highest value reaching 0.012 m/m2 (12.32 km/km2) while the grid based values of rill densities on the slopes reached 0.036 m/m2. The concentration of rills and gullies on slopes of south eastern and south western aspect was related to the dip of the sandstone and marl strata of the Brown- stones and Senni Beds and weakly related to the distribution of terraces. This indicates that ground water runoff is possibly the pre- dominant factor controlling the pattern of hillslope erosion (at least in the initiation of rill and gully erosion) with gullying facilitated by the erodibility of the terrace deposits. Slopes of northern aspect have very few rills and gullies, which can be explained by the movement of ground water away from those slopes down the dip of the strata towards the south south east. Soil and vegetation patterns are related to the slope, terrace and drainage conditions (Crampton, 1967) and may explain, with the history of land use (Crampton, 1967), some of the details of the hill- slope erosion both spatially and temporally­×in particular the significance of different methods of management in hill farming in terms of the development of soil profiles and consequent drainage conditions. In terms of present erosion processes the rills and gullies form a dense drainage network capable of carrying surface runoff rapidly to the main tributary channels, thus reducing the response time of the catchment. The response time will probably increase however with afforestation of slopes in the catchment, which is altering the drainage pattern and will eventually affect soil moisture levels and increase the interception of precipitation. References BARRETT, E. C. and CURTIS, L. F. (1976) Introduction to Environ- mental Remote Sensing. Chapman and Hall, 215. CARSON, M. A. and KIRKBY, M. J. (1972) Hillslope Form and Process. Cambridge, 192-4. CRAMPTON, C. B. (1967) Historic and present aspects ofsoils, vegetation and land use in the region of the Brecon Beacons, in Upland Soils (ed. D. JENKINS) Welsh Soils Discussion Group Rept. 8 Appendix, 23-32, 48-50. EVANS, D. EMLYN (1973) Brecon Beacons National Park Scenery: a geological interpretation. National Museum of Wales, Cardiff, 4-6, 8-9, 33. GARDINER, V. (1975) Drainage basin morphometry. British Geo- morphological Research Group Tech. Bull. 14, 4-15.