| Popis: |
This paper attempts to interpret the relationship between geology, slope form and process in parts of the 20 km square study area of the Royal Society/Royal Geographical Society's Xavantina-Cachimbo Expedition. In the light of the initial analysis of slope form by Dr. A. Young it was decided to focus attention on the valleyside slopes of Classes II and 777 (see Young, above p. 385), where it was thought likely that geomorphological activity was greatest. Steps have been found in slope profiles which appear, at least in part, to result from the seepage of water at a junction between a shale and a sandstone. Measurements of sediment transport across the steps suggest that they are being actively formed at present. Laterite horizons were found to be closely related to slope form at the steps. Geology of the area.?The geological situation is a relatively simple one. A series of near horizontally-bedded, fine-grained rocks, varying from siltstones to shales of Carboniferous age, are overlain unconformably by a highly quartzose sandstone apparently dipping gently to the west. The unconformity is marked by an uncemented quartz pebble band usually from 0-30 cm thick which dips gently in an approximately easterly direction. Generally the weathered sandstones are very unconsolidated. The weathered sandstones form the bedrock of the majority of the area, the fine-grained rocks only outcropping in the valley bottoms some way downstream from the head? |
