EVIDENCE OF BASAL EROSION AND SHEARING AS MECHANISMS CONTRIBUTING THE DEVELOPMENT OF LATERAL RIDGES IN MUDSLIDES, FLOW-SLIDES, AND OTHER FLOW-LIKE GRAVITATIONAL MOVEMENTS

The origin of lateral ridges in flow-like gravitational movements is controversial. Four examples of movements showing lateral ridge development are examined: Bedwellty in south Wales, U.K.; La Coma and Gosol in the eastern Pyrenees, Spain; and Los Olivares in the Baetic range, southern Spain. Several trenches dug in these sites showed that lateral ridges appear related to erosion of the pre-existing ground surface and that shear surfaces develop at both lateral and bottom boundaries of the moving masses. Textural characteristics of the basal debris at Bedwellty show that pre-existing ground material has been incorporated within the slide material. No erosion was found where lateral ridges disappear at the toe of the movements. An additional mechanism is proposed to explain the origin of the lateral ridges in relation to these erosional features. Erosion of the pre-existing ground during the downward progression of the sliding or flowing mass contributes to both the generation and isolation of lateral ridges. As the outer edges of the flowing mass move more slowly than the main mass, they become progressively detached. Either basal erosion or failure of the ground causes the subsidence of the surface of the moving mass, leaving the outer boundary materials as abandoned standing lateral ridges. This mechanism of lateral ridge generation may occur simultaneously with other mechanisms. In the examples presented here, however, the amount of basal erosion and the fact that ridges are only present where erosion occurs, suggest that subsidence of the whole moving mass due to ground erosion or shearing is a critical factor for the ridge development.