Debris flow near Revelstoke, British Columbia, Canada. Mass Movement in Cohesive Materials Slopes formed from clays and silts sediments display somewhat unique mass movement processes. Clay and silt particles have a degree of cohesion which gives them potentially more internal strength than non-cohesive sediments. This cohesion occurs because of electrochemical bonds which operate between particles and the surface tension effects of water films that forms around particles. Both of these sources of cohesion are dependent upon moisture content.
Maximum cohesiveness takes place when mositure conditions are moderate.
Too much or too little water reduces the strength of the cohesion. Two common types of mass movements in cohesive materials are rotational slips and mudflows . Both of these processes occur over very short time periods. Rotational slips can be caused by a variety of factors. The most common mechanism reason for them to occur is erosion at the base of the slope which reduces the support for overlying sediments.