3. KINDS OF MASS WASTING
3.1 Enough of theories and generalities. What kinds of mass-wasting processes are important? Mass downslope movement of bedrock and/or regolith is a complexly (almost hopelessly) multidimensional phenomenon. Here is a list of some of the factors in this complexity (I’m not pretending that it’s a complete list):
slope of the land surface
composition of the surficial material
nature and degree of fracturing of bedrock
degree of weathering of bedrock
rate of downward transition from regolith to bedrock
size, shape, and sorting of regolith particles
degree of water saturation of medium
extent of excess pore pressure
fauna and flora
undercutting at base of slope
3.2 Accordingly, the mode, speed, and volume of downslope mass movements varies enormously. Below is a list of some types of commonly recognized modes of movement (in alphabetical order). I think that an appropriate adjective in this situation is “bewildering”. Many, if not most, of these grade into one another, without clear boundaries.
The most general term in the foregoing list is landslide. That term applies to all perceptible movement of rock or regolith down a slope. Two further qualifications are still needed, though:
(1) The adjective “perceptible” leaves out a class of very important mass-wasting processes—creep and solifluction—that are of great importance but are too slow to stand around and watch.
(2) There’s a phenomenon called debris flows, whose nature lies somewhere in between what’s generally considered to be landslides, on the one hand, and sediment-transporting streamflow (the topic of an earlier chapter) on the other hand.
3.3 Many classifications of types of mass wasting have been proposed. It’s generally agreed that the definitive criteria for such classifications are as follows:
• type of material in motion (particle size, degree of coherence)
• mode of motion (falling, toppling, sliding, flowing)
• speed of motion
Rather than trying to present to you various technical classifications, I’m going to deal in the following sections with several of the most important kinds of mass wasting. Given what’s said above, it seems natural to do this in the form of three sections: creep and solifluction; landslides; and debris flows.