40 



GEOMORPHOLOGY: 



Although the ultimate effect of erosion is creation 

 of a peneplain, along the way to flattening a terrain, 

 erosion is said to produce three types of destructional 

 land forms; erosional, residual, and depositional. 

 Ewsional forms are empty spaces produced by the 

 removal of materials, and include gullies, canyons, 

 and caves. Residual forms are remnants of eroded 

 topography, such as rounded peaks or ridges, sharp 

 ridges, needlelike peaks, and arches. Depositional 

 forms consist of materials that were transported from 

 their original sites and that accumulated in a new 

 location, e.g., deltas, beaches, and dunes. These 

 depositional land forms are often considered con- 

 structional rather than destructional because they 

 are elevations. 



One should keep the general pattern of erosion 

 and the erosion cycle in mind while considering the 

 individual erosion factors. Throughout the discus- 

 sions of individual agents of erosion it should be 

 remembered that no factor is ever the only one op- 

 erating in a given locality. 



WEATHERING 



The consequences of weathering are much the 

 same as those of streams; however, weathering is 

 not nearly as important a factor of degradation as is 

 water. 



EROSION MECHANISMS 



The causes of weathering are the atmosphere, 

 plants, and animals, working mechanically or 

 chemically. The mechanical processes are those 

 which use force to erode. Rocks are broken by 

 being expanded and contracted by temperature; by 

 being wedged by crystals forming, by plant roots 

 growing, or by ice expanding from the liquid state 

 of water; and by being undermined by burrowing 

 animals. These mechanical processes are most 

 effective in arid areas and on exposed mountains 

 (i.e., places having climatic extremes.) 



The chemial processes of weathering often cause 

 the weakening of rocks by adding particular chem- 

 icals to the rocks. Such processes include the ad- 

 dition of oxygen, carbon dioxide, and water. Also 

 there are processes in which liquids (mostly water) 

 dissolve and transport particles and in which acids of 

 plant or animal origin break up rocks into smaller 

 particles. 



MASS MOVEMENTS 



The products of weathering are transported by 

 gravity in mass movements. The movements are of 

 four main types: slow flowage, rapid flowage, land- 

 slides, and subsidence. Slow flowage consists of im- 

 perceptible transportation of fine to large rock 

 particles; rapid flowage, of barely visible travel of 

 fine rock particles. Landslides are very rapid move- 

 ments of fine to large rock particles, and subsidence 

 is the slow collapse or settling of a small to large land 



SLOPE LIFE HISTORY 



The various mass movements are associated in a 

 sequence of changes, or life cycle, of slopes (see the 

 many geomorphic cycle figures in this chapter). This 

 cycle shows some, but not complete, relationship to 

 the life history of the surrounding landscape. In 

 youth, slopes are steep, and both erosion and mass 

 movements are rapid. In maturity, slopes are 

 reduced to a point where erosion and downhill 

 movement of particles are minimized. As always, old 

 age is a peneplain with no more than scattered rem- 

 nants of past slopes and elevations. 



LAND FORMS 



Weathering, like any other agent of erosion, 

 produces three kinds of land forms: erosional, 

 residual, and depositional (Figure 4.3). The erosional 

 forms are various kinds of holes and pits. The 



exfoliated dome 



''l'-';!^''<r~- -St.^t'r'V'-',,^- '''''. L-" 



p:<^fg?>^^^'^r^,;v;^ landslid 



:^^?;;:'' pinnacles:: ''^^^y'j^"-:,^ _.^j ^ 



' r r ■ J^' i ^ -f -'V^- "r rr i joints toi^^r 



e scar 



Figure 4.3 Land forms produced by weathering. Tlie consequences of 

 weathering often are clearest in arid climates. 



