LAND FORMS, THEIR DEFORMATION AND FORMATION 



41 



residual forms are domes, mushroom or pedestal 

 rocks, pinnacles, jointed rocks, and land scars. The 

 weathered domes form by the "peeling oflf" of rock in 

 fiat but curved or rounded slabs, resulting in rounded 

 summits on mountains. Such summits are called ex- 

 foliated dnmes. The umbrella-shaped mushroom or 

 pedestal rocks grade into tall, slender, and pointed 

 pinnacles. Jointed rocks have the general appear- 

 ance of boxes or other flat-sided geometric figures. 

 Landslide scars are the depressions remaining after 

 landslides. Finally, the depositional forms are regu- 

 lar to irregular structures of variable composition, 

 such as cones. 



WATER 



Although not as destructive locally as glaciation, 

 water on a worldwide basis is the greatest factor of 

 erosion. This is the consequence of a global average 

 annual precipitation of 28.5 inches. However, the 

 average is based on extremes of about 450 inches on 

 the slopes of the Himalayas and less than 2 inches 

 in a number of very arid places in the world. In 

 spite of such variation, the world is molded by rain- 

 fall. Rainfall striking the surface of the earth has 

 one of three possible fates in the hydrologic cycle. 

 It may evaporate and return to the atmosphere, it 

 may run off in streams that eventually reach the 

 ocean, or it may seep into the ground and join the so- 

 called underground water. The history of water in 

 the atmosphere was already discussed. Its history as 

 underground water and as streams is deeply involved 

 with the eroding of landscapes. 



UNDERGROUND WATER 



The amount of underground water often varies 

 according to the annual cycle of precipitation and 

 drought and the structure of underlying rocks. Dur- 

 ing the wet season, the upper surface, or water table, 

 is high or near the ground surface; during the dry 

 season, especially after a long period of drought, the 

 water table is low. Over long periods of time, how- 

 ever, there exists a point below which the water table 

 does not drop. This is the level of the permanent 

 water table. Structurally, the occurrence of this water 

 is related to the porosity of subsurface rock layers, 

 primarily the distribution of porous and impervious 

 layers. For example, below the ground surface there 

 are, first a porous layer and then an impervious layer. 



The impervious layer acts as a basin or container for 

 the water which is in the porous layer. Actually, only 

 the lower part of the porous layer contains the water 

 and for this reason is called the zone of saturation. 

 Above the zone of saturation but still in the porous 

 layer there is air in the zone of aeration. 



The distribution of surface water is related to that 

 of underground water (Figure 4.4). Any surface area 

 that is depressed below the level of the water table 

 will contain water. Moreover, seasonal fluctuations 

 in water-table level will be accompanied by seasonal 

 fluctuations in associated surface waters. These con- 



~ ~- ^ temporary stream 



geyser 



i 



w 



'L*TrT 



fissure 



abridge 



Saturated 



Figure 4.4 Some land forms related to underground water. Full de- 

 velopment of features molded by such waters occurs in limestone oreos. 



ditions explain the seasonal variations and occurrence 

 of many lakes. In addition, water-table changes may 

 determine why temporary streams are dry part of the 

 year and flow at other times. Also, the relationship 

 between subsurface waters and a small opening at the 

 impervious surface explains the pipe-like flow of water 

 from artesian wells. Finally, if there is an associa- 

 tion between underground water and the escape of 

 hot gases from beneath the surface, the land form 

 produced is a hot spring or geyser. 



Erosion Mechanisms. Circulation of underground 

 water leaves its mark upon a landscape. Its efTects 

 are most pronounced where the surrounding rocks 

 are limestone, because limestone is especially soluble 

 and is afifected by the slight acidity ground water 



