42 



GEOMORPHOLOGY: 



often obtains from humus, the organic matter in soil. 

 When rainfall strikes limestone topography, water 

 moves downward through the more porous portions 

 of these rocks. This downward movement is asso- 

 ciated with water dissolving the limestone and caus- 

 ing surface cracks, deeper fissures, and finally caves 

 and caverns. Other geomorphic features form from 

 water seeping through cave roofs and into caves. Be- 

 cause such water tends to collect on the roofs and 

 dehydrate there, calcium carbonate accumulates into 

 icicle-shaped rocks (stalactites) hanging from the 

 ceiling. In addition, some calcium carbonate water 

 may drip to the floor and dehydrate; the calcium 

 carbonate then accumulates as elongate and conical 

 rocks (stalagmites). 



The presence of underground excavations leads 

 to subsidence and a low but very rough topography 

 on the surface. Due to erosion of underground fea- 

 tures, local sinking leaves surface sink holes and 

 natural bridges. In this topography there are few 

 streams and most that do exist are large. There are 

 few lakes or smaller bodies of permanent water. 

 However, during brief periods of the wet season, rain- 

 fall can collect in small depressions and dissolve small 

 sink holes. Therefore, the total effect is a much pitted, 

 but generally flat, landscape. 



Land Forms. The features produced by under- 

 ground water are generally quite distinctive. Ero- 

 sional forms are caverns, sink holes, and fissures. 

 Residual forms are mostly sunken valleys and natural 

 bridges. Depositional forms include cave rocks (sta- 

 lactites and stalagmites), fossils (those produced by 

 limestone replacing or casting organic remains), 

 alkali flats, mineral deposits, cemented rocks, and 

 geyser deposits. 



STREAMS 



Any movement of water over the surface of the 

 land leads to wearing down of the land. Erosion by 

 streams most closely approximates the activities of 

 weathering. 



Stream erosion is related to the amount of rainfall, 

 soil porosity, and vegetation in the area; and the fac- 

 tors determining the erosion power are water volume 

 and stream slope. Rainfall is directly associated with 

 water volume; the greater the rainfall, naturally, the 

 greater the water volume in an individual stream. 

 Soil porosity and vegetation, however, reduce stream 

 water volume because an increase in either soil po- 

 rosity or vegetation causes less runofTinto streams. 



Erosion power is directly allied to water volume 

 and steepness of stream slope. Increase in either 

 volume or steepness of slope increases water velocity. 

 Also, the greater the water velocity, the greater the 

 erosion rate and sediment-carrying capacity of a 

 stream. Actually, doubling of a stream's velocity may 

 increase its erosion power four times and its rock- 

 carrying capacity even more. This relationship helps 

 explain the devastating effects of large flooding 

 streams, because at such times water velocities in- 

 crease ten to twenty times above normal.. 



Erosion Mechanisms. Actual removal of materials 

 involves four processes: corrasion, impact, quarrying, 

 and dissolution. Corrasion is similar to the work of a 

 file and is the scraping and scratching action of par- 

 ticles suspended in the moving water. The ultimate 

 product of corrasion is fine, water-worn materials. 

 Impact is the striking (and breaking) of large rocks 

 against one another. It, too, produces fine materials. 

 Quarrying is downstream movement of materials by 

 the lifting and carrying force of water. It transports 

 the larger, at first less worn rocks downstream. Dis- 

 solution reduces rocks to the fine particles usually 

 found suspended only in mature and old streams. 

 All of the four processes work together in such a way 

 that it is difficult to segregate the actions of any single 

 factor. 



Life History. Youth is the period of greatest ero- 

 sion rate of streams. Young streams (Figure 4.5) 

 are usually straight and have a steep, uneven slope 

 which includes rapids, rock channels, pot holes, 

 plunging pools, falls, and lakes, the uneven slope be- 

 ing the consequence of erosion acting upon rock 



Figure 4.5 Young stream. Note the straight path, waterfall, and V- 

 shoped valley. (After a Ward's Geomorphic model). 



