LAND FORMS, THEIR DEFORMATION AND FORMATION 



57 



growth of different plants occurs. Eventually, the 

 continued activities of beavers, alluvial deposition, 

 and plants produce a very thick layer of soil so rich 

 that streams are sometimes diverted by man in order 

 that he can use the land. 



Burrowing rodents cause subsoil to be aerated 

 either in place or after it is moved to the surface. The 

 aeration favors more rapid soil formation and, in 

 situations of slow to moderate runoff, causes more 

 water to move underground; in conditions of rapid 

 runoff, burrows may lead to even greater erosion 

 than normal. Similar effects are accomplished by 

 earthworms, but their activity is not likely to con- 

 tribute greatly to erosion. Finally, termites and ants 

 in tropical areas construct mounds or hills, sometimes 

 about 25 feet high. 



Certain extremely important, but not easily ob- 

 served, geomorphic changes are brought about by 

 microorganisms. For example, certain sulfur bacteria 

 are responsible for the presence of sulfuric acid in 

 many waters. This sulfuric acid is a primary agent 

 in the leaching effects of many streams. 



Coral and other rock deposits are formed by 

 monerans and algae. Red algae are more important 

 reef formers than are corals. 



Coral Deposits. Corals, animals related to sea 

 anemones and jellyfishes, have the ability to extract 

 calcium carbonate and various other calcium com- 

 pounds from sea water and deposit this material as 

 coral around their bodies. Because these coral de- 

 posits remain after individual animals die, and living 

 animals build upon the remains of the dead, large 

 masses, called coral reefs, may accumulate. How- 

 ever, corals have definite temperature and salt water 

 requirements which prevent their being found 

 throughout the oceans. Because conditions below 

 18°C. are unfavorable, the reef-builders live from the 

 surface to depths near 125 feet between lati- 

 tudes near 28° north and 28° south of the equator. 

 Their salt water requirements allow them to occur 

 adjacent to land, but they may be killed by excess 

 sediments or fresh water. Although these general re- 

 quirements are often reflected by the shape of reefs, 

 the three kinds of coral reefs are classified on the basis 

 of their relation to other land masses (Figure 4.20). 

 A fringing reef is adjacent to, or on the fringe of, a land 

 mass. A barrier reef occurs some distance from the 

 mainland, generally parallel to the shore but sepa- 

 rated by an expanse of ocean. For example, the Great 

 Barrier Reef, the world's largest, which is from a few 



fringing reef 



barrier reef 



atol 



Figure 4.20 Coral island development. Subsidence of a volcanic 

 islond causes reefs to proceed from a fringing reef to a barrier reef and, 

 finally, to a corol atoll containing a lagoon. (After a set of Ward's 

 Geomorphic models.) 



to about 90 miles offshore, begins at the northeast 

 coast of Australia and extends about 1200 miles. An 

 alnll is an essentially circular reef containing an in- 

 terior lagoon. Atolls are generally remote from other 

 land masses. 



Many hypotheses exist as to how these reefs form. 

 However, most of the hypotheses present the three 



