BIOGENIC LANDFORM

any topographic feature that can be attributed to the activity of organisms. Such features are diverse in both kind and scale. Organisms contribute to the genesis of most topography involving rock weathering; they play an auxiliary role, as demonstrated by bacterial and lichen activity, the effects of root wedging, and solutional erosion made possible by humic acid produced by rapid organic decay. The latter is responsible for much tropical karst. On an entirely different level are features that constitute what may be termed micro topography. Some of these are produced by individual creatures or groups of such creatures, as, for example, the cylindrical mud towers that stand 4050 centimetres high atop crayfish burrows in the southern part of the United States; badger and bear den burrows; elephant waterholes on the veld (grasslands of Africa); and quarries and open-pit mines dug by humans. Other topographic features are attributable to colonial organisms. In various parts of the world such as the semiarid plains of the Western Sahara, colonies of termites build large conical mounds that reach several metres high. The interaction of corals, algae, and bryozoa is largely responsible for the framework of features known as organic reefs, which abound in tropical marine settings. Some of these reefs have given rise to entire insular land areas many kilometres in diameter. The largest example is the Great Barrier Reef of Australia, which covers an area of about 207,000 square kilometres. Though nearly submerged today, it was an island during the Pleistocene glaciation. With the possible exception of the Great Barrier Reef, all major biogenic landforms produced in recent times are attributable to the activities of humankind. The construction of modern superhighways involves some of the most extensive terrain changes on Earth, having in some cases resulted in the removal of mountains or at least large portions thereof. Many human effects are not necessarily tied to particular construction projects. On a subtler level, the removal of fluids from the ground, principally water and petroleum, have dropped water tables and reduced pore pressure so greatly that extensive areas have experienced subsidence, collapse, and shrinkage. Terrain changes due to groundwater removal are extremely severe in such regions as the southwestern United States or the area near Mexico City. To the foregoing human effects on topography must be added the bomb craters left by war that are ever so slowly being obliterated from Europe and Asia, and the erosional gullying of terrain where uncontrolled deforestation has been allowed. Finally, there are the engineering modifications of waterways and coasts practiced nowhere more intensively than in the United States and Europe. River flow patterns have been drastically altered, usually by channel straightening, and the construction of large dams has converted entire valleys, gorges, and canyons into lakes. In fact, dams are among the largest biogenic landforms produced. Hessle Filmore Garner