Further Reading

Abrahams, Athol D., and Anthony J. Parsons. Geomor-phology of Desert Environments. London: Chapman and Hall, 1994. Bagnold, Ralph A. The Physics of Blown Sand and Desert

Dunes. London: Methuen, 1941. Burke, Kevin, and G. L. Wells. "Trans-African Drainage System of the Sahara: Was It the Nile?" Geology 17 (1989): 743-747. El-Baz, Farouk. "Origin and Evolution of the Desert." Interdisciplinary Science Reviews 13 (1988): 331-347. El-Baz, Farouk, Timothy M. Kusky, Ibrahim Himida, and Salel Abdel-Mogheeth. Ground Water Potential of the Sinai Peninsula, Egypt. Cairo, Egypt: Ministry of Agriculture and Land Reclamation, 1998. Guiraud, R. "Mesozoic Rifting and Basin Inversion along the Northern African-Arabian Tethyan Margin: An Overview." In Petroleum Geology of North Africa, edited by D. S. MacGregor, R. T. J. Moody, and D. D. Clark-Lowes, 217-229. Geological Society of London Special Publication 133 (1998). Haynes, C. Vance, Jr. "Great Sand Sea and Selima Sand Sheet: Geochronology of Desertification." Science 217 (1982): 629-633. Haynes, C. Vince, Jr., C. H. Eyles, L. A. Pavlish, J. C. Rotchie, and M. Ryback. "Holocene Paleoecology of the Eastern Sahara: Selima Oasis." Quaternary Science Reviews 8 (1989). Klitzsch, E. "Geological Exploration History of the Eastern Sahara." Geologische Rundschau 83 (1994): 1437-3254.

Kusky, Timothy M., Mohamed A. Yahia, Talaat Ramadan, and Farouk El-Baz. "Notes on the Structural and Neotectonic Evolution of El-Faiyum Depression, Egypt: Relationships to Earthquake Hazards." Egyptian Journal of Remote Sensing and Space Sciences 2 (2000): 1-12.

McCauley, J. F., G. G. Schaber, C. S. Breed, M. J. Grolier, C. Vince Haynes, Jr., B. Issawi, C. Elachi, and R. Blom. "Subsurface Valleys and Geoarchaeology of the Eastern Sahara Revealed by Shuttle Radar." Science 218 (1982): 1004-1020. McKee, E. D., ed. A Study of Global Sand Seas. United States Geological Survey Professional Paper 1052, 1979.

Pachur, H. J., and G. Braun. "The Paleoclimate of the Central Sahara, Libya, and the Libyan Desert." Paleoecol-ogy Africa 12 (1980): 351-363.

Sestini, G. "Tectonic and Sedimentary History of the NE African Margin (Egypt/Libya)." In The Geological Evolution of the Eastern Mediterranean, edited by J. E. Dixon, and A. H. F. Robertson, 161-175. Oxford: Blackwell Scientific Publishers, 1984. Szabo, B. J., W. P. McHugh, G. G. Shaber, C. Vince Haynes, Jr., and C. S. Breed. "Uranium-Series Dated Authigenic Carbonates and Acheulian Sites in Southern Egypt." Science 243 (1989): 1053-1056. Walker, A. S. "Deserts: Geology and Resources." United States Geological Survey Publication 60 (1996): 421-577.

Webster, D. "Alashan, China's Unknown Gobi." National

Geographic (2002): 48-75. Wendorf, F., and R. Schild. Prehistory of the Eastern Sahara. New York: Academic Press, 1980.

Devonian The Devonian is the fourth geological period in the Paleozoic Era, spanning the interval from 408 to 360 million years ago. It was named after exposures in Devonshire in southwest England. British geologists Adam Sedgwick (1785-1873) and Roderick I. Murchison (1792-1871) first described the Devonian in detail in 1839. The Devonian is divided into three series and seven stages based on its marine fauna.

Devonian rocks are known from all continents and reflect the distribution of the continents grouped into a large remnant Gondwanan fragment in the Southern Hemisphere, and parts of Laurasia (North America and Europe), Angaraland (Siberia), China, and Kazakhstania in the Northern Hemisphere. The eastern coast of North America and adjacent Europe experienced the Acadian orogeny, formed in response to subduction and eventual collision between Avalo-nian fragments and ultimately Africa with Laurasia. Other orogenies affected North China, Kazakstania, and other fragments. These mountain-building events shed large clastic wedges, including the Catskill delta in North America and the Old Red sandstone in the British Isles.

The Devonian experienced several eustatic sea-level changes and had times of glaciation. There was a strong climatic gradation with tropical and monsoonal conditions in equatorial regions, and cold water conditions in more polar regions.

Marine life in the Devonian was prolific, with brachiopods reaching their peak. Rugose and tabulate corals, stromatoporoids, and algae built carbonate reefs in many parts of the world including North America, China, Europe, North Africa, and Australia. Crinoids, trilobites, ostracods, and a variety of bivalves lived around the reefs and in other shallow water environments, whereas calcareous foramin-ifera and large ammonites proliferated in the pelagic

Devonian Glaciation

Middle Devonian coral reef construction, including different metazoans such as the coelenterates (Tom McHugh/Photo Researchers, Inc.)

realm. The pelagic conodonts peaked in the Devonian, and their great variety, widespread distribution, and rapid changes make them useful biostratigraphic markers and form the basis for much of the biostratigraphic division of Devonian time. Bony fish evolved in the Devonian and evolved into tetrapod amphibia by the end of the period.

The land was inhabited by primitive plants in the Early Devonian, but by the middle of the period great swampy forests with giant fern trees (Archae-opteris) and spore-bearing plants populated the land. Insects, including some flying varieties, inhabited these swamps.

The end of the Devonian brought widespread mass extinction of some marine animal communities, including brachiopods, trilobites, conodonts, and corals. The cause of this extinction is not well known, with models including cooling caused by a southern glaciation, or a meteorite impact.

The Devonian saw the climactic development of the Appalachian Mountain belt in eastern North America. The Appalachians extend for 1,600 miles (1,000 km) along the east coast of North America, stretching from the st. Lawrence River valley in Quebec Canada, to Alabama. many classifications consider the Appalachians to continue through New foundland in maritime Canada, and before the Atlantic ocean opened, the Appalachians were continuous with the Caledonides of Europe. Home to many of America's great universities, the Appalachians are one of the best-studied mountain ranges in the world, and understanding of their evolution was one of the factors that led to the development and refinement of the paradigm of plate tectonics in the early 1970s.

Rocks that form the Appalachians include those that were deposited on or adjacent to North America and thrust on the continent during several oro-genic events. For the length of the Appalachians, the older continental crust consists of Grenville Province gneisses, deformed and metamorphosed about 1 billion years ago during the Grenville orogeny. The Appalachians grew in several stages. After Late Precambrian rifting, the Iapetus ocean evolved and hosted island arc growth, while a passive margin sequence was deposited on the North American rifted margin in Cambrian-ordovician times. in the middle ordovician the collision of an island arc terrane with North America marks the Taconic orogeny, followed by the mid-Devonian Acadian orogeny, which probably represents the collision of North America with Avalonia, off the coast of Gondwana. This orogeny formed huge molassic fan delta complexes of the Catskill Mountains, and was followed by strike-slip faulting. The Late Paleozoic Alleghenian orogeny formed striking folds and faults in the southern Appalachians, but was dominated by strike-slip faulting in the Northern Appalachians. This event appears to be related to the rotation of Africa to close the remaining part of the open ocean in the southern Appalachians. Late Triassic-Jurassic rifting reopened the Appalachians, forming the present Atlantic Ocean.

See also North American geology; Paleozoic.

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