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Buffalo National River Geology

Physical time travel to the past is still in the realm of science fiction, but a float trip down the Buffalo River can take you back to ancient times when the Ozarks were being formed and shaped. The Ozarks are commonly referred to as the Ozark Physiographic region. The Ozarks are generally not considered true mountains by geologists because they did not form from compressive folding or block faulting. Instead, the Ozarks topography was formed on a doming up of the 1.5 billion year old Precambrian basement rock. These basement rocks are mostly granites, but there are some basalts and rhyolites. They are exposed at the surface in the St. Francios mountains in southeast Missouri and can be seen at places such as Johnson Shut-In's and Elephant Rocks State Parks.

The Ozark Plateau is composed chiefly of shallow marine limestones, dolomites, sandstones and shales from the Paleozoic era. It is generally characterized by karst topography with many caves, sinkholes, losing streams and springs. The plateau can be subdivided into four subunits. The St. Francois mountains, in southeast Missouri, which consist of Pre-Cambrian igneous basement rocks. The Salem Plateau dolomites, limestones, and sandstones, ranging in age from Cambrian to Silurian, form the first ring around the igneous core. The Springfield Plateau limestones of Mississippian and Devonian age form the second ring. The Boston Mountains sandstones, limestones, and shales of Mississippian and Pennsylvanian ages form a high escarpment on the southern edge of the area. Along the course of the Buffalo River, the Salem and Springfield Plateaus are generally not well developed. This is because of the steep and heavy dissection of the area by the river and tributary springs. Some areas of the watershed have good examples of the Springfield Plateau. Most notably, this can be seen around Marble Falls, St. Joe, and Big Flat. The best example of the Salem Plateau in the area is between Yellville and Cotter.

The Buffalo River source is in the Boston Mountains at an elevation of approximately 2400 feet above mean sea level. From here, it flows northerly for nearly twenty miles (32 km) in a narrow canyon like course until it reaches the broad, flat-floored Boxley Valley. Boxley Valley is formed in the Mississippian Boone limestone. The Boone is the premier cave forming strata in Arkansas. Nearly all of the state's largest and most complex cave systems are found in this unit. After flowing another eight miles in Boxley Valley, the waters of Buffalo River first encounter the Ordovician strata of the Everton formation, near the town of Ponca. These rocks tend to be more resistant to erosion than the Boone limestone, and once again the river takes on more of a canyon like morphology. The bluffs in this section of the river are among the highest in the central highlands of the United States. The river continues to flow through pre-Mississippian rocks, except for some short stretches near Sneed's Creek and Pruitt for the next forty-eight miles until it once again encounters rocks from the Boone formation near Richland CreekOnce again the river valley becomes broad and flat-bottomed. There are still spectacular bluffs on the outside of bends on this section. A couple of miles above the town of Gilbert, after flowing for seventeen miles through the Mississippian strata, the waters once again carve into Silurian and Ordovician rocks. The Buffalo River finishes its traverse to the White River at 400 feet above mean sea level mostly in these older strata. The bluffs which line the outside of bends on the Buffalo River offer a chance to look at various rock strata which cover a time period of over two hundred million years. There are many interesting features in this section. There are several faults alongside the river and some nice unconformity surfaces. Ancient sinkholes, algal stromatolites, and orthocone cephalopod fossils can be seen in the Ordovician and Silurian formations. Crinoids, Blastoids, and many other shallow marine fossils are abundant in the Mississippian limestones. Plant fossils can be seen in some of the Pennsylvanian formations. The various colors of stains running down the bluffs make the river especially scenic. Any float trip down the Buffalo River can be a very rewarding experience for those interested in geology.

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