Friday, December 07, 2007

Uplifting!


The geologic history of the Ozarks is so complex and vast that geologists explain the forces that gave rise to the Uplift in terms of theories rather than facts. It is widely accepted, however, that the formation of the Ozark Mountains began during the latter part of the Precambrian Period, between 1.65 and 1 billion years ago. In the 19th century, geologists were unable to locate fossils earlier than those found in Cambria, Wales, which dated to 600 million years ago. Anything older than the fossils in Wales were described as Precambrian. Thanks to soft-bodied fossils found in Australia and Canada, evidence is conclusive that life began almost three billion years earlier than the Cambrian Period.

Formation of the Ozarks began when North America was still located south of the equator. In the Precambrian Period, a combination of outpourings of lava from volcanoes, layers of ash and cinders piled up, eventually forming the base of the Ozarks. Over the course of 500 million years, volcanic eruptions below the surface caused blister-like projections to rise up out of the sea. The lava floes created a small chain of islands in the primordial sea. These ancient igneous rocks formed the St. Francois Mountains, the core of the modern Ozarks.

After millions of years of volcanic activity, the molten rocks of the St. Francois Mountains may have been as tall as 10,000 feet above the ocean floor, although no more than a mile or so above the water level. The strong forces of continental drift had not yet formed the supercontinent, Pangaea, but the drifting was directly impacting the formation of the rest of the Ozarks, causing a bending of the ocean floor, raising the seafloor almost above the water. By the end of the Cambrian Period, the St. Francois Mountains were balmy desert islands, much like the Galapagos. The rest of the Ozarks were still forming under the sea.

About 520 million years ago, the earth's crust warped, causing successive rising and falling sea levels. During this period, several layers of sediments were deposited and quickly eroded, leaving behind carbonate and sandstone deposits on the ocean floor. Finally, by the beginning of the Ordovician Period, a major uplift related to previous volcanic activity and continental shifting brought the Ozarks out of the sea. At the same time, the northern and central Appalachians were forming. Some geologists link the burgeoning mountain ranges to the same continental collisions. Nevertheless, and regardless of similarities today, the Ozarks were still in the tropics during the Ordovician Period; the fossil record shows snails, crustaceans and other marine life from this period, all creatures of limestone-based mud.

In its infancy, based on geologic records and careful speculation, the Ozark Mountains may have been as tall as the Himalayas. The erosive forces that began in the Silurian Period (440-410 million years ago) formed rivers and streams, many cutting deeply through the Ordovician bedrock. Sea levels changed once again as early as 360 million years ago during the Mississippian Period, placing the Ozarks underwater. So, on top of the Ordovician layers, limestone-based muds and sandstone layers were deposited. Meanwhile, the Appalachians continued to grow as plates collided back east. The Ozarks, on the other hand, grew thanks to the addition of new sediment layers.

Finally, fast forward to the Cretaceous Period (146-65 million years ago). Shifting plates caused the Ozarks to uplift one last time, sending torrents of recent sediments and gravels into southeast Missouri. Since the last major uplift, the erosive forces of wind and rain have weathered this ancient mountain range, leaving behind an easily accessible, stunning timeline of Missouri's geologic history.

The Ozarks are separated into four distinct ecoregions: the Salem Plateau (dominated by sedimentary rocks like limestone, dolomite and sandstone), the Springfield Plateau (represented best by rolling hills and vast stretches of prairie), the Boston Mountains of Arkansas, and the St. Francois Mountains (composed of rhyolite, granite, and basalt).

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