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Upper Table Rock and Lower Table Rock are two prominent volcanic plateaus located just north of the Rogue River in Jackson County, Oregon, US. Created by an andesitic lava flow approximately seven million years ago and shaped by erosion, they now stand about above the surrounding Rogue Valley. The Table Rocks are jointly owned; The Nature Conservancy is responsible for , while the Bureau of Land Management is responsible for . The Takelma tribe of Native Americans inhabited the Table Rocks for at least 15,000 years prior to European American settlement. Starting in the mid-19th century during a gold rush, the settlers forced the Takelma away from the Table Rocks and into reservations. The surrounding area was quickly developed. The Table Rock post office was established in 1872, an airstrip was built atop Lower Table Rock in 1948, and a very high frequency omni-directional range (VOR) aviation tower was constructed on Upper Table Rock in the 1960s. The Table Rocks were not protected until the 1970s. The rocks are home to over 70 species of animals and 340 species of plants, which includes over 200 species of wildflowers. Vernal pools atop the plateaus fill during the rainy season in winter and spring because the andesite is impermeable. The dwarf woolly meadowfoam, a species of wildflower, grows around these pools, and is endemic to the rocks. The pools are also one of only a few places where the federally threatened species of fairy shrimp, ''Branchinecta lynchi'', can be found. To protect these and other threatened species, the Bureau of Land Management has listed the rocks as an Area of Critical Environmental Concern since 1984. The Table Rocks are one of the most popular hiking locations in the Rogue Valley, with over 45,000 visitors annually. Two trails, Lower Table Rock Trail and Upper Table Rock Trail, were cut across the plateaus' slopes in the early 1980s by the Youth Conservation Corps, Boy Scouts, and the Oregon Department of Forestry. This effort was coordinated by John Ifft, a forester for the Medford Office of the BLM. The plateaus are named for their relatively flat tops. Upper and Lower refer to their location along the Rogue River, not their height. Upper Table Rock, above sea level at its highest point, is located upstream, while Lower Table Rock is farther downstream, with an elevation of . ==Geology and climate== Starting approximately 40 million years ago in the middle Eocene, a braided river system called the "Ancestral Rogue River" flowed through the region where the Rogue Valley is now carved. For about 2.1 million years, the river deposited what is now known as the Payne Cliffs Formation by laying down a thin conglomerate, followed by arkosic sandstone and siltstone. Between 10 and 20 million years ago, the uplift that created the nearby Klamath Mountains caused an incision that formed the Rogue River valley. Vertical erosion, or downcutting of the Rogue River continues to keep pace with the recent uplift, with about of erosion occurring in the past seven million years. Approximately seven million years ago in the upper Miocene, a long trachyandesitic lava flow that likely came from Olson Mountain near present-day Lost Creek Lake flowed down the Ancestral Rogue River and its tributaries and spread throughout the valley. This lava formed a hard cap over the Payne Cliffs Formation. At Lost Creek Lake, the lava attained its maximum thickness of and thinned to about to the north of Medford. Since the Olson eruption, the Rogue River has eroded 90 percent of the solidified lava. Though the andesite prevented much erosion to the caps of the Table Rocks, the andesite-capped cliffs eroded from the side as the softer sedimentary units of the Payne Cliffs Formation gave way. This erosion created expansive talus fields which surround the plateaus on all sides, creating slopes capable of supporting abundant plant and animal life. Upper and Lower Table Rock both stand above the valley floor, and just over above sea level. There are approximately of level ground on Lower Table Rock, and on Upper Table Rock. The Table Rocks offer an example of inverted relief, in which previous topographic lows are filled with a resistant rock and become new topographic highs after the erosion of the surrounding region. Each plateau is shaped roughly like a horseshoe because the lava followed the meanders of the Ancestral Rogue River. Two caves and two former gold mines are located at the base of the andesite cap on Upper Table Rock. The caves were created by natural fractures in the cap, and the gold mines were excavated by prospectors searching for gold in the 19th century. Three are large enough to walk into, with an average width of , while one is a small pit, dropping vertically into a pond of water. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Upper and Lower Table Rock」の詳細全文を読む スポンサード リンク
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