The Big Thicket is the name given to a somewhat imprecise region of a heavily forested area of Southeast Texas in the United States. This area represents a portion of the mixed pine-hardwood forests of the Southeast US. The National Park Service established the Big Thicket National Preserve (BTNP) within the region in 1974 and it is recognized as a biosphere reserve by UNESCO. The diversity of animals in the area is high, with over 500 vertebrates, and it is the complex mosaic of ecosystems and plant diversity that is particularly remarkable. Biologists have identified at least eight, and up to eleven, ecosystems in the Big Thicket area. More than 160 species of trees and shrubs, 800 herbs and vines, and 340 types of grasses are known to occur in the Big Thicket, and estimates as high as over 1000 flowering plant species and 200 trees and shrubs have been made, plus ferns, carnivorous plants, and more. The Big Thicket has historically been the most dense forest region in what is now Texas.
Native Americans are known to have lived and hunted in the area nomadically, but did not establish permanent settlements there before the Alabama–Coushatta settled in the northeast about 1780. Spanish explorers and missionaries generally avoided the area and routed their roads around it. Logging in the late 19th and 20th centuries dramatically reduced the forest concentration. Efforts to save the Big Thicket from the devastation of oil and lumber industries started as early as the 1920s with the founding of the East Texas Big Thicket Association by Richard Elmer Jackson.
Conservatively the area occupies all of Hardin County, most of Polk, and Tyler Counties, and parts of Jasper, Liberty and San Jacinto Counties, including areas between the Neches River on the east, the Trinity River on the west, Pine Island Bayou on the south, to the higher elevations and older Eocene geological formations to the north. Broader interpretations have included the area between the Sabine River on the east and the San Jacinto River on the west including much of Montgomery, Newton, Trinity, and Walker Counties, as well. Several attempts to define the boundaries of the Big Thicket have been made, including a biological survey in 1936 which included over covering 14 counties. A later botanical based study in 1972 included a region of over . This same habitat extends into Louisiana and eastward.
Geography
Physical geography
Geology: What makes the Big Thicket a remarkable place is the unusually high diversity of plants concentrated in a relatively small area. A major factor enabling this diversity is an unusually high diversity of soils. The exposed surface soils are relatively recent, late Cenozoic Era, predominantly Miocene, Pliocene, Pleistocene, and Holocene Epoch formations. Other than some slight and uniform tilting, the strata are not greatly deformed. The strata were laid down in several cycles of glaciations and warmer interglacial interludes. Although glaciers never extended as far south as Texas, it was during these cold periods that enough water was frozen in polar regions to lower sea levels and the Big Thicket area was dry land well above sea level. Conversely, during the warmer interludes, enough ice melted to increase sea levels and submerge Southeast Texas in the sea. It was during those warmer periods of submersion that alluvial and delta deposits from the streams and rivers of the continent laid down the succession of strata, each new layer burying and compacting the previous. At some point there was an approximately 1% tilting of the layers. Some say it was caused by the rising of the Rocky Mountains, while others say it was the weight of the successive deposits that eventually caused the Gulf of Mexico to subside. The older Pliocene, Miocene, and Oligocene formations are exposed in the north (Polk, San Jacinto, and Tyler counties), while the more recent Quaternary Period formations are exposed in the south (Hardin and Liberty counties).