While no exact boundaries exist, 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 everything 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 .