Archeological investigations since the 1970s provide much new information about the Southern Ozark Highlands. Excavations at mound centers and associated residential sites in northwest Arkansas, southwest Missouri, and northeast Oklahoma have revealed the emergence of Mississippi period trade and ceremonial practices connecting the people inhabiting those areas with Arkansas Valley populations centered around Spiro. Marvin Kay and colleages have hypothesized that during periods when Harlan and Spiro were the dominant centers, a three-level hierarchy of settlements extended across the Arkansas Valley/Southern Ozark region (Arkansas Basin). At the top were major centers presided over by elite functionaries, below which were regional satellite mortuary centers, and local residential sites. At present, the evidence of such a structured stettement system is weak, at best. Perishable materials preserved in dry rockshelters have shown that the domestication of starchy and oily-seeded Eastern Agricultural Complex plants (such as goosefoot and marsh elder) during the Late Archaic led to the development of horticultural economies. By Middle Woodland times, the region's occupants were making and using shell-tempered ceramics in response to the need for thin-walled, fire-resistant pottery in which dried grains -- including corn -- could be boiled. Woodland era burials also document the emergence of mortuary practices that were later elaborated at regional mound centers.